Technical Field The invention relates to an offshore platform comprising a semi-submersible frame structure having a first side and a second side as seen in the direction of a longitudinal axis of the frame structure, a rotating mooring system for mooring the frame structure to the sea-bed at an offshore deployment site, wherein the rotating mooring system allows for rotation of the frame structure around a vertical axis of the frame structure, and one or more wave absorber elements attached to the frame structure, the one or more wave absorber elements being adapted to absorb wave energy by interaction with incoming waves, said interaction resulting in a wave induced force acting on the frame structure, wherein the one or more wave absorber elements are arranged such with respect to the vertical axis that the combined wave induced forces provide an alignment force rotating the frame structure around the vertical axis so as to drive the longitudinal axis towards the direction of propagation of the waves, wherein the first side faces towards the incoming waves and the second side faces away from the incoming waves.

Background

Offshore operations are important for the exploitation of traditional energy sources, such as gas and oil fields. Also renewable energy sources, such as wind farms and/or wave energy plants are exploited in operations that are placed further and further away from the shore. However, operations at locations far away from the shore are often challenged by the fact that marine environments are particularly harsh environments, where a need for frequent maintenance and repair, or limitations to the accessibility of the offshore sites can seriously affect the operational up-time of offshore facilities. Furthermore, considerable safety issues arise when a transfer of personnel between a vessel and the offshore facility is required under conditions of heavy sea. Similar haz- ards may also occur when goods are to be transferred from or to the offshore facility. For example, loading/offloading of oil or gas from a floating storage and offloading (FSO) or floating storage, production and offloading (FSPO) facility to a transport vessel under conditions of heavy sea can be made impossible, due to the difficulties involved in docking to the facility in such situations. DK 174 463 discloses a wave energy plant comprising a partially submerged base structure comprising a hanger to which wave energy absorbers are pivotally attached. The base structure is moored to the sea bed by means of a rotating mooring system allowing the base structure to weathervane. However, while detailed instructions for on- site maintenance and repair of the hydraulic system on the deployed wave energy plant are disclosed, these procedures are not intended to be carried out under harsh conditions. In particular, no indications are given on how to access the wave energy plant under conditions of heavy sea and for example safely transfer personnel and/or goods for carrying out any emergency or repair procedure.

A platform which is stabilised to float essentially at rest with respect to an average level of the body of water, in which it is submerged, is advantageous for many offshore operations. However, this does not solve the problem of performing a safe transfer of personnel and/or goods between the stable platform and a supply/transport vessel subject to heave, roll, pitch, and/or yaw-movements of the waves.

Disclosure of the invention

An object of the present invention is therefore to provide a stable platform for offshore operations that is accessible by a supply/transport vessel and allows for safe transfer of personnel and/or goods between the platform and the vessel even under conditions of heavy seas at the site of deployment of the platform.

According to one aspect of the invention, the object is achieved by an offshore platform according to claim 1. Advantageous embodiments of the offshore platform are disclosed in the dependent claims referring to claim 1. Further advantageous embodiments for the offshore platform are recited in the description below. According to further aspects of the invention, advantageous uses of an offshore platform according to any of the disclosed embodiments are also disclosed.

According to one aspect of the invention, an offshore platform comprises a semi- submersible frame structure having a first side and a second side as seen in the direction of a longitudinal direction, docking means for transferring personnel and/or goods between a vessel and the offshore platform, wherein the docking means are arranged on the second side of the frame structure, a rotating mooring system for mooring the frame structure to the sea-bed at an off-shore deployment site, wherein the rotating mooring system allows for rotation of the frame structure around a vertical axis of the frame structure, and one or more wave absorber elements attached to the frame structure. The one or more wave absorber elements are adapted to absorb wave energy by interaction with incoming waves. The interaction results in a wave induced force acting on the frame structure i. The one or more wave absorber elements are arranged such with respect to the vertical axis that the combined wave induced forces provide an alignment force rotating the frame structure around the vertical axis so as to drive the longitudinal axis towards the direction of propagation of the waves, wherein the first side faces towards the incoming waves and the second side faecs away from the in- coming waves. The frame structure is thereby brought in a trailing position with respect to the vertical axis and the direction of propagation of the incoming waves.

The semi-submersible design ensures that the platform rests in a stabilised position in the body of water, which due to a deep draft is less affected by perturbations from waves at or close to the surface of the body of water.

The rotating mooring system comprises a mooring buoy secured to the sea-floor, e.g. by mooring chains anchored in a known manner to the sea-floor. On the mooring buoy, rotatable attachment means are provided for connecting the platform to the mooring buoy in a rotatable manner with a vertical axis of rotation, which is arranged before the platform as seen in a longitudinal direction from a fore end on the first side of the platform to an aft end on the second side of the platform. The rotating mooring system may further comprise in a known manner feed-through connections for electrical power cables, pipelines/piping, and the like. The rotating mooring system allows the platform to weather vane under the influence of the impact of wind, ocean currents and waves, i.e. following the influences of wind, waves and local currents, the platform rotates around the vertical axis to always trail the mooring buoy. While the directions of these influences often occur in a more or less similar direction, deviations may and will occur, e.g. due to local and/or transient conditions under operation at a particular offshore de- ployment site.

The wave absorber elements have a front end for receiving incoming waves and a rear end from which damped waves may be discharged, wherein the front end is oriented towards the first side of the platform, and the rear end is oriented towards the second side of the platform. The wave absorber elements are designed to strongly interact with the waves in order to absorb energy from the incoming waves on the first side of the platform and to suppress or at least reduce the waves that are transmitted to the second side of the platform. The forces resulting from the interaction of the wave absorber elements with the waves are transferred to the frame structure. Thereby, the influence of the wave component of the forces rotating the platform into a trailing alignment behind the mooring buoy is promoted to dominate over the influence of wind and/or ocean currents. The platform therefore primarily aligns with its first side pointing wave- wards, i.e. towards the direction of the incoming waves, whereas the influence of wind and ocean currents that may occur in somewhat deviating directions is reduced/suppressed. The platform thus may be said to 'wave vane' rather than to weath- er vane.

An important objective according to one aspect of the present invention, namely in an offshore operation providing a region of calm sea where a safe transfer can be performed, requires the combination of efficient means for damping of the incoming waves and a proper alignment of these damping means with the incoming waves. As dis- cussed further below, an improved wave absorber element design with an enhanced absorption efficiency serves to achieve an improved damping of the waves. In addition thereto, due to the reaction force that is transferred to the frame structure to which the wave absorbers are attached, an improved absorption efficiency of the wave absorber elements also contributes to improve the wave vaning behaviour.

Also other parts of the platform interact with the incoming waves and contribute to improved damping and alignment of the longitudinal axis of the platform with the direction of propagation of the incoming waves. As discussed further below, such parts may e.g. be sidewalls fixed to the frame structure and arranged laterally sidewards of the ab- sorber elements so as to form an absorber chamber confining and forcing the incoming waves to interact with the absorber elements. Also one or more bottom plates fixed to the framestructure and arranged in an essentially horizontal orientation underneath the wave absorber element enhance the pressure build-up against the absorber element, thereby enhancing absorption efficiency. Also e.g. pontoons, which are provided in the frame structure between adjacent wave absorbers, and which project out at the first side in a wave-ward direction catch and interact with the incoming waves, thereby contributing further to the wave-vaning behaviour.

An alignment with the direction of the incoming waves also has the beneficial effect that wave induced pitch, roll and yaw movements of the platform are even further reduced as compared to a platform with an arbitrarily oriented semi-submersible frame struc- ture. As also discussed further below, the stability of the frame structure can be further improved by means of heave plates - either in the form of one or more bottom plates arranged underneath the wave absorber elements that simultaneously work as heave plates, and/or in the form of other heave plates attached to the semisubmersible frame structure.

In combination of all features, a particularly stable platform is achieved that by means of the wave absorbers interacts with the waves such as to rotate the platform into alignment with the direction of the incoming waves and to dampen the waves, thus al- ways providing a lee-ward region of calm sea on the second side of the platform. A vessel accessing the platform through the lee-ward region from the second side therefore experiences much less movements with respect to the stable platform. Consequently, the vessel can easily dock to the docking means arranged on the second side of the platform to transfer personnel and/or goods to and from the platform. The plat- form is therefore easily accessible for all kinds of operations even under very harsh conditions and heavy seas.

The wave vaning behaviour may be achieved/improved by numerous equivalent arrangements of wave absorbers with respect to the vertical axis. In one embodiment, the wave absorber elements are arranged side-by-side in a straight alignment extending in a transverse direction perpendicular to the longitudinal axis of the frame structure, wherein the vertical axis is located in front of the frame structure before the first side as seen in a longitudinal direction from the first side towards the second side. In this embodiment, the frame structure may be considered in perfect alignment with in- coming waves when the transverse direction of the framestructure is perpendicular to the direction of propagation of the waves in a trailing position behind the vertical axis, wherein the first side faces towards the incoming waves. Alternatively, according to further embodiments, the wave absorbers may also be placed in a staggered arrangement such that the first side aligns at a slant angle with respect to the direction of prop- agation of the incoming waves. The staggered arrangement may be configured as a V- shape when seen from the top, wherein the first side of the frame structure and the tip of the V points towards the incoming waves when the frame structure is aligned with the waves. It is noted that the vertical axis of the rotating mooring system around which the platform turns for alignment with the waves does not necessarily have to be placed exterior of the frame structure, but may also be provided as a turret inetgrated with the frame structure between the first and second side, or even as a turret placed behind the second side. The requirement for placing the vertical axis with respect ti the frame structure is that a trailing behaviour has to be achieved, where the frame structure under the influence of the incoming waves acting on the wave absorber arrangement moves to a stable equilibrium position with respect to the vertical axis, wherein the first side faces towards the incoming waves and the second side faces away from the incoming waves.

Further according to one embodiment of an offshore platform, the one or more wave absorber elements are arranged symmetrically with respect to a mirror plane compris- ing the vertical axis. Under operation, the interaction of the symmetrically arranged wave absorbers with the incoming waves provides a symmetric force balance acting on the platform to align it with the wave direction. Due to the symmetry, the balance of the forces and thus the alignment of the platform with the waves is achieved more easily for a broader variety of sea states. Advantageously, the wave absorbers are arranged essentially symmetrically with respect to the mirror plane comprising the vertical axis, and in the transverse direction aligned with respect to each other in a linear formation.

Further according to one embodiment of an offshore platform, the one or more wave absorber elements are of the front pivot type, a front pivot axis being arranged at the front end of the wave absorber element, wherein the frame structure pivotally supports the one or more absorber elements from the front pivot axis such that the front end of the wave absorber element is oriented towards the first side of the frame structure, and a rear end of the wave absorber element is oriented towards the second side of the frame structure, wherein in an idle position the front pivot axis is located at a predeter- mined front pivot axis height Fa above still water, and the absorber element is partially submerged with an idle draught Fd. Wave absorber elements of the front pivot type are particularly efficient in a floating platform. An efficient absorption of wave energy provides a good damping of the waves to provide a region of calm sea aft of the platform. An increased interaction with the waves also enhances the wave-vaning behaviour of the platform over potentially competing influences such as wind and/or ocean currents.

Further according to one embodiment of an offshore platform, the absorber element at the front side comprises a concave portion as seen in a direction from the front end to the rear end. In a yet further embodiment, the concave portion at the front side of the absorber element comprises at least two concave sub-portions separated by a ridge. A concave portion on the front side further improves the absorption efficiency and thus the ability of the absorber elements to dampen the incoming waves and, due to the enhanced interaction, improves the alignment with the direction of propagation of the waves (wave vaning). Advantageously, the front pivot axis of the one or more wave energy absorber elements of the front pivot type are oriented essentially perpendicular to the longitudinal direction of the platform. Advantageously, the front pivot absorber element is defined by features as further detailed below. Advantageously, the front pivot absorber element is configured according to the method further detailed below. Further advantageously, the wave absorber element is operated in an absorber unit as further detailed below, the absorber unit being included in the frame structure. If appropriately shaped and dimensioned/configured according to the wave climate of the site of deployment as further detailed below, the use of front pivot wave absorber elements may reduce the energy of the transmitted waves as compared to the incoming waves by 70% or even more.

