Damping motion of a floating body

Abstract

Motion of a floating body 10 is damped by anchoring a piston 34 with a sea anchor 38 to restrict movement of the piston, permitting greater movement of a chamber 32 that surrounds the piston and is fi

Description

This invention relates to the challenges of controlling or damping the motion of a floating body, for example to stabilise a floating offshore wind turbine or other platform that is moored to the seabed. Floating wind turbines comprise a buoyant foundation or platform, also known in the art as a floater, that floats at the surface and is anchored to the seabed. Commonly, the floater is anchored to the seabed by a distributed mooring pattern comprising taut, semi-taut or catenary mooring lines. Being located offshore, the floater is subject to sea dynamics and so has a tendency to pitch, roll, heave and yaw. To ensure stability and optimal performance of the wind turbine, it is important to control movement and orientation of the floater and hence of the tower, nacelle and rotor of the wind turbine structure that surmounts the floater. Not only must yaw be controlled to keep the rotor oriented on a desired heading to suit the incident wind direction but also pitch, roll and heave motions must be kept within acceptable limits. In this respect, the floater will tend to tilt not only due to the pitching or rolling effect of wave or swell loadings but also under wind loadings due to gusts of wind impinging on the wind turbine structure above the surface. Damping of tilting and heaving motions of the floater is desirable to ensure the stability of the wind turbine system and to maintain optimal orientation of the rotor to the horizontal. Various approaches to damping the motion of a floating body are known in the art. For example, some mooring patterns use one or more clump weights fixed along a mooring line, whereas EP 3129277 describes connecting mooring lines directly to dampers. EP 2472105 discloses an adjustment system for adjusting the orientation of a floating wind turbine platform to maximise the efficiency of the wind turbine. The system comprises sensors for detecting an effective rotation axis angle formed between a horizontal plane and a rotation axis of the wind turbine rotor. The system further comprises a means for orienting the platform to modify the effective rotation axis angle by adjusting the lean of the wind turbine tower. In CN 104806457, a stabilising weight is suspended from a buoy. The weight can be displaced horizontally relative to the buoy to adjust the uprighting moment to compensate for variations in the sea and wind state. The complexity of active adjustment or stabilisation systems such as those of EP 2472105 and CN 104806457 is undesirable, as is their inability to react quickly enough to damp relatively high-frequency oscillations such as those driven by waves, swells and wind gusts. An alternative solution is preferred to stabilise floating wind turbines or other floating equipment or installations. Against this background, the invention resides in a motion damper for a floating body. The damper has a brake structure comprising a submerged sea anchor, such as a water-filled envelope, that is suspended in the water column and connected to a piston by a link, for example suspended beneath the piston via the link. The piston is movable along an upright axis within an elongate chamber that is in fixed relation to the body. The relative movement between the piston and the body is braked by displacement of fluid in the chamber, for example water that is displaced within the chamber and in or out of the chamber by virtue of the relative movement between the piston and the chamber. The brake structure may be of substantially neutral buoyancy overall but the sea anchor could be negatively buoyant and the piston could be positively buoyant. Alternatively, the sea anchor could be positively buoyant and suspended above a negatively buoyant piston. In either case, the link can be held in tension between the sea anchor and the piston. This enables the link to be flexible but if the link is instead rigid, it can also act in compression between the sea anchor and the piston. Conveniently, the link can emerge from the chamber through a first end of the chamber, the chamber also having a second end opposed to the first end about the piston. The first end may be at a level below or above the second end. A flow restrictor may permit restricted water flow into or out of the chamber through the first end, or a closure can seal the first end around the link. Similarly, the second end can be closed to fluid flow or open to fluid flow, in the latter case through an optional flow restrictor that permits restricted water flow into or out of the chamber through the second end. The piston may be partially or fully submerged in water within the chamber. Similarly, the chamber itself