MARINE FOUNDATIONS COMPRISING SUCTION PILES

Abstract

A marine foundation such as a jacket or a tripod foundation for a wind turbine comprises suction piles that are subjected, in service, to cyclical loading of compression phases and tension phases in alternation. Each pile has a one-way valve that opens and closes autonomously in response to pressure differentials between the internal chamber and the surrounding water. The valve opens during the compression phases to effect fluid communication between an internal chamber of the pile and surrounding water. Water is thereby ejected from within the chamber through the valve. Conversely, during the tension phases, the valve closes and water is admitted into the pile only through soil within a skirt of the pile. Thus, a unidirectional, generally upward flow of water is driven through the soil within the skirt during the compression and tension phases, maximising water flow friction and reducing the risk of liquefaction of the soil.

Claims

1. A method of operating a marine foundation during cyclical loading that subjects a suction pile of the foundation to compression phases and tension phases in alternation, the method comprising: during the compression phases, opening a one-way valve to effect fluid communication between an internal chamber of the pile and surrounding water, thereby ejecting water from within the chamber through the valve; and during the tension phases, closing the valve and admitting water into the pile through soil within a skirt of the pile.

2. The method of claim 1, comprising opening and closing the valve autonomously in response to pressure differentials between the internal chamber of the pile and the surrounding water.

3. The method of claim 1, comprising driving an upward flow of water through the soil within the skirt during the compression phases and the tension phases.

4. The method of claim 1, comprising ejecting water from the internal chamber through an external wall of the pile.

5. The method of claim 4, wherein the valve is arranged to close an aperture in the external wall.

6. The method of claim 4, wherein the external wall is a top plate of the pile.

7. The method of claim 1, comprising ejecting water from the internal chamber through a plug within the pile, atop the soil within the skirt.

8. The method of claim 7, wherein the valve is arranged to close an aperture in the plug.

9. The method of claim 1, comprising passing the water being ejected through at least one porous barrier disposed upstream and/or downstream of the valve.

10. The method of claim 1, comprising a preliminary step of opening the valve while lowering the pile through water toward the soil, allowing water to flow out of the valve after entering an open bottom of the skirt.

11. The method of claim 1, comprising a preliminary step of opening the valve while pre-loading the pile after embedding the skirt in the soil, allowing water draining from the soil within the skirt to exit through the valve.

12. The method of claim 10, comprising holding a movable valve element of the valve in an open position during the preliminary step, and subsequently freeing the valve element to move into a closed position.

13. The method of claim 1, comprising, preliminarily, pumping water from within the pile after embedding the skirt in the soil.

14. The method of claim 13, comprising keeping the valve closed during the pumping step.

15. The method of claim 1, comprising, preliminarily, depositing ballast material over the pile and then holding the deposited ballast material clear of the valve.

16. The method of claim 1, comprising closing the valve with assistance of gravity.

17. A marine installation comprising a structure supported by at least one suction pile having a skirt embedded in soil beneath a body of water, the at least one pile having a one-way valve arranged to effect fluid communication between an internal chamber of the pile and water surrounding the pile to allow ejection of water from within the chamber through the valve, wherein the valve is enabled to open autonomously when there is overpressure in the chamber due to the pile being under a compression load and to close autonomously when there is underpressure in the chamber due to the pile being under a tension load.

18. The installation of claim 17, wherein the structure extends to a level above the body of water.

19. The installation of claim 18, wherein the structure is a wind turbine foundation.

20. The installation of claim 17, comprising at least two suction piles embedded in the soil with mutual horizontal spacing.

21. The installation of claim 20, wherein the suction piles are under respective legs of a jacket or tripod foundation.

22. The installation of claim 17, wherein the valve is arranged to close an aperture in an external wall of the pile.

23. The installation of claim 22, wherein the external wall is a top plate of the pile.

24. The installation of claim 17, wherein the valve is arranged to close an aperture in a plug within the pile, atop the soil within the skirt.

25. The installation of claim 17, further comprising at least one porous barrier disposed upstream and/or downstream of the valve.

26. The installation of claim 25, comprising at least one foraminous shroud over the valve.

27. The installation of claim 26, further comprising a layer of ballast material lying upon the pile and separated from the valve by the shroud.

