Patent Description:
Suction piles - also known in the art as suction anchors, suction cans, suction caissons or suction buckets - are commonly used in the renewable energy industry and in the oil and gas industry to anchor large offshore installations to the seabed. To do so, they are designed to engage soft seabed soil that typically comprises marine sediments such as sand or soft clay.

A suction pile is usually fabricated from steel and typically comprises an open-bottomed hollow straight tube defining a deep cylindrical skirt. The skirt may be several metres in length, for example ten metres. A major bottom portion of the skirt engages the seabed soil by friction or cohesion upon being embedded axially into the soil. The top of the skirt is closed by a top plate. A suction chamber is defined between the top plate, the skirt and the seabed soil trapped within the embedded skirt.

The top plate is penetrated by a suction vent or port through which water can be pumped out of the suction chamber. The resulting underpressure in the suction chamber promotes engagement of the suction pile with the soil. <CIT> describes various examples of ports or vents for suction piles. Another example of a vent is shown in <CIT>, where a mobile plate is driven by a screw and a mechanism holds the plate at the required opening position.

Installation of a suction pile involves firstly allowing the pile to self-penetrate under its own weight into the seabed and secondly, after a short period of settlement, pumping water out of the resulting suction chamber to create a pressure differential.

Specifically, when a suction pile is landed on the seabed in an upright orientation, the skirt embeds partially into the seabed soil under the self-weight and momentum of the pile. Self-penetration of the pile ends when resistance to relative sliding movement between the skirt and the seabed soil balances the weight of the pile. The soil within the embedded skirt closes the bottom of the pile to create the suction chamber.

When seawater is subsequently pumped out of the suction chamber, underpressure in the chamber draws the top plate toward the seabed as the chamber contracts under external hydrostatic pressure. This forces the skirt to sink further into the soil, hence effecting fuller engagement of the suction pile with the soil. Thus, suction overcomes the resistance of friction or cohesion to force the skirt deeper into the seabed, hence enabling the pile to resist forces that will be applied after installation by equipment subsequently anchored to or supported on the pile.

By virtue of the piston effect of the closed top of the pile, any longitudinal movement of the installed pile requires water to flow into or out of the open bottom of the pile. Water flow friction, being resistance to that flow of water through the pores of the soil within the pile, is significantly greater - potentially more than five times greater - than skirt friction, being friction or cohesion between the skirt and the embedding seabed soil. A suction pile therefore engages with the seabed by virtue of water flow friction combined with skirt friction.

The top plate may comprise openable hatches or valves that are left open while lowering the suction pile through the water. This mitigates added mass by allowing water to flow freely through the pile as the pile moves downwards, hence reducing drag and improving stability. Once the skirt of the pile has been embedded into the seabed soil, the hatches or valves are closed so that a pressure differential can be created in the suction chamber. Any such hatches or valves are kept closed thereafter to enhance the water flow friction effect that resists longitudinal movement. For example, <CIT> discloses a suction pile foundation in which check valves are designed to open only during lowering and are closed in service.

Once embedded into the seabed soil, a suction pile can serve as an anchor or as a support for various types of subsea or surface equipment. For example, suction piles may be used for mooring or tethering a platform, a surface vessel or a buoy. Mooring lines and tethers act in tension and so apply upward and lateral traction forces to a pile. In that case, the pile must resist being pulled up out of engagement with the seabed or being pulled away from the vertical, or capsizing. Suction piles are also commonly used to support the weight of a structure such as a manifold. In that case, the pile must resist downward compression forces that tend to bury the pile deeper into the seabed.

In many cases, suction piles are subjected to only one type of loading in use, being either tension or compression. For example, where a suction pile supports the weight of a structure located deep underwater below the influence of surface dynamics, compressive loading dominates. However, where a suction pile supports a structure that is located close to or at the surface, cyclical loading becomes more significant. For example, cyclical loading may arise due to periodic application and relaxation of tension as a tethered floating structure moves under the influence of surface dynamics.

Cyclical loading becomes particularly significant where a suction pile supports the weight of a rigid structure or frame that extends above the surface, such as a wind turbine base or a jacket. This is because a structure exposed to the effects of weather tends to rock from side to side about an upright axis on encountering intermittent lateral forces from swells, waves and wind gusts, hence pivoting around horizontal axes as the structure tilts to-and-fro. This is a particular concern where a structure is supported by a group of laterally-spaced suction piles, for example positioned under respective legs of a tripod base structure or a jacket.

