FIELD OF THE INVENTION

This invention relates to the field of scour around marine structures, including, bridges and piers, seabed mounted foundations and devices and submarine cables and pipelines, and especially monopile (or tri-pile) mounted structures such as off-shore wind turbines. In particular, it relates to a method of repairing, rectification, controlling and/or preventing recurrence of scour about seabed mounted foundations and, in particular, of tidal scour about a monopile (or tri-pile) mounted marine structure or sub-marine cables or pipelines, to a scour repair (or rectification) system or device for use in such a method and to a method of assembling and installing such a scour repair system.

BACKGROUND OF THE INVENTION

Renewable Energy is of increasing importance to national and global energy policies and methods for harnessing renewable energies are attracting increasing amounts of investment. Marine renewable energy devices are an important part of the energy mix, particularly for countries with a good marine resource. Such marine renewable devices include tidal energy devices, wave energy devices and off-shore wind turbines.

Wind turbines (or wind-powered electricity generators or other wind energy capture devices) are typically multi-blade devices (usually three blades) mounted on a tower which is fixed in place with a monopile (or, increasingly for larger sized turbines, tri-pile foundations). The wind turbine tower is subject to various stresses arising from the movement of the substantial blades and from exposure to variable wind conditions. These stresses can cause movement of the towers which can then cause the towers to vibrate. Such vibrations can cause structural failure of the wind turbine or lead to increased maintenance requirements. Monopile mounted devices vibrate according to the modes of natural frequency of the system (especially the first and second modes), which is a function of the height of the monopile and the weight distribution across the device (as well as other factors). It is known to minimise the vibrations in a system and to dampen the vibration of a wind turbine tower by suspending a certain mass within or in association with the tower at a certain height (which damping mechanism may capture the energy of vibration by oscillation of the suspended mass). Off-shore wind turbines, however, which are monopile mounted are susceptible to sea-bed scour (e.g. by tidal variations or due to stormy weather), which removes an area of sea-bed about the base of the wind turbine monopile. This can, in effect, cause the height of the wind turbine to vary according to the degree and severity of sea-bed scour with the result that the optimal position and configuration of vibrational damping systems may vary (with resultant increase maintenance to review and vary damping configurations and/or increased risk of structural damage to the device).

It is therefore of utmost importance to ensure that the scouring about the monopile base and related structural integrity, vibrational variations, maintenance and possible structural damage are minimised.

Tidal energy devices designed to capture the regular and predictable tidal energy may be sea-bed mounted, optionally via an anchor or one or more seabed piles. Since tidal energy devices are typically located in areas of high tidal currents (to maximise energy capture), their anchor arrangements are particularly susceptible to tidal scour which may cause loosening of the anchor or sea-bed mount and resultant movement and damage or hazard from the device. The process of scouring may also result in inefficiencies in the operation of tidal energy devices.

Again, ensuring that the scour about seabed anchors for tidal or wave energy capture devices is important to prevent instability in the system and resultant damage.

Methods and materials for seabed scour protection exist and attempts have been made to address scoured seabed problems associated with seabed mounted structures.

Around wind turbine monopiles without scour protection, scour depths about the monopile of more than twice the diameter of the monopile have been observed, which presence of scouring requires monopiles of an extra 8- 10m in length to be utilised to ensure adequate structural stability and risks the vibrational variations, maintenance costs and damage referred to above. Scour can also leave a cable leading from a wind turbine exposed to turbulence and damage.

Scour protection for seabed-mounted off-shore wind turbines is a recognised problem and a significant cost of construction of such wind turbines is in the scour protection systems, which are recognised as being inadequate.

Typical offshore wind turbine scour protection may be approached in two ways: installing the wind turbine and then repairing scour that forms around the base (dynamic scour protection); or forming the scour protection before or immediately after installation (static scour protection). The scour protection typically takes the form of armour protection (e.g. a layer of rocks or large concrete elements), typically with rocks of diameter of the order of 50-100 cm, and a filter layer beneath of smaller stones or rocks having a diameter of the order of 10cm. The protection may be of a metre or two in depth and may extend several metres in radius from the monopile.

However, it has been found that these traditional methods result in local scour at the extreme boundary of the scour protection area and the surrounding seabed, which can lead to undermining of the limits of the scour. It has also been found that the foundation of the scour protection has lowered about the base of the monopile, due to erosion of the sea-bed beneath the layer of scour protection (see, for example, Hansen et al, "Scour Protection around Offshore Wind Turbine Foundations, full-scale Measurements", EWEC 2007). Erosion about the monopile or tripod/tri-pile foundations of wind turbines has been shown to have a significant impact on the natural frequency of vibration of a wind turbine, particularly on the second mode of vibration (see, for example, M.B. Saaijer, "Tripod support structure - pre-design and natural frequency assessment for the 6MW DOWEC", doc. No. 63, TUD, Delft, May 2002).