28. The installation of claim 17, wherein the valve comprises a valve element that is free to move relative to a valve seat between a lower, closed position and an upper, open position.

29. The installation of claim 28, wherein the valve element comprises a plate that is movable relative to the valve seat along at least one upright guide.

30. The installation of claim 29, wherein the at least one upright guide comprises an upper enlargement that limits upward movement of the plate along the guide.

31. The installation of claim 28, wherein the valve element comprises a flap that is pivotable relative to the valve seat.

Description

[0046] To put the invention into context, reference has already been made to FIGS. 1a and 1b of the accompanying drawings, which are schematic side views of an offshore installation subjected to reversing lateral loads. In order that the invention may be more readily understood, reference will now be made, by way of example, to the remainder of the drawings in which:

[0047] FIGS. 2a and 2b are schematic sectional side views of a suction pile of the invention during, respectively, compression and tension phases of cyclical loading;

[0048] FIG. 3 is a schematic sectional side view of a variant suction pile of the invention;

[0049] FIG. 4 is a schematic sectional side view of a further variant suction pile of the invention, shown during a compression phase;

[0050] FIG. 5 is a schematic sectional side view of a suction pile of the invention while being lowered through water toward the seabed during installation;

[0051] FIG. 6 is an enlarged detail view showing provisions for holding a one-way valve open while lowering a suction pile of the invention through water or during a preloading phase of installation;

[0052] FIG. 7 is a schematic sectional side view of a conventional pile as the surrounding seabed soil consolidates during a preloading phase; and

[0053] FIG. 8 corresponds to FIG. 7 but shows a suction pile of the invention during the same phase of installation.

[0054] FIGS. 2a and 2b show a suction pile 18 of the invention in use, embedded in the soil 24 of the seabed 20, which is exemplified here by sand. The suction pile 18 is shown in FIGS. 2a and 2b during, respectively, compression and tension phases of cyclical loading. In successive cycles, alternating downward and upward loads Fc, Fr are applied to the suction pile 18 by a supported structure such as a jacket leg 16 like that shown in FIGS. 1a and 1b.

[0055] Some seabed soil 24 is encircled by the tubular skirt 26 of the suction pile 18. As noted above, a suction chamber 28 is defined in the space within the skirt 26 between that soil 24 and the top plate 30. Water occupies the suction chamber 28 and fills pores between grains of sand in the soil 24, in fluid communication with the suction chamber 28.

[0056] In this example, the top plate 30 of the suction pile 18 comprises a conventional suction valve 32 through which water can be pumped out of the suction chamber 28 during a suction phase of installation. The suction valve 32 remains closed thereafter while the pile 18 remains in service.

[0057] In accordance with the invention, the suction pile 18 comprises a one-way valve 34 in a fluid communication path between the exterior of the pile 18 and an internal chamber of the pile 18 that communicates with the pores in the soil 24. In this case, that internal chamber is the suction chamber 28 located directly beneath the top plate 30. Conveniently, the valve 34 is mounted on or in the top plate 30, as in this example, although that location is not essential.

[0058] The valve 34 is enabled to open and close in response to reversal of pressure differentials between the exterior and the interior of the suction pile 18 when the pile 18 is in service and exposed to cyclical loads Fc, Fr. This is distinguished from a conventional suction valve 32, which is always closed except when pumping water out of the suction chamber 28 during installation. It is also distinguished from check valves or hatches of the prior art that are kept open only while lowering a suction pile to the seabed during installation and then are kept closed.

[0059] In this example, the valve 34 comprises a tubular housing or sleeve 36 mounted in a corresponding aperture 38 that penetrates the top plate 30. The sleeve 36 is open at its top and bottom ends to effect fluid communication, through the aperture 38, between the exterior of the suction pile 18 and the suction chamber 28 within the pile 18.

[0060] The valve 34 further comprises a valve element 40 in the form of a movable plate that defines a closure cooperable with the top end of the sleeve 36. The sleeve 36 therefore provides a seat for the valve element 40. The valve element 40 is guided in its movement by parallel upright guides 42 such as rods or bolts along which the valve element 40 can slide up and down. Upward excursion of the valve element 40 is limited by enlarged heads 44 at the upper ends of the guides 42 beyond which the valve element 40 cannot slide, thus defining a limited range of vertical movement of the valve element 40 relative to the top end of the sleeve 36.