By virtue of lateral loads, a group of suction piles supporting an offshore structure such as a wind turbine may have at least one pile in tension and at least one other pile in compression at any given moment. This situation will reverse as swells or waves act on the structure from different lateral directions, with a period of typically six to twelve seconds between reversals. Thus, each pile will experience high-frequency cycles of compression and tension alternating in rapid succession as the structure supported by the group of piles tends to rock from side to side.

To illustrate this, <FIG> show an offshore installation <NUM> exemplified by a wind turbine <NUM> supported by a jacket structure <NUM>. The jacket <NUM> comprises legs <NUM> that surmount respective suction piles <NUM> embedded in the seabed <NUM>. There may be three, four or more such legs <NUM> and at least a corresponding number of suction piles <NUM>.

The jacket <NUM> extends from the seabed <NUM> to above the surface <NUM>, where it and the wind turbine <NUM> are subject to a fluctuating lateral load L arising primarily from swells and waves impinging on the legs <NUM> of the jacket <NUM>. By virtue of its vertical offset from the seabed <NUM>, the load L generates a moment M that tends to tilt the installation <NUM> about a horizontal axis and therefore to sway the tower of the wind turbine <NUM> away from the vertical. Thus, a suction pile <NUM> on the side of the jacket <NUM> facing away from the load L is subjected to a downward compressive force FC whereas a suction pile <NUM> on the side of the jacket <NUM> facing toward the load L is subjected to an upward tension force FT.

Where the suction piles <NUM> are embedded in sand, the compressive force FC and the tension force FT cause water to flow in the pores of the sand trapped within each suction pile <NUM>. The suction pile <NUM> subjected to an upward tension force FT will try to move upwards and therefore water will be sucked in through the open bottom of the suction pile <NUM> as shown. Conversely, the suction pile <NUM> subjected to a downward compressive force FC will try to move downwards and therefore water will be pushed out through the open bottom of the suction pile <NUM> as shown.

As will be apparent from a comparison of <FIG>, the load L reverses in direction periodically. Thus, the moment M also reverses, as do the vertical forces FC and FT acting on the suction piles <NUM> and hence also the direction of water flow through the open bottom of each suction pile <NUM>. This cycle may repeat every six to twelve seconds as noted above. Rapid cyclical loadings are particularly problematic as bidirectional water flow, with little time for settling between flow reversals, can lead to liquefaction of the soil and hence potentially catastrophic failure of the foundation.

More generally, when a suction pile in subjected to compression, water builds up and flows within the volume of soil surrounded by the skirt. This build-up and flow of water reduces skirt friction and therefore reduces the performance of the pile when the pile is subsequently subjected to tension in a cyclical loading scenario. Generally, this is not a concern where a suction pile can be over-designed or over-sized to compensate for a slight loss of friction or cohesion with the surrounding soil. However, over-designing is costly and inefficient and may not always be possible to a necessary extent. For example, if the seabed comprises a shallow layer of soil overlying solid rock, it may not be possible to make the skirt long enough to ensure good performance in tension. In such cases, cyclical loadings that tend to loosen the seabed soil can weaken the foundation to an unacceptable extent, hence requiring a different and more expensive foundation solution.

<CIT> relates to a system for preventing the build-up of excessive pore water pressure in permeable foundations. The system operates in response to the pressure differentials created by sea swell, with a one-way valve opening when the sea pressure differs from a datum pressure by a specified value to allow fluid communication with a network of drainpipes, allowing the pore water to drain away into the sea.

<CIT>, on the other hand, discloses an underwater sediment evacuation system. The system comprises a suction pile and uses a series of pumps and valves to evacuate sediment from the internal volume of the suction pile.

<CIT> discloses a segmented suction pile with an attachment system to allow attachment of external equipment to the suction pile, particularly a subsea structure or an anchor line. The suction pile also includes a hole in the top surface of the pile for attaching a ventilation system for ejecting water, mud and air trapped in the pile.

<CIT> provides a foundation structure that uses the wave action of the sea to generate added stability against overturning forces. The foundation includes flap valves below the surface of the sea and just below the lowest wave trough level that open to allow water to escape through a central column of the foundation when the trough of a wave passes the flaps.

<CIT> discloses a tripod foundation for a wind turbine. The structure comprises foundations and legs that connect the foundations to a central column. The interiors of the legs and the column define ballasting compartments. Ballast water is forced into the compartments by the pressure gradient formed due to the relative heights of the surface of the water inside the compartments and of the sea, with valves controlling the ingress of the ballasting water. Water can be pumped out of the compartments to aid penetration of the foundation into the seabed.

