Technical field of the invention

The present invention concerns a heave compensated marine vessel, in particular a semi-submersible marine vessel as set out by the preamble of claim 1, a semisubmersible platform as set out by the preamble of claim 12, and associated methods as set out by the preamble of claims 14 and 15.

Background of the invention

Floating wind power plants are generally supported by floating platforms such as spar platforms, tension-leg platforms, or semi-submersible platforms.

A spar platform comprises a large-diameter, vertical buoyant cylinder that supports a deck or other structure. The cylinder is balanced and maintained in a stable vertical orientation in the water by means of solid ballast at the bottom and ballast tanks (for adjusting buoyancy) near the waterline. The center of gravity is always below the center of buoyancy, such that the spar will float in an upright orientation. The spar is permanently anchored to the seabed by a plurality of anchor lines or chains. A spar platform requires substantial water depths and a complex installation procedure.

A tension-leg platform is a floating platform that is moored to the seabed by a plurality of vertically tensioned cables or steel pipes grouped at each comer of the platform. The platform. When installed, stability is maintained by the taut tethers, but buoyancy and ballast devices are required for transport and installation. Fabrication and installation are complex and time-consuming operations.

Semi-submersible marine vessels are well known. Examples include offshore drilling rigs, safety vessels, oil production platforms, heavy lift cranes and ships, and support structures for various structures (e.g. wind turbines, bridges), and may include a single column or a multi-column platform. A semi-submersible platform comprises a deck which is supported by structural columns (usually four) that in turn are supported by ballasted, watertight pontoons located below the waterline and wave action. A semi- submersible platform has a small water-plane area and is thus sensitive to load changes, and must therefore comprise trimming systems (e.g. ballast tanks and pumps) to maintain stability.

The prior art floating wind power plants generally require complex mooring and/or ballasting systems in order to accommodate environmental parameters (e.g. waves, currents) and the loads induced by the wind turbine.

The prior art includes EP 3 960 617 Al, which describes an offshore semi-submersible platform which comprises at least three stabilizing columns, truss structures securing the stabilizing columns to one another. Each one of the stabilizing columns comprise a substantially horizontal perforated plate and a fastening arranged for fastening the perforated plate to the stabilizing column in a working position below a bottom surface of the stabilizing column, creating a wave load attenuation chamber defined between the perforated plate in said working position and the bottom surface of the stabilizing column.

The prior art also includes US 5363788 A, which discloses a controlled-heave floating oil platform comprising a deck and a floatation assembly. In order to control platform response to sea movement, the floating assembly comprising a rising and falling compartment open to the sea and connected to a gas reservoir by means of a circulation pipe provided with a restriction.

The prior art also includes US 2021/0269126 Al, which discloses a motion absorbing system and method for a structure that includes the coupling of a container to a structure. The container has a liquid disposed therein wherein an ullage is defined above a surface of the liquid. An elastic element is positioned in the liquid. The elastic element has a natural frequency tuned to damp motion of the liquid.

The prior art also includes US 2022/0128035 Al, which describes a semi-submersible floating wind power generator comprising a wind power generator set, a post device, a load carrying device and a mooring device. The wind power generator set is disposed at a first end of the post device. The load carrying device is disposed at a second end of the post device. The mooring device is disposed at the second end of the post device. The post device includes a main post and multiple auxiliary posts. The main post is disposed in parallel with the multiple auxiliary posts, and second ends of the multiple auxiliary posts are aligned such that the second ends of the multiple auxiliary posts form a first plane, and the second end of the main post is disposed at a position closer to the first end of the main post than the first plane.

The prior art also includes WO 2022/098286 Al, which describes a semi-submersible wind power platform having a tower carrying a wind generator housed in a nacelle and a plurality of arms. The tower comprises a main float and each arm comprises a secondary float to stabilize the tower. Each arm consists of a strut element connected between the tower in a main position and the secondary float. A first catenary element is connected between the secondary float and a first position of the tower, and a second catenary element is connected between the secondary float and a second position of the tower.