Further according to one embodiment of an offshore platform, the frame structure further comprises at least one heave plate arranged below the one or more wave absorber elements and preferably further heave plates arranged lee-wards of the wave ab- sorber elements. The heave plates improve stability of the semi-submersible frame structure against influences of the waves and counteracts heave due to forces transferred to the frame structure as a consequence of the interactions of the wave absorber elements with the waves. By improving the stability of the frame structure with respect to the body of water two effects are achieved.

Firstly, the platform itself is further stabilized minimizing or at least reducing movements induced by interaction with the waves even under heavy sea conditions. This further improves the usability of the platform as a foundation or base for any offshore operations, such as those mentioned above, in particular in heavy sea conditions. Sec- ondly, by providing a further stabilized reference frame, the absorption efficiency of wave absorber elements moving with respect to the frame structure of the platform is increased, thereby improving the wave breaking effect of the wave energy absorbers.

Advantageously according to further aspects of the invention, the offshore platform may be used as foundation/base for one or more wind turbines, floating storage and offloading (FSO) or floating production storage and offloading (FPSO) units for oil field and gas field exploitation, offshore fish farms, offshore wave energy harnessing or any combination thereof. The high accessibility of the platform facilitates the operation as well as any maintenance and repair, and thereby considerably increases the up-time and reduces cost of such offshore operations. The high accessibility of the platform fa- cilitates the operation as well as any maintenance and repair, and thereby considerably increases the up-time and reduces cost of any of the recited offshore operations.

According to one aspect, an offshore platform according to any of the above-mentioned embodiments is used in a wave energy plant.

According to a further aspect, an offshore platform according to any of the above- mentioned embodiments is used as floating foundation for one or more wind turbines.

According to a further aspect, an offshore platform according to any of the above- mentioned embodiments is used as floating storage and offloading (FSO) or floating production storage and offloading (FPSO) unit.

According to a further aspect, an offshore platform according to any of the above- mentioned embodiments is used as floating base for an offshore fish farm.

As mentioned above, particularly advantageous embodiments of the offshore platform comprise wave absorber elements that are appropriately shaped and dimensioned/configured according to the wave climate of the site of deployment as further detailed below, so as to improve the wave energy absorption efficiency, and thus the efficiency for damping the waves that are transmitted to the second side of the platform. An enhanced absorption efficiency also means an enhanced interaction with the waves, which is transferred to the frame structure and transformed into a combined alignment force, thereby improving the wave-vaning behaviour of the platform. Additional contributions to the wave vaning behaviour can also be derived from the interac- tion of the frame structure and parts fixed to the farme structure with the incoming waves.

According to one aspect, a wave absorber element of the front pivot type for the absorption of wave energy from a body of water is provided. The absorber element allows for the efficient absorption of wave energy in irregular waves and under varying wave conditions. In a further aspect, a method of providing a wave absorber element of the front pivot type for the absorption of wave energy from a body of water is disclosed, wherein the method comprises configuring the absorber element for operation in a wave climate at a given deployment site. A wave absorber element of the front pivot type for the absorption of wave energy from a body of water has a front end comprising a front pivot axis around which the absorber element swings under operation, and an absorber element body being substantially defined by a front side extending from the front end to a lower rear edge, a rear side extending from an upper rear edge to the lower rear edge, a top side extending from the front end to the upper rear edge, and lateral sidewalls defining the width of the absorber body in an axial direction parallel to the front pivot axis, wherein the lower rear edge is located in a first radial direction at a first distance from the pivot axis, and the upper rear edge is located in a second radial direction at a second distance from the pivot axis, wherein the first and second radial directions define an acute tip angle of the ab- sorber element, wherein the first distance defines the absorber element length, and the distance of the upper rear edge from the first radial direction defines the absorber element height, the absorber element having a cross-sectional profile as seen in a cut- plane perpendicular to the front pivot axis, wherein said absorber element profile encloses a profile of the absorber element body and the front pivot axis, wherein the ab- sorber element profile at the front side comprises a concave portion as seen in a direction from the front end to the rear end.

The term "vertical" refers to a direction parallel to gravity and the term "horizontal" refers to directions perpendicular thereto. The terms "top" and "bottom" of the absorber are defined with respect to the position of the absorber when in use or at least when deployed in a body of water, wherein an "upward" direction from the bottom towards the top points out of the water and a "downward" direction points from the top towards the bottom into the water. The terms "front" and "rear" of the absorber are defined with respect to the position of the absorber when in use, wherein the direction of wave propagation is from the front end pointing towards the incoming waves to the rear end pointing away from the incoming waves. The term "front" refers to the end of the absorber element, which, under operation, points towards the direction from which the waves are coming, whereas the absorber element body floats "downstream" of the pivot axis with regard to the direction of wave propagation. An absorber element of the front pivot type is thus an element, which is configured to be pivotally supported for rotary motion around a pivot axis arranged at the front end, i.e. "upstream" with respect to the direction of propagation of the waves, the actual body of the absorber element being arranged aft of the pivot axis, i.e. the rear end of the absorber element is arranged "downstream" with respect to the direction of propagation of the waves. The front pivot axis is arranged in an essentially horizontal direction allowing the front pivot type ab- sorber element to reciprocate up and down in a rotary motion of the absorber element body around its front pivot axis, thereby absorbing kinetic and potential energy from the waves driving the motion. The reciprocating motion of the absorber element body is performed aft of the front pivot axis with respect to the propagation direction of the waves.

The absorber element is operated in an absorber unit comprising a frame pivotally supporting the absorber element from the front pivot axis at a given pivot axis height above the body of water. The front pivot axis is on the one hand chosen sufficiently close to the water surface to allow for an efficient interaction of the absorber element with the incoming waves, and on the other hand sufficiently high above the water surface to avoid loss of energy due to submersion of the top side and/or due to wave impact on the absorber unit thus interacting with the frame instead of transferring the energy to the absorber element. Continuing direct wave impact on the absorber unit structure and the front pivot axis may cause excessive wear or even damage the absorber unit. The absorber unit's frame may be part of a floating platform that is moored to the sea floor or may be immobilised by means of a foundation fixed to the sea floor. Preferably, the absorber unit is rotatable about a vertical axis, e.g. by means of a rotatable mooring system, so as to be able to align the absorber unit with the prevalent direction of propagation of the incoming waves. When used in a wave energy plant, the recipro- eating motion of the absorber element is harnessed by a power take-off system comprising conversion means for converting the harnessed energy into a desired form of useful energy, such as an electrical generator.

An idle position of the absorber may be defined with respect to the body of water under still water conditions, wherein an average level of the surface of the body of water defines a still water reference. Distances from the still water reference in a direction normal to the surface and out of the body of water may be referred to as height above still water, whereas distances from the still water reference in a direction normal to the surface and into the body of water may be referred to as depth below still water. In its idle position, the absorber element is configured to be suspended from the front axis at a predetermined axis height above still water, and is partially submerged. The front pivot axis height is thus the height of the front pivot axis above still water. An idle draught of the absorber element is defined by the depth below still water of the absorber element's submerged lower rear edge. Typically, a principal portion of the absorber element is under water with only a minor top portion of the absorber element sticking out of the water.

The front side surface faces towards the incoming waves thus forming the pressure side of the absorber element. The rear surface faces away from the incoming waves thus forming the wake side of the absorber element. The front side forming the pres- sure surface for interaction with the incoming waves extends from the front end to the bottom of the rear end of the absorber element. The front side length is the distance of the lower rear edge from the front pivot axis, i.e. equal to the first distance. The front side inclination under idle conditions is the angle of the first radial direction with respect to the still water level. An absorber element operating angle may be defined as the an- gle of the first radial direction with respect to the still water level. An absorber element top side angle may be defined as the angle of the second radial direction with respect to the still water level and is equal to the difference between front side inclination and the tip angle of the absorber element. The length of the absorber element is measured from the front end to the rear end in a direction parallel to the first direction, perpendicular to the pivot axis. The height is measured from top to bottom in a direction perpendicular to the first direction and in a plane perpendicular to the pivot axis. The width of the absorber refers to the overall width of the absorber element body as measured in an axial direction, i.e. parallel to the pivot axis. A cross-section taken in a plane perpendicular to the pivot axis may be referred to as the profile of the absorber element.

Providing the front side of the profile of the absorber element with a concave portion improves capture of and interaction with the waves rolling in towards the absorber element, thereby increasing the efficiency of absorption.

As mentioned above, a principal portion of the absorber element's body is typically submerged so as to interact not only with the waves at the surface, but also to capture energy from the wave induced recirculating motion of the water particles deep below the surface. Typically, the absorber is submerged up to a level close to the top. The top side of the absorber element typically projects out of the water. Also during operation, it is preferably avoided that the top side is submerged - apart from spill over that may commonly occur in high waves or under conditions of extreme swell or storm.

The submerged portion of the absorber element yields buoyancy to the absorber ele- ment. The buoyancy provides a lifting force in an upward direction, which in the rising phase of a wave lifts the absorber element to swing around the front pivot axis in an upward direction. Furthermore, the submerged portion of the front side provides a pressure surface of the absorber element extending from the pivot axis downward into the water where it interacts with the incident wave to absorb kinetic energy of the wave. The rising wave thus accelerates the absorber element in an upward direction to an upper turning point, thereby absorbing both potential energy and kinetic energy from the wave. As the wave falls again, the absorber element recovers from the upper turning point to a lower turning point mainly under the influence of gravity, assisted by further downward acting forces, such as adhesion of the outer surfaces of the absorber element to the retracting body of water. Driven by a subsequent wave, the absorber element rises again from the lower turning point to the upper turning point. Thereby, the incident wave field transfers a substantial portion of its energy to the absorber by driving the reciprocating motion of the absorber element with respect to the absorber unit's frame. This energy may be harnessed by means of a power take-off means driving electrical generator means for converting the absorber motion into useful electrical energy. The power take-off means may e.g. be a hydraulic system comprising pumps mounted between the absorber element and the absorber unit frame, wherein the pumps are used to generate a pressure for driving a hydraulic turbine. Alternatively, the power take-off system may be a direct energy conversion system, wherein the me- chanical motion of the absorber element is converted mechanically and linked to directly drive the input shaft of an electrical generator. Alternatively or in addition to converting the absorber element motion into useful energy, such as electrical energy, the absorber may also be used in a break water system. By absorbing a substantial portion of the energy of incoming waves over a broad spectral range, the absorber element acts as efficient break water for calming/protecting the waters located aft of the absorber.

As further detailed below, preferably the absorber element is configured according to the wave climate prevalent at a given deployment site at which the absorber element is to be operated. Further, according to one embodiment of the wave absorber element, a front side portion of the convex envelope to the absorber element profile is a straight line extending from the front end to the lower rear edge. The convex envelope to a shape may be visualized as the contour defined by a rubber band stretched around the shape. A con- cave portion of the contour of the shape means a portion bulging away from the convex envelope in an inward direction. In the region of the concave portion, the convex envelope to the shape is a straight line. A profile of the absorber element includes both the front pivot axis and the profile to the body of the absorber element. Providing an absorber element that has a convex envelope with a straight line extending from the front end to the lower rear edge means the front side bulges inwardly and away from the first direction to form a generally concave pressure surface for capturing the incoming waves. By this configuration, an efficient absorption is achieved.