There have been a number of efforts to improve scour protection. Fronded concrete mattresses have been proposed and found use in deepwater oil installations. However, this solution suffers from a number of disadvantages including a high cost of installation, the development of local scouring about the boundary of the concrete mattress, depression of the elements of the concrete mattress due to erosion of the seabed beneath and the inadequate performance of such devices in high energy shallow waters. Fronded fibre or textile mattresses are utilised about oil platform supports. These devices have fronds that are buoyant and extend upwards from the textile mat. Whilst having some effect in relatively low sea current environments, they suffer from certain disadvantages. In particular, in high current environments, the fronds are forced to a very shallow angle to the mat and lose a significant amount of their sediment trapping capability. In addition, in strong current flows, the seabed material about the edges and beneath the mat can be undermined leading ultimately to disturbance of the mat anchors and the fronded mat being unsecured and moving away with the currents.

WO-A-2010/0097199 discloses a method and system for preventing seabed scour about a seabed-mounted foundation or renewable energy device having a single layer arrangement of interconnected sedimentation elements for placing about the seabed-mounted foundation or renewable energy device, which sedimentation elements have a specific gravity of substantially similar to the seabed material on which they are placed so as to prevent further scour or seabed material displacement whilst capturing mobilised seabed material particles.

A particular problem has been identified which affects efforts to install surface scour prevention measures in the case where a tidal scour has already formed. Re-filling the scour hole with seabed material presents difficulties since the replacement seabed material placed in the scour is relatively mobile and in vigorous scouring conditions can cause the scour to re-form in too short a time to enable a surface scour prevention measure to be installed or implemented.

It would be desirable to provide a method and/or system and/or apparatus for repairing scour about a sea-bed mounted foundation such as a bridge support or a monopile, e.g. in association with an offshore wind turbine, or anchoring of other marine renewable energy device, which method and/or apparatus overcame the aforementioned problems in a cost effective and readily applicable manner. PROBLEM TO BE SOLVED BY THE INVENTION

There is a need for improved methods and devices for the repair of scour about the base of bridges, marine energy devices (such as oil rigs) and marine renewable energy devices, especially the foundations such as monopiles of offshore wind turbines, in a cost-effective and readily applicable manner.

It is an object of this invention to provide a method for scour repair including scour inhibition and/or correction, which is readily applicable and cost- effective.

It is a still further object to provide a method and manner of manufacture and installation of such a device and/or implementation of such a method.

SUMMARY OF THE INVENTION

Accordingly, there is provided in a first aspect of the invention a method for the repair or reduction and/or prevention (e.g. prevention of recurrence) of scour about a riverbed or seabed-mounted foundation or renewable energy device or anchor thereof, the method comprising disposing in a scour formed in an area surrounding and/or adjacent to said riverbed or seabed-mounted foundation or renewable energy device or anchor thereof a plurality of

sedimentation elements each having at least one sediment trapping component, wherein the plurality of sedimentation elements is disposed in the scour, preferably in a random or disordered arrangement.

In a second aspect of the invention, there is provided a scour rectification (and/or prevention) apparatus for use alone or in combination in the rectification or reduction of scour about a riverbed or seabed-mounted foundation or renewable energy device or anchor thereof, the apparatus comprising a collection of sedimentation elements retained by a collective retention means, each sedimentation element having at least one sediment trapping component

In a third aspect of the invention, there is provided a scour rectification (and/or prevention) system for the rectification or reduction of scour about a riverbed or seabed-mounted foundation or renewable energy device or anchor thereof, the system comprising one or more scour rectification apparatus as defined above.

In a fourth aspect of the invention, there is provided a use of an apparatus as defined above in the rectification of a riverbed or seabed scour by disposing one or a plurality of said apparatus in the seabed scour and, preferably, demobilizing said apparatus relative to any other of a plurality of said apparatus.

In a fifth aspect of the invention, there is provided a use of tyres in the rectification of seabed scour, by forming a collection or cluster of tyres by use of a collective retention means and disposing the collection of tyres in a scour.

ADVANTAGES OF THE INVENTION

The method, apparatus and system of the present invention enables rectification and subsequent inhibition or prevention of riverbed or seabed scour about marine foundations such as bridges and piers, oil rigs or wind turbine monopiles and tri-piles at low cost using collections of retained sedimentation elements, preferably in random or disordered arrangement for disposal into a riverbed or seabed scour and optionally covered with an anti-scour mat. The sedimentation elements (and anti-scour mat) may be manufactured from readily available materials. The invention thereby addresses the significant and recognized problem of rectification of riverbed and seabed scour about seabed mounted structures and, further, in the case of wind turbines, enables wind turbine tower vibrational dampening devices to work effectively in continuous operation due to control of any scour-related change in natural frequency of vibration, thereby reducing maintenance and risk of damage and turbine downtime.

BRIEF DESCRIPTION OF THE DRAWINGS

Figures la to lh are plan view illustrations of optional portions of a sedimentation element for use in the present invention;

Figures 2a to 2f are cross-sectional side aspects of optional portions of a sedimentation element for use in the present invention Figure 3 a and 3b are illustrations in a perspective view of sedimentation elements or a portion thereof for use in accordance with the present invention.

Figure 4 is an illustration of a preferred sedimentation element according to the present invention.

Figure 5 illustrates one embodiment of the scour rectification apparatus of the present invention;

Figure 6 illustrates an anti-scour mat for use in one embodiment of a system of the present invention;

Figure7& is a side aspect of an anti-scour mat for use in one embodiment of a system of the present invention.