The prior art also includes WO 2020/009588 Al (PCT/N02019/050142, by the present inventor), which describes an active control system for a multi-column semisubmersible platform. This control system is configured for heave motion neutralisation of a semi-submersible vessel and may be built into any conceivable configuration of such vessel. The control system comprises a rise canister with one or more openings in a lower part, wherein the water surface level in the rise canister is balanced by a confined volume of air or other gas at a slight overpressure. The top level of the riser canister is below the lowest water surface around the column at the bottom of the passing wave. The air volume is connected to an appropriately dimensioned reservoir tank. With rising wave height, the hydrostatic pressure in the rise canister increases and drives the water surface upwards and compresses the confined air volume. The rising water level that encompasses the column leads to increasing buoyancy, and at the same time the increasing water volume in the rise canister under the splash zone leads to correspondingly reduced buoyancy, as the air volume is compressed. These two volumes offset each other at any time and thereby neutralise the heave forces of the floating rig. The rise canister comprises a space between the outer walls of the column and curved bulkheads that form a quadratic - or circular - core inside the column, and the rise canister volume increases with increasing height. Scuppers connect the rise canister to the sea, and a deck isolates the wet area in the rise canister from the internally dry column below. The deck constitutes the roof of the rise canister and the air volume above the water surface coupled to a pipe and a gas (air) reservoir. The pipe is further connected to an arrangement comprising a first valve connected to a first tank containing air at a higher pressure than the highest pressure in the rise canister. The pipe is also connected to a second valve which is connected to a second tank containing air at a lower pressure than the pressure in the rise canister. A third valve provides ventilation to the atmosphere. A low-pressure compressor with a high-volume capacity maintains an overpressure in the first tank. The valves are controlled and operated based on data from geostationary references. When, due to hydrodynamic inertia effects and viscous hydrodynamic frictions, the platforms are not kept entirely calm, the additional valves may be operated to adjust for this. The system may comprise suitable algorithms that can function predicatively and individually independently for the four columns to keep the platform stable horizontally and heave-neutral.

The compressors required to pressurize the air reservoir tanks of the prior art systems require separate control systems and supply of electrical power. An object of the present invention is to provide a system which is less dependent on external power systems and which has faster response times than the prior art system.

Summary of the invention

The invention is set forth and characterized in the main claim, while the dependent claims describe other characteristics of the invention.

It is thus provided a heave compensated vessel configured for floating in a body of water, comprising

- a chamber of which at least a portion is arranged below the water surface and comprising one or more first fluid ports in a chamber lower region arranged below the water surface; characterized by

- a second fluid port arranged below the water surface, said second fluid port fluidly connected to a piston movably arranged in a cylinder;

- the cylinder above the piston being fluidly connected to a gas reservoir container via a first gas conduit and a first valve device and to the atmosphere outside the vessel via second gas conduit and a second valve device, wherein the first and second valve devices are check valves (non-return valves) or valves having similar operational oneway flow characteristics;

- whereby movement of water entering the second fluid port drives the piston such that air from the atmosphere outside the vessel is drawn into the cylinder and compressed air is fed into the into the gas reservoir container from the cylinder;

- an opening between the gas reservoir container and the chamber, said reservoir opening comprising a reservoir valve;

- an opening between the chamber and the atmosphere outside the vessel, said chamber opening comprising a chamber valve; and

- control means configured for controlling at least the reservoir valve and the chamber valve.

In one embodiment, the control means comprises a control system configured to receive and process data retrieved from sensors providing environmental data, and/from sensors or devices providing operational data. The control means may comprise a machinelearning algorithm.

In one embodiment, the second fluid port is fluidly connected to the piston via a vesselinternal shaft.

In one embodiment, water from the first fluid port is allowed to rise and fall inside a portion of the chamber, and that at least this portion of the chamber comprises an inner surface that is curved and forms a compartment in which the volume that increases with increasing height. Horizontal cross-sections of the portion may increase with increasing height.

In one embodiment, the vessel comprises an elongate, cylindrical body, and the second fluid port is arranged in the region of a lower end of the vessel when the vessel is arranged in the body of water. The vessel may further comprise a ballast tank in the lower region of the vessel when the vessel is arranged in the body of water. In one embodiment, the first valve device and the second valve device are check-valves, and the reservoir valve and chamber valve are remotely operated butterfly valves.

A plurality of first fluid ports may be arranged circumferentially around the vessel body. In one embodiment, the vessel is a semi-submersible vessel.

It is also provided a semi-submersible platform comprising a plurality of the vessel according to the invention, characterized by:

- a main vessel configured as a central support member for carrying a structure such as a wind turbine, and

- a plurality of vessels arranged at a distance from the main vessel, connected to the main vessel via respective arm structures and configured for serving as outrigger stabilizing vessels to stabilize the main vessel.

