GB2621598A - Installation of suction piles underwater - Google Patents

Abstract

A system and method comprising a suction pile 10 and an impermeable mat 34 for placing on the seabed 12 adjacent the suction pile. The mat is used to prevent piping and seepage effects occurring during installation of a pile in loose, gravel or sand seabed. The pile may include a pump 26, a non-return valve 24 and a suction port 22 for drawing fluid out of the chamber 20 to create a pressure differential to embed a pile into the seabed. Piping may occur creating ratholes 30 under skirt 16 which allows fluid to flow into chamber 20 and be pumped out, preventing the pump creating sufficient pressure differential. A preferably elastic and flexible e.g. rubber, mat 34 is deployed e.g. from a rolled or folded stowed position, to cover the rathole mouth preventing further water being sucked into the hole. Mats may be discrete and deployed as necessary, and removed after use, or may be an annular ring like skirt surround the pile, remaining in place after installation as an anti-scour device.

Description

Installation of suction piles underwater This invention relates to the installation of suction pile foundations that are designed to be embedded into the seabed. In particular, the invention relates to the problem of suction failure due to an effect known as 'piping that may be encountered when installing suction piles, especially in sandy or otherwise granular seabed soil.

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 for anchoring or supporting purposes offshore. To do so, they are designed to engage soft seabed soil that typically comprises marine sediments such as sand or soft clay. Once embedded into the soil, a suction pile can serve as an anchor or as a support for various types of equipment, structures or installations located underwater or above the surface. For example, one or more suction piles can be used for mooring or tethering a platform, a surface vessel or a buoy or to support the weight of a structure such as an offshore wind turbine or a manifold.

Figures la and lb show a conventional suction pile 10 during a pumping or suction phase of its installation into the seabed 12, partially embedded in sandy seabed soil 14 The pile 10 shown in these and other accompanying drawings is represented schematically for simplicity and is not to scale: for example, a suction pile will usually be considerably taller than it is wide.