may be partially or fully submerged in water. The inventive concept also embraces a buoyant body comprising at least one motion damper of the invention. The body may be anchored to a fixed point located upon or embedded in the seabed. The chamber may be disposed within a structure of the body, such as a buoyancy structure, or may be attached externally to a structure of the body. In either case, the or each motion damper can be offset laterally from a centre of buoyancy of the body. The inventive concept also extends to a corresponding method of damping motion of a floating body. The method comprises: anchoring a piston with a sea anchor to restrict movement of the piston along an upright axis; permitting greater movement along the upright axis of an elongate chamber surrounding the piston, the chamber being in fixed relation to the body; and by displacement of fluid in the chamber, braking relative movement between the chamber and the piston. For example, a fluid such as water can be displaced out of the chamber, and that flow can be restricted. Also, or instead, fluid can flow around or through the piston, for example between the piston and an internal wall of the chamber. Thus, the invention provides a passive damper system for an offshore floating structure. A sea anchor, which can be neutrally buoyant or slightly negatively buoyant, is positioned subsea and coupled to a main floater structure at the surface. The floater may, for example, support a wind turbine or other energy production or transmission facility. The sea anchor captures a water mass in the water column, for example in a bag or other encapsuled volume, and may be shaped as a parachute or a drogue to increase hydrodynamic drag that resists its movement through water. The sea anchor is coupled to the floater via a connection or link. The link may be flexible or articulated, such as a cable, wire or chain, or may be rigid such as a tube or a meshed or latticed bracing arm. The link extends to an encapsuled secondary floater that serves as the piston of a damper. The damper comprises the piston and a chamber defined within a surrounding tube, cylinder or well forming part of, or attached to, the main floater structure. The inertia and drag of the sea anchor slow down or damp heave, pitch and roll motions of the main floater structure when that structure moves upwardly and / or downwardly in the water. The damping effect is supplemented by limited or restricted water flow within, into or out of the chamber that contains the piston, via which the sea anchor is coupled to the main floater structure. In low, calm sea conditions, the main floater structure and the sea anchor adopt an equilibrium position. Being substantially neutrally buoyant, the brake assembly of the sea anchor, the piston and the link does not add any further vertical load to the main floater structure. However, when heave, pitch or roll motions of the main floater structure create relative vertical movement between the main floater structure and the peripheral floater serving as the piston, part of the load is transmitted to the sea anchor via the compressed water flow that slows movement of the piston within the chamber. Thus, without requiring any active ballasting or damping mechanism, the invention controls heave, roll or pitch movement of a surface floater structure by virtue of a passive submerged water mass that does not add any significant weight load or complexity to the floater structure. The invention can not only reduce movement of the floater structure but also modify its frequency motion response. The dynamic response of the floater structure is improved and better controlled in a passive mode. By improving stability and reducing motion amplitude, the floater structure can be made smaller and simpler than otherwise, with less reinforcement and therefore less weight and cost. The invention therefore provides a beneficial alternative to providing an active ballasting system, or making the main floater structure larger or heavier, or including a mechanical active damper in the mooring pattern or as part of the main floater structure. Embodiments of the invention provide a stabilising system for a floater. The system comprises, at least: a well extending through or attached to the floater structure, the well having an opening at the bottom and being closed or open at the top; a piston free to move vertically inside the well; and a sea anchor suspended from the piston. The well may, for example, be located within or be attached to a buoyancy column of the floater. The sea anchor may, for example, be a water-filled envelope such as a balloon or a heavy parachute. The sea anchor may be neutrally buoyant or slightly negatively buoyant. The floater can be anchored to the seabed. For example, the floater may be anchored via a distributed or spread mooring pattern of taut, semi-taut or catenary