Advantageously, the absorber element profile is at the front side provided with a sub- stantial concave portion, i.e. a major portion of the front side portion of the absorber element profile deviates from the straight line bulging inwardly with respect to the convex envelope to the absorber element profile. Thereby, the cross-sectional area of the absorber element enclosed by the absorber element profile is reduced as compared to the area enclosed by the convex envelope to the absorber element profile. Advanta- geously, the difference in area is at least 10%, alternatively at least 20%, alternatively at least 30%, said difference being attributed to the front side of the absorber element profile deviating from the straight line extending from the front end to the lower rear edge. By the concavely shaped front side surface absorption efficiency is increased as compared to a planar front side surface that essentially follows a straight line from the front end to the rear end. Furthermore, by using a concavely shaped front side, buoyancy of the absorber element may be distributed in such a way that the concave front side reaches deeper into the water than for an absorber element with the same giv- en/desired buoyancy and different shape, such as an absorber element with a flat front side or an absorber element that is configured as a wave follower essentially riding on top of the waves, i.e. following the wave induced movement of the uppermost portion of the body of water. By reaching deeper into the water than other shapes, a larger portion of the energy carried by the waves may be absorbed/harvested. Further, according to one embodiment of the wave absorber element, the concave portion at the front side of the profile comprises at least two adjacent concave sub-portions separated by a ridge. By structuring a concave front side surface into sub-portions, the efficiency can be further enhanced. Furthermore, the front side may be further shape- optimized for interaction with different wave conditions, such as very deep waters at off-shore deployment sites or more shallow waters close to a shore or bank.

Advantageously, according to one embodiment, the concave sub-portions are arranged in a radial direction in extension of each other, wherein a first, proximal sub-portion is located close to the pivot axis and one or more further sub-portions are placed distally thereto in a direction towards the rear end.

Further, according to one embodiment of the wave absorber element, one or more, preferably all, of the concave sub-portions follow essentially circular arcs.

Further, according to one embodiment of the wave absorber element, the rear side portion of the profile follows a circular arc around the front pivot axis. The circular rear end of the profile minimizes wave generation aft of the absorber when the absorber element moves up and down in the body of water. Thereby loss due to wake generation is avoided or at least reduced.

Advantageously, the dimensions of the absorber element may be selected from the following ranges of dimensions in order to provide efficient absorbers for a large number of potential deployment sites. Upper limits for length and height are typically given in view of the complexity and the elevated construction cost associated with excessively large absorber elements and the frame structures required for supporting and operating such large absorber elements. Lower limits for length and height are typically given with regard to a minimum size and frequency of waves in order to be relevant for exploitation.

Further, according to one embodiment of the wave absorber element, the absorber element length is in the range from 5-50 m, alternatively in the range from 10-40 m.

Further, according to one embodiment of the wave absorber element, the minimum absorber element height is 2 m, alternatively 3 m, or alternatively 4 m, and wherein the maximum absorber element height is 30 m, alternatively 20 m, or alternatively 10 m. Further, according to one embodiment of the wave absorber element, the tip angle is between 10-70 degrees, alternatively between 20-60 degrees, preferably between 25- 50 degrees. An advantageous value for the tip angle, and accordingly for an associated length to height ratio, is preferably determined according to the wave conditions under which the absorber element predominantly is operated. Long period shallow waves, e.g. in shallow waters, may require a very small tip angle, whereas high amplitude waves coming in with a high frequency may require a relatively short wave absorber with a large tip angle.

Advantageously, according to one embodiment, the absorber element has a tip angle alpha of about 30 degrees, and a length to height ratio of about 2.

Further, according to one embodiment of the wave absorber element, the absorber el- ement comprises one or more interior hollow spaces for ballasting the absorber element. Providing interior hollow spaces for ballasting the absorber element allows for trimming the draught of the absorber element at rest by controlling its total weight. Different configurations can be conceived. By placing the same ballast in a different ballast tank located at a larger distance from the front pivot axis, the draught may be in- creased. Furthermore, subdividing the interior hollow spaces into a plurality of separate ballasting tanks arranged adjacent to each other in a radial direction also allows for adjusting the moment of inertia of the absorber element, i.e. controlling the absorber element's inertia for rotational motion around the front pivot axis, e.g. for a given total weight.

According to a further aspect, a method of providing a wave absorber element according to any of the above-mentioned embodiments comprises configuring the absorber element for operation in a wave climate at a given deployment site, the configuration comprising the steps of

- obtaining statistical wave data describing the wave climate of the deployment site,

- deriving from the statistical wave data one or more characteristic parameters that are representative of the wave climate, the characteristic parameters comprising a characteristic wave height H and/or a characteristic wave period T, and

- dimensioning the absorber element according to the one or more characteristic parameters, so as to optimise productivity for conversion of available wave energy to use- ful energy when operating the absorber element in the wave climate of the given deployment site, wherein an idle draught Fd of the absorber element is dimensioned to be larger than the characteristic wave height, said idle draught being defined under still water conditions as the depth of submersion of the lower rear edge below still water level when pivotally suspending the absorber element from the front axis at a predetermined axis height Fa above said still water level, and/or the length Fl of the absorber element is chosen to be smaller than a characteristic wavelength corresponding to the characteristic wave period. The irregular nature of waves in realistic applications, such as those mentioned above, requires a high level of efficiency over a broad range of sea-states. In order to fulfil these requirements, the absorber element is configured according to the wave climate prevalent at a given deployment site. Advantageously, the geometry of the absorber element is therefore configured in terms of characteristic values representative of that wave climate. The wave climate may be derived from wave data of that deployment site, such as statistical data on the wave height, wavelength and wave directions.

A given sea-state may be described as a superposition of wave components, wherein a sea-state of irregular waves with an arbitrary directional spread may be described in terms of the occurrence of regular wave components throughout a given observation period. The distribution of the energy content over these regular wave components may be summarised in an irregular wave spectrum describing the sea-state of irregular waves. Throughout the present application, the term "wave spectrum" refers to a distribution representing a sea-state of irregular waves. The spectral distribution of the wave components in the (irregular) wave spectrum may be characterised by key figures, such as the wave energy period Te, a peak wave period Tp, a mean zero-crossing period Tz, and/or a significant wave height Hs. Te, Tz, and Hs may be defined in terms of the frequency moments mn of the wave spectrum:

with The peak wave period Tp may be defined as the wave period where the wave spectrum exhibits a maximum. A wave period may be associated with a corresponding wavelength being the length, which the wave travels within the wave period. The wave- length λ is approximately

2 π ' wherein g is the gravitational constant, and T is the appropriate wave period. Wave data may be gathered from several sources, which are often combined. Examples for wave data sources comprise: Direct measurements, Acoustic Doppler Current Profiling (ADCP), buoys, etc.; Direct Measures from land, radar, etc.; Conversion of wind data to site wave data; Global and/or local wave and weather models. If necessary, these data are adjusted to the specific site characteristics. For example, if the da- ta are not measured precisely at the site they must be adjusted to the site in regards to water depths and other site specific conditions. Thereby, a pool of site specific wave climate data is created and may be archived.

The statistical distribution of the sea states year round may be analysed in terms of these key figures to provide a scatter chart of how often sea-states falling within intervals of the key figures, occur within the scope of the wave-climate to be established (monthly, seasonal, annual, or any other period of interest, such as the life time of the absorber). The intervals are sliced to cover the full range of observed sea-states with an appropriate resolution. For example, a commonly used bin size for intervals of the significant wave height Hs is 0.5 metres, and for intervals of the wave energy period Te is 1 second. The occurrence of sea-states and the wave components comprised in these sea-states may be normalised/binned to a pre-determined observation period serving as a time base for the statistical description of the wave climate. A commonly used time base is hourly.

From the wave climate description, characteristic parameters may be derived, for example a prevalent wave height, a prevalent wave period, and/or a prevalent directional spread. The characteristic parameters may relate to a maximum of the distribution with respect to the underlying parameter. Alternatively, the characteristic parameters may be derived from a combination of moments that may be calculated from the distribution, or otherwise derived according to a theoretical model.

The above-mentioned statistical distribution of the sea-states may be scaled with the energy content in the respective intervals of sea-states to obtain a distribution of the available energy over the different sea-states of the wave climate. Using the absorber element, the available energy may be converted into useful energy, wherein the conversion efficiency of a given absorber element depends on the incoming wave. Productivity may be defined as the ratio of useful energy divided by available energy. For a wave energy conversion system producing electrical energy to a grid, the productivity may thus be defined as the energy output delivered to the grid as compared to the available energy. Alternatively, e.g. for the purpose of optimising the absorber element configuration, the productivity may be defined as the energy uptake of the absorber element as compared to the available energy.

Starting with a given absorber element profile, preferably the following dimensions of the absorber element may be specified in order to configure the absorber element for operation in a wave climate of a given deployment site: length, height, pivot axis height, idle draught, and width. Specifying a length and a height implies a certain tip angle. Specifying a tip-angle implies a certain length to height ratio. One of the merits of the present invention is to realize that a wave absorber element of the front pivot type to a large extend can be optimised for operation at a given deployment site merely by configuring the absorber element length and/or the absorber element idle draught according to the deployment site's prevalent wavelength and/or wave height, respectively.

The idle draught may be varied e.g by changing the shape/dimensions of the submerged portion so as to re-distribute the buoyancy providing volume in order to achieve a different draught of the absorber element. For an absorber element of a given shape/dimensioning, the idle draught may be varied by adjusting a ballast carried by the absorber element. A given absorber element is configured for operation in an absorber unit. The configuration comprises determining a value for the front pivot axis height Fa and the idle draught Fd. The front pivot axis height Fa and the idle draught Fd determine in combination the angle of operation of the absorber element as e.g. expressed by the front side inclination angle with respect to the water surface, in the idle position and under still water conditions. Typically, the front pivot axis height Fa corre- sponds to about 5%-30% of the sum of the axis height Fa and the idle draught Fd, (Fa + Fd), and preferably Fa is about 10% of (Fa + Fd).

It is noted that the skilled person knows that draught will be dependent on a number of parameters, such as salinity and temperature of the surrounding body of water. The draught may therefore preferably be determined for a set of standardised conditions, such as in freshwater at a temperature of 20 degrees Celsius, or alternatively using the salinity and/or average/prevalent temperature at the given deployment site. The skilled person also knows how to convert a draught value determined for a given set of stand- ardized conditions into a draught value for different conditions.

It is also noted, that the absorber element length and/or the idle draught may be expressed in terms of equivalent dimensioning parameters that for a given absorber element by means of a well-defined relation may be converted into the absorber length and/or the idle draught, respectively. For example, an active length may be defined as the length of the waterline of the absorber element in the idle position. An active height of the absorber element may be defined as a depth of interaction between the absorber element and the body of water, and may be measured as the draught of the absorber element, wherein the idle draught is a special case selected, because it is a convenient parameter for configuration purposes. In combination with a given front side length and front pivot axis height, the idle draught also determines the front side inclination with respect to the body of water. Varying the front side inclination for a given profile affects the interaction between the incoming waves and the pressure surface formed by the front side, thereby affecting the conversion efficiency of the absorber element.

Optimising the idle draught may be performed by deriving from the statistical wave data a characteristic wave height that is representative of the wave climate, and configuring the absorber element idle draught according to the characteristic wave height, so as to maximise the production of useful energy when operating the absorber element in a wave energy conversion system at the given deployment site.

Accordingly, optimising the absorber length may be performed by deriving from the statistical wave data a characteristic wave period that is representative of the wave climate, determining the corresponding characteristic wavelength, and dimensioning the absorber element length according to the characteristic wavelength, so as to maximise the production of useful energy when operating the absorber element in a wave energy conversion system at the given deployment site.