A suction pile 10 is typically fabricated from steel and comprises an open-bottomed hollow tubular wall of constant cross-section defining a deep cylindrical skirt 16. The skirt 16 may be several metres in length, for example ten metres. A major bottom portion of the skirt 16 engages the seabed soil 14 by friction or cohesion upon being embedded axially into the soil 14. Some seabed soil 14 is then encircled by the skirt 16. Thus, the soil 14 engages the skirt 16 on its inner side in addition to its outer side. *30 ;The top of the skirt 16 is closed by a top plate 18. A suction chamber 20 is defined between the top plate 18, the skirt 16 and the seabed soil 14 encircled by the embedded skirt 16. The top plate 18 is penetrated by a suction vent or port 22 through which water can be pumped out of the suction chamber 20 through a non-return valve 24 For this purpose, a pump 26 communicating with the port 22 via the valve 24 can be mounted on the pile 10 as shown here or temporarily coupled to the pile 10, for example by being implemented on a skid of an ROV that couples to the valve 24 during installation. ;Installation of a suction pile 10 involves firstly allowing the pile 10 to self-penetrate under its own weight into the seabed 12 and secondly, after a short period of settlement, pumping water out of the resulting suction chamber 20 to create a pressure differential. Specifically; 'when the pile 10 is landed on the seabed 12 in an upright orientation, the skirt 16 embeds partially into the seabed soil 14 under the self-weight and momentum of the pile 10. Self-penetration of the pile 10 ends when the resistance of friction or cohesion to sliding motion of the skirt 16 relative to the seabed soil 14 balances the weight of the pile 10. ;When the pump 26 is activated to pump seawater out of the suction chamber 20 through the valve 24, the resulting under-pressure in the suction chamber 20 causes the pile 10 to embed more fully in, and hence to engage more completely with, the seabed 12. Specifically, under-pressure in the chamber 20 draws the top plate 18 toward the seabed 12 as the chamber 20 contracts under a surplus of external hydrostatic pressure. Thus; suction overcomes the resistance of friction or cohesion to force the skirt 16 deeper into the seabed 12, hence enabling the pile 10 to resist forces that will be applied by whatever equipment, structure or installation is anchored to or supported on the pile 10 after installation. ;Once installed, the pile 10 engages with the seabed 12 by a combination of water flow friction and skirt friction. Water flow friction is resistance to a flow of water through the pores of the plug of soil 14 within the skirt 16, in particular water driven to flow into or out of the open bottom of the skirt 16 due to the piston effect of upward or downward longitudinal movement of the pile 10. Conversely; skid friction is friction or cohesion between the skirt 16 and the seabed soil 14 that is in contact with the inner and outer surfaces of the skirt 16. Friction predominates in sandy soils and cohesion predominates in clay soils. ;It will be apparent that successful installation of a suction pile relies upon a thorough understanding of the structure and composition of the seabed where it is proposed to locate the pile. Surveying and sampling operations are therefore performed routinely before installation. However, the unpredictable and inhomogeneous nature of the seabed soil and of the subsurface geology means that some installation problems are only revealed when installation is already underway. At best, such problems can lead to potentially costly delays in completing installation of a suction pile. At worst; such problems can lead to abandonment of the installation and relocation or even scrapping of the pile, which is inevitably very costly in monetary terms and in its impact on a wider installation project. ;Various failure modes involving suction piles are known, for example: sinking or tilting, in which the pile sinks too far into the soil or tilts substantially away from the vertical during or after installation; sticking, in which downward progression of the pile during installation is stopped by a subsurface barrier such as a harder layer of soil or rock, or an isolated boulder; loosening and erosion, causing the soil to weaken structurally and to suffer from inadequate frictional and/or cohesive engagement with the skirt of the pile; plugging, in which a harder layer of soil or other blockage is uplifted within the pile and hinders pumping: and seepage, being a lack or loss of sealing of the suction chamber causing an inward flow of water during pumping due to insufficient self-penetration or incomplete contact between the skirt and the soil before pumping begins. ;The invention is primarily concerned with mitigating another failure mode experienced during installation of a suction pile, namely an effect known in the art as piping. Piping is a particular risk in granular, freely draining soils such as sands or gravels, especially in coarse or gravelly sands. In this respect, reference is made to Figures 2a and 2b in which like numerals are used for like features. ;Piping is a hydraulic failure in which small particles of seabed soil 14 are fluidised or liquefied and entrained in a flow of water through the pores of the soil 14, which leads to loosening, erosion and displacement of the soil 14. Piping initiates within the skirt 16 of the pile 10 where the soil 14 is subjected to an inward, upward flow of water toward the suction chamber 20, driven by the pressure differential arising from pumping. ;The effect of piping may initially be diffuse but tends quickly to become localised because any location of relatively high permeability will attract a greater rate of flow. This creates an adverse feedback loop in which increased flow will further loosen the soil 14, which will further increase permeability, flow and loosening, in this way, piping can lead rapidly to the formation of one or more channels 28 of loose, highly permeable soil 14 that regress downwardly through the soil 14 within the skirt 16 as shown in Figure 2a. ;Correspondingly, low pressure within the suction chamber 20 of the pile 10 drives a downward flow of water through the soil 14 outside the pile 10 toward the bottom edge of the skirt 10, also promoting the formation of channels 28 of loose permeable soil outside the pile 10. ;Eventually, as shown in Figure 2b, channels 28 of loose permeable soil 14 within and outside the pile 10 may unite to form a generally U-shaped piping channel, known in the art as a rathole 30, that extends from the seabed 12 outside the pile 10 and beneath the skirt 16 to the suction chamber 20 within the pile 10. The rathole 30 behaves as a pipe, tube or tunnel in which water can flow quite freely through the loose soil 14 during the pumping phase of installation. As a result, the rathole 30 acts as a bypass channel that circumvents the skirt 16 to effect fluid communication between the suction chamber 20 and the body of water surrounding the pile 10. ;When a rathole 30 forms, pumping becomes ineffective because water pumped out of the suction chamber 20 is replaced with water drawn into the suction chamber 20 through the rathole 30. This tends to equalise pressure within the pile 10 with hydrostatic pressure outside the pile 10, at least to the extent that any surplus of external pressure over internal pressure is insufficient to overcome the resistance of friction or cohesion to axial motion of the pile 10. Consequently, downward progression or penetration of the pile 10 eventually ceases. Thereafter, further pumping is likely to achieve nothing more than enlargement of the rathole 30 and further loosening and weakening of the soil 14 that is in contact with the skirt 16. ;A localised patch of turbid swirling water 32 may become visible above the seabed 12 outside the skirt 16, marking the location of the outer end or mouth of the rathole 30 where pumping draws water into the rathole 30 and then into the suction chamber 20. ;Pressure losses in porous seabed soil 14 mean that ratholes 30 typically follow the shortest path and so lie close to or against the inner and outer faces of the skirt 16 as shown. Consequently, the loose, permeable soil 14 of a rathole 30 will undermine the effects of both water flow friction and skirt friction on which the pile 10 depends for its engagement with the seabed 12. ;Piping is especially likely to arise early in the pumping phase of installation if the suction applied exceeds the capacity of the soil to resist the resulting pressure differential, noting that the capacity of the soil will increase with the depth to which the skirt of the pile is embedded However, piping can arise at any point in the pumping phase. ;Piping is akin to seepage, both being detrimental to sealing of the suction chamber, but those terms are not, strictly, interchangeable even if they are sometimes used synonymously in the art. For example! piping can arise even where sufficient self-penetration had previously been achieved. ;Conventionally, mitigation of piping involves depositing a pile of sandbags on the seabed outside the skirt of a pile to block the mouth of a rathole. However, piled sandbags leave gaps or spaces between them and between the sandbags and the skirt. Even small gaps could allow enough water to flow into the rathole that pumping will remain ineffective. Consequently, a large number of sandbags may be required before the measure is effective, if indeed it will be effective at all unless sandbags can be placed underwater with sufficient accuracy to block all significant gaps. At best, depositing sandbags is a slow and imprecise solution to the problem of piping. ;CN 113404078 proposes injecting grout to seal a rathole, which should be more effective than