configuration. A sea anchor may be attached to the mooring pattern in the water column, with or without a piston arrangement, instead of being embedded in or attached to the floater itself. In summary, the invention damps motion of a floating body by anchoring a piston with a sea anchor to resist or restrict movement of the piston, permitting greater movement of a chamber that surrounds the piston and is fixed to the body, but braking the resulting relative movement between the chamber and the piston by displacement of fluid in the chamber. Thus, a motion damper of the invention has a brake structure that comprises a submerged sea anchor suspended in a water column and connected to a piston. The piston is movable within an elongate chamber that is in fixed relation to the floating body and that contains a fluid such as water. In order that the invention may be more readily understood, reference will now be made, by way of example, to the accompanying drawings in which: Figure 1 is a schematic side view of a floater substructure for a floating offshore wind turbine moored to the seabed, the substructure being fitted with motion dampers of the invention; Figure 2 corresponds to Figure 1 but shows how the motion dampers respond to motion of the substructure, exemplified here by a roll or pitch motion; and Figures 3 to 12 are schematic side views of various motion dampers of the invention. Referring firstly to Figures 1 and 2 of the drawings, a buoyant floater 10 for a floating offshore wind turbine 12 is shown here floating at the surface 14 of a sea and moored to the seabed 16. In this example, the floater 10 is moored by a distributed mooring pattern 18 that comprises an array of mooring lines 20 radiating from the floater 10 and anchored by piles 22 embedded in the seabed 16. The floater 10 is equipped with at least one motion damper 24 of the invention and is preferably equipped with two or more such dampers 24 that are positioned in mutual opposition about a central upright axis 26 of the floater 10 as shown here. Inset images in Figures 1 and 2 show the dampers 24 in more detail. One of the dampers 24 is also shown, further enlarged, in Figure 3 of the drawings. More generally, it is advantageous for the or each damper 24 to be offset laterally from the central axis 26 of the floater 10, or more particularly from the centre of buoyancy of the floater 10 that may lie on that central axis 26. As it is conventional to connect mooring lines 20 to the floater 10 at laterally offset positions, it is also advantageous for the mooring lines 20 to connect to the floater 10 at connection points that are adjacent to the dampers 24. The laterally offset positioning of the dampers 24 maximises the vertical movement or excursion that each damper 24 experiences as the floater 10 rolls or pitches and hence tilts away from the horizontal as shown in Figure 2. Correspondingly, their offset positioning maximises the damping moment that the dampers 24 can apply to the floater 10. As is conventional, the floater 10 exemplified here comprises buoyancy columns 28 that are spaced apart horizontally by a connecting frame structure 30 and are spaced angularly, in plan view, around the tower of the wind turbine 12. In this example, the tower of the wind turbine 12 is on the central axis 26 of the floater 10 but this is not essential. Conveniently, as shown here, the dampers 24 are integrated with the buoyancy columns 28 of the floater 10 and lie within the buoyancy columns 28. In other examples, the dampers 24 could be attached to the buoyancy columns 28, for instance on outer sides of the buoyancy columns 28 or at another partially- or fully-submerged location on the floater structure. This allows a possibility of retrofitting the dampers 24 to a floater 10 that is already in use. Potentially, such a retrofitting operation could be performed offshore with the floater 10 in situ and even with the wind turbine 12 remaining operational. As best shown in Figure 3, the damper 24 comprises an upright well or tube defining a hollow cylindrical cavity or chamber 32 that contains a piston 34. The piston 34 is movable freely up and down within the chamber 32 through a longitudinal stroke. There is a radial clearance between the piston 34 and the surrounding internal wall of the chamber 32 in this example but in other examples, the piston 34 could be a close sliding fit with, or slidably sealed to, the internal wall of the chamber 32. The chamber 32 contains water that surrounds the piston 34. Movement of the piston 34 relative to the chamber 32 is resisted by churning, pumping or friction losses in water that is displaced by the piston 34 and / or by compression of water that is trapped by the piston 34. The piston 34 can therefore act as a plunger within the chamber 32, which serves as a hydraulic dashpot. In other words, the piston 34 acts as a brake by moving water in