Figure 8 is an illustration of a system comprising a modular array of sedimentation elements about a wind turbine monopile according to one embodiment of the present invention.

Figure 9 illustrates a side aspect of Figure 8.

Figure 10 is a further illustration of a system comprising a modular array of sedimentation elements about a wind turbine monopile according to one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention provides a method, apparatus and system for rectification, repair and reduction of scour about riverbed and seabed mounted foundations, such as for bridges, piers, rigs or renewable energy devices or for renewable energy devices themselves or anchors therefor by utilizing a plurality of sedimentation elements that have certain structural properties, preferably collections or clusters of such sedimentation elements whereby a riverbed or seabed scour may be rectified.

Sedimentation elements have the capability to trap sediment or seabed material by way of at least one sediment trapping component. The plurality of sedimentation elements are preferably organized into one or more

collections/clusters of sedimentation elements in which the sedimentation elements are retained by a collective retention means. The collections may be formed of any suitable arrangement of sedimentation elements, for example organized, ordered stacks, semi-ordered arrangements or a disordered arrangement or a random arrangement of sedimentation elements.

According to one embodiment, for example, there is a semi-ordered arrangement of sedimentation elements linked by collective retention means.

Preferably, according to this embodiment, the apparatus comprises a plurality of sedimentation elements (such as tyres) arranged in a mat arrangement (such as those mats of sedimentation elements defined below) and folded, crumpled or rolled into a secondary formation and secured in the secondary formation by collective retention means. An apparatus comprising a collection of sedimentation elements made up from a mat configured in a secondary formation by folding, crumpling or rolling may comprise any suitable number of elements, such as described below.

Preferably, the collection is formed of a disordered arrangement or a random arrangement of sedimentation elements. By utilizing a disordered arrangements, the plurality or collections of sedimentation elements disrupt currents and are less likely to cause further scour at the boundaries of the disposed plurality of sedimentation elements. Further, a disordered arrangement of elements may be less efficiently stacked, thus fewer elements are required to infill a scour (thus leaving more room for captured mobilized sediment to settle).

The collections of sedimentation elements are retained by a collective retention means. Any suitable collective retention means, which is capable of retaining the sedimentation elements making up such a collection in some pre-defined special relationship with other sedimentation elements in the collection may be used. Typically, the collective retention means will be a physical retention means. The collective retention means may be, for example one or a combination of an interconnection means or a flexible or rigid containment means (which preferably should be capable of allowing the transport of riverbed or seabed material therethrough). An interconnection means may be, for example, a rope which is passed through apertures in the sedimentation elements (or otherwise engaged therewith) to form the collection of elements, which are preferably loosely tied together. A containment means, which is preferably capable allowing the passage of mobilized sediment or seabed material so maybe defined as porous, may be rigid or flexible. For example a rigid containment means a self-supporting rigid element such as a cage element, having a space therein for disposing sedimentation elements and which cage may be disposed in a scour, through the apertures or holes in the cage which mobilized sediment may pass before being trapped and demobilized by the sedimentation elements therein. A flexible containment means may be substantially absent self-supporting structure but capable of allowing passage of mobilized sediment. A flexible containment means may be resiliently elastic or may be of a substantially inelastic material.

In one embodiment, a flexible containment means may be a bag or purse formed by a porous mat. Preferably, the porous mat may be formed of a plurality of interconnected sedimentation elements, such as a mat of sedimentation elements formed in a manner such as that set out below, or a portion thereof. Preferably, the purse or mat may be configured to retain therein a plurality of sedimentation elements. A purse or mat formed of a plurality of interconnected sedimentation elements (which are preferably as further defined below) preferably is sized to comprise and contain a number of sedimentation elements desired in the apparatus or collection/cluster, as set out below.

Preferably, the flexible containment means is formed of a netted material such as a netted fibre (e.g. a netted rope) that forms a bag which may be closed, tied or drawn. The netted fibre may be elastic but is preferably substantially inelastic. Preferably, the containment means is a flexible netted bag.

The collective retention means and/or the number and/or size of the sedimentation elements thereby retained may be sized and arranged to form a tight fit whereby the sedimentation elements are largely immobilized relative to one another, but are preferably sized and arranged so that the sedimentation elements are relatively loosely retained such that they are capable of at least some degree of movement relative to one another. In an embodiment where the collective retention means is a flexible netted bag loosely retaining sedimentation elements, the slack in the net allows the sedimentation elements to move within the net thereby enabling the collection of sedimentation elements formed to mould to fit the shape of the scour hole and to mould to form a continuum with other collections disposed therewith in a scour hole.

Preferably the collection of sedimentation elements comprises a interconnection means (such as a rope linking the sedimentation elements or multiple links between individual sedimentation elements) and a containment means (such as a netted bag within which the roped or otherwise linked sedimentation elements may be placed). This gives an added level of security that individual sedimentation elements may not escape into the environment.