depositing sandbags to achieve a seal. However, the grouting operation is also slow because it requires multiple stages, including curing of the grout, and is costly because it requires a dedicated grouting spread to be provided and for grout to be prepared ready for use. Those expensive provisions would be redundant if pile installation proceeds successfully without encountering piping, as intended. ;Against this background, the invention resides in a method of mitigating suction failure due to a piping or seepage effect when installing a suction pile underwater. The method comprises laying a substantially impermeable mat at a seabed location to restrict or block a flow of water entering a bypass channel that extends under a skirt of the pile from the seabed to a suction chamber of the pile. Suction may he suspended in response to appearance of the bypass channel and then resumed after the mat has been laid on the seabed. ;The laid mat can he forced against the seabed under suction applied to the mat via the bypass channel. The laid mat can conform pliantly to contours of the seabed; and may be positioned to abut the pile. For example, a concave-curved side or edge of the mat could abut or adjoin the skirt of the pile. ;An entrance of the bypass channel can be identified before the mat is laid on the seabed to cover that entrance. Advantageously, the laid mat can extend across the seabed beyond the entrance of the bypass channel. This defines an annular or part annular peripheral sealing interface between the mat arid an underlying area of the seabed around that entrance. Two or more mats could be overlapped or abutted to cover the entrance of the bypass channel. ;Conveniently, the mat can be stored, transported and/or lowered to the seabed in a compact configuration and then opened out into a wider and/or longer deployment configuration underwater before being laid on the seabed. For example, the mat can be unrolled or unfolded from the compact configuration into the deployment configuration. ;Preliminarily, the pile can be lowered through water toward the seabed with the mat stowed on the pile. More generally, the mat may be stowed on the pile and deployed from the pile vaen being laid on the seabed. For example, the mat can be deployed from the pile while a portion of the mat remains attached to the pile. The mat can be lowered relative to the pile to a seabed level and then deployed from the pile, for example by being pivoted away from the pile. ;The mat can be lifted from the seabed after resuming or completing installation of the pile, or can be left on the seabed after completing installation of the pile. In the latter case, the mat could be used to mitigate scouring of the seabed during operational use of the pile. Thus, the mat can support anti-scour features being one or more of dumped rocks; dumped bags of granular material; or fronds upstanding from the mat. ;The pile could be encircled by the mat or by a circumferential array of such mats. Alternatively, the mat or a plurality of such mats may cover only a miner angular segment of the seabed around a central longitudinal axis of the pile. ;The invention can also be expressed as a combination of a suction pile with at least one substantially impermeable mat, the or each mat comprising a concave-curved side or edge having a radius of curvature substantially matching a radius of curvature of a convex skirt of the pile. Two or more of such mats can be in mutually overlapping or abutting relation when laid on the seabed. The or each mat may comprise a web that is pliantly conformable with contours of the underlying seabed under self-weight and/or suction. ;Where the or each mat is supported on and deployable from the pile; the skirt of the pile may have at least one upright rail that supports the or each mat for downward movement relative to the pile. The or each mat may be pivotably mounted to the or each rail to pivot away from the skirt. ;In summary, the invention recognises that piping can occur where highly permeable seabed soil erodes near the boundary of a pile; especially where only shallow self-penetration of the pile was achieved before applying suction. The invention also recognises the risk of seepage, being an inadequate seal between the skirt of the pile and underlying soil of any type due to excessively shallow self-weight penetration. To deal with such piping arid/or seepage, the invention proposes laying one or more mats onto the seabed beside a pile as a barrier to water flow through or under the mat. The or each mat may be made of rubber or of any other material or construction that can block water flow through the mat. ;The or each mat is positioned to prevent or reduce water being drawn into the pile preferentially through a piping or seepage channel that presents much lower resistance to water flow than the soil surrounding the channel. Such a channel; or rathole, may be regarded as a bypass channel because it circumvents the skirt of the pile and enables a rapid localised flow of water to bypass the desired slow bulk flow of water through the entire plug of soil within the skirt in addition to the annular body of soil adjoining the exterior of the skirt. The pressure differential that is required to drive penetration of the pile during the suction phase of installation depends upon the resistance of the soil to that slow percoiative flow of water. Thus, preventing or reducing water flow through a bypass channel enables pump pressure to be effective to drive further penetration of the pile. ;Mats of the invention may, for example, be mounted around the suction pile and could be detached from the pile to be laid on the seabed when required or could remain attached to the pile while being lowered to the seabed, for example on a rail mechanism. The arc, radius and/or length of each mat around the pile can be small relative to the overall size of the pile, for example about two metres. ;Mats of the invention could be made of recyclable or decomposable materials to be left on the seabed and to degrade over time, or they can remain on the seabed indefinitely for anti-scour purposes, or they can be recovered after installation. For example, mats could be fitted a short distance of, for example, about one metre above the bottom of the skirt or pile tip. If not required to block a bypass channel or for anti-scour purposes, the mats can be released from the pile by a diver or ROV and recovered before or after completion of installation It is known to place mats on the seabed beside a subsea structure such as a pile for the purpose of scour protection. The role of anti-scour mats is to prevent wave action or subsea currents excavating soil from around the structure, potentially affecting its stability or structural integrity. Anti-scour mats work by reducing the velocity of a current at the interface between the seabed and the water flowing above. The reduction of current velocity discourages transportation of seabed soil away from the mat and encourages soil particles already entrained in the water to fall onto and collect on the mat. ;The requirements of scour mitigation contradict the requirements of piping mitigation. in particular, anti-scour mats are advantageously permeable or foraminous so that they can reduce the velocity of a current flowing over them, and so that they can engage, entrap and retain seabed soil on which they are laid and that is deposited on them. ;Gaps or holes traverse the mats not only to improve scour mitigation once installed but also to reduce drag resistance when lowering them through the water column during installation. Consequently, anti-scour mats are not water-tight across a sufficient area to mitigate piping and so are not suitable for the purposes of the invention. ;Anti-scour mats are commonly made of an array of interconnected discrete blocks as exemplified by EP 1440208. The blocks are individually rigid but are flexibly or pivotably connected to each other across gaps between them so that the mat can drape under its own weight to conform to a seabed contour. The individual blocks may also be penetrated by apertures, as shown in EP 1440208, to increase the permeability of the mat beyond that provided by the gaps. ;It is also known to mount anti-scour mats on a suction pile. For example, the Scour Protection System offered by SPT Offshore of the Netherlands, as described at https://www.sptoffshore.ccimiscour-protection-system-scpsi (trade marks acknowledged), comprises a self-deploying umbrella-like system of mats encircling the pile. The mats comprise arrays of buoyant fronds secured to a base of polyester webbing mesh that is anchored to the seabed. Again, therefore, the mats are necessarily permeable. ;CN 110886326 also discloses an anti-scour device surrounding a subsea pile. Again, the anti-scour device comprises a polymer curtain that supports fronds of artificial seaweed. The curtain encircles the pile in a trusto-conical arrangement that extends outwardly and downwardly from a ring mounted around the pile. The curtain is ballasted by sandbags that are attached to the curtain at angularly-spaced intervals. ;By virtue of its trusto-conical shape, the curtain of CN 110886326 is held clear of the seabed other than at its periphery. Also, there is no requirement, provision or benefit for the curtain to be impermeable or for sealing the curtain to the pile or to the seabed. ;Whilst CN 110886326 is silent as the permeability of the curtain, it can be assumed that the curtain is permeable or foraminous as is conventional in the anti-scour art. In this respect, it is notable that the sandbags are tied or stitched to the curtain, that the fronds are attached to the curtain after the curtain is mounted to the ring, and that the curtain is lowered through the water column when already in its frusto-conical shape. *30