the chamber 32 to resist relative movement between the piston 34 and the structures that define and support the chamber 32. In this example, the piston 34 is neutrally or positively buoyant with respect to the surrounding water. The piston 34 may be partially submerged in water within the chamber 32 as shown in Figures 1 and 2 and also in Figure 4, noting that Figures 1 and 2 show the chamber 32 only partially submerged beneath the level of the surface 14. The piston 34 could, however, be fully submerged in water within the chamber 32, for example if the chamber 32 is filled entirely with water as shown in Figure 5. In this respect, the chamber 32 could instead be located on the floater 10 at a level entirely beneath the surface 14. In either case, the chamber 32 can be in fluid communication with the body of water that surrounds and supports the floater 10, hence also containing seawater. The piston 34 forms part of a brake structure 36 that further includes a submerged sea anchor 38 and a link 40 that connects the piston 34 to the sea anchor 38. In this example, the sea anchor 38 is a mass of water contained within a balloon-like envelope. The sea anchor 38 is neutrally or negatively buoyant with respect to the surrounding water. The link 40 may be long enough that the sea anchor 38 is at a depth less effected by waves and swell than near the surface 14, for example ten metres. The sea anchor 38 need not be coupled to the seabed 16 or to any other subsea structure but instead resists movement of the piston 34 via the link 40 due to inertia and hydrodynamic drag. Thus, the sea anchor 38 shown here is not drawn to scale and could be substantially larger than is shown in comparison with the other parts of the damper 24. Also, the envelope of the sea anchor 38 could have a non-spherical shape, for example a flat or concave shape that is oriented to increase hydrodynamic drag, hence resisting vertical movement of the sea anchor 38 through the water. However, the main factor that resists movement of the sea anchor 38 is the substantial inertia of the encapsulated mass of water itself. The link 40 is exemplified here by a flexible elongate element such as a wire, rope or cable that acts in tension between the piston 34 and the sea anchor 38. The link 40 is substantially inextensible but could instead be extensible, for example elastically. In this example, the link 40 extends through a flow restrictor 42 in the lower end of the chamber 32. The flow restrictor 42 permits but hinders a flow of water through the lower end of the chamber 32 that the piston 34 displaces from within the chamber 32 into the surrounding sea. Thus, the flow restrictor 42 cooperates with the piston 34 to enhance the braking effect of the piston 34 on the chamber 32 and hence on the structures that define and support the chamber 32. Conversely, the upper end of the chamber 32 is closed in this example. More specifically, when the chamber 32 of the damper 24 rises in the water as represented on the right side of the tilted floater 10 shown in Figure 2, the inertia and drag of the sea anchor 38 applies tension to the link 40 to resist upward movement of the piston 34. Under the tension of the taut link 40, the piston 34 cannot move upwardly as quickly as the chamber 32 that is fixed to the floater 10. This results in upward movement of the chamber 32 relative to the piston 34, which has the effect of downward movement of the piston 34 within the chamber 32. Movement of the piston 34 within the chamber 32 forces water from the lower portion of the chamber 32 through the flow restrictor 42 in the lower end of the chamber 32. To the extent that there is clearance between the piston 34 and the internal wall of the chamber 32, that movement also displaces and churns water around the piston 34. Both of these effects resist upward movement of the chamber 32 and hence of the floater 10 relative to the piston 34. In this way, the resistance to motion of the piston 34 relative to the chamber 32 couples the sea anchor 38 to the floater 10 to apply damping resistance to the movement of the floater 10. It will be apparent that the damper 24 on the left side of the floater 10 represented in Figures 1 and 2 will similarly resist upward movement of that side of the floater 10 when the floater 10 rolls with opposite inclination to that shown in Figure 2. However, the dampers 24 may have unidirectional effect if the links 40 are flexible and therefore are not configured to act in compression, as exemplified here. In this respect, the image inset to the left of Figure 2 shows the link 40 slackening and bending under compression as the left side of the tilting floater 10 moves downwardly through the water. The brake structure 36 also comprising the piston 34 and the sea anchor 38 will provide only minimal damping effect in that instance. Viewed overall, the brake structure 36 has substantially