The rope may be, for example, a twisted or woven rope. The rope may be manufactured from any suitable material, e.g. polypropylene, polyethylene, polyester or polyamide fibres or a combination thereof, preferably polyethylene, polyester or polyamide. The rope may be any suitable diameter, such as 4 mm to 20 mm, e.g. 2 cm to 1 1.5 cm (e.g. 8 to 10 mm) or 12.5 to 20 mm (e.g. 14-20 mm).

According to a preferred embodiment of the method of the present invention, the collections of sedimentation elements may be disposed in a scour and then prior to or subsequently, the collections of sedimentation elements may be interconnected or linked (e.g. with a rope or other means) to secure them in place.

Any suitable sedimentation element may be used in accordance with the present invention.

Preferably, the sedimentation element is, in situ, of substantially similar average specific gravity to the seabed material on/in which the scour rectification apparatus is to be disposed. Alternatively or additionally, the sedimentation element may be partially buoyant or capable of partial suspension in the seabed material on which it is to be disposed or in a suspension of mobilized seabed particles in water, by virtue of the material's average specific gravity (being substantially similar to that of the seabed material) and/or by virtue of the suspensive effect of each sedimentation element being interconnected or retained with other sedimentation elements in a collection. Without being bound by theory, it is believed that by interconnecting a sedimentation element with one or more other elements or by containing a plurality of sedimentation elements in a flexible containment means, when a portion of seabed material located by or beneath said element is temporarily mobilized by local sea currents, the connection or retention with other sedimentation elements may render the undermined element temporarily suspended or may slow its occupation of the vacated foundation, thereby allowing time for the foundation to be reoccupied by trapped sediment. It is further believed that by utilizing a sedimentation element having an average specific gravity of substantially similar to the seabed material on/in which it is disposed, the element is inhibited from displacing (or slow to displace) seabed material beneath it even when seabed material is mobilized by local sea currents. Preferably, the material from which the element is formed is of a specific gravity substantially similar to that of the seabed material. Optionally, the sedimentation element is configured in such a manner as to trap and contain a substantial amount of seabed material (relative to the size of the element) such that in situ the seabed material- filled element has a specific gravity substantially similar to the seabed material on which it is disposed.

Preferably, the sedimentation element has a specific gravity in the range from 1.05 to 2.5, more preferably 1.05 to 2 (where water has a specific gravity in standard conditions of 1 and sea water typically has a specific gravity of about 1.03), still more preferably 1.2 to 1.5, either in situ or, preferably, inherent.

The scour problems that the present invention solves are caused by river currents about bridge foundations or pier foundations and sea currents about piers or seabed mounted structures in an offshore environment exposed to strong tidal currents and/or exposed to storm energy.

The apparatus, system and method of the present invention find utility in a range of offshore applications, including seabed-mounted foundations or seabed mounted or anchored devices. Seabed-mounted foundations include for example the foundations of offshore oil platforms or exploration and drilling platforms and the foundations of renewable energy devices such as wind turbines. Offshore wind turbines are typically mounted upon piles sunk into the seabed. These piles are usually monopiles, but increasingly for larger-sized turbines may be for example tri-piles. Seabed scour about the foundations of offshore wind turbines is a particular problem since the piles are sunk typically in seabed area in which a pile can readily be sunk. Such seabed types are particularly susceptible to scour by nature of the material. Seabed-mounted or anchored devices may include other renewable energy capture devices such as seabed-mounted wave power devices, seabed-anchored wave power devices (or power lines drawn therefrom) and seabed-mounted tidal power devices.

Preferably, the method, apparatus and system of the present invention is for rectifying and then inhibiting scour about a foundation (e.g.

monopile or tri-pile) of an offshore wind turbine.

A sedimentation element as used herein is an arrangement of one or more sedimentation trap component(s) which provide a sedimentation trapping capability in situ in two substantially differing, preferably substantially opposing, directions of liquid flow. Thus a sedimentation element will typically comprise at least two opposing trap components separated by a gap diameter or a single trap component with sufficient curvature to provide a sedimentation capability in two substantially opposing directions of flow.

A sedimentation trap component in plan view preferably has a curved lateral capture shape, by which it is meant that the component defines a bay through its curved or angular concave arrangement. Examples of the shape of a sedimentation trap component in plan view are shown in Figure 1 (a-h).

A sedimentation trap component preferably has a vertical capture shape, by which it is meant that in cross-section in the side aspect it has a shape inclined to cause sediment capture, e.g. by a concave or angled arrangement. Preferably the side aspect cross-sectional configuration of the element is angled or curved to form a wall portion and extending therefrom one or more radial elements which may be designated a base portion and/or a roof portion. A wall portion is defined as that part of the element, when in its orientation in use, having an angle within 45 degrees of the vertical. A roof portion or base portion is defined as that part of the element, when in its orientation in use, having an angle within 45 degrees of the horizontal. Unless the context requires otherwise, the base portion and roof portion may be interchangeably used and it should be understood that unless otherwise clear from the context, the roof portion may be the base portion simply by reversing the orientation of the element (especially in symmetrical elements).

Preferably, the sedimentation trap component and the sedimentation element have the ability to contain material, such as seabed material. Typically, the containment capability of, for example, a sedimentation trapping component having a wall portion and one or more radial elements such as a base portion and a roof portion is enhanced by a retaining lip positioned on the base and/or roof portion. Where there is a base and roof portion, there is preferably a pair of convergent retaining lips.