Additionally; the pile of CN 110886326 is a hammer-driven pile that is not susceptible to the problem of piping because its installation does not involve suction. Even if the pile of CN 110886326 were a suction pile, the curtain is lowered through the water column after installation of the pile and is then mounted on or assembled around the pile. It follows that the curtain is not present during installation of the pile and so could not assist with mitigating piping even if the curtain was impermeable.

The invention proposes a much simpler solution to the problem of piping than is offered by the prior art. Aspects of the invention exploit the lirnited horizontal distance, if any, between the mouth of a rathole and the outer face of the pile skirt.

Embodiments of the invention implement a method to mitigate piping if that phenomenon is experienced during installation of a suction pile underwater. The method comprises: effecting suction; when piping occurs, identifying the location of an entry point into a piping channel; and laying a mat temporarily or permanently on the seabed over the entry point of the piping channel. It would also be possible to install a mat on the seabed extending outwardly from the pile before piping occurs, as a precaution to discourage initiation of piping.

The mat should be impervious or impermeable to water, at least to the extent of slowing water flow into the piping channel to a degree necessary for penetration of the pile to resume during the pumping phase of installation. For this purpose, the ma: may be sufficiently pliant and preferably elastic to be sucked at least partially into the entry point of the piping channel, hence being deformed into sealing conformity with the mouth of the piping channel and the surrounding seabed.

When in use or when required for use, the mat may be attached to the pile, for example on top of the pile or on a side wall of the pile emerging from the seabed. The mat may be stored provisionally on the top or a side wall of the pile. Whether stored on the pile or otherwise, the mat can be folded or unfolded, or rolled or unrolled, by an RCN/ or by divers.

The mat is preferably of a watertight pliant material such as a flexible polymer material.

At least one face or side of the mat may be smooth or continuous, such that the structure of mat defines a substantially imperireable barrier to passage of water through the mat. However, at least one face of the mat could have a textured surface.

For example, a textured under-surface could improve engagement with the soil when deployed to prevent the mat sliding across the seabed under the action of a current. Similarly, a textured top surface could help to retain soil if the mat is left on the seabed for anti-scour purposes as suggested below.