An iterative optimization of the absorber element configuration for a given deployment site may comprise the steps of

(a) deriving the energy content distribution for a representative wave climate description of the deployment site in terms of wave height and/or wave period (or corresponding wavelength),

(b) for a given absorber element with a specified idle draught and/or length, deriving a conversion efficiency distribution in terms of wave height and/or wave period (or corresponding wavelength),

(c) combining the energy content distribution and the conversion efficiency distribution to obtain a productivity distribution for the given absorber element in the given wave climate (if applicable when operated in a given wave energy conversion system) (d) varying the idle draught and/or length keeping remaining parameters for defining the operational absorber element constant, and

(e) repeating the steps (b) through (d) until an optimisation criterion is satisfied.

Multiplying the conversion efficiency distribution of the obtained absorber element with the distribution of the available energy content of the wave climate at the deployment site gives the potential energy production/output power that may be achieved with that absorber element.

An optimisation criterion may be directed to maximising energy production during the period of interest at the given deployment site. An optimisation criterion may also comprise further factors, maximizing, minimizing and/or balancing these factors together with a mere maximisation of energy production. Further factors may comprise construction costs for a system using the absorber element, service friendliness, life-cycle analysis, environmental considerations, etc.

Other parameters defining the absorber element, such as profile shape, width, pivot axis height, and the like, are kept constant for the purpose of optimising the length and draught of the absorber element. However, also any of these parameters may be optimised using the above routine, wherein instead in step (d) the parameters to be opti- mized are varied keeping remaining parameters constant. Advantageously, the width of the absorber element is dimensioned according to a dimensionality/directional spread of the waves: the larger the prevalent directional spread, i.e. the directional spread of the wave components comprised in the prevalent sea-states of the wave climate, the narrower the absorber element may be dimen- sioned. A minimum width of the absorber element to be at least one or a few metres wide may be determined according to practical considerations. On the other hand, if the incoming waves most of the times are predominantly unidirectional, i.e. the wave components comprised in the prevalent sea-states have a narrow directional spread, then the absorber element may be made wider. In the case of low directional spread, but where the direction of the incoming waves varies a lot between different sea-states, the wave energy system is advantageously aligned with the prevalent wave direction of the different sea-states by means of a rotating mooring system.

Advantageously, configuring the absorber element comprises determining a total mass according to a desired net lift force, wherein the net lift force is the difference between the gravitational force acting on the absorber element in a downward direction and the buoyancy force acting in an upward direction. Adjusting the total mass of an absorber element with a given geometry allows adjusting the absorber element operation angle, and thereby the front side inclination angle. Varying the absorber element operation angle / front side inclination influences the conversion efficiency of the absorber element under operation. By adjusting the mass in a configuration phase, the absorber element operation angle / front side inclination may be adapted, and if desired optimised, for the general wave conditions of the wave climate of a given deployment site. The mass of the absorber element may also be adjusted during operation by ballasting means in order to dynamically optimise the absorber element conversion efficiency under varying wave conditions.

Further, according to one embodiment of a method of providing a wave absorber element, the statistical data is an energy content distribution over intervals of wave heights H, preferably significant wave heights Hs, and/or intervals of wave periods, preferably wave energy periods Te, for sea-states at the deployment site.

Further, according to one embodiment of a method of providing a wave absorber element, the one or more characteristic parameters representing the wave climate are as- sociated with a maximum in energy content. When describing the wave energy content distribution in terms of the wave periods and/or the wave heights, the characteristic wave period and the characteristic wave height are the wave period and/or wave height values, where the energy content distribution is a maximum. Selecting the peak positions in the energy content distribution as the characteristic parameters is a simple way of estimating the region of the distribution where most of the available energy is cen- tred.

Further, according to one embodiment of a method of providing a wave absorber element, dimensioning/optimisation is performed on the basis of statistical data in a production window selected from the statistical data representing the wave climate.

The production window is a subset of the statistical data describing the wave climate. Advantageously, the optimisation may be performed within a production window of wave heights and/or wave periods selected from the range of wave heights and/or wave periods covered by the complete wave climate data, thereby disregarding the most improbable/extreme sea-states for the purpose of configuring the absorber element.

Further advantageously, the production window may be selected taking into account energy production cost. The production window may be selected balancing the desire of covering as much of the available energy against the difficulty of designing an efficient absorber element that is responsive over the full range of sea-states contained in the production window: if the same energy content can be achieved with a narrower production window, such a narrower production window may be preferable. Alternatively or in addition thereto, the production window may be selected by balancing energy content against construction cost for the absorber element.

Advantageously, the production window is the smallest possible group of waves that constitute between 85% and 95% of the available wave energy. A requirement of an energy content of about 85%-95% of the total available energy is found to provide a good basis for obtaining an optimised productivity at a reasonable energy production cost.

Further, according to one embodiment of a method of providing a wave absorber element, the idle draught Fd is the characteristic wave height times a height scaling factor D in the range 2-5, preferably in the range 2.2-4, more preferably between 2.5-3.5, or about 2.9. By reaching deep into the body of water, a high fraction of the energy carried by the wave can be exploited, thereby increasing the conversion efficiency of the absorber element. Preferably, the characteristic wave height is the significant wave height, where the energy content of the wave climate is a maximum. Using an idle draught which is scaled with a factor selected from the above sequence of ranges, in- creasingly improved conversion efficiency is achieved.

Further, according to one embodiment of a method of providing a wave absorber element, the absorber element length Fl is the characteristic wavelength times a length scaling factor L in the range 0.1-0.4, alternatively in the range 0.1 1-0.25, alternatively in the range 0.12-0.20, or about 0.15. Preferably, the characteristic wavelength is the wavelength associated with the peak wave period where the energy content of the wave climate is a maximum. Using an absorber element length which is scaled with a factor selected from the above sequence of ranges, increasingly improved conversion efficiency is achieved.

Further, according to one embodiment of a method of providing a wave absorber element, the length of the absorber element and/or the idle draught Fd of the absorber element is reduced so as to optimise productivity for conversion of available wave energy to useful energy with respect to cost.

Reducing the dimension allows reducing cost for construction and operation of the absorber element. A reduction in size of the absorber element also entails a reduction in size, and thereby cost, of a wave energy conversion system comprising the absorber element. Dimensions may thus be reduced so as to balance absorber element con- struction cost against energy production efficiency, thereby minimising the overall cost for the production of useful energy. In particular if the maximum in productivity with respect to the length and/or height is relatively flat, a decrease in construction cost may be bought at the expense of a relatively small decrease in productivity, thereby reducing the overall cost of energy production.

Further, according to one embodiment of a method of providing a wave absorber element, the configuration further comprises the step of

- dimensioning a height Fh of the absorber element to be larger than the idle draught Fd such that the upper rear edge in the idle position of the absorber element is above still water level. Preferably, the height of the absorber element body height is configured such, that the top side / upper rear edge of the absorber does not submerge during operation. Thereby, counterproductive resistance to the motion of the absorber element as well as un- desired wave generation in the wake of the absorber element is avoided.

Based on a fluid static analysis, the absorber element body height may be configured such that the upper rear edge in the idle position of the absorber element is above still water level, i.e. such that an upper portion of the rear edge projects out of the water. Typically, the absorber element height is chosen to be approximately the sum of pivot axis height and idle draught.

Further according to one embodiment of a method of providing a wave absorber element the configuration further comprises the steps of

- performing a motion analysis on the absorber element under the influence of irregular wave fields representative of the wave climate, and

- determining a height of the absorber element Fh so as to prevent submerging of the upper rear edge during operation of the absorber element in the wave climate.

As mentioned above, the height of the absorber element body height is preferably con- figured such, that the top side / upper rear edge of the absorber does not submerge during operation in order to avoid losses.

Alternatively or in combination with the above-mentioned fluid static analysis, an absorber element height may be configured based on a motion analysis. In this approach, the absorber element height may be determined iteratively starting with a motion analysis for a given absorber element profile in a representative wave field, preferably taking operational loads and/or load variations into account. The height of the starting profile may e.g. be the fluid statically determined absorber element height. A motion analysis may include observations on full scale absorber elements, motion data from model experiments and/or computer simulations, such as CFD-based calculations. The motion analysis may use statistical wave distribution data representative of the wave climate at a given deployment site as input to an experimental and/or computational analysis. In case the motion analysis determines a critical submersion that may affect wave energy absorption efficiency, the height of the absorber element is increased ac- cordingly. The iterative process may be repeated for the thus obtained absorber element profile until satisfactory absorption performance is verified. An excessive height of the absorber element profile is preferably avoided in order to reduce construction/installation cost, and to avoid undesired interaction of the absorber element with wind.

Advantageously, an absorber unit for the absorption of wave energy from a body of water comprises an absorber element according to any of the above-mentioned embodiments, the absorber unit further comprising a frame structure suspending the absorber element from the front axis at a predetermined axis height Fa above still water, wherein the frame structure is configured to essentially stay at rest with respect to the average level of the body of water. To a good approximation, the reference of the still water level may be mapped to a level on the frame structure of the absorber unit, which frame structure level may thus be taken as an equivalent reference for the configuration and/or operation of the absorber element.

The axis height is sustained by a frame structure, which is essentially immobile with respect to the body of water, essentially unaffected by the incoming waves. The axis height may be determined to reduce wave impact yet improving absorption efficiency, wherein advantageous values for the axis height are in the range about 5%-30% of the sum of the axis height Fa and the idle draught Fd, (Fa + Fd), as mentioned above.

Wave energy is exploited by power take-off means harnessing the motion of the absorber element with respect to the frame structure. Typically, the frame structure is part of an offshore floating platform, which is moored using a rotating mooring system allow- ing the absorption unit to be oriented such that the front end of the absorber element faces towards the incoming waves. Furthermore, the floating platform is typically configured and dimensioned so as to rest in the body of water when assuming a given wave climate. To that end, active and passive stabilizing means may be provided on the platform. A plurality of absorber units, each comprising a front pivot absorber ele- ment, may be combined in the same platform.

Alternatively, in particular for locations close to a shore with low sea depth, the frame structure may also be supported from a foundation fixed to the sea floor. Furthermore, a combination of floating modules anchored between fixed foundations may be con- ceived. Advantageously, an absorber element/unit according to any of the above mentioned embodiments may be used for driving a hydraulic power take-off system and/or means for direct energy conversion in a wave energy plant for converting wave energy into electrical energy.

Advantageously, an absorber element/unit according to any of the above mentioned embodiments may be used as an open wave breaker. The absorption element/unit according to the invention may have a surprisingly high absorption efficiency of up to 70% or even more. The energy contained by the outgoing waves aft of the absorber ele- ment/unit may thus be effectively reduced as compared to incoming waves. At the same time, such a wave breaker system is open for fluid communication and the exchange of marine life, while on the leeward side providing coastal protection, protecting marine structures/installations, such as wind farms or fish farms, protecting spawning areas, or the like. Thereby, the environmental impact of the wave breaker is minimised while providing an efficient protection against waves. Further advantageously, a plurality of absorber elements/units is arranged in parallel and next to each other along a protection line. Furthermore, a plurality of absorber elements/units may be cascaded in series in order to increase the total wave absorption and improve the protection. In a cascaded arrangement, the downstream absorber elements should be dimensioned small- er than the upstream absorber elements, in order to account for the reduced size of the waves aft of the upstream absorber elements/units. Further advantageously, the absorber elements/units of the open wave breaker are used for driving power take-off means for producing useful energy from the motion of the absorber elements with respect to the frame structure supporting them.