It is also provided a method of operating the vessel according to the invention, characterized by operating the reservoir valve and chamber valve to control the pressure inside the chamber in order to control the water surface level in the portion of the chamber.

It is also provided a method of controlling heave, pitch, and/or yaw motions of the semisubmersible platform according to the invention, by selectively controlling heave, pitch, and/or yaw motions of the main vessel and the plurality of outrigger vessels. For each vessel, said control may be performed by the above-mentioned method of operating the vessel and coordinated by a common control system.

Brief description of the drawings

These and other characteristics of the invention will become clear from the following description of embodiments of the invention, given as non-restrictive examples, with reference to the attached schematic drawings, wherein:

Figure l is a schematic perspective view of an embodiment of a semisubmersible platform according to the invention, utilizing an embodiment of the invented floating vessel; Figure 2 and figure 3 correspond to figure 1 and are perspective views of an embodiment of a semi-submersible platform according to the invention, utilizing an embodiment of the invented floating vessel;

Figure 4 is a longitudinal cross-sectional schematic drawing of an embodiment of the invented floating vessel as illustrated in figures 2 and 3; and

Figure 5 is a schematic illustration of an embodiment of a system for controlling heave motion of one or more floating semi-submersible vessels.

Detailed description of embodiments of the invention

The following description may use terms such as “horizontal”, “vertical”, “lateral”, “back and forth”, “up and down”, ’’upper”, “lower”, “inner”, “outer”, “forward”, “rear”, etc. These terms generally refer to the views and orientations as shown in the drawings and that are associated with a normal use of the invention. The terms are used for the reader’s convenience only and shall not be limiting.

Figure 1 illustrates an embodiment of the semi-submersible platform 40 according to the invention. In the illustrated embodiment, the platform 40 comprises a central support member lb and two stabilizing members la, 1c. The stabilizing members la, 1c are connected to the central support member lb via respective support structures - or arms - 3a, 3b. In the illustrated embodiment, the angle between arms 3a, 3b in the horizontal plane is 90°, but other angles are conceivable. The support structures 3a, 3b may comprise internal bulkheads and buoyancy compartments (e.g. controllable ballast compartments) and comprise a solid ballast 12, for example iron ore or gravel. Each stabilizing member la, 1c, together with its respective arm 3a, 3b, therefore effectively comprises stabilizing outriggers for the central support member lb. The support structures 3a, 3b are dimensioned according to applicable design criteria.

Supported by the central support member lb is a column 5 and a payload 6 (schematically illustrated). The payload 6 may for example be a wind turbine generator having a center of gravity CoGp. In figure 1, CoGi is an exemplary indication of the center of gravity for the entire platform without the solid ballast 12, whereas C0G2 is an exemplary indication of an operational center of gravity for the entire platform with the solid ballast 12 included in the arms 3a, 3b. The solid ballast 12 is thus dimensioned such that the operational center of gravity C0G2 is directly above the center of buoyancy CoB (the location of the CoB being determined by the geometry and displaced volume of the submerged structures).

Figure 2 is another illustration of the invented semi-submersible vessel 40. The internal solid ballast 12, the support column 5 and wind turbine generator 6 - cf. figure 1 - are not illustrated in this figure. The central support member lb and the stabilizing members la, 1c are similar floating vessels, although the dimensions of the central support member lb may be different from (e.g. larger) than the dimensions of the stabilizing members la, 1c. This floating vessel is described in more detail below, designated by reference number 1. Reference number 4 indicates a foundation, for example for carrying the above-mentioned support column 5 and wind turbine 6, and reference number 2 indicates the water surface of the body of water in which the vessels are floating, as seen in figure 3.

In the illustrated embodiment, the central support member lb and the stabilizing members la, 1c are floating vessels, shaped as elongate cylindrical columns which are connected to its respective support structure 3a, 3b in a lower region. In the illustrated embodiment, the vessel 1 comprises an outer housing having plurality of circumferentially arranged fluid ports 7. It should be understood that the vessel 1 may have other shapes than cylindrical and may comprise fewer or more (i.e. one or more) fluid ports 7 than illustrated.