The mat may have any suitable shape, for example part-circular, annular, part-annular or rectangular, and may have a concave inner side or edge to interface with and complement the convex outer curvature of a skirt of the pile. An outer side or edge may be straight or may be curved, for example coaxially and hence convexly with a concave inner side or edge. The mat may have a length or width of at least 2m projecting radially from the skirt of the pile or from the concave inner side or edge to an outer edge. The mat could extend around all or most of the circumference of the pile or may only cover a minor sector of the circumference of the pile, for example less than 180', or less than 900, or less than 60'.

The mat may be removed and/or recovered to the surface after the pumping phase if a pressure balance is sufficient to negate fluid interchange between the suction chamber and the surrounding sea. Alternatively, the mat could be left in place after the pumping phase. In that case, scour protection such as rock dumping, sandbags or gravel bags can be placed on top of the mat when pile installation is complete. In this respect, an impermeable mat will provide good protection against winnowing, in which small particles of seabed soil tend to migrate up through rock or other elements that are conventionally dumped for scour protection.

Thus, in accordance with the invention, suction failure due to a piping effect encountered when installing a suction pile underwater is mitigated by laying at least one substantially impermeable mat on the seabed. The or each mat is laid to restrict or block a flow of water entering a piping channel that extends under a skid of the pile from the seabed to a suction chamber of the pile.

One or more mats of the invention may be laid at a discrete location in response to the formation of a piping channel. In that case, an entrance of the piping channel can be identified and the or each mat can then be laid on the seabed to cover that entrance. Alternatively, one or more mats of the invention could be laid on a precautionary basis to inhibit fon-nation or propagation of a piping channel, for example by being laid around the full circumference of the pile to extend radially from the pile in plan view.

To put the invention into context, reference has already been made to Figures la, lb, 2a and 2b of the accompanying drawings, in which: Figures la and lb are schematic sectional side views of a suction pile being installed normally during the pumping phase; and Figures 2a and 2b are schematic sectional side views of a suction pile suffering from a piping effect that interrupts the pumping phase of installation.

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: Figures 3a and 3b are schematic sectional side views showing a mat of the invention being unrolled or unfoide.g and placed on the seabed beside a suction pile to mitigate the piping effect shown in Figure 2b, allowing the pumping phase of installation to resume; Figure 4 is a schematic sectional side view showing various arrangements for storing mats on a suction pile in a rolled configuration, one of those mats being unrolled onto the seabed to mitigate piping; Figure 5 corresponds to Figure 4 but shows various arrangements for storing mats on a suction pile in a folded configuration, one of those mats being unfolded onto the seabed to mitigate piping; Figures 6a and 6b are schematic sectional side views showing deployment of a mat from a suction pile; Figure 7 is a schematic top plan view of a suction pile showing various mat configurations deployed on the seabed beside the pile to mitigate piping; Figure 8 is an enlarged sectional side view showing a pliant mat deployed on the seabed beside the skirt of a suction pile, the pliancy of the mat helping the mat to mitigate piping by conforming to contours of the seabed; Figure 9 is a schematic sectional side view showing an annular mat encircling a suction pile, the mat being shown in stored and deployed configurations; and Figure 10 is a schematic top plan view of a suction pile showing the annular mat of Figure 9 deployed to mitigate piping to provide a base membrane for various anti-scour measures.

Referring next, then, to Figures 3a and 3b of the drawings, a rathole 30 is shown here having formed in the soil 14 during the pumping phase of installing a suction pile 10 in the seabed 12. The situation in Figure 3a corresponds to that shown in Figure 2b except that the pump 26 has been deactivated and pumping has been suspended, having been rendered ineffective by fluid communication between the suction chamber 20 and the water surrounding the pile 10.

In accordance with the invention, a mat 34 is shown in Figure 3a being laid on the seabed 12 over the mouth of the rathole 30 outside the skirt 16 of the pile 10. The mat 34 could be lowered into that position when suspended from the surface but is preferably laid in that position by divers or by an RON/. Once laid, the mat 34 substantially blocks or at least significantly hinders an inflow of water into the rathole 30. In this way, the mat 34 enables resumed pumping to recreate sufficient under-pressure in the suction chamber 20 for penetration of the pile 10 to continue, as shown in Figure 3b.

Advantageously, soon after penetration of the pile 10 resumes as shown in Figure 3b, the bottom edge of the skirt 16 cuts through the lower apex of the rathole 30. The wall of the skirt 16 may thereby help to disrupt fluid communication between the suction chamber 20 and the water surrounding the pile 10.

The mat 34 comprises an integral body that, initially or as manufactured, is generally planar or flat-bottomed across substantially its full width. The mat 34 is suitably moulded of an elastic polymer, elastomer or rubber that could be reinforced with fibre or other embedded elements. Thus, the mat 34 is preferably flexible and may have some elasticity, being pliant enough to confolin to the contours of the seabed 12 under its own weight and under suction applied to its underside via the rathole 30, as described in more detail below with reference to Figure 8. Yet, the mat 34 has enough stiffness to be handled easily and not to collapse into the rathole 30 under suction. The mat 34 is neutrally or negatively buoyant so that it can remain on the seabed 12 once laid, without requiring additional ballasting.

