Abstract
Suction failure due to a piping effect when installing a suction pile underwater is mitigated by laying at least one substantially impermeable mat on the seabed. An entrance of a piping channel extending under a skirt of the pile from a seabed location to a suction chamber of the pile is identified. The mat is laid to cover the entrance to restrict or block a flow of water entering the piping channel.
Claims
1-21. (canceled)
22. A method of mitigating suction failure due to a piping or seepage effect when installing a suction pile underwater, the method comprising identifying an entrance of a bypass channel that extends under a skirt of the pile from a seabed location to a suction chamber of the pile, and laying a substantially impermeable mat having a pliant, flexible body at the seabed location to cover the entrance and so restrict or block a flow of water entering the bypass channel, wherein a radially inner side of the mat lies against and extends partially up a side of the skirt.
23. The method of claim 22, 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.
24. The method of claim 22, comprising forcing the laid mat against the seabed under suction applied to the mat via the bypass channel.
25. The method of claim 22, comprising overlapping or abutting two or more mats to cover the entrance of the bypass channel.
26. The method of claim 22, wherein the laid mat conforms pliantly to contours of the seabed.
27. The method of claim 22, comprising abutting the laid mat with the pile.
28. The method of claim 22, comprising suspending suction in response to appearance of the bypass channel and then resuming suction after laying the mat on the seabed.
29. The method of claim 22, 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.
30. The method of claim 29, comprising unrolling or unfolding the mat from the compact configuration into the deployment configuration.
31. The method of claim 22, comprising supporting the mat on the pile and deploying the mat from the pile when laying the mat on the seabed.
32. The method of claim 31, comprising deploying the mat from the pile while a portion of the mat remains attached to the pile.
33. The method of claim 32, comprising lowering the mat relative to the pile to a seabed level and then deploying the mat from the pile.
34. The method of claim 33, comprising deploying the mat by pivoting the mat from the pile.
35. The method of claim 31, comprising preliminarily lowering the pile through water toward the seabed with the mat stowed on the pile.
36. The method of claim 22, comprising lifting the mat from the seabed after resuming or completing installation of the pile.
37. The method of claim 22, comprising leaving the mat on the seabed after completing installation of the pile.
38. The method of claim 37, comprising using the mat to mitigate scouring of the seabed during operational use of the pile.
39. The method of claim 38, comprising supporting anti-scour features on the mat, those features being one or more of: dumped rocks; dumped bags of granular material; or fronds upstanding from the mat.
40. The method of claim 22, comprising encircling the pile with the mat or with an array of such mats.
41. The method of claim 22, 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.
Description
[0057] To put the invention into context, reference has already been made to FIGS. 1a, 1b, 2a and 2b of the accompanying drawings, in which:
[0058] FIGS. 1a and 1b are schematic sectional side views of a suction pile being installed normally during the pumping phase; and
[0059] FIGS. 2a and 2b are schematic sectional side views of a suction pile suffering from a piping effect that interrupts the pumping phase of installation.
[0060] 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:
[0061] FIGS. 3a and 3b are schematic sectional side views showing a mat of the invention being unrolled or unfolded and placed on the seabed beside a suction pile to mitigate the piping effect shown in FIG. 2b, allowing the pumping phase of installation to resume;
[0062] FIG. 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;
[0063] FIG. 5 corresponds to FIG. 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;
[0064] FIGS. 6a and 6b are schematic sectional side views showing deployment of a mat from a suction pile;
[0065] FIG. 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;
[0066] FIG. 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;
[0067] FIG. 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
[0068] FIG. 10 is a schematic top plan view of a suction pile showing the annular mat of FIG. 9 deployed to mitigate piping to provide a base membrane for various anti-scour measures.
[0069] Referring next, then, to FIGS. 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 FIG. 3a corresponds to that shown in FIG. 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.
[0070] In accordance with the invention, a mat 34 is shown in FIG. 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 ROV. 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 FIG. 3b.
[0071] Advantageously, soon after penetration of the pile 10 resumes as shown in FIG. 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.
[0072] 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 fibres or other embedded elements. Thus, the mat 34 is preferably flexible and may have some elasticity, being pliant enough to conform 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 FIG. 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.
[0073] 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.
[0074] 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.
[0075] 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.
[0076] 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.
[0077] Optionally, as shown in FIG. 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 performed underwater, and so is also apt to be performed by divers or by an ROV.
[0078] FIGS. 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. FIG. 4 shows the mats 34 stowed in a compact rolled configuration, with one of those mats 34 on the right of FIG. 4 shown unrolled or unfurled and deployed over the mouth of a rathole 30. Conversely, FIG. 5 shows the mats 34 stowed in a compact folded configuration, with one of those mats 34 on the right of FIG. 5 shown unfolded and deployed over the mouth of a rathole 30.
[0079] FIGS. 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.
[0080] 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 FIGS. 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.
[0081] In FIGS. 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 FIG. 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 FIG. 6b before or after reaching the level of the seabed 12.
[0082] 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.
[0083] FIG. 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. [0084] 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. [0085] 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. [0086] The side edges 48 of mats 34A and 34C are parallel, whereas the side edges 48 of mats 34B and 34D splay apart in a radially outward direction. [0087] Each of mats 34A to 34C 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 34B 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. [0088] Mat 34E is a discrete mat that may be deployed separately from the pile 10 over the mouth of 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 34C, 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. [0089] 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.
[0090] FIG. 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.
[0091] 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.
[0092] Finally, FIGS. 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.
[0093] The left side of FIG. 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 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 FIG. 9, to form an annulus that covers the mouth of a rathole 30 or inhibits such a rathole 30 from forming, expanding or propagating. The mat 34 shown in FIG. 9 could instead be mounted on a circumferential array of rails 36 like that shown in FIGS. 6a and 6b to be lowered to the level of the seabed 12 before or after unfolding.
[0094] An advantage of the circumferentially continuous mat 34 shown in FIGS. 9 and 10 is that a rathole 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, FIG. 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.
[0095] 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 FIG. 9, whether or not that mat 34 was deployed to mitigate piping.
[0096] 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.
[0097] 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 be 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.