[0061] In an upper, open position shown in FIG. 2a, the valve element 40 is disposed above and clear of the top end of the sleeve 36, thereby allowing fluid flowing through the aperture 38 to pass through the gap between the sleeve 36 and the valve element 40. Conversely, in a lower, closed position shown in FIG. 2b, the valve element 40 bears against and closes the top end of the sleeve 36, thereby blocking fluid flow through the aperture 38.

[0062] The valve element 40 is movable between the open and closed positions by pressure differentials between the exterior of the suction pile 18 and the suction chamber 28. Specifically, an overpressure in the suction chamber 28 relative to the exterior of the pile 18, characteristic of a compression phase shown in FIG. 2a, lifts the valve element 40 into the open position. Conversely, an underpressure in the suction chamber 28 relative to the exterior of the pile 18, characteristic of a tension phase shown in FIG. 2b, forces the valve element 40 into the closed position. The valve element 40 is also biased toward the lower, closed position by gravity.

[0063] It will be apparent from FIG. 2a that when the suction pile 18 is subject to downward load Fc during a compression phase, overpressure in the suction chamber 28 lifts the valve element 40 into the upper, open position. The overpressure is thereby relieved by an upward flow of water 46 from the suction chamber 28 through the valve 34. Water 48 also flows upwardly toward the suction chamber 28 through the pores of the soil 24 within the skirt of the pile 18, in addition to some water 50 being expelled downwardly through the pores of the soil 24 via the open bottom of the skirt 26 in a conventional manner.

[0064] It will also be apparent from FIG. 2b that when the suction pile 18 is subject to upward load Fr during a tension phase, underpressure in the suction chamber 28 pulls the valve element 40 down into the lower, closed position. The underpressure draws an upward flow of water 52 into the pile 18 via the open bottom of the skirt 26 and through the pores in the soil 24. As water cannot now enter the top of the pile 18 through the closed valve 34, the flow of water is essentially upward-only and unidirectional.

[0065] In the next compression phase, much of the water drawn into the suction pile 18 during the tension phase is expelled through the now-reopened valve 34 as shown in FIG. 2a. Thus, in this respect, the flow of water through the pores of the soil 24 within the skirt 26 is predominantly upward and substantially unidirectional throughout successive compression-tension-compression cycles. This maximises the beneficial effect of water flow friction and minimises the risk of liquefaction of the soil 24.

[0066] Turning next to FIG. 3, this shows various porous barriers that allow water to flow through them while preventing soil or other debris such as rocks 54 from jamming, or otherwise disrupting, autonomous operation of the valve 34 in service of the suction pile 18. In this respect, FIG. 3 shows the option of a berm of rocks 54 deposited on top of the installed pile 18 as ballast. Grouting of the pile 18 after installation is also a conventional possibility.

[0067] One such barrier is a cage 56, or other foraminous shroud, that surrounds the external side of the valve 34 to keep rocks 54 away from the valve 34. Another barrier is a filter mesh 58 that spans the aperture 38 within the sleeve 36 of the valve 34. A further barrier is a filter mesh 60 that spans the interior of the skirt 26 between the valve 34 and the top of the soil 24 within the skirt 26. The filter meshes 58, 60 keep soil 24 within the pile 18 away from the underside of the valve 34. The cage 56 and the filter meshes 58, 60 can be used individually or in any combination of two or more such barriers.

[0068] Moving on to FIG. 4, this shows another embodiment of the suction pile 18 in which a concrete layer or plug 62 is cast or otherwise placed on top of the soil 24 within the skirt 26, under the top plate 30. In this case, fluid communication between the exterior of the pile 18 and the soil 24 within the skirt 26 is effected by an upper aperture 64 in the top plate 30 and a lower aperture 66 in the plug 62. At least one of those apertures 64, 66 can be closed by a one-way valve 68 in accordance with the invention.

[0069] In this example, the valve 68 comprises a tubular housing or sleeve 70 around the lower aperture 66 extending through the plug 62. The upper aperture 64 is always open but may be protected by a barrier mesh 72, as shown, that permits water flow but prevents rocks or other debris from falling into the pile 18 and potentially jamming the valve 68.