Furthermore, as mentioned above, according to advantageous embodiments of the offshore platform the one or more wave absorber elements are each operated in an absorber unit as further detailed below. Preferably, the frame structure of the absorber unit is fixed to or integrally included in the frame structure of the offshore-platform. However, in one embodiment, the frame structure of the absorber unit is configured as a detachable module that is detachably fixed to the frame structure of the offshore platform. Such an embodiment allows for easy replacement of the absorber unit and is advantageous for service, repair and maintenance operations. As mentioned above, means for enhancing the absorption efficiency promote the objective of the invention by improve the efficiency for damping waves and improving the wave vaning behaviour over competing influences such as wind or ocean currents that are not aligned with the direction of the waves.

According to one aspect, an absorber unit for the absorption of wave energy from a body of water is provided. The absorber unit allows for the efficient absorption of wave energy under varying wave conditions. According to a further aspect, a wave absorber unit for use in a wave energy plant is provided.

According to one embodiment of the absorber unit, the absorber unit comprises an ab- sorber element of the front pivot type with a front end comprising a front pivot axis around which the absorber element under operation reciprocates between a lower turning position and an upper turning position, and with a rear end extending from a lower rear edge to an upper rear edge, wherein under operation the front end faces towards the incoming waves, and the rear end faces away from the incoming waves, wherein the lower rear edge is located in a first radial direction at a first distance from the pivot axis, and the upper rear edge is located in a second radial direction at a second distance from the pivot axis, wherein the first and second radial directions define an acute tip angle of the absorber element, wherein the first distance defines the absorber element length, and a frame structure configured to be essentially at rest with respect to the body of water, the frame structure pivotally supporting the absorber element from the essentially horizontal front pivot axis at an axis height Fa above an average surface level S of the body of water, wherein the absorber element in an idle position under still water conditions is partially submerged, with an idle draught Fd of the absorber element being determined by the depth of submersion of the lower rear edge below still water.

According to one embodiment of the absorber unit for the absorption of wave energy from a body of water is provided, the absorber unit comprising - an absorber element of the front pivot type with a front end comprising a front pivot axis around which the absorber element under operation reciprocates between a lower turning position and an upper turning position, and with a rear end extending from a lower rear edge to an upper rear edge, wherein, under operation, the front end faces towards the incoming waves, and the rear end faces away from the incoming waves, wherein the lower rear edge is located in a first radial direction at a first distance from the pivot axis, and the upper rear edge is located in a second radial direction at a se- cond distance from the pivot axis, wherein the first and second radial directions define an acute tip angle of the absorber element, wherein the first distance determines the absorber element length, and - a frame structure defining an absorber chamber and being configured to be essentially at rest with respect to the body of water, the frame structure pivotally supporting the absorber element within the absorber chamber from the essentially horizontal front pivot axis at an axis height Fa above an average surface level S of the body of water, wherein the absorber element in an idle position under still water conditions is partially submerged, with an idle draught Fd of the absorber element being determined by the depth of submersion of the lower rear edge below the average surface level S, wherein the frame structure comprises sidewalls defining the absorber chamber in an axial direction parallel to the front pivot axis, and a bottom plate essentially extending in the axial direction from sidewall to sidewall, wherein a front portion of the bottom plate has a minimum distance and a maximum distance from a level of the front pivot axis, the minimum distance corresponding to the sum of the pivot axis height Fa and the idle draught Fd of the absorber element, and the maximum distance not exceeding the absorber element length Fl. According to a broader aspect of the above-mentioned absorber unit, the frame structure defines an absorber chamber and is configured to be essentially at rest with respect to the body of water, the frame structure pivotally supporting the absorber element within the absorber chamber from the essentially horizontal front pivot axis at an axis height Fa above an average surface level S of the body of water, wherein the ab- sorber element in an idle position under still water conditions is partially submerged, with an idle draught Fd of the absorber element being determined by the depth of submersion of the lower rear edge below the average surface level S, and comprises a bottom plate arranged underneath the absorber element extending from a front edge to a rear edge, wherein the rear edge is placed behind the front edge as seen in a longi- tudinal direction from the fore end to the aft end of the absorber unit, and wherein, under still water conditions, the front edge is placed at a level below the absorber element idle draught, and the rear edge is placed at a level above the idle draught and below the average surface level of the water, and wherein the distance of the rear edge from the front pivot axis is larger than the absorber element length. The average surface level S of the body of water is the planar level obtained by averaging out variations in surface elevation, e.g. resulting from wind waves. The average surface level corresponds to a still water level of the body of water, i.e. the surface level under still water conditions. To a good approximation, the reference of the average sur- face level S may be mapped to a level on the frame structure of the absorber unit, which frame structure level may thus be taken as an equivalent reference for the configuration and/or operation of the absorber unit. For floating structures, slow changes of the average sea level on a time scale exceeding the operation regime of wave energy harvesting, e.g. tidal changes in sea level, do not affect the reference of the average surface level S, because the floating structure follows such slow changes in the average sea level. The frame structure may thus be considered at rest with respect to the average surface level of the body of water at time-scales corresponding to the wave periods of the waves to be harvested. Also, frame structures attached to fixed foundations may comprise means for following changes in sea-level on a time-scale exceed- ing the operation regime of wave energy harvesting.

As mentioned above, the term "vertical" refers to a direction parallel to gravity and the term "horizontal" refers to directions perpendicular thereto. The vertical direction is essentially perpendicular to the average surface of the body of water. The terms "top" and "bottom" of the absorber are defined with respect to the position of the absorber when in use or at least when deployed in a body of water, wherein an "upward" direction from the bottom towards the top points out of the water and a "downward" direction points from the top towards the bottom into the water. The term "level" refers to the vertical position of a horizontal plane. Levels above the average surface level S may be used to specify heights above still water. Levels below the average surface level S may be used to specify depths below still water. Two horizontal directions may be defined as directions parallel to the average surface level of the body of water, wherein an axial direction is parallel to the front pivot axis, and a longitudinal direction is perpendicular to both the axial direction and the vertical direction. The longitudinal direction goes in the direction from the front end to the rear end of the absorber element and from fore to aft of the absorber unit. The terms "front" and "rear" as well as "fore" and "aft" are derived from the position of the absorber under conditions of normal use for which it is configured, wherein the direction of wave propagation is from the fore/front end facing towards the incoming waves to the aft/rear end facing away from the incoming waves. An idle position of the absorber may be defined with respect to the body of water under still water conditions. In its idle position, the absorber element is configured to be suspended from the front pivot axis at a predetermined axis height above still water, and is partially submerged. An idle draught of the absorber element is determined by the depth below still water of the absorber element's submerged lower rear edge. Typically, a principal portion of the absorber element is under water with only a minor top portion of the absorber element sticking out of the water. A front side of the absorber element extends from the front end to the bottom of the rear end of the absorber element. The front side surface faces towards the incoming waves thus forming the pressure side of the absorber element. The front side inclination under idle conditions is the angle of the first radial direction with respect to the still water level and may be referred to as the operating angle of the absorber element.

The front pivot axis is arranged in an essentially horizontal direction allowing the front pivot type absorber element to reciprocate up and down in a rotary motion of the absorber element body around its front pivot axis, thereby absorbing kinetic and potential energy from the waves driving the motion. The reciprocating motion of the absorber element body is performed aft of the front pivot axis with respect to the idle position, thereby covering a stroke volume between the upper and lower turning positions. Typi- cally, the front pivot axis of the absorber element is located close to the fore end of the absorber unit. The rear end of the absorber element faces in the direction of the aft end of the absorber unit, the main body of the absorber element floating aft of the front pivot axis. The absorber element is operated in the absorber unit comprising a frame pivotally supporting the absorber element from the front pivot axis at a given pivot axis height above the average surface of the body of water. The submerged portion of the absorber element yields buoyancy to the absorber element. The buoyancy provides a lifting force in an upward direction, which in the rising phase of a wave lifts the absorber ele- ment to swing around the front pivot axis in an upward direction. Furthermore, the submerged portion of the front side provides a pressure surface of the absorber element extending from the pivot axis downward into the water where it interacts with the incident wave to absorb kinetic energy of the wave. The rising wave thus accelerates the absorber element in an upward direction to an upper turning position, thereby ab- sorbing both potential energy and kinetic energy from the wave. As the wave falls again, the absorber element recovers from the upper turning position to a lower turning position mainly under the influence of gravity, assisted by further downward acting forces, such as adhesion of the outer surfaces of the absorber element to the retracting body of water. Driven by a subsequent wave, the absorber element rises again from the lower turning position to the upper turning position. Thereby, the incident wave field transfers a substantial portion of its energy to the absorber by driving the reciprocating motion of the absorber element with respect to the absorber unit's frame. This energy may be harnessed by means of a power take-off means driving electrical generator means for converting the absorber motion into useful electrical energy. The power take-off means may e.g. be a hydraulic system comprising pumps mounted between the absorber element and the absorber unit frame, wherein the pumps are used to generate a pressure for driving a hydraulic turbine. Alternatively, the power take-off system may be a direct energy conversion system, wherein the mechanical motion of the absorber element is converted mechanically and linked to directly drive the input shaft of an electrical generator. Alternatively or in addition to converting the absorber element motion into useful energy, such as electrical energy, the absorber may also be used in a break water system. By absorbing a substantial portion of the energy of incoming waves over a broad spectral range, the absorber element acts as efficient break water for calming/protecting the waters located aft of the absorber. The absorber unit is at the fore end open for receiving the incoming waves in the absorber chamber, and the aft end is at least partially open for fluid communication between the absorption chamber and the body of water aft of the absorber unit. The reciprocating motion of the absorber element is driven by the repetitive pressure build-up and pressure release within the absorber chamber. The absorber unit comprises in a bottom region of the absorber chamber a bottom plate. A front portion of the bottom plate is arranged underneath the absorber element, wherein the front portion of the bottom plate has a minimum distance and a maximum distance from a level of the front pivot axis. The bottom plate is arranged close to the lowest turning position that limits the maximum stroke volume for motion of the absorber element in a downward direc- tion. Thereby, the vertical confinement of the incident waves is improved, and an enhanced build-up of the pressure is achieved, which drives the movement of the absorber element with respect to the frame of the absorber unit. As a result, an improved absorption efficiency is achieved. The minimum distance should at least at the rear end of the absorber element exceed the sum of the pivot axis height Fa and the idle draught Fd in order to allow for a minimum of downward motion of the absorber element. Preferably, the maximum distance does not exceed the absorber element length Fl. The maximum distance reflects that an advantageous stroke volume of the absorber element is arranged rearward of the longitudinal position of the front pivot axis.

Preferably, the bottom plate is essentially horizontal in the axial direction.

In a longitudinal direction, the bottom plate is placed between the fore end and the aft end of the absorber unit. A rear portion of the bottom plate may extend in a rearward direction beyond the rear end of the absorber element, wherein the rear portion defines a rear end of the absorber chamber.

A front edge of the bottom plate is located upstream, a rear edge of the bottom plate is located downstream with respect to the general direction of propagation of the waves through the absorber unit. The front edge is typically at the front end of the absorber chamber as defined by the sidewalls. In one embodiment, the front egde of the bottom plate is in a longitudinal direction located roughly at the vertically projected longitudinal position of the front pivot axis. Preferably, the front egde of the bottom plate is in a longitudinal direction located at the vertically projected longitudinal position of the front pivot axis within 10%, alternatively within 20%, alternatively within 30% of the absorber element length.