Examples of the shape in side-aspect cross-section of components of the element are shown in Figures 2a to 2f.

Preferably, the sedimentation element is formed from a curved wall portion forming a cylinder from which extend radially annulus roof and base portions.

Optionally, the element has a hollow torus shape having a coequitorial cylindrical cavity in the internal wall thereof

Since the weight of the sedimentation element, in situ, is in large part made up of the material of the seabed in/on which it is disposed, the relative density (or specific gravity) of the element in situ may be substantially similar to the seabed material and thus is in a dynamic equilibrium. An element and collection or cluster of elements having significantly sized gaps between the sedimentation trap components (gaps) and gaps between the sedimentation elements themselves (interstitial sites) allows movement of flowing sand with and to an extent between the elements and enables captured or trapped sediment to be readily returned to the seabed, which seabed remains accessible and is not immediately occupied by displacement with the sedimentation elements themselves.

In a preferred embodiment of the invention, the sedimentation element comprises a vehicle tyre or modified vehicle tyre. Typically, the tyres are re-used tyres (i.e. tyres that have served their purpose as vehicle tyres and require disposal, recycling or reuse). It is particularly advantageous to utilize used vehicle tyres for this purpose for a number of reasons. Used tyres represent a significant waste- management problem and vast numbers of used tyres are accumulating in dumps and storage sites in the UK and in many other countries. Whilst alternative uses for used tyres are being sought, such as the formation of rubber chips for use as ground material in paths and playgrounds, or incorporation into concrete for structural materials or as sea defenses, their use remains insignificant compared to the waste management problem. In addition, used tyres for use in the seabed represent relatively low pollution risk as they degrade slowly over many years without leaching of noxious or harmful substances.

Any suitable material may be utilized in the sedimentation element of the present invention. If, for example, the sedimentation element is configured to trap and contain seabed material, it may be manufactured from thinly cast or lightweight metal such as aluminium, provided, preferably, that the element in situ (i.e. when positioned in normal operation and containing seabed material) has a specific gravity substantially similar to that of the seabed material (e.g. from 1.05 to 2) and that the element itself preferably has a specific gravity of greater than 1.03. This applies for any material from which the elements may be made, including thinly case or lightweight metals, molded plastics or reinforced fibrous materials (e.g. reinforced glass fibre or carbon fibre materials), or rubber.

Preferably, the material from which the sedimentation element is composed itself has a specific gravity (or average specific gravity) of substantially similar to the seabed material and in any case preferably from 1.05 to 2, preferably 1.2 to 1.5.

Optionally, the sedimentation element may be manufactured from modified cement, concrete or ceramic materials which are modified to have a closed cell structure (containing air or other foaming gas) or to contain a significant proportion of less dense particles (e.g. rubber fragments made from used tyres), which cause the cement, concrete or ceramic material to have a suitable specific gravity (e.g. within the range referred to above). Alternatively, the material may be composed of a silicon or other particulate-containing expanded polymer material having the desired specific gravity characteristics.

Preferably, the sedimentation element is manufactured from a rubber, vulcanized rubber or synthetic rubber or plastic material, preferably such materials that are utilized in tyre manufacture. Preferably, these materials have a specific gravity within the ranges referred to above.

The dimensions of the sedimentation element may be selected according to the particular requirements of the application.

In one embodiment of the method and system of the invention, a sedimentation mat may be disposed in an area surrounding and/or adjacent to the riverbed or seabed-mounted foundation or renewable energy device or anchor thereof and/or over at least a portion of the plurality or collection of sedimentation elements (i.e. over one or more apparatus) disposed in the scour.

Any suitable sedimentation mat may be used. For example, a fronded mat or mattress which may comprise, for example, a fibre or textile mat having buoyant fronds extending upwards therefrom, whereby the fronds may cause mobilized seabed particles to demobilize and become deposited on the seabed. Preferably, the sedimentation mat comprises an arrangement of interconnected sedimentation elements, which sedimentation elements may be as defined above. Preferably, the arrangement of interconnected sedimentation elements comprises a close-packed arrangement, e.g. square close-packed or hexagonal close-packed. More preferably, the arrangement of interconnected sedimentation elements is an arrangement of sedimentation elements in a single layer.

For a monopile or tri-pile or such like foundation (e.g. for a wind turbine), the mat may be selected to be of a size to prevent or inhibit scour formation or worsening. Preferably, the mat is sized to provide protection for at least a distance of lx the diameter (D) of the monopile or tri-pile foundation about each side of thereof, more preferably at least 2.5x D and up to an optional distance of lOx D, more preferably 5x D. In providing scour protection about a 3.5m diameter wind turbine monopile, for example, the anti-scour mat according to the invention preferably has a diameter of from about 10 m (including the aperture through which the monopile itself will pass) to about 75 m, more preferably of from about 20 m to about 40 m. This would be the preferred range of mat size for any monopile or tripile up to about 5m in diameter (with 20 m dimension being the preferred minimum size for foundations up to about 10m in diameter).

The mat may be selected to be any suitable shape, e.g. square, rectangular or elliptical, but preferably approximates to circular.