neutral buoyancy so as to remain substantially stationary in the water column 28. For this purpose, the components of the brake structure 36 could all be of substantially neutral buoyancy, or positive buoyancy of one component could compensate for negative buoyancy of another component. For example, the piston 34 could have positive buoyancy sufficient to counterbalance negative buoyancy of the sea anchor 38, that buoyancy differential being useful to keep a flexible link 40 taut between them. Where the piston 34 is only partially immersed when the brake structure 36 is in an equilibrium position as shown in Figure 4, it will be apparent that the piston 34 will displace more water if the water level rises within the chamber 32 due to sea movement. In that case, the piston 34 will contribute additional buoyant upthrust that tends to lift the piston 34 up the chamber 32. Whilst Figure 2 shows the response of the dampers 24 to roll or pitch movements of the floater 10, it will be apparent that dampers 24 of the invention can also act against heave where the floater 10 rises or falls in the water without necessarily tilting. Figure 6 exemplifies how a link 40 between the piston 34 and the sea anchor 38 could be rigid, for example a rigid tube or rod. A rigid link 40 could be used in any embodiment of the invention. As a rigid link 40 is capable of acting in compression in addition to acting in tension, this allows a damper 24 to have a bidirectional damping effect on the floater 10. The example shown in Figure 6 also has a closure 44 in the lower end of the chamber 32, forming a sliding seal around the link 40. In this way, the chamber 32 can be sealed from the surrounding sea, hence relying upon churning or friction losses in water that is displaced by the piston 34 within the chamber 32. In principle, a fluid other than seawater could be present in the chamber 32. However, a flow restrictor 42 like that shown in Figures 3 to 5 could be used instead at either or both ends of the chamber 32 if it is preferred to pump water out of or into the chamber 32. The damper 24 of Figure 7 shows a converse possibility in which the upper end of the chamber 32 remains closed but the lower end of the chamber 32 is wholly open, save for any retaining provisions that may be appropriate to prevent the piston 34 sliding out of the chamber 32. The damper 24 shown in Figure 8 goes further by also having an open upper end of the chamber 32. Figure 9 is similarly open at the upper end of the chamber 32 but the link 40 extends through a flow restrictor 42 at the lower end of the chamber 32. A flow restrictor 42 can also be present at the upper end of the chamber 32 as shown in Figure 10, either with a flow restrictor 42 at the lower end of the chamber 32 as shown or without such an additional flow restrictor 42. Figure 11 shows that the sea anchor 38 need not necessarily encapsulate a mass of water but could instead be shaped to increase hydrodynamic drag that resists vertical movement of the sea anchor 38 through the water. In the example shown, the sea anchor 38 is shaped as an underwater parachute or drogue, which again is not drawn to scale. Turning finally to Figure 12 of the drawings, this shows a variant of the damper 24 in which the arrangement is inverted to exemplify how the sea anchor 38 could instead be disposed above the piston 34 of the brake structure 36. In this case, if a flexible link 40 is present, the sea anchor 38 may conveniently have positive buoyancy to offset negative buoyancy of the piston 34, hence keeping the link 40 taut. The link 40 extends through a flow restrictor 42 in the upper end of the chamber 32 and there is another flow restrictor 42 at the lower end of the chamber 32 in this example. More generally, flow restrictors 42 or closures 44 could be present at one or both ends, or at neither end, of the chamber 32 shown in Figure 12. Many other variations are possible within the inventive concept. For example, dampers of the invention could be attached to mooring lines of a floater instead of, or in addition to, the floater itself. Valves could control flows of water into either or both ends of the chamber either unidirectionally or bidirectionally, and such valves may be adjustable to control the damping effect. An opening or an associated flow restrictor or valve communicating with the interior of the chamber is not necessarily positioned at an end of a chamber but could instead penetrate a side wall of the chamber. A floater damped in accordance with the invention need not necessarily float at the surface but could, for example, be a sub-surface buoy that floats suspended in the water column between the surface and the seabed. It would also be possible, in principle, for dampers of the invention to act on a floating structure that is not moored but is instead free-floating at the surface or in the water column. 30 10 25