Preferably, the absorber element is essentially wedge shaped having a front side extending from the front end to the lower rear edge, a rear side extending from the lower rear edge to the upper rear edge, and a top side extending from the upper rear edge to the front end. The front side forms the pressure surface interacting with the incoming waves. Advantageously, the front side is concavely shaped as seen in the direction from the front end to the back end. Preferably the front side bulges inwardly with respect to a straight line extending along the first direction from the front end to the lower rear edge. The rear side surface faces away from the incoming waves thus forming the wake side of the absorber element. The shape of the rear side surface thus affects the wave generation in the wake of the absorber unit. Advantageously, the rear end of the absorber element as seen in a cross-sectional plane perpendicular to the front pivot axis is shaped to follow a circular arc around the front pivot axis extending from the lower rear edge to the upper rear edge, wherein the radius of the circular arc is equal to the absorber element length. Thereby, the undesired generation of loss generating waves in the wake of the absorber unit is avoided. Further advantageously, the tip angle of the absorber element is between 10-70 degrees, alternatively between 20-60 degrees, preferably between 25-50 degrees. An advantageous value for the tip angle, and accordingly for an associated length to height ratio, is preferably determined according to the wave conditions, under which the absorber element predominantly is operated. Long period shallow waves, e.g. in shallow waters, may require a small tip angle, whereas high amplitude waves coming in with a high frequency may require a relatively short wave absorber with a large tip angle. Advantageously, according to one embodiment, the absorber element has a tip angle alpha of about 30 degrees, and a length to height ratio of about 2. Further, according to one embodiment of the wave absorber unit, the absorber unit is configured for a given value of the idle draught Fd of the absorber element, and the frame structure supports the front portion of the bottom plate at a level below the average surface level S, wherein the depth is in the range between 1.1-1.7 times the given value of the idle draught Fd, alternatively 1.2-1.5 times the given value of the idle draught Fd, or about 1.3 times the given value of the idle draught Fd. The given ranges for the depth below still water where the bottom plate front portion is placed reflect advantageous values for the stroke volume range of the absorber element that should be covered with regard to absorption efficiency. Further, according to one embodiment of the wave absorber unit, the front portion of the bottom plate is essentially planar. Preferably, the bottom plate is arranged essentially horizontal in the axial direction, such that the front edge of the bottom plate is essentially parallel to the front pivot axis. The front portion of the bottom plate may be inclined in the longitudinal direction within the above-mentioned limits for the distance of the bottom plate from the level of the front pivot axis and/or from the average surface level of the body of water.

Further, according to one embodiment of the wave absorber unit, the front portion of the bottom plate is arranged essentially horizontally. According to this embodiment, the bottom plate is arranged essentially horizontal in the axial direction. Furthermore, the front portion of the bottom plate is essentially horizontal also in the longitudinal direction to within a few degrees. An essentially horizontal planar front portion of the bottom plate improves stability of the frame structure with respect to the body of water under the influence of varying wave fields. Further, according to a preferred embodiment of the wave absorber unit, the bottom plate further comprises a rear portion projecting from a rear end of the front portion in an upward direction, wherein the minimum radial distance between the front pivot axis and the rear portion of the bottom plate is larger than the absorber element length. The rear portion of the bottom plate is arranged aft of the absorber element, and close to the stroke volume defined by the reciprocating motion of the absorber element. The upwardly projecting rear portion of the bottom plate closes a lower portion of the absorber chamber in a rearward direction and thereby improves pressure build-up in the absorber chamber during the upward stroke.

Further, according to a preferred embodiment of the wave absorber unit, the rear portion of the bottom plate extends from a bottom edge level at the rear end of the front portion to a top edge level above the bottom edge level and below the average surface level S such that the absorber chamber above said top edge is in fluid communication with the portion of the body of water aft of the absorber unit. The rear end of the absorber chamber is only partially closed so as to allow for fluid communication across a rear plane defined by the rear portion of the bottom plate. Thereby, pressure release during the downward "recovery" stroke is improved such that the energy stored in the absorber element may be extracted by a power take-off system instead of being con- sumed by work to be performed on the body of water for removing water from the absorber chamber.

Further, according to one embodiment of the wave absorber unit, a height of the rear portion of the bottom plate as measured in a vertical direction is at least 10%, prefera- bly at least 20%, most preferably at least 30% of the distance of the bottom edge level from the average surface level S, and at most 80%, preferably at most 60%, and most preferably at most 40% of the distance of the bottom edge level from the average surface level S. The limitation to a minimum height reflects the goal to increase the pressure build-up during the upward stroke of the absorber element. This is balanced against the goal of facilitating the pressure release during the downward stroke of the absorber element as reflected by the limitation to a maximum height.

Further, according to one embodiment of the wave absorber unit, the minimum radial distance between the front pivot axis and the rear portion of the bottom plate exceeds the absorber element length by at least 0.5%, preferably by at least 1 %, most preferably by about 2% and by at most 20%, preferably by at most 15%, most preferably by at most 10%. These values specify upper and lower limits for the minimum distance between the upwardly extending rear portion of the bottom plate and a rear end of the stroke volume covered by the absorber element's reciprocating motion. The upper limit for the minimum distance reflects the goal to increase the pressure build-up during the upward stroke of the absorber element. This is balanced against the goal of facilitating the pressure release during the downward stroke of the absorber element as reflected by the lower limit for the minimum distance. Preferably, the optimisation of the minimum radial distance of the rear portion is performed in combination with the optimisation of the height of the rear portion.

Further, according to one embodiment of the wave absorber unit, the rear portion of the bottom plate is a planar plate projecting from the rear end of the front portion of the bottom plate in a rearward direction so as to form on the aft-side an acute inclination angle with respect to a horizontal level. The upwardly projecting rear portion of the bottom plate is inclined in a backward direction. Preferably, the plane defined by the rear portion of the bottom plate is tangent to a circular arc within the angular range defined by the reciprocating motion of the absorber element.

Advantageously, the angle of inclination measured on the backside of the rear portion of the bottom plate and with respect to the horizontal is between 50 and 80 degrees, preferably between 60 and 70 degrees, when using an absorber with a tip-angle of about 30 degrees.

Further, according to one embodiment of the wave absorber unit, the position of the rear portion of the bottom plate and/or the area covered by the rear portion of the bottom plate are adjustable. Adjusting the flow through the rear end of the absorber chamber allows for adjusting the absorber unit to cope with production in a broader range of sea-states. For example, by reducing the area blocked at the rear end of the absorber chamber or by increasing the distance between the absorber element and the rear portion of the bottom plate, the pressure build-up may be reduced, thereby allowing for production at bigger waves. Furthermore, an adjustable rear portion of the bottom plate may contribute to the storm protection of the absorber unit.

Further, according to one embodiment of the wave absorber unit, the rear portion of the bottom plate is adjustable by releasing means that are automatically activated when a threshold of a value representing the energy contained in the incoming wave is ex- ceeded. A quantity monitored in order to determine if a threshold is exceeded may be a pressure exerted on the rear portion, a pressure measured in the absorber chamber, measured or forecast wave data (e.g. significant wave height), or the like. Triggering a releasing means may open the rear end of the absorber chamber in order to reduce pressure build up to a minimum. The trigger may also be activated by another safety related event, such as catching the absorber element in a storm protection position when an upper limit for the upper turning position is exceeded. Thereby, a safety device is provided for the physical protection of the absorber unit against excessive loads, e.g. during a storm. A safety device increases the reliability and the survivability of the absorber unit under harsh wave conditions.

Further, according to one embodiment of the wave absorber unit, the frame structure comprises sidewalls defining the absorber chamber in an axial direction parallel to the front pivot axis. Preferably, the bottom plate essentially extends in the axial direction from sidewall to sidewalk Sidewalls of the absorber chamber may in an axial direction confine the wave propagation to the absorber chamber, and contribute to directing the incoming waves onto the absorber element. The sidewalls of the absorber chamber may be placed closely adjacent to sidewalls of the absorber element so as to increase confinement of the incoming waves in the axial direction, increase pressure build-up in- side the absorber chamber, and enhance interaction of the incident waves with the absorber element to increase the absorption efficiency.

Advantageously, according to one embodiment of the wave absorber unit, the absorber unit comprises a limit stop for limiting the motion of the absorber element at an upper and/or lower limit position, the limit stop preferably being provided with shock absorption means. Advantageously, the maximum angular amplitude of the absorber element motion with respect to the idle position is ±30 / ±20 / ±15 degrees depending on the wave conditions, under which the absorber unit is to be operated. A limit stop is provided to mitigate any damages to the absorber element and/or the structure of the ab- sorber unit when such operation angles are exceeded.

Advantageously, according to one embodiment of the wave absorber unit, the frame structure is attached to or part of a floating platform. A floating platform is particularly useful for offshore operation. Typically, the absorber unit is part of an offshore floating platform, which is moored using a rotating mooring system allowing the platform to "wave vane", i.e. to follow the prevalent direction of the incident waves such that the front end of the absorber element faces towards the incoming waves. Furthermore, the floating platform is typically configured and dimensioned so as to rest in the body of water when assuming a given wave climate. To that end, active and passive stabilizing means may be provided on the platform. Advantageously, a plurality of absorber units, each comprising a front pivot absorber element, may be combined in the same platform.

Advantageously, according to one embodiment of the wave absorber unit, the frame structure is supported by a foundation fixed to the seabed. Fixed foundations may be useful for deployment close to the shore at low sea depths. Furthermore, a combination of floating modules anchored between fixed foundations may be conceived.

Further, according to one embodiment of the wave absorber unit, the absorber unit is configured as a detachable module, wherein the frame structure is provided with re- leasable attachment means for attaching the frame structure to cooperating receiving means on a docking structure. This embodiment allows for rapid exchange of modules, thereby facilitating easy service/maintenance and reducing down-time of a given installation/platform. Advantageously, according to one embodiment of the wave absorber unit, the detachable module comprises power take-off and energy conversion means such that the module is self-contained and/or has an easily detachable interface comprising mechanical fasteners and an electrical power connector. According to a further aspect, a wave energy plant comprises one or more absorber units according to any of the preceding claims. When using an absorber unit of the above mentioned type in a wave energy plant, the reciprocating motion of the absorber element with respect to the absorber unit's frame is harnessed by a power take-off system comprising conversion means for converting the harnessed energy into a desired form of useful energy, such as an electrical generator. In a wave energy plant, typically a plurality of absorbers is arranged in parallel next to each other.

According to a further aspect, a wave breaker comprises one or more absorber units according to any of the preceding embodiments. Advantageously, an absorber ele- ment/unit according to any of the above mentioned embodiments may be used as an open wave breaker. The absorption element/unit according to the invention may have a surprisingly high absorption efficiency of up to 70% or even more. The energy contained by the outgoing waves aft of the absorber element/unit may thus be effectively reduced as compared to incoming waves. At the same time, such a wave breaker system is open for fluid communication and the exchange of marine life, while on the lee- ward side providing coastal protection, protecting marine structures/installations, such as wind farms or fish farms, protecting spawning areas, or the like. Thereby, the environmental impact of the wave breaker is minimised while providing an efficient protection against waves. Further advantageously, a plurality of absorber elements/units is arranged in parallel and next to each other along a protection line. Furthermore, a plu- rality of absorber elements/units may be cascaded in series in order to increase the total wave absorption and improve the protection. In a cascaded arrangement, the downstream absorber elements should be dimensioned smaller than the upstream absorber elements, in order to account for the reduced size of the waves aft of the upstream absorber elements/units. Further advantageously, the absorber elements/units of the open wave breaker are used for driving power take-off means for producing useful energy from the motion of the absorber elements with respect to the frame structure supporting them.

Brief description of the drawings

The invention is in the following further discussed with reference to exemplifying embodiments, wherein the drawings show on

Fig. 1 schematically, a top elevational view of a platform according to one embodi- ment of the invention,

Fig. 2 A cross-sectional view of an absorber element according to one embodiment,

Fig. 3 schematically, geometrical parameters of the absorber element of Fig. 2 under operational conditions,

Fig. 4 a top elevational view of the absorber element of Fig. 2, and

Fig. 5 a side elevational view of the absorber element of Fig. 2.