Preferably, the mat is formed of a single layer of sedimentation elements or is of a substantially constant depth (i.e. it may comprise of portions or incidences of multilayer elements provided the elements in multilayer portions are substantially shallower than those elements in the single layer, with the result that they act as effectively single layer elements). Optionally, incidences of multilayer elements of different depth to provide deepened protrusions from the mat may be incorporated as 'anchorage' elements. A substantially constant depth of mat enables the mat to better retain its dynamic integrity with the seabed material.

The mat is preferably formed of close-packed (i.e. touching) sedimentation elements. Preferably, the mat is formed of square close-packed or hexagonal close-packed elements.

Optionally a mat may be fixed to the foundation or monopole about which it is disposed and/or further anchored by anchorage means to secure the mat to the seabed (e.g. at its extremities) or to one or a plurality of the apparatus disposed in the scour.

The characteristic dimensions of a sedimentation element itself may be selected according to the requirements of the environment in which it is placed. The element width, element depth (i.e. depth of trap component or portion), width of trap component or portion, and gap diameter are inter-related dimensions which together determine the effectiveness of the sediment trapping and scour prevention actions of an apparatus or mat formed from the elements in a particular environment. The element width is typically about twice the width of a trap component or portion plus the gap diameter (distance between the trap diameter). In the case of a tyre-shaped element, for example, the element width is the total width of the element, the width of the trapping portion is the radial width of a base or roof portion of the element, the gap diameter is the diameter of the concentric hold in the centre of the tyre and the depth of the element is the height of the end wall of the tyre (the road gripping, tread bearing surface of the tyre, for the avoidance of doubt).

Preferably, the gap diameter is from 0.5 to 5 x the width of the trapping component or portion (which for a tyre-shaped element would mean the gap diameter is 0.25x the diameter of the element to 2.5x the diameter of the element), more preferably from 1 to 3 x the width of the trapping component, still more preferably 1.5 to 2.5 x the width and preferably 2x the width.

Preferably, the aspect ratio of the sedimentation element (i.e. the aspect ratio of the trapping components), being the width or radial width r of the trapping component divided by the section depth d of the trapping component, is from 0.5 to 2, preferably 0.75 to 2.25 and most preferably about 2.

Preferably, the element has a width from 10cm to 2m, more preferably 20 cm to 1 m, still more preferably, 30 cm to 70 cm (e.g. to 50 cm) and most preferably 35cm to 45 cm or from 55 cm to 65 cm. For example, a 630 mm width element (diameter tyre) works well, although a range of sizes (e.g. within the above ranges) can be used in a single collection/cluster.

Preferably the radial width of the trapping component or portion is from 0.1 to 1 x the element width, preferably 0.25 x element width and a depth of 0.1 to 1 x the element width, preferably 0.25 x the element width.

Preferably, the element has a depth in the range from 1 m to 5 cm, more preferably 50 cm to 10 cm and still more preferably 25 cm to 15 cm.

Sedimentation elements for use in accordance with the present invention, for example used vehicle tyres, are preferably modified to have apertures formed in the upper part of the side wall of the element or in the roof portion to allow escape of trapped air during the installation. Submergence of an apparatus or mat or portion thereof without air-release apertures would render the procedure more difficult and the mat undesirably buoyant. There may be at least two and preferably several such apertures formed in each element. Typically, such apertures are of a size from 1 cm ² to about 5 cm ² and may be of any suitable shape, e.g. circular. Apertures may also be formed in the side walls or in the base or roof portions.

An apparatus according to a preferred embodiment of the invention preferably comprises at least 10 sedimentation elements, e.g. from 10 to 1000, more preferably from 15 to 500 and still more preferably 20 to 200. In a more preferred embodiment, the apparatus comprises a collection of from about 25 to 100, more preferably 30 to 70 and still more preferably about 40 to 60 (e.g. about 50) sedimentation elements. In a particularly preferred embodiment the

sedimentation elements (being preferably of a preferred size as defined above) are used tyres and comprises 30 to 70, more preferably 40 to 60 and still more preferably about 50 tyres per collection/cluster.

In one optional embodiment, for example, in which smaller collections or clusters are desirable (for example to reduce the size/weight of each cluster to allow smaller scours to be filled or a smaller installation vessels to be used), an apparatus may comprise a collection of from 5 to 49 sedimentation elements, e.g. 5 to 45 elements, preferably 20 to 40 elements, e.g. from 30 to 35 or from 35 to 40 elements.

In another optional embodiment, for example, in which larger apparatus/collections or clusters are desired (e.g. to enable more rapid infill of a large scour, where a large installation vessel is available or where the installation window of operation is narrow, an apparatus may comprise a collection of from 51-1000 sedimentation elements, preferably 55 to 200, preferably 60 to 90 sedimentation elements (e.g. 55 to 60 or 60 to 65).