Against this background, the invention resides in 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. During the compression phases, a one-way valve is opened to effect fluid communication between an internal chamber of the pile and surrounding water, thereby ejecting water from within the chamber through the valve. Conversely, during the tension phases, the valve is closed and water is admitted into the pile through soil within a skirt of the pile. Thus, a predominantly upward flow of water can be driven through the soil within the skirt during the compression phases and the tension phases. Advantageously, the valve may open and close autonomously in response to pressure differentials between the internal chamber of the pile and the surrounding water.

Water may be ejected from the internal chamber through an external wall of the pile, such as a top plate of the pile that may partially define the chamber. In that case, the valve may be arranged to close an aperture that penetrates the external wall. Water may also, or instead, be ejected from the internal chamber through a plug within the pile, atop the soil within the skirt, that similarly may partially define the chamber. In that case, the valve may be arranged to close an aperture extending through the plug. In each case, water being ejected may flow through at least one porous barrier such as a foraminous filter, mesh, shroud or cage that is disposed upstream and/or downstream of the valve.

The method may comprise various preliminary steps. One such step involves opening the valve while lowering the pile through water toward the soil, thus allowing water to flow out of the valve after entering an open bottom of the skirt. Another such step involves opening the valve while pre-loading the pile after embedding the skirt in the soil, hence allowing water draining from the soil within the skirt to exit through the valve. In each of those steps, a movable valve element of the valve may be held in an open position and subsequently freed to move into a closed position, advantageously with the assistance of gravity.

Another preliminary step of pumping water from within the pile after embedding the skirt in the soil may be performed while keeping the valve closed. A further preliminary step may involve depositing ballast material over the pile and holding the deposited ballast material clear of the valve.

The inventive concept also embraces 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 or each pile has 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. 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.

The structure may extend to a level close to or above the body of water, where the structure is subject to wave action. For example, the structure could be a wind turbine foundation.

At least two suction piles may be embedded in the soil with mutual horizontal spacing. For example, those suction piles could be under respective legs of a jacket or tripod foundation.

The valve may comprise a valve element that is free to move relative to a valve seat between a lower, closed position and an upper, open position. For example, the valve element could comprise a plate that is movable relative to the valve seat along at least one upright guide. In that case, the or each upright guide may have an upper enlargement that limits upward movement of the plate along the guide. Alternatively, the valve element could comprise a flap that is pivotable relative to the valve seat.

In summary, the invention ensures that water flows substantially unidirectionally in pores of the soil within a suction pile, and only flows when needed. This may be achieved by using a one-way valve or an equivalent mobile plate. If there is suction inside the pile under tension, then no water can exit from the top of the pile and the flow path for water flow is through the soil and upwardly into the open bottom of the pile. A few seconds later, when the pile is under compression, there will be an overpressure under the top plate of the pile. Water is then allowed to flow out of the top of the pile instead of being forced downwardly through the soil.

A benefit of the invention is that it creates a one-way flow inside a suction pile. Specifically, pore water in much of the soil within the pile will only flow one way, namely upwardly. This reduces the risk of liquefaction of the soil within the suction pile and so allows designers to rely upon a higher proportion of water flow friction. The result is an increase in the tension capacity of suction piles subject to fast cyclical loading, especially in sand.

The invention can also have benefit in a preloading phase, where seabed soil inside and around a suction pile will start to consolidate once a load is placed on the top of the pile. The invention beneficially adds to the normal consolidation path for excess pore water to escape from the soil within the pile.

The design of a wind turbine foundation supported by a group of suction piles is governed by the performance of the piles when in tension. Often, calculations are made on the assumption that resistance to movement relies solely or predominantly upon skirt friction. In contrast, optimising pore water flow in accordance with the invention allows water flow friction within a suction pile to be taken into greater account in the design of the pile. In this way, it may be possible to shorten the skirt substantially, for example to halve its length, without unduly limiting the performance of the pile. This reduces the cost of such piles, eases their transportation and installation and enables them to be installed in locations where larger conventional suction piles could not be accommodated.

Thus, the invention addresses the problem of improving water flow paths within suction piles and mitigates internal water consolidation under cyclical loading. This allows the invention to depart from the conventional practice of over-sizing suction piles, instead allowing shorter piles to be used.

Embodiments of the invention implement a method for preventing loss of adherence of a suction pile subjected to vertical tension/compression cycles. The method comprises: installing at least one one-way valve in a suction pile, which pile may for example be positioned to support a leg of a jacket; embedding the suction pile in the seabed by self-penetration and by suction; and leaving the one-way valve free to open in service. For example, the one-way valve may be forced closed during a suction phase and reopened after the suction phase. In particular, the one-way valve may be forced open during a compression phase.