Fig. 6 an example of a wave spectrum, Fig. 7 a diagrammatic representation of a method for configuring an absorber element, Fig. 8 a scatter chart of sea state distribution,

Fig. 9 a scatter chart of the energy content distribution, and

Fig. 10 a graph comparing the performance of absorber elements with different di- mensions.

Fig. 11 a a schematic cross-sectional view of an absorber unit,

Fig. 11 b a schematic cross-sectional view illustrating the geometry of the absorber unit of Fig. 1 a,

Fig. 12 A, 12 B, 12 C examples of different bottom plate profiles, and

Fig. 13 a graph comparing the efficiency of the different bottom plate profiles of Fig.

12 A-12 C.

Detailed description of the invention

Fig. 1 shows schematically an offshore platform 100 according to one embodiment in a top view. The offshore platform 100 comprises a semi-submersible frame structure 101 having a first side 102 and a second side103, wherein the first side 102 and the and second side 103 both extend in a transverse direction perpendicular to a longitudinal direction 105 when seen from the top. When the offshore platform 100 is deployed at an offshore deployment site, the frame structure 101 is moored to the sea bed by means of a rotating mooring system 106. The rotating mooring system 106 allows for rotation of the frame structure 101 around a vertical axis 107 arranged in front of the first side 102 of the frame structure 101 as seen in the longitudinal direction 105 from the first side 102 towards the second side 103. The offshore platform 100 is further provided with one or more wave absorber elements 108 attached to the frame structure 101 such that a front end 109 of the wave absorber element 108 is oriented towards the first side 102 of the frame structure 101 , and a rear end 1 10 of the wave absorber element 108 is oriented towards the second side 103 of the frame structure 101. The one or more wave absorber elements 108 are adapted to absorb wave energy by interaction with incoming waves 200 propagating in a direction 201 from the front end 109 to the rear end 1 10. The interaction of each of the wave absorber elements 108 with the waves 200 results in a wave induced force acting on the frame structure 101 in the longitudinal direction 105 from the first side 102 towards the second side 103. The one or more wave absorber elements 108 are arranged such that the combined wave induced forces provide an alignment force driving the longitudinal direction 105 towards the direction 201 of propagation of the waves 200 and bring the frame structure 101 in a trailing position with respect to the vertical axis 107 of the rotating mooring system 106. In the embodiment of the offshore platform 100 shown in Fig.1 , the wave absorber elements are arranged symmetrically with respect to a mirror plane defined by the vertical axis 107 and a centre line parallel to the longitudinal direction 105. Accordingly, by the alignment force, the transverse direction is pushed towards an essentially parallel alignment with the wave fronts of the incoming waves 200, wherein the first side 102, which is proximal to the rotating mooring system 106, is the wave- ward side, i.e. the first side faces towards the incoming waves 200, and the second side 103, which is distal to the rotating mooring system 106, is the lee-ward side, i.e. the second side 103 faces away from the incoming waves 200. The offshore platform 100 further comprises docking means 1 11 for transferring personnel and/or goods between a vessel 202, 203 and the offshore platform 100, wherein the docking means 1 11 are arranged on the second side 103 of the frame structure 101. As shown in Fig. 1 , the docking means may be provided on a portion of the semi-submersible frame structure 101 that extends in a rearward direction on the second side of the main portion of the frame structure 101. The docking means may comprise personnel transfer devices/gangways 113, mooring devices and fenders 114, storage and production facilities 1 15, transfer devices for loading/offloading goods (1 16), such as cranes, pumps and hoses, and similar devices required for connecting a vessel to the platform and performing a transfer.

By the combination of the alignment of the offshore platform 100 with the waves 200 and the damping of the incoming waves 200 by absorption in the wave energy absorbers 108, a region of calm sea 204 is created aft of the offshore platform 100, i.e. on the second side of the platform 100. Due to the semi-submersible design of the frame structure 101 , the offshore platform 100 is stable with respect to an average level of the body of water in which it is deployed - even under conditions of heavy seas. Additional stabilizing means (not shown in Fig. 1) are placed at the bottom of the frame structure 101. Preferably, the stabilizing means are of a passive nature, such as heave plates arranged at a fore end of the offshore platform 100, preferably underneath the wave ab- sorbers, and further heave plates arranged at an aft end of the offshore platform 100, preferably in combination with a rear pontoon. The heave plates underneath the wave absorber elements 108 may act as a bottom plate defining an absorber chamber of an absorber unit in a downward direction. This bottom plate may be designed and configured to improve/optimise wave energy absorption as further detailed below. As a con- sequence of the stable position of the platform, and the calm sea provided on the second side, vessels 202, 203 approaching the offshore platform from the second side experience a considerably reduced heave, pitch and roll movements relative to the platform. Thereby, docking of the vessels 202, 203 to the offshore platform and a safe transfer of personnel and/or goods is facilitated for a larger range of sea states.

Referring to Figs. 2-5, an embodiment of the absorber element is described. Fig. 2 shows a cross-sectional view along line I-I as indicated in Fig. 4; Fig. 3 illustrates geometrical parameters of the absorber element under operation; and Figs. 4 and 5 show top and side elevational views, respectively. The absorber element 1100 has a front end 1101 comprising a front pivot axis 1 around which the absorber element 1100 swings up and down under the influence of incoming waves travelling in the direction W from the front end to a rear end 1102 of the absorber element 1 100. An absorber element body 1103 is defined by a front side 1004 extending from the front end 1 101 to a lower rear edge 1002 at the rear end 1 102, a rear side 1005 extending from an upper rear edge 1003 at the rear end 1102 to the lower rear edge 1002, a top side 1006 extending from the front end 1 101 to the upper rear edge 1003, and lateral sidewalls 1007, 1008 defining the width Fw of the absorber body 1103 in an axial direction parallel to the front pivot axis 1001. The lower rear edge 1002 is located in a first radial direction 101 1 at a first distance from the pivot axis 1001 , the upper rear edge 1003 is lo- cated in a second radial direction 1012 at a second distance from the pivot axis 1001 , and the first and second radial directions 101 1 , 1012 define an acute tip angle a of the absorber element 1100. The first distance defines the absorber element length Fl, and the distance of the upper rear edge 1003 from the first radial direction 1012 defines the absorber element height Fh. The embodiment shown in Fig. 2 has a length to height ratio Fh/FI of about 2 and a tip angle a of about 30 degrees. The cross-sectional view of Fig. 2 shows the profile of the absorber element 1100 in a cut-plane I-I perpendicular to the front pivot axis 1001 , wherein the absorber element profile comprises a profile of the absorber element body 1103 and the front pivot axis 1001. The profile of the absorber element body 1103 is shown as the hatched area in Fig. 2. Seen in a direction from the front end 1101 to the rear end 1 102, the absorber element profile comprises at the front side 1004 a concave portion with two concave sub-portions 1013, 1014 separated by a ridge 1015. A convex envelope to the absorber element profile in the cut plane I-I may be considered as a rubber band stretched around the absorber element to enclose the front pivot axis and the profile of the ab- sorber element body 1 103. A front side portion of the convex envelope to the absorber element profile is a straight line extending from the front end 1102 to the lower rear edge 1002.

The sidewalls 1007, 1008 essentially follow the convex envelope, and provide addi- tional stiffness to the absorber element, in particular for absorber elements with considerable concave portions at the front side. Optional interstitial walls (not shown) that may be arranged in between and essentially parallel to the sidewalls 1007, 1008 may further increase the stiffness of the absorber element 1100. For operation, the absorber element 1100 is pivotally supported from the front pivot axis 1001 arranged at a pivot axis height Fa above the average surface of the body of water equal to the level S of the surface under still water conditions. The absorber element is configured such that the rear end 1 102 is partially submerged, wherein the lower rear edge 1002 is under water and the upper rear edge 1003 is above water. When suspended at the axis height Fa above the water surface S in an idle position under still water conditions, the lower rear end 1002 is located beneath the water surface S at a depth Fd defining the idle draught of the absorber element 1 100.

An absorber element pitch may be defined as the front side inclination measured as the angle β the first direction encloses with the horizontal. Alternatively, an operation angle γ of the absorber element may be defined as the angle between the surface S and the second direction 1012, wherein operation angles γ where the upper rear edge is below the front pivot axis are defined as negative. The rear side 1005 of the absorber element profile essentially follows a circular arc around the front pivot axis 1 with a radius equal to the absorber element length Fl. Consequently, the first distance of the lower rear edge 1002 and the second distance of the upper rear edge 1003 from the front pivot axis are equal to each other and equal to the absorber element length Fl. Under operation, the circular shape avoids that the rear surface 1005 excites waves in the wake of the absorber element 1100 as it moves up and down in the water.

The absorber element body comprises arms 1017, 1018 connecting the buoyancy portion of the body to the pivot axis 1001. The buoyancy portion may comprise ballasting means (not shown), such as one or more hollow interior spaces that may be filled with e.g. water, wherein the ballasting means may comprise inlet and outlet openings and pressurising means allowing the absorber element mass to be adjusted during operation.

The absorber element may further be provided with means for coupling power take-off means to the absorber element (not shown), and/or further accessories (not shown) attached to the outside of the absorber element, such as brackets for use with a limit stop for limiting the angular span of the absorber element motion.

Advantageously, an absorber element is configured for operation at a given deploy- ment site with a given wave climate by dimensioning the absorber element according to these characteristic parameters. When operating the absorber element in a given sea- state, the absorber element is exposed to a wave-train of irregular waves, which is incident from the front-end and drives the reciprocating motion of the absorber element. A given sea-state of irregular waves may be described as a superposition of sinusoidal waves of different frequencies, phases, amplitudes and directions. The energy content of a sea-state may thus be described by a wave spectrum S(f), i.e. a frequency dependent energy distribution S(f)- To a good approximation, the shape of a wave spectrum may be described by a model well-known in the field of wave science, such as a PM-spectrum (Pierson-Moscowitz), or a JONSWAP-spectrum (Joint North Sea Wave Program). A possible wave spectrum is illustrated in Fig. 5. The spectrum of the irregular sea-state may be represented by key figures derived from the moments of the spectral distribution S(f), as discussed above. These key figures comprise the significant wave height Hs, the wave energy period Te, the average wave period Tz, and the peak wave period Tp, wherein wave periods T are the inverse of the corresponding wave frequency f : T = 1/f. In order to determine the wave climate at a given location, wave data are gathered over a longer period of time, wherein several sources may be combined to obtain a useful set of wave data. The wave climate thus comprises an ensemble of sea-states occurring within said longer period of time, wherein the sea-states may be defined as wave data gathered within a predetermined observation period serving as a time base for the statistical description of the wave climate. A commonly used time base is hourly. The wave climate may thus be represented as a time based statistical distribution of how often certain sea-states occur. The occurrence of the sea-states may be analysed in terms of key figures to provide a scatter chart of the wave climate. Depending on the application, the time period scope for such a representation of the wave climate may be monthly, seasonal, annual, or any other time period of interest, such as the life time of the absorber. The statistical distribution may further be scaled/weighted by the energy content of the different sea-states. From the wave climate description, characteristic parameters may be derived that characterise the statistical distribution of waves occur- ring throughout the time period of the wave climate, such as the wave height and/or the wave period for which the overall energy content is a maximum.

Example

Referring to Figs. 7-10 in the following, configuration of an absorber element is de- scribed by way of example for a given deployment site, wherein the configuration is performed for a pre-determined shape of the absorber element profile. Configuring of the absorber element for operation in the wave climate of the deployment site essentially amounts to determining the wave climate at the specific site and dimensioning the absorber element accordingly so as to ensure an efficient harvesting of the available wave energy at a commercially viable cost level.