In a particularly preferred embodiment, the apparatus of the invention comprises a flexible containment means such as a flexible netted bag. Preferably the flexible containment means contains a plurality of tyres as sedimentation elements. Preferably, the flexible containment means (e.g. flexible netted bag) is from about 1 m ³ to about 20 m ³ in volume, more preferably about 2 m ³ to about 10 m ³ , still more preferably about 4 m ³ to about 6 m ³ , most preferably about 5 m ³ . Preferably, taking an opening as the 'top' of the apparatus, a flexible containment means (e.g. flexible netted bag) has a width of about 2 to 3 x the height. In the case of a netted bag of 5 m ³ volume, it is preferred to have a width of about 2.5 m to a height of about 1 m.

In one optional embodiment, such as that mentioned above, in which smaller apparatus is desirable, the volume of the flexible containment means (e.g. the flexible netted bag) is configured to be in the range from 1 to 4.5 m ³ , preferably from 3 to 4 m ³ (e.g. from 3-3.5 m ³ or from 3.5-4 m ³ ). This may typically be utilized, for example, when using from 30 to 40 sedimentation elements, e.g. tyres, (e.g. of size 55-65 cm).

In another optional embodiment, such as that mentioned above, in which larger apparatus is desirable, the volume of the flexible containment element (e.g. the flexible netted bag) is configure to be in the range from 5.5 to 20 m ³ , preferably 6 to 10 m ³ (e.g. from 6 to 8 m ³ or 8 to 10 m ³ ).

In a preferred embodiment, where the flexible containment means is a flexible netted bag (or indeed where the containment means is a rigid cage), the net preferably has a mesh size that is large enough not to substantially encumber transport of mobilized seabed material. Preferably the mesh size is such as to prevent escape of sedimentation elements. More preferably, the mesh diameter is up to 0.5x the diameter of the sedimentation elements (e.g. tyres), more preferably up to 0.4x and more preferably up to 0.2x and optionally up to O.lx the diameter of the sedimentation elements. Preferably the mesh diameter is at least 1 cm more preferably from 1 cm to about 30 cm, more preferably, 2 cm to about 20 cm, still more preferably up to about 15 cm and most preferably in the range 5 to 10 cm.

Optionally, in a preferred embodiment of the invention in which the flexible containment means is a netted bag, such as a fibre woven net, the yarn diameter (and yarn material) is such that the integrity of the netted bag is retained when the bag is lifted with a quantity of sedimentation elements (e.g. 10-100, or 10-50 tyres) and optionally also when the bag is lifted with a quantity of sedimentation elements in sediment. Typically, the yarn diameter is from 2 mm to about 20 mm, preferably about 5 to 15 mm and more preferably about 8 to 12 mm. Optionally, the yarn diameter is about 4 to 10 mm (e.g. 4, 6, 8 or 10mm) [e.g. for smaller apparatus/clusters] or about 14 to 40 mm (e.g. 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34, 36, 38 or 40 mm) [e.g. for larger apparatus/clusters].

According to a method of the invention, in one embodiment, a scour may be about 300 m ³ in volume and the apparatus of a preferred embodiment mentioned above may be used to fill the scour. For example, using a 5 m ³ netted bag of about 50 tyres would require about 60 bags (about 3000 tyres). Preferably, depending upon the size of the scour, the size of elements, bags, etc, are arranged to allow the scour to be filled with about 10 to 500 netted bags of sedimentation elements, preferably about 20 to 150, more preferably about 30 to 100 and still more preferably about 40 to 80 bags. By way of another example, using a 4 m ³ netted bag of about 40 tyres would require about 75 bags (about 3000 tyres). And using a 6 m ³ netted bag of about 60 tyres would require about 50 bags (about 3000 tyres).

The flexible containment means when provided by netted bag may be made of wound or woven fibre nets comprising a polypropylene fibre, a polyethylene fibre, a polyester fibre or a polyamide fibre or a combination thereof. Preferably, it is one of a polyethylene fibre, a polyester fibre or a polyamide fibre, preferably a polyethylene fibre. Ropes may be used to secure the ends of the net bags. Any suitable ropes may be used.

In the embodiments of the invention in which the apparatus is a netted bag, preferably the netted bags are formed by a net bag having an open end, which is drawn together by a head rope which itself is affixed to a lifting eye. The netted bags may be, for example, up to 5 m high and from 0.5 to 3 m in diameter (e.g. about 2m high and 1.5m in diameter). Optionally, a smaller netted bag of up to 1.8 m (e.g. 1.5-1.8 m) high and from 0.5 to 3 m in diameter (e.g. 0.5 to 1.3m) or a larger netted bag of at least 2.5 m, e.g. 2.5 to 3.5 m in height and a diameter of 0.5 to 3 m (e.g. 1.8 to 2.5m) is provided depending upon specific requirements.

In one embodiment, the apparatus may be assembled by providing a netted bag held open about an assembly frame (e.g. a circular frame) whilst the sedimentation elements (e.g. tyres) are placed in the bag. A head rope may then be secured to a lifting eye and the bag secured and the apparatus lifted out of the frame and stored for deployment.

Optionally, according to one embodiment of the method of the invention, a quantity of sand (e.g. seabed sand taken from a nearby location not suffering from a scour problem) may be disposed on top of the system comprising a plurality of apparatus and, optionally, a mat disposed thereon. Typically, the sand will be disposed into the scour on top of the plurality of apparatus prior to disposing a mat thereon.