The invented floating vessel 1 will now be described in more detail, with reference to figure 4, which illustrates the vessel 1 floating in a body of water W at an arbitrary water level. The vessel 1 comprises a ballast tank 8 by means of which the vessel may be positioned at a suitable draft, by pumping ballast water 9 into and out of the ballast tank. Pipes, valves and other devices required to control the buoyancy (and hence the draft), are commonly known in the art and need therefore not be illustrated. The vessel may also comprise fixed ballast members (not shown). The vessel 1 comprises an internal chamber 11, of a volume VRC, which is in communication with the water W via the above-mentioned fluid port 7 (one or more such ports; two fluid ports 7 are illustrated in figure 4). The fluid port (or ports) 7 is arranged such that it is always below the water surface 2 when the vessel is in operation and appropriately ballasted in the water. The chamber 11 extends upwards inside the vessel, in the illustrated embodiment, to the top of the vessel 1. The chamber 11 comprises an opening 30 to ambient air (i.e. the atmosphere outside the vessel), and this chamber opening 30 is controlled by a valve (hereinafter referred to as a chamber valve) 25 which may be remotely operated and controlled. In one embodiment, the chamber valve 25 is a fast-acting, remotely controlled, butterfly valve. When the chamber valve is in an open position, pressurized air may be evacuated from the chamber 11 and into the atmosphere surrounding the vessel. The chamber valve 25 position shown in figure 4 is arbitrarily illustrated and does not necessarily reflect an operational state.

The lower part of the chamber 11, between the fluid port 7 and a restriction 12, comprises an inner surface (e.g. bulkhead) that is curved and forms a compartment in which the volume increases with increasing height. This part is referred to as a rise canister 11’ and is described in more detail in the above-referenced WO 2020/009588 Al. When the vessel is in operation and appropriately ballasted, the water level 10 inside the chamber may move between a lower position, above the fluid port 7, and the restriction 12. The water level 10 inside the chamber is controlled by the pressure PRC in the chamber 11, as described in the above-referenced WO 2020/009588 Al.

A fluid channel 13 extends from a fluid port 13’ in the lower region of the vessel, and upwards in the vessel. In the illustrated embodiment, the fluid channel 13 is in the form of a vertical shaft centrally arranged in the vessel, but other configurations are conceivable as long as the fluid port 13’ is arranged such that it is always below the water surface 2. Water from the body of water W surrounding the vessel may thus freely flow into and out from the fluid channel 13. A float 15 is arranged inside the fluid channel 13 and is configured to move with the water inside the channel, inside a cavity 14, corresponding to the level of the water surface 2. The float 15 is connected to a piston 16 via a connection member 18, and the piston 16 is movably arranged inside a cylinder 17. The term “cylinder” shall not exclude other geometrical shapes, as the piston and cylinder need not necessarily have cylindrical shapes. An upward movement of the float 15, caused by water movement inside the fluid channel, will thus cause the piston 16 to compress the gas (i.e. air) inside the cylinder 17. Gaskets, seals, etc., that may be required, are not illustrated as such items are known in the art.

The cylinder 17 is connected to a gas reservoir (e.g. a tank) 21, of a volume VR, via a first valve 23 and a first gas conduit 22, and is also connected to an air intake 29 opening (for ambient air) via a second valve 20 and a second gas conduit 19. The first and second valves 23, 20 are check valves (non-retum valves) or valves having similar operational one-way flow characteristics. A piston 16 downstroke (retraction) will thus draw ambient air into the cylinder 17, and a piston 16 upstroke (extension) will force pressurized air from the cylinder 17 and into the reservoir 21. The reservoir 21 comprises an opening (hereinafter referred to as a reservoir opening) 28 into the chamber 11, and this reservoir opening is controlled by a reservoir valve 24 which may be remotely operated and controlled. In one embodiment, the reservoir valve 24 is a fast-acting, remotely controlled, butterfly valve. When the reservoir valve 24 is in an open position, pressurized air may be evacuated from the reservoir 21 and into the chamber 11. The reservoir valve 24 position shown in figure 4 is arbitrarily illustrated and does not necessarily reflect an operational state.

The reservoir 21 may conveniently be arranged in the support structure 3a or 3b , as illustrated in figure 4, but it may also be arranged inside the chamber 11 or external to the vessel 1.

Although not illustrated, it shall be understood that the vessel 1 comprises sensors for monitoring operational parameters, such as piston movement, reservoir tank 21 pressure PR, and chamber 11 pressure PRC.