Fig. 7 illustrates the steps performed. Note that the steps related to determining the wave climate may have been performed beforehand, and may at least partially be available from archives. Wave climate data may include the monthly, seasonal and an- nual statistics of wave power as well as a consideration of the variability of wave power on monthly, seasonal, annual and inter-annual timescales. Gathering wave data is quite complex and expensive. Therefore several sources are often combined (1601 A - 1601 D). The wave data are, if necessary/possible, adjusted to the specific characteristics of the deployment site (1602). This creates a pool of wave climate date for the spe- cific deployment site. The site specific wave data is then transformed into a wave scatter diagram 1700 with the purpose of providing a time based statistical description of the sea states in terms of the wave heights and periods, more specific the distribution of significant wave heights Hs, the wave energy period Te, and optionally the wave directions/directional spread (not shown) of the sea-states for the entire lifetime of the project, distributed on an hourly basis (1603). After having obtained a suitable repre- sentation of the wave climate, preferably within a production window selected to disregard the most extreme sea-states (1604), the absorber element is dimensioned accordingly (1605).

Fig. 8 shows a scatter chart 1700 describing the wave climate of a given deployment site. The scatter chart 1700 is subdivided into cells 1701 defined by intervals 1702, 1703 of the significant wave height Hs and the wave energy period Te, here labelled by their centre value in units of metres and seconds, respectively. Sea states falling within the (Hs, Te)-intervals of a cell 1701 are counted in this cell 1701. Considering a time period of one year and a time base of one hour, scatter chart 1700 shows the hourly distribution of the occurrence of sea-states throughout a year.

The distribution of sea states may then be scaled/weighted by an optimisation parameter, which in the present example is the energy content of the (Hs, Te) cells. Preferably, for the purpose of dimensioning, a production window is chosen. Depending on site specific variations the production window is the smallest possible group of sea-states that constitutes typically between 85%-95% of the available wave energy. In practice, this means the smallest, shortest, longest and highest waves are disregarded from a dimensioning point of view. The yearly hourly distribution of the available wave energy is corresponding to the scatter chart 1700 of occurrences of sea states is shown in Fig. 9. The distribution of the available energy is obtained in the form of an energy content scatter chart 1800 by calculating the wave energy content in each (Hs, Te) cell and multiplying with the number of occurrences of sea-states within this cell. The (Hs, Te) intervals 1802, 1803 defining the cells 1801 of the energy content scatter chart 1800 correspond to the intervals 1702, 1703 of the sea-state distribution chart 1700. Note that the re-scaling with the optimization parameter shifts the position of the cell 1804 with maximum energy content with respect to the position of cell 1704 with the most frequent sea-states. The significant wave height Hs(peak) and the wave energy period Te(peak) characterizing the position of the cell 1804 with peak energy content is then used as the characterizing parameters of the wave climate at the deployment site for the purpose of dimensioning the absorber element. The peak position values are Hs = 3,25 m, and Te = 8,5 s.

As mentioned above, the idle draught Fd may be scaled according to a characteristic wave height of the wave climate using a height scaling factor D, wherein the characteristic wave height is preferably a significant wave height where the energy content of the wave climate is a maximum. Also, the absorber element length Fl may be scaled according to a characteristic wavelength of the wave climate using a length scaling factor L, wherein the characteristic wavelength is preferably a wavelength corresponding to a peak wave period Tp where the energy content of the wave climate is a maximum. Tests have shown that an advantageous height scaling factor D is in the range 2-5, preferably in the range 2.2-4, more preferably between 2.5-3.5, or about 2.9, and an advantageous length scaling factor L is in the range 0.1-0.4, alternatively in the range 0.1 1-0.25, alternatively in the range 0.12 - 0.20, or about 0.15. By way of example, Fig. 9 shows test results for the conversion efficiency of a given wave energy conversion system as a function of the absorber element length and for a number of different absorber element heights. For the sake of comparison, the data is normalized, wherein the absorber element length is expressed by the dimensionless length scaling factor L, and the absorber element height is expressed by the dimensionless height scaling fac- tor D. The length scaling factor L is normalized with respect to the wavelength corresponding to the key figure Tp, and the height scaling factor D is normalized with respect to the significant wave height Hs. Approximately in the above case, T _p = 1,17 T _e and the corresponding wavelength λρ is to a good approximation equal to λ _ρ = Using a length scaling factor of L = 0,15 , the absorber element length is advanta- geously configured to Fj = 0,15 ^g ^'^ ^ = 23,2 m . Accordingly, a preferred idle draught is determined as F _d = 2,9 ^■ 3,25m = 9,4 m .

The values obtained by an optimisation of the absorber element dimensions with respect to energy conversion efficiency may be balanced against the construction costs that are increasing with increasing size of the absorber element, wherein trading a slight decrease in conversion efficiency for a substantial decrease in construction cost reduces the total cost of energy production. Construction cost or similar considerations may already be implemented in the optimisation parameter for weighting the sea-state distribution. Alternatively, a correction of the dimensions may be performed after de- termining the absorber element dimensions for maximum conversion efficiency. Fig. 11 shows an embodiment of an absorber unit 2100 for the absorption of wave energy from a body of water 2099, wherein Fig. 11 a shows a cross-sectional view of an absorber unit, and Fig. 1 1 b illustrates geometry parameters of the absorber unit of Fig. 1 1 a.

The absorber unit has a fore end 2101 and an aft end 2102, wherein, under operation, the fore end 2101 faces towards the incoming waves 2103, and the aft end 2102 faces away from the incoming waves 2103 towards a wave field 2104 in the wake of the ab- sorber unit 2100. A side facing towards the incident wave field 2103 may be referred to as the wave-ward side and a side facing towards the outgoing wave field may be referred to as the lee-ward side of the absorber unit 2100.

The absorber unit 2100 comprises an absorber element 21 10 of the front pivot type with a front end 21 1 1 and a rear end 2112. The front end 21 11 comprises a front pivot axis 21 13 around which the absorber element 21 10 under operation reciprocates between a lower turning position and an upper turning position. The rear end 2112 has a lower rear edge 21 14 and an upper rear edge 21 15. The lower rear edge 21 14 is located in a first radial direction 21 16 at a first distance Fl from the pivot axis 2113, and the upper rear edge 21 15 is located in a second radial direction 21 17 at a second distance from the pivot axis 2113, wherein the first and second radial directions define an acute tip angle alpha of the absorber element 2100. The first distance Fl determines the absorber element length. Under operation, the front end 21 11 faces towards the incoming waves 2103, and the rear end 21 12 faces away from the incoming waves 2103 towards the outgoing waves 2104 in the wake of the absorber unit 2100. The absorber element 21 10 has a rear side 21 18 extending at the rear end 21 12 from the lower rear edge 21 14 to the upper rear edge 21 15, a top side 21 19 extending from the front end 211 1 to the upper rear edge 2115, and a front side 2120 facing the incoming waves at an angle beta with respect to the average surface level. In the idle position the front side is inclined at an idle position angle βθ. In the embodiment shown in Fig. 1 1 a, the tip angle is about 30 degrees, the front side 2120 is preferably concavely shaped (not visible) bulging inwardly with respect to the straight line from the front pivot axis 21 13 to the lower rear edge 21 14, and the rear side 21 18 of the absorber element 21 10 is shaped to follow a circular arc around the pivot axis 2113, i.e. with a radius corresponding to the absorber element length Fl, so as to avoid generation of waves due to a reciprocat- ing radial displacement of the rear side surface as the absorber element 21 10 moves up and down in the body of water 2099.

The absorber unit 2100 further comprises a frame structure 2121 defining an absorber chamber 2122. The frame structure 2121 is configured to be essentially at rest with respect to the body of water 2099, such that the motion of the absorber element 2110 with respect to the frame structure 2121 at rest can be harnessed to produce useful energy. The frame structure 2121 pivotally supports the absorber element 21 10 within the absorber chamber 2122 from the essentially horizontal front pivot axis 2113 at an axis height Fa above an average surface level S of the body of water 2099. In an idle position under still water conditions, the absorber element 21 10 is partially submerged, and an idle draught Fd of the absorber element 21 10 is determined by the depth of submersion of the lower rear edge 21 14 below the average surface level S. The frame structure 2121 comprises sidewalls defining the absorber chamber 2122 in an axial di- rection parallel to the front pivot axis 21 13, and a bottom plate 2130 essentially defining the absorber chamber 2122 in the downward direction. In the axial direction the bottom plate 2130 essentially extends from sidewall to sidewalk A front portion 2131 of the bottom plate 2130 provides stability to the frame structure 2121 by damping any heave, yaw or roll due to the added mass of the portions of the body of water above and below the stabilizing plate 2130 that have to be displaced when performing any such motion. A rearward extending horizontal portion 2133 of the bottom plate 2130 further contributes to the stabilizing effect. The front portion 2131 of the bottom plate 2130 is essentially horizontal to within a few degrees at a level Fb below the average surface S of the body of water 2099. Placing the first portion 2131 at a level close to the level of the lower rear edge 21 14 of the absorber element 21 10 in the lowest possible turning position of the absorber element 21 10 has the advantage of increasing the absorption efficiency over a broad range of wave conditions. In the embodiment shown in Fig. 1 1 , the depth of the level Fb of the front portion 2131 of the bottom plate 2130 below the average surface level S is about 1.3 times the idle draught Fd of the absorber element 21 10. A rear portion 2132 of the bottom plate 2130 projects in an upward direction up to a level Fc corresponding to or above the idle draught level Fd of the absorber element 21 10. The rear portion 2132 of the bottom plate 2130 extends from a level of a bottom edge 2134 at the rear end of the front portion 2131 of the bottom plate 2130 to a level of a top edge 2135 above the bottom edge level and below the average surface level S such that the absorber chamber 2122 above said top edge 2135 is in fluid communication with the body of water 2104 aft of the absorber unit 2100. The upwardly projecting rear portion 2132 of the bottom plate 2130 further enhances pressure buildup during the rising phase of a wave while allowing for an efficient pressure release during the falling phase of the wave, thereby increasing absorption efficiency. In the embodiment shown in Fig. 11 , the rear portion 2132 is a planar plate arranged at an rearward inclination of 65 degrees with respect to the horizontal, and the level Fc of the top edge 2135 of the rear portion 2132 of the bottom plate 2130 is about 40% to 50% of the idle draught Fd above the level Fb of the front portion 2131. The rear portion 2132 thereby impedes the outflow in the lowest portion of the absorber chamber 2122 in a rearward direction by blocking about 30% to 40% of the area of the rearward cross-section.

Fig. 12 shows three configurations of the bottom portion of an absorber chamber 2222 defined within a frame structure 2221 of an absorber unit 2200. The configurations of Fig. 12 A and Fig. 12 B show two different bottom plate profiles 2230, 2240. The pre- ferred bottom plate profile 2230, type A, has a horizontal front portion 2231 and an upwardly projecting rear portion 2232 analogue to the configuration of Fig. 1 1 discussed above. Bottom plate profile 2240, type B, is equivalent to profile type A apart from the upwardly projecting rear portion 2232. The configuration of Fig. 12 C has no bottom plate and is considered as a reference. Performing tests on all three configurations un- der otherwise identical conditions demonstrate the increase in absorption efficiency achieved by adding a bottom plate. Representative results of such tests on the different configurations of Figs. 12 A-12 C are compared to each other in Fig. 13, wherein the results are normalised to the reference efficiency achieved by configuration C, i.e. in the absence of any bottom plate. Addition of a bottom plate 2240 (type B) improves the absorption efficiency by about 20%, whereas the further addition of an upwardly projecting rear portion 2232 to a horizontal front portion 2231 of bottom plate 2230 (type A) improves the absorption efficiency by more than 100% as compared to the configuration without bottom plate (type C).