Another aspect of the invention concerns a method of installing a scour rectification system in a scour, e.g. about a riverbed or seabed mounted foundation, oil rig or seabed mounted marine energy device, foundation or monopile/tri-pile for an offshore wind turbine. The method preferably comprises the step of providing a plurality of apparatus (comprising providing a plurality of sedimentation elements, e.g. tyres, and retaining certain numbers of sedimentation elements in collections by means of a collective retention means such as a netted bag for containing a collection of retention means), disposing a plurality of apparatus into a scour to be rectified, preferably interconnecting the apparatus and then optionally disposing a quantity of seabed material or sand onto the apparatus- filled scour and/or disposing a sedimentation mat over at least a portion of the apparatus disposed in the scour.

Preferably in installing a mat over the apparatus disposed within the scour, and optionally for interconnection therewith, sections of a mat are formed on shore or on a vessel for installation and a series of sections installed in place. The sections preferably have dimensions of a desired length up to say 20 m and a width of up to say 10 m. In the case of used tyre sedimentation elements, the section may be composed of, for example, 150 to 300 tyres, typically about 200 tyres (for a wind turbine monopile protection system comprising from 800-1200 tyres). Preferably, the sections for transportation and installation have a width of up to about 8 m, more preferably up to about 5 m (e.g. in the range 3 - 5 m) and preferably a length of up to about 12 m, more preferably up to about 8 m (e.g. in the range 4 to 8 m). The preferred dimensions should utilize up a number of interconnected elements in the range 25 to 100, preferably 40 to 80. The preferred dimensions are ideal for maximizing the production of substantial sections on land whilst facilitating transport and installation about an offshore wind turbine, which sections will then be interconnected in situ.

Sections of a mat may thereby be disposed in the desired location about, e.g. a wind turbine foundation and connected together in situ (starting, for example, with the elements adjacent the base of the wind turbine tower and working radially outward).

The invention provides, in another aspect, the use of rubber tyres fastened together in collections (to form apparatus) to provide scour rectification apparatus and system.

Advantages of the invention include: ease of construction using readily available materials; ease of installation; minimal impact on marine environment and ecology; lifetime of the tyres of approximately 100 years so will at least outlive the working lifetime of a monopole.

In another aspect of the invention, there is provided a a scour mitigation apparatus comprising a plurality of elements housed within a net; the plurality of elements each comprising at least one through hole being arranged irregularly within the net such that the plurality of elements within the net form a plurality of channels within the scour mitigation apparatus. Preferably, the elements are made of rubber. Preferably, the elements are tyres. Preferably, the plurality of elements are connected together (e.g. in series together, to form a string of elements). Preferably, the plurality of elements are each connected to at least one other element. Preferably, the plurality of elements are connected together by rope, which rope is preferably a laid rope.

In one embodiment, a portion of the plurality of elements are arranged into a string of elements and the portion of the plurality of elements in the string elements are connected to one another. In a further embodiment, a portion of the plurality of elements are arranged into a string of elements and the portion o the plurality of elements in the string of elements are connected in series to one another. Preferably, a number of strings of elements are provided. Preferably, each string of elements comprises a substantially equal number of elements and each of the elements in a respective string of elements is connected to at least one other element in that string of elements.

The invention will now be described in more detail, without limitation, with reference to the accompanying Figures.

The sedimentation elements may be of any suitable configuration. An example of a sedimentation trap component is shown in Figure 3a, which is formed with a plan according to Figure lc and a cross-sectional profile according to Figure 2d. A preferred sedimentation element (Figure 3b) has a trap component plan according to Figure lh and a cross-sectional profile according to Figure 2d, or preferably 2e with retaining lip.

In Figure 4, a preferred embodiment utilizes a ring-shaped element 1 , such as a tyre, has an element width e, a wall 3 (in the form of a cylinder) with depth a, a roof/base portion 5 with radial width b and a concentric central hole 7 with gap diameter c.

In Figure 5, an apparatus 501 comprises a netted bag 503 with an opening 505 drawn together to be secured, the apparatus containing a plurality of tyres 507 as sedimentation elements, which are disposed in a random or disordered arrangement as shown. The mesh 509 is sized so as to be too small for tyres 507 to escape but large enough not to encumber flow of mobilized seabed material through the net.

Figure 6 shows an embodiment of a mat for use in preferred embodiments of the method and system the invention in which a mat or portion thereof 9 is formed of a plurality of ring-shaped sedimentation elements 1 connected by fixings 11 to form a square close-packed planar arrangement. The sedimentation elements 1 are provided with apertures 13 to allow the escape of air during submergence of the mat 9 into the desired location, for disposal about a foundation on top of the apparatus-filled scour. The mat 9, as shown in Figure 7, comprises a single layer of elements 1. In Figure 8, a mat 9 of multiple sedimentation elements 1 is located about a monopile 15 (see Figure 9) of, for example, a wind turbine 17 (Figure 9), in a single layer typically on top of a plurality of apparatus 501 (not shown) and provides protection about a potential scour area 19 defined by the illustrative boundary 21 (Figure 10).

The invention has been described with reference to preferred embodiments. However, it will be appreciated that variations and modifications can be effected by a person of ordinary skill in the art without departing from the scope of the invention.