[0070] FIG. 4 shows an alternative arrangement for the valve 68, which could also be applied to the preceding embodiment. Conversely, the valve 34 of the preceding embodiment could be applied to this embodiment. In this case, the valve element is a flap 74 that is hinged to the top of the sleeve 70. The sleeve 70 is spanned by a filter mesh 76 to keep the soil 24 away from the flap 74.

[0071] In an upper, open position shown in FIG. 4, the flap 74 is hinged away from the top of the sleeve 70, thereby allowing fluid flowing upwardly through the lower aperture 66 to pass through the gap between the sleeve 70 and the flap 74. Conversely, in a lower, closed position, the flap 74 bears against and closes the top of the seeve 70, thereby blocking fluid flow downwardly through the lower aperture 66. Thus, as in the preceding embodiment, upward, unidirectional flow of water through pores of the soil 24 within the skirt 26 is encouraged during both the compression phase shown in FIG. 4 and a subsequent tension phase when load on the pile 18 reverses.

[0072] It would be possible to reverse the arrangement of FIG. 4 by positioning the valve 68 in the upper aperture 64 and leaving the lower aperture 66 open, save for the option of a filter mesh 76.

[0073] FIGS. 5 and 6 show that the invention may also have benefit when lowering a suction pile 18 toward the seabed 20. Here, a one-way valve 34 in the top plate 30 of the pile 18 is open when lowering, as shown in FIG. 5, so that water can flow along and through the pile 18 to the benefit of stability.

[0074] The valve element 40 may assume the upper, open position shown in FIG. 5 in response to differential pressure or drag forces as the pile 18 falls through the water column. Alternatively, or additionally, the valve element 40 may be held open temporarily during the lowering operation. For example, FIG. 6 shows removable pins 78 such as beta pins that are received in transverse bores 80 extending through the guides 42. When engaged with the guides 42, the pins 78 bear against the underside of the valve element 40 to prevent the valve element 40 dropping into the closed position against the top of the sleeve 36. When the pile 18 reaches the seabed 20, the pins 78 can be removed, for example by an ROV, to allow the valve 34 to close. The valve 34 is then enabled to open and close automatically and autonomously in response to cyclical compression and tension loads F.sub.C, F.sub.T applied to the pile 18 as shown in FIGS. 2a and 2b.

[0075] In principle, the pins 78 could be replaced or repositioned above the valve element 40 after the valve 34 closes so as to hold the valve element 40 in the closed position against the top of the sleeve 36. This may be beneficial to ensure the integrity of the suction chamber 28 during a suction phase of installation in which water is pumped out through the suction valve 32. However, an underpressure applied to the suction chamber 28 via the suction valve 32 will tend to hold the valve 34 closed in any event.

[0076] Turning finally to FIGS. 7 and 8, these drawings show the behaviour of a suction pile 18 during a pre-loading phase of installation. In that phase, as the pile 18 settles into the seabed 20 under downward load, the soil 24 of the seabed 20 consolidates around the skirt 26 of the pile 18 as water drains through the pores of the soil 24. The behaviour of a conventional pile 18 shown in FIG. 7 may be compared with that of a pile 18 of the invention as shown in FIG. 8. In FIG. 7, water can only drain downwardly from the soil 24 within the skirt 26 and out through the open bottom of the skirt 26. This limited flow of water out of the pile 18 slows the process of consolidation. In contrast, in FIG. 8, water can escape from the soil 24 within the skirt 26 both upwardly through the one-way valve 34 and downwardly through the open bottom of the skirt 26. Beneficially, this enhanced flow of water out of the pile 18 by virtue of plural drainage paths accelerates the process of consolidation.

[0077] The one-way valve 34 could be held open during the pre-loading phase shown in FIGS. 7 and 8, for example using an arrangement of pins 78 like that shown in FIG. 6. Alternatively, an overpressure in the suction chamber 28 can open the one-way valve 34 to an extent sufficient to promote drainage of water through the top plate of the pile 18.

[0078] Many other variations are possible within the inventive concept. For example, a one-way valve of the invention could be integrated with, or also serve as, a suction valve so that one valve performs both functions.

[0079] Upward excursion of the valve element could be limited in other ways, for example by a cage or other protective barrier structure surrounding the one-way valve.

[0080] The valve element of the one-way valve could be biased into the closed position, for example by a spring acting downwardly from above the valve element.