The mat 34 has generally parallel major faces on its upper and lower sides joined by a thin peripheral edge, such that the width or length of the mat 34 is much greater than its thickness.

The body of the mat 34 is a continuous, solid and substantially imperforate web or membrane and therefore is substantially impermeable to water, hence presenting a barrier to flow of water through its thickness. Also, the mat 34 overlaps the mouth of the rathole 30 to form an annular or part-annular peripheral sealing interface between the underside of the mat 34 and the underlying area of seabed 12 around the rathole 30.

That peripheral sealing interface blocks or hinders any flow of water between the mat 34 and the seabed 12 that could otherwise circumvent or undermine the mat 34 and then enter the suction chamber 20 via the rathole 30.

In this example, as is common, the mouth of the rathole 30 is close to or adjoining the skirt 16 of the pile 10. Consequently, an inner edge of the mat 34 may abut and seal against the skirt 16 as shown.

As a perfect seal around the mat 34 may not be achievable in practice and as the seabed 12 is itself saturated with water, some water could still enter the rathole 30 as the pump 26 expels water from the suction chamber 20. However, the objective of the mat 34 is significantly to hinder an inflow of water into the suction chamber 20 through the rathole 30. This enables the pump 26 to expel water from the suction chamber 20 at a greater flow rate than the aggregate flow rate of water entering the suction chamber 20, whether through the rathole. 30 or otherwise. That surplus of outflow versus inflow enables the pump 26 to recreate the under-pressure that is necessary to drive penetration of the pile 10 into the seabed 12.

Optionally, as shown in Figure 3a, the mat 34 can be stored, transported and/or lowered to the seabed 12 in a compact configuration before being opened out into a wider and/or longer deployment configuration. Thus, when deployed, the mat 34 acquires a greater surface area to ensure full coverage of the mouth of the rathole 30 and a wide and effective sealing interface around the periphery of the rathole 30. For example, the mat 34 could be unrolled from a rolled, coiled or furled configuration or unfolded from a folded configuration as shown. This makes a large mat 34 easier to store, to transport and to lower through the water column. Thus, unrolling or unfolding of the mat 34 may be perfomied underwater, and so is also apt to be performed by divers or by an ROV.

Figures 4 and 5 show that one or more mats 34 can be stowed compactly on a pile 10 for convenient and quick deployment over the mouth of a rathole 30 in the event that piping occurs during installation. Figure 4 shows the mats 34 stowed in a compact rolled configuration, with one of those mats 34 on the right of Figure 4 shown unrolled or unfurled and deployed over the mouth of a rathole 30. Conversely, Figure 5 shows the mats 34 stowed in a compact folded configuration, with one of those mats 34 on the right of Figure 5 shown unfolded and deployed over the mouth of a rathole 30.

Figures 4 and 5 both illustrate the possibility of mounting the stowed mats 34 externally on the top plate 18 and/or on an upper portion of the skirt 16. Two or more mats 34 can be spaced angularly from each other in plan view around the circumference of the pile 10.

When deployed, a radially inner portion of a mat 34 can remain attached to and sealed to the pile 10 as shown to the right in Figures 4 and 5. Conversely, a radially outer portion of the mat 34 can drop downwardly and radially outwardly, conveniently through its self-weight under gravity, to lie over the mouth of a rathole 30 beside the skirt 16. In that case, the width of the mat 34 should be such that enough of the mat 34 can extend radially to cover the mouth of a rathole 30 even if penetration of the pile 10 stalls at an early stage with a relatively large portion of the skirt 16 still above the seabed 12.

In Figures 6a and 6b, a mat 34 is shown supported on at least one upright rail 36 that is fixed to the exterior of the skirt 16. The mat 34 could be attached directly to the rail 36 or to a carriage 38 that runs along the rail 36 as shown. The rail 36 allows the mat 34 to be lowered from an upright stowed position shown in Figure 6a to the level of the seabed 12, regardless of the depth of penetration of the pile 10. The mat 34 can pivot about the rail 36 and/or the carriage 38 into the generally horizontal orientation shown in Figure 6b before or after reaching the level of the seabed 12.

Conveniently, the mat 34 can be released to fall down the rail 36 under gravity. For example, the mat 34 may be held in a raised stowed position against the skirt 16 by a latch or pin 40. The latch or pin 40 can be released or removed by an ROV or a diver to free the mat 34 for downward translational and pivotal movement relative to the skirt 16 as shown.

Figure 7 shows a range of options for the shape and size of mats 34 and how such mats 34 can be arranged to cover the mouth of a rathole 30. The various illustrated options for the mats 34 are designated with suffixes A to E, as follows.