The one-way valve may, for example, extend through the top of the pile or may be located inside the pile beneath a removable top plate or lid.

An additional installation step of grouting and/or ballasting the pile with concrete and/or with rocks may take place after suction of the pile.

Embodiments of the invention also comprise a one-way fluid flow mechanism for a suction pile, the mechanism comprising: a transverse closure of an inner volume of the suction pile; a traversing bore through the transverse closure; and a one-way valve in fluid relation with the traversing bore.

The transverse closure could, for example, be a top plate of the pile. In that case, the one-way valve may comprise: an opening through the top plate; at least one vertical guide such as a rod, a bolt or a shaft; a mobile plate sliding on the vertical guide and arranged to close the opening completely when the plate is in a lower position; and an upper stopper that limits upward excursion of the plate.

The transverse closure could instead be a concrete plug atop the seabed soil within the pile. In that case, the one-way valve could comprise a tubular casing through the concrete plug. Such a tubular casing could have a filter at a lower end in fluid communication with a lower volume of the pile and/or a one-way flap or plate at an upper end.

Whilst sand and small debris may be pushed away by a flow of exiting water, the one-way valve may, for example, be enclosed inside a cage or a net preventing larger debris such as rocks entering and potentially jamming the mechanism during opening. For similar purposes, the valve may comprise a filter or the pile itself may contain a filtering mesh, located within the skirt of the pile.

A mechanism may be provided for forcing the one-way valve into an open or closed position, or for holding the one-way valve in either or both of those positions. For example, a removable pin such as a beta pin may pass through or otherwise engage the vertical guide to block movement of the mobile plate along the guide.

Thus, the invention is concerned with a marine foundation such as a jacket or a tripod foundation for a wind turbine. The foundation 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.

To put the invention into context, reference has already been made to <FIG> 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:.

<FIG> show a suction pile <NUM> of the invention in use, embedded in the soil <NUM> of the seabed <NUM>, which is exemplified here by sand. The suction pile <NUM> is shown in <FIG> during, respectively, compression and tension phases of cyclical loading. In successive cycles, alternating downward and upward loads FC, FT are applied to the suction pile <NUM> by a supported structure such as a jacket leg <NUM> like that shown in <FIG>.

Some seabed soil <NUM> is encircled by the tubular skirt <NUM> of the suction pile <NUM>. As noted above, a suction chamber <NUM> is defined in the space within the skirt <NUM> between that soil <NUM> and the top plate <NUM>. Water occupies the suction chamber <NUM> and fills pores between grains of sand in the soil <NUM>, in fluid communication with the suction chamber <NUM>.

In this example, the top plate <NUM> of the suction pile <NUM> comprises a conventional suction valve <NUM> through which water can be pumped out of the suction chamber <NUM> during a suction phase of installation. The suction valve <NUM> remains closed thereafter while the pile <NUM> remains in service.

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

The valve <NUM> is enabled to open and close in response to reversal of pressure differentials between the exterior and the interior of the suction pile <NUM> when the pile <NUM> is in service and exposed to cyclical loads FC, FT. This is distinguished from a conventional suction valve <NUM>, which is always closed except when pumping water out of the suction chamber <NUM> 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.

In this example, the valve <NUM> comprises a tubular housing or sleeve <NUM> mounted in a corresponding aperture <NUM> that penetrates the top plate <NUM>. The sleeve <NUM> is open at its top and bottom ends to effect fluid communication, through the aperture <NUM>, between the exterior of the suction pile <NUM> and the suction chamber <NUM> within the pile <NUM>.

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

In an upper, open position shown in <FIG>, the valve element <NUM> is disposed above and clear of the top end of the sleeve <NUM>, thereby allowing fluid flowing through the aperture <NUM> to pass through the gap between the sleeve <NUM> and the valve element <NUM>. Conversely, in a lower, closed position shown in <FIG>, the valve element <NUM> bears against and closes the top end of the sleeve <NUM>, thereby blocking fluid flow through the aperture <NUM>.

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

It will be apparent from <FIG> that when the suction pile <NUM> is subject to downward load FC during a compression phase, overpressure in the suction chamber <NUM> lifts the valve element <NUM> into the upper, open position. The overpressure is thereby relieved by an upward flow of water <NUM> from the suction chamber <NUM> through the valve <NUM>. Water <NUM> also flows upwardly toward the suction chamber <NUM> through the pores of the soil <NUM> within the skirt of the pile <NUM>, in addition to some water <NUM> being expelled downwardly through the pores of the soil <NUM> via the open bottom of the skirt <NUM> in a conventional manner.

