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, and a method of operating said marine vessel, as set out by the preamble of claim 20.

Background of the invention

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.

The prior art 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 claims, while the dependent claims describe other characteristics of the invention.

It is thus provided a 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;

- 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 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, 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 reservoir 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.

The control means may comprise 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 machine-learning 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 at least this portion of the chamber comprises a volume that increases with increasing height above the first fluid port. In one embodiment, horizontal cross-sections of the portion increases with increasing height above the first fluid port.

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 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.

In one embodiment, a plurality of first fluid ports are arranged circumferentially around the vessel body. The vessel may comprise a plurality of upper scuppers and a plurality of lower scuppers, in the form circumferentially arranged openings in the vessel, and the upper and lower scuppers are fluidly connected via an internal passage, external of the chamber. The chamber may comprise a narrow portion in the vessel axial direction, approximately the same region of the internal passage. In one embodiment, the chamber comprises a first chamber and a second chamber, fluidly interconnected by a third valve, and the first chamber is in communication with the water in its respective rise canister, and may be evacuated to ambient air via a corresponding chamber valve and chamber opening. In one embodiment, the second chamber has a greater volume than the first chamber. The third valve may be a quick response valve that may be operated instantly when a rapid pressure increase or decrease is required.

In one embodiment, a float is arranged inside a vessel-internal shaft connected to the second fluid port and configured to move with the water inside the vessel-internal shaft and connected to the piston via a connection member, and the piston is movably arranged inside the cylinder, wherein the buoyancy capacity of the float determines the force to drive the piston, and wherein the piston diameter is determined by the highest pressure needed. In one embodiment, the length of the connection member is dimensioned such that the piston reaches its top dead end when the float reaches the top of the vessel-internal shaft.

In one embodiment, the vessel is a semi-submersible vessel, and one or more vessels may be used as one or more foundations for a support structure to form a floating platform.

It is also provided a method of operating the invented vessel, the reservoir valve and chamber valve are operated to control the pressure inside the chamber in order to control the water surface level in the portion of the chamber.

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 perspective view of a semi-submersible platform utilizing an embodiment of the invented floating vessel;

Figure 2 is a longitudinal cross-sectional schematic drawing of an embodiment of the invented floating vessel as illustrated in figure 1; Figure 3 is an enlarged view of an upper portion of figure 2;

Figure 4 is a perspective and partly cut-away view of an embodiment of the invented floating vessel illustrated in figure 1, and illustrates a still water line (SWL), e.g. a water surface with no waves;

Figure 5 corresponds to figure 4, but illustrates the water surface being at a minimum (wave trough);

Figure 6 corresponds to figure 4, but illustrates the water surface being at a maximum (wave crest);

Figure 7 is a schematic illustration of an embodiment of a system for controlling heave motion of one or more floating semi-submersible vessels;

Figure 8 is a schematic illustration of a semi-submersible platform utilizing a second embodiment of the invented floating vessel;

Figures 9 and 10 are enlarged views of areas A and B, respectively, in figure 8; and

Figure 11 is a schematic illustration of an embodiment of a float-and-piston configuration, corresponding to the float-and-piston configuration illustrated in figure 9.

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 a semi-submersible platform 40 supported by three similar floating vessels 1 (indicated as la, lb, 1c) according to the invention. The platform 40 comprises a support structure in the form of a deck 3, which also interconnects the three floating vessels 1. Reference number 4 indicates a foundation, for example for a wind turbine tower, and reference number 2 indicates the water surface of the body of water in which the vessels are floating. It should be understood that a platform may comprise more or fewer floating vessels 1 and may have other shapes and functions than what is illustrated in figure 1.

The floating vessel 1 in the illustrated embodiment is shaped as an elongate cylindrical column which is connected to the support structure 3 in an upper region. Pontoons (not illustrated) may be connected to the vessel 1 lower end. In the illustrated embodiment, the vessel 1 comprises an outer housing having plurality of circumferentially arranged fluid ports 7, a plurality of circumferentially arranged upper fluid openings (scuppers) 5, and a plurality of circumferentially arranged lower fluid openings (scuppers) 6. 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 and scuppers 5, 6 than illustrated.

The invented floating vessel 1 will now be described in more detail, with reference to figures 2 and 3. Figure 2 illustrates the vessel 1 floating in a body of water W at an arbitrary water level. The lower scuppers 6 are below the water line 2. 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 comprises an internal passage 27 between the upper and lower scuppers 5, 6, i.e. in the splash zone.