Each semi-submersible vessel 1 thus comprises a built-in system for maintaining constant buoyancy at varying sea levels due to waves and tidal movement. The system provides energy-neutral generation and storage of energy in the form of superatmospheric pressure for use in predictive advanced regulation and control of heave motion neutralization (HMN). The water level 10 in the rise canister 11’ is controlled by the air pressure PRC. This pressure PRC is controlled by manipulation of the reservoir valve 24 - to allow a desired quantity of pressurized air from the reservoir 21 to flow into the chamber 11 - and by the chamber valve 25 - to evacuate a desired quantity of pressurized air from the chamber 11. Pressurization of the reservoir 21 is maintained by continuous operation of the piston 16 in the cylinder 17, as described above.

This manipulation of the reservoir valve 24 and the chamber valve 25, and hence the ability to adequately control pac, is based on real-time sensor data and/or information stored in a database. This information may obtained by machine-learning algorithms, enabling predictions as to the operation of the valves. Such algorithms and systems are per se known from DP (dynamic positioning) of ships and oil rigs, and from AHC (active heave compensated) cranes on deep subsea service vessels. Advanced software with predictive control algorithms may thus be used to control the air pressure PRC above the water surface 10 in the rise canister 11’ sufficiently in advance of the calculated vertical movement of the platform so that the movement is neutralized.

Scale model tests have shown that the HMN system functions in harmony with the wave motions at long waves and low frequency. However, in shorter waves with higher frequency, there is a delay in the pressure build-up of the controlling air volume in the chamber 11. To control this, stored energy is required. The invented energy-neutral generation and storage system is thus able to cope with shorter, high-frequency, waves. This is achieved by instant and rapid operation of the reservoir valve 24 - for bleeding compressed air from the gas reservoir container 21 and into the chamber 11 - and by instant and rapid operation of the chamber valve 25 - for bleeding compressed air from the chamber 11 and out into the surrounding atmosphere. The operation of these valves 24, 25 may be based on environmental data and operational data, such as ambient wind velocity, sea wave characteristics (e.g. height, frequency, direction), vessel motion (e.g. pitch, roll, yaw, heave), and global position.

Figure 5 is a schematic illustration of an embodiment of a system for controlling the valves 24, 25 (or, in general: actuators 42) in a vessel 1 as described above. The figure illustrates three vessels la-c, each having valves and actuators. This arrangement may correspond to the arrangement illustrated in figure 2, and shows how the valves in each vessel may be controlled independently of the valves in other vessels. It should, however, be understood that the invention shall not be limited to this quantity.

Figure 5 illustrates how a control system 41 is connected to vessels la-c and to the individual valves 24, 25. The control system 41 may be configured for receiving data from the individual vessel and for controlling devices in or on the vessel, such as the above-mentioned valves 24, 25. The control system may be arranged on a platform common to a plurality of vessels, or on a vessel itself. The control system may be arranged at a distal location, and will preferably comprise a user interface. The control system 41 is connected to environmental sensors 43 (wind, waves, etc.) and to operational sensors 44 (position, vessel motion, etc.). The control system may also be connected to external data and/or control devices (45), such as a remotely located computer or control device. The connections shown in figure 4 may be by any means known in the art, such as wire, fiberoptic cable, wireless communication, via satellite, etc.

The control system may comprise, or be connected to, computers that are configured to provide fast control actions to the actuators and valves. Such computers may incorporate machine-learning algorithms that are able to provide predictive control commands based on data obtained previously and/or on other parameters.

The buoyancy and vertical movement may thus be controlled independently for each vessel la-c, to counter and neutralize heave, pitch and roll movements, and thus balance the main vessel lb (i.e. the central support member carrying the wind turbine). The control system may be configured and operated such that the vessels la, 1c, act as outriggers and provide stability for the main vessel. More specifically, referring to figure 1, the buoyancy and vertical movement of each vessel la-c may be controlled such that the operational center of gravity C0G2 for the platform 40 always is directly above the platform center of buoyancy CoB.

Although the invention has been described and illustrated (in figure 2) as being used to support a semi-submersible platform 40 with three buoyancy members in the form of the vessel 1 according to the invention, it should be understood that other applications are conceivable. The invented vessel and control system may be retrofitted to existing semi-submersible platforms.