Claims

1. A passive motion damper for a floating body, the damper having a brake structure comprising a submerged sea anchor that is suspended in a water column and connected to a5 piston by a link, the piston being movable along an upright axis within an elongate chamber that is in fixed relation to the body and that movement being braked by displacement of fluid in the chamber by relative movement between the chamber and the piston.

2. The damper of Claim 1, wherein the brake structure is of substantially neutral buoyancy10 overall.

3. The damper of Claim 1 or Claim 2, wherein the sea anchor is suspended beneath the piston via the link.15 4. The damper of Claim 3, wherein the sea anchor is negatively buoyant.

5. The damper of Claim 3 or Claim 4, wherein the piston is positively buoyant.

6. The damper of Claim 1 or Claim 2, wherein the sea anchor is suspended above the piston 20 by positive buoyancy.

7. The damper of Claim 6, wherein the piston is negatively buoyant.

8. The damper of any preceding claim, wherein the link acts in tension between the sea25 anchor and the piston.

9. The damper of Claim 8, wherein the link is flexible.

10. The damper of Claim 8, wherein the link also acts in compression between the sea30 anchor and the piston.

11. The damper of Claim 10, wherein the link is rigid.

12. The damper of any preceding claim, wherein the link emerges from the chamber through35 a first end of the chamber, the chamber also having a second end opposed to the first end about the piston.30 10 2513. The damper of Claim 12, wherein the first end is beneath the second end.

14. The damper of Claim 12 or Claim 13, further comprising a flow restrictor that permits but restricts water flow into or out of the chamber through the first end.

515. The damper of Claim 12 or Claim 13, further comprising a closure that seals the first end around the link16. The damper of any of Claims 12 to 15, wherein the second end is closed to fluid flow.1017. The damper of any of Claims 12 to 15, wherein the second end is open to fluid flow.

18. The damper of Claim 17, further comprising a flow restrictor that permits but restricts water flow into or out of the chamber through the second end.1519. The damper of any preceding claim, wherein the piston is fully submerged in water within the chamber.

20. The damper of any preceding claim, wherein the chamber is fully submerged in water.2021. The damper of any preceding claim, wherein the sea anchor comprises a water-filled envelope.

22. A buoyant body comprising at least one motion damper of any of Claims 1 to 21.2523. The body of Claim 22, wherein the chamber is disposed within a structure of the body.

24. The body of Claim 22, wherein the chamber is attached externally to a structure of the body.3025. The body of Claim 23 or Claim 24, wherein said structure of the buoyant body is a buoyancy structure.

26. The body of any of Claims 22 to 25, wherein the or each motion damper is offset laterally 35 from a centre of buoyancy of the body.30 10 2527. The body of any of Claims 22 to 26, being anchored to a fixed point upon or in the seabed.

28. A method of passively damping motion of a floating body, the method comprising:5anchoring a piston with a sea anchor to restrict movement of the piston along an upright axis;permitting greater movement along the upright axis of an elongate chamber10 surrounding the piston, the chamber being in fixed relation to the body; andbraking relative movement between the chamber and the piston by displacing fluid in the chamber by that relative movement.15 29. The method of Claim 28, comprising displacing fluid out of the chamber.

30. The method of Claim 29, comprising restricting a flow of fluid being displaced out of the chamber.20 31. The method of any of Claims 28 to 30, comprising permitting fluid flow through the pistonor around the piston between the piston and an internal wall of the chamber.

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