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). 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, at least beyond the level of the upper scuppers 5. In the illustrated embodiment, the chamber 11 extends to the top of the vessel 1. The chamber comprises an opening 30 to ambient air (i.e. the atmosphere outside the vessel at pressure PA), 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 figures 2 and 3 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 above the fluid port 7. 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 pac 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. Water from the body of water W surrounding the vessel may thus freely flow into and out from the fluid channel. A float 15 is arranged inside the fluid channel and is configured to move with the water inside the channel, inside the 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 float 15 movement, 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 reservoir (here: 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-return valves) or valves having similar operational one-way flow characteristics. A piston 16 downstroke will thus draw ambient air (from the air intake opening 29) into the cylinder 17, and a piston 16 upstroke 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 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 figures 2 and 3 is arbitrarily illustrated and does not necessarily reflect an operational state.

The reservoir 21 may conveniently be arranged in the support structure (e.g. platform deck) 3, as illustrated in figures 2 and 3, 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 comprises sensors for monitoring operational parameters, such as piston movement, reservoir 21 pressure pa, and chamber 11 pressure PRC.

Figure 4 shows an embodiment of the vessel 1 in which the water surface 2 is at a still waterline (SWL) level. The upper scuppers 5 are above the water surface, while the lower scuppers 6 are submerged in the water. The vessel has been ballasted to a draft in which the SWL is approximately halfway between the upper and lower scuppers.

Figure 5 corresponds to figure 4, but illustrates a wave trough, in which the water surface 2 is at its lowest level. The upper and lower scuppers are above the water surface, and ventilated to air. The fluid ports 7 are submerged in water.

Figure 6 corresponds to figure 4, but illustrates a wave crest, in which the water surface 2 is at its highest level. The upper and lower scuppers are submerged in water, as are the fluid ports 7.

The semi-submersible vessel 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 PRC, 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 reservoir 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 7 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 1, 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 7 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 la-c 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, mooring line tension, 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 7 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.

A second embodiment of the invention will now be described with reference to the schematic drawings in figures 8-11. Unless otherwise noted, the principles, features and functions described above with reference to figures 1-7 shall apply also to this second embodiment. Figure 8 illustrates a floating semi-submersible platform 40’ comprising first and second vessels la, lb and a payload vessel 33. The vessels are interconnected by a pontoon 32 and may be designed as columns and arranged in a triangular configuration, a V-shaped configuration, or any other configuration suitable for the intended purpose. Mooring lines 31 extend between the platform 40’ and seabed anchors or other mooring means (not shown). In the illustrated embodiment, the payload vessel 33 is configured for carrying a payload (e.g. a wind turbine tower). It should be understood that other configurations are conceivable. In this second embodiment, the first and second vessels la, lb comprise an arrangement as described above, but a plurality of reservoirs (tanks) 21 are arranged in the pontoon 32 and connected to the first valve 23 as explained above. The vessels la, lb and the payload vessel 33 each comprise a first chamber I la and a second chamber 1 lb, fluidly interconnected by a third valve 34. The first chambers 1 la are in communication with the water in the respective rise canisters, and may be evacuated to ambient air via corresponding chamber valves 25 and chamber openings 30, as described above. Air pressurized by the piston movement is fed into the first chamber 1 la in each of the vessels. The second chamber 1 lb has a greater volume than the first chamber I la, and the third valve 34 is normally open. However, the third valve 34 is a quick response valve that may be operated instantly when a rapid pressure increase or decrease is required. For example, when the third valve 34 is closed, water inside the rise canister is affected only by pressurized air in the first chamber I la, and not by air in the second chamber 1 lb. This operation may be useful to counter small-amplitude (i.e. choppy) and irregular waves.

Referring additionally to figure 11, the float 15 in this embodiment is designed to the largest diameter which practically can be accommodated in the column. This is because it is desired to keep the float pressure draft at a minimum in order to get maximum utilization of the water oscillation. The float 15 is designed as shallow and light as possible to take maximum energy out of the oscillating water surface. The buoyancy capacity of the float determines the force to drive the air compression piston 16. The diameter of the piston is determined by the highest pressure needed. The length of the connection member 18 is dimensioned such that the piston 16 reaches its top dead end when the float 15 reaches the top of the fluid channel 13 (see figure 9), which corresponds to SWL which is the middle amplitude of the wave.

Although the invention has been described and illustrated as being used for supporting 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 semisubmersible platforms. In the embodiments described above, various features and details are shown in combination. The fact that several features are described with respect to a particular example should not be construed as implying that those features by necessity have to be included together in all embodiments of the invention. Conversely, features that are described with reference to different embodiments should not be construed as mutually exclusive. As a person skilled in the art readily will understand, embodiments that incorporate any subset of features described herein and that are not expressly interdependent have been contemplated by the inventor and are part of the intended disclosure. However, explicit description of all such embodiments would not contribute to the understanding of the principles of the invention, and consequently some permutations of features have been omitted for the sake of simplicity or brevity.