It will also be apparent from <FIG> that when the suction pile <NUM> is subject to upward load FT during a tension phase, underpressure in the suction chamber <NUM> pulls the valve element <NUM> down into the lower, closed position. The underpressure draws an upward flow of water <NUM> into the pile <NUM> via the open bottom of the skirt <NUM> and through the pores in the soil <NUM>. As water cannot now enter the top of the pile <NUM> through the closed valve <NUM>, the flow of water is essentially upward-only and unidirectional.

In the next compression phase, much of the water drawn into the suction pile <NUM> during the tension phase is expelled through the now-reopened valve <NUM> as shown in <FIG>. Thus, in this respect, the flow of water through the pores of the soil <NUM> within the skirt <NUM> 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 <NUM>.

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

One such barrier is a cage <NUM>, or other foraminous shroud, that surrounds the external side of the valve <NUM> to keep rocks <NUM> away from the valve <NUM>. Another barrier is a filter mesh <NUM> that spans the aperture <NUM> within the sleeve <NUM> of the valve <NUM>. A further barrier is a filter mesh <NUM> that spans the interior of the skirt <NUM> between the valve <NUM> and the top of the soil <NUM> within the skirt <NUM>. The filter meshes <NUM>, <NUM> keep soil <NUM> within the pile <NUM> away from the underside of the valve <NUM>. The cage <NUM> and the filter meshes <NUM>, <NUM> can be used individually or in any combination of two or more such barriers.

Moving on to <FIG>, this shows another embodiment of the suction pile <NUM> in which a concrete layer or plug <NUM> is cast or otherwise placed on top of the soil <NUM> within the skirt <NUM>, under the top plate <NUM>. In this case, fluid communication between the exterior of the pile <NUM> and the soil <NUM> within the skirt <NUM> is effected by an upper aperture <NUM> in the top plate <NUM> and a lower aperture <NUM> in the plug <NUM>. At least one of those apertures <NUM>, <NUM> can be closed by a one-way valve <NUM> in accordance with the invention.

In this example, the valve <NUM> comprises a tubular housing or sleeve <NUM> around the lower aperture <NUM> extending through the plug <NUM>. The upper aperture <NUM> is always open but may be protected by a barrier mesh <NUM>, as shown, that permits water flow but prevents rocks or other debris from falling into the pile <NUM> and potentially jamming the valve <NUM>.

<FIG> shows an alternative arrangement for the valve <NUM>, which could also be applied to the preceding embodiment. Conversely, the valve <NUM> of the preceding embodiment could be applied to this embodiment. In this case, the valve element is a flap <NUM> that is hinged to the top of the sleeve <NUM>. The sleeve <NUM> is spanned by a filter mesh <NUM> to keep the soil <NUM> away from the flap <NUM>.

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

It would be possible to reverse the arrangement of <FIG> by positioning the valve <NUM> in the upper aperture <NUM> and leaving the lower aperture <NUM> open, save for the option of a filter mesh <NUM>.

<FIG> show that the invention may also have benefit when lowering a suction pile <NUM> toward the seabed <NUM>. Here, a one-way valve <NUM> in the top plate <NUM> of the pile <NUM> is open when lowering, as shown in <FIG>, so that water can flow along and through the pile <NUM> to the benefit of stability.

The valve element <NUM> may assume the upper, open position shown in <FIG> in response to differential pressure or drag forces as the pile <NUM> falls through the water column. Alternatively, or additionally, the valve element <NUM> may be held open temporarily during the lowering operation. For example, <FIG> shows removable pins <NUM> such as beta pins that are received in transverse bores <NUM> extending through the guides <NUM>. When engaged with the guides <NUM>, the pins <NUM> bear against the underside of the valve element <NUM> to prevent the valve element <NUM> dropping into the closed position against the top of the sleeve <NUM>. When the pile <NUM> reaches the seabed <NUM>, the pins <NUM> can be removed, for example by an ROV, to allow the valve <NUM> to close. The valve <NUM> is then enabled to open and close automatically and autonomously in response to cyclical compression and tension loads FC, FT applied to the pile <NUM> as shown in <FIG>.

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

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

The one-way valve <NUM> could be held open during the pre-loading phase shown in <FIG>, for example using an arrangement of pins <NUM> like that shown in <FIG>. Alternatively, an overpressure in the suction chamber <NUM> can open the one-way valve <NUM> to an extent sufficient to promote drainage of water through the top plate of the pile <NUM>.

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.

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.

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