Mats 34A to 34D are shaped to be attached to, or to abut, the skirt 16 of the pile 10. Consequently, mats 34A to 34D each have a concave inner edge 42 whose curvature matches and complements the convex external curvature of the skirt 16. In other words, the curvature of the inner edge 42 is centred on a central longitudinal axis 38 of the skirt 16.

The outer edge 46 of mat 34A is straight and tangential relative to a circle centred on the central longitudinal axis 44 of the skirt 16, whereas the outer edges 46 of mats 34B to 34D are curved about that axis 44, hence coaxially with the curvature of the inner edge 42.

The side edges 48 of mats 34A and 340 are parallel, whereas the side edges 48 of mats 348 and 34D splay apart in a radially outward direction.

Each of mats 34A to 340 occupies a minor sector of the circumference of the skirt 16, each covering less than 60' of arc in this example. Individual mats 34 could, however, cover a greater range of arc. Also, mats 34D each correspond to mat 348 but exemplify that two or more mats 34, of any shape, could be deployed in adjoining or overlapping relation to increase their effective surface area. In this case, the side edges 42 of mats 34D overlap so that their overlapping portions cooperate to cover the mouth of a rathole 30.

Mat 34E is a discrete mat that may be deployed separately from the pile 10 over the mouth al' a rathole 30 not necessarily adjoining the skirt 16. Mat 34E could have any desired shape in plan view, such as a generally rectangular shape as shown here, or a generally circular or elliptical shape. Indeed, mat 34E could have a concave edge akin to the inner edge 42 of mats 34A to 340, hence being shaped to complement the curvature of the skirt 16 to facilitate use abutting the pile 10. Conversely, any of mats 34A to 34C could be deployed separately from the pile 10, like mat 34E.

Each of mats 34A to 34E preferably has a width of at least two metres, for example extending radially by at least two metres from the inner edge 42 to the outer edge 46 in the case of mats 34A to 34C.

Figure 8 is a side view that shows a benefit of a mat 34 having a pliant, flexible body. The flexibility of the mat 34 is such that the mat 34 can drape under its own weight and can be pulled down by suction acting through a rathole 30 into closer sealing engagement with the seabed 12 in and around the mouth of the rathole. 30. Preferably, the mat 34 also has some elasticity so that it can be stretched into the mouth of the rathole 30 as shown. In this way, the mat 34 conforms to the contours of the seabed 12 beneath the mat 34 across substantially its full width and therefore seals more effectively to the seabed 12 around its periphery. This minimises leakage of water under the mat 34 and into a rathole 30 that is covered by the mat 34.

In this case, the radially inner side of the mat 34 lies against the skirt 16 of a pile 10 in sealing relationship, here extending partially up the side of the skirt 16 to increase the interface area and hence the effectiveness of sealing. That side of the mat 34 can lie against the skirt 16 so that the resilience of the mat 34 promotes effective sealing at their mutual interface, or the mat 34 can be attached to the skirt 16 via a seal at that location. However, the flexibility of the mat 34 is advantageous even if the mat 34 is deployed at a location away from the skirt 16.

Finally, Figures 9 and 10 shows another possible configuration for a mat 34, in this case a continuous annular arrangement that surrounds the full circumference of the skirt 16 of a pile 10. The mat 34 could be deployed over the pile 10 as a ring when required but is more conveniently stowed on the skirt 16 of the pile 10 to be installed with the pile 10.

The left side of Figure 9 shows the mat 34 stowed compactly, folded back on itself in a U-section, invaginated configuration An inner limb of the U-section adjoins the skirt 16 whereas an outer limb of the U-section can be radially pleated or otherwise folded. The outer limb of the U-section can be unfolded down onto the seabed 12, as shown on the right side of Figure 9, to form an annulus that covers the mouth of a rathole 30 or inhibits such a rathoie 30 from forming, expanding or propagating. The mat 34 shown in Figure 9 could instead be mounted on a circumferential array of rails 36 like that shown in Figures 6a and 6b to be lowered to the level of the seabed 12 before or after unfolding, An advantage of the circumferentially continuous mat 34 shown in Figures 9 and 10 is that a rathoie. 30 can be blocked or inhibited at any angular position around the skirt 16 of a pile 10. Another advantage is that a continuous mat 34 lends itself to being used for anti-scour purposes after installation of the pile 10, whether or not that mat 34 was also used to mitigate piping. In this respect, Figure 9 shows the possibility of the deployed mat 34 being used to support bags 50 of sand or gravel, or dumped rocks 52, or fronds 54 of artificial seaweed, any of which may be used to slow the current flowing over them and to trap sediments. In practice, such measures will extend around the full circumference of the mat 34 rather than in discrete sectors or being mixed as shown schematically here.

In principle, any other mat 34 of the invention could be deployed as a base membrane for supporting anti-scour measures like those shown in Figure 9, whether or not that mat 34 was deployed to mitigate piping.

Many other variations are possible within the inventive concept. For example, it would be possible for a mat stowed on a pile to be detached from the pile by a diver or an ROV for deployment. This enables the mat to be deployed over the mouth of a rathole formed at any position on the nearby seabed, not necessarily in angular alignment with the stowage position of the mat or adjoining the skirt of the pile.

Other annular mat arrangements are possible. For example, a petaloid array of circumferentially-distributed mats could extend around the skirt of a pile. The mats could be spaced apart from each other angularly or the mats of the array could overlap or abut with neighbouring mats of the array to form a substantially continuous annular mat arrangement around the pile. Such an arrangement could he used as a base for anti-scour measures after installation of a pile in addition to mitigating piping, if piping is encountered during installation of the pile.

Claims (32)

1. Claims 1. A method of mitigating suction failure due to a piping or seepage effect when installing a suction pile underwater, the method comprising laying a substantially impermeable mat at a seabed location to restrict or block a flow of water entering a bypass channel that extends under a skirt of the pile from the seabed to a suction chamber of the pile.
2. 2. The method of Claim 1, comprising identifying an entrance of the bypass channel and laying the mat on the seabed to cover that entrance.
3. 3. The method of Claim 2, wherein the laid mat extends across the seabed beyond the entrance of the bypass channel to define an annular or part-annular peripheral sealing interface between the mat and an underlying area of the seabed around that entrance.
4. 4. The method of Claim 2 or Claim 3, comprising forcing the laid mat against the seabed under suction applied to the mat via the bypass channel.
5. 5. The method of any of Claims 2 to 4, comprising overlapping or abutting two or more mats to cover the entrance of the bypass channel.
6. 6. The method of any preceding claim, wherein the laid mat conforms pliantly to contours of the seabed.
7. 7. The method of any preceding claim, comprising abutting the laid mat with the pile.
8. 8. The method of Claim 7, comprising abutting a concave-curved side or edge of the mat with the skirt of the pile.
9. 9. The method of any preceding claim, comprising suspending suction in response to appearance of the bypass channel and then resuming suction after laying the mat on the seabed.
10. 10. The method of any preceding claim, comprising storing, transporting and/or lowering the mat to the seabed in a compact configuration and then opening the mat out into a wider and/or longer deployment configuration underwater before laying the mat on the seabed.
11. 11 The method of Claim 10, comprising unrolling or unfolding the mat from the compact configuration into the deployment configuration.
12. 12. The method of any preceding claim, comprising supporting the mat on the pile and deploying the mat from the pile when laying the mat on the seabed.
13. 13. The method of Claim 12, comprising deploying the mat from the pile while a portion of the mat remains attached to the pile.
14. 14. The method of Claim 13, comprising lowering the mat relative to the pile to a seabed level and then deploying the mat from the pile.
15. 15. The method of Claim 14, comprising deploying the mat by pivoting the mat from the pile.
16. 16. The method of any of Claims 12 to 15, comprising preliminarily lowering the pile through water toward the seabed with the mat stowed on the pile.
17. 17. The method of any preceding claim, comprising lifting the mat from the seabed after resuming or completing installation of the pile.
18. 18. The method of any of Claims 1 to 16, comprising leaving the mat on the seabed after completing installation of the pile.
19. 19. The method of Claim 18, comprising using the mat to mitigate scouring of the seabed during operational use of the pile.
20. 20. The method of Claim 19, comprising supporting anti--scour features on the those features being one or more of: dumped rocks; dumped bags of granular material; or fronds upstanding from the mat.
21. 21. The method of any preceding claim, comprising encircling the pile with the mat or with an array of such mats.
22. 22. The method of any of Claims 1 to 20; comprising laying the mat or a plurality of such mats to cover a minor angular segment of the seabed around a central longitudinal axis of the pile.
23. 23. In combination, a suction pile with at least one substantially impermeable mat; the or each mat comprising a concave-curved side or edge having a radius of curvature substantially matching a radius of curvature of a convex skirt of the pile.
24. 24. The combination of Claim 23, comprising two or more of the mats in mutually overlapping or abutting relation.
25. 25. The combination of Claim 23 or Claim 24, wherein the or each mat comprises a web that is pliantly conformable with contours of a supporting surface under self-15 weight.
26. 26. The combination of any of Claims 23 to 25, wherein the or each mat abuts or adjoins the pile.
27. 27. The combination of any of Claims 23 to 26, wherein the or each mat is supported on and deployable from the pile.
28. 28. The combination of Claim 27, wherein a portion of the mat remains attached to the pile when deployed.
29. 29. The combination of Claim 27 or Claim 28, wherein the skirt of the pile has at least one upright rail that supports the or each mat for downward movement relative to the pile and the or each mat is pivotably mounted to the or each rail to pivot away from the skirt.
30. 30. The combination of any of Claims 23 to 29; wherein the or each mat supports anti--scour features; those features being one or more of: dumped rocks; dumped bags of granular material; or fronds upstanding from the mat.
31. 31. The combination of any of Claims 23 to 30; wherein the pile is ncircled by the mat or by an array of the mats.
32. 32. The combination of any of Claims 23 to 30; wherein the or each mat covers a minor angular segment around a central longitudinal axis of the pile.