Monitoring soil plug hazards when installing suction caissons

Abstract

A suction caisson 10 comprises a suction chamber 22 and at least two plug level sensors 34 mutually spaced across the suction chamber, for example about a central longitudinal axis of a skirt that sur

Description

This invention relates to suction caissons, also known in the art as suction anchors, suction cans, suction piles or suction buckets. The invention relates particularly to monitoring soil plug hazards, such as soil plug heave and soil plug uplift, when installing suction caissons in seabed soil. Suction caissons are commonly used in the renewable energy industry and in the oil and gas industry for anchoring or supporting purposes offshore. They are designed to engage soft soil that typically comprises marine sediments such as sand or soft clay. Once embedded into the soil, a suction caisson 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 caissons can be used for mooring or tethering a platform, a surface vessel such as an FPSO, or a buoy, or to support the weight of a structure such as an offshore wind turbine or a manifold. Figure 1 shows a conventional suction caisson 10 during a pumping or suction phase of its installation into the seabed 12, partially embedded in sandy seabed soil 14. The caisson 10 shown in these drawings is represented schematically for simplicity and is not to scale. A suction caisson 10 is typically fabricated from steel and comprises an open-bottomed hollow tubular wall defining a deep cylindrical skirt 16 that is rotationally symmetrical about an upright central longitudinal axis 18. The skirt 16 may be several metres in length, for example ten metres. A major bottom portion of the skirt 16 engages the soil 14 by friction or cohesion upon being embedded axially into the soil 14. A plug 20 of 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. The top of the skirt 16 is closed by a lid or top plate 22. A suction chamber 24 is defined between the top plate 22, the skirt 16 and the plug 20 of soil 14 encircled by the embedded skirt 16. The top plate 22 is penetrated by a suction port or vent hole 26 through which water can be pumped out of the suction chamber 24. For this purpose, a pump 28 communicating with the vent hole 26 can be mounted on the caisson 10 or temporarily coupled to the caisson 10, for example by being implemented on a skid of an ROV that couples to the vent hole 26 during installation. The pump 28 is powered via an umbilical 30 that extends to the surface 32. Installation of a suction caisson 10 involves firstly allowing the caisson 10 to selfpenetrate under its own weight into the seabed 12 and secondly, after a short period of settlement, pumping water out of the resulting suction chamber 24 to create a pressure differential. Specifically, when the caisson 10 is landed on the seabed 12 in an upright orientation, the skirt 16 embeds partially into the soil 14 under the self-weight and momentum of the caisson 10. Thus, a bottom edge of the skirt 16 embeds beneath the mudline, being the boundary between the seabed 12 and the water column above the seabed 12. Self-penetration of the caisson 10 ends when the resistance of friction or cohesion to sliding motion of the skirt 16 relative to the soil 14 balances the weight of the caisson 10. When the pump 28 is activated to pump seawater out of the suction chamber 24, the resulting under-pressure in the suction chamber 24 causes the caisson 10 to embed more fully in, and hence to engage more completely with, the seabed 12. Specifically, under-pressure in the chamber 24 draws the top plate 22 toward the seabed 12 as the chamber 24 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 caisson 10 to resist forces that will be applied by whatever equipment, structure or installation is to be anchored to or supported on the caisson 10 after installation. By interfering with installation of a suction caisson 10, soil plug hazards can cause costly refusal events. For example, plug heave occurs when a plug 20 rises within the caisson 10 due to suction pressure as soil 14 displaced by the skirt 16 enters the caisson 10 around the bottom edge of the skirt 16. Plug heave can develop in cohesionless or low-cohesion soil 14 due to seabed sediment loosening inside the caisson 10. Conversely, plug uplift occurs if a plug 20 detaches from the soil 14 beneath due to high suction pressures and variations in soil conditions. Soil plug hazards such as plug uplift are a particular risk in layered soils, for example where a low-permeability layer, such as clay, overlays a high-permeability layer, such as sand. The difference in permeability between the two layers can cause plug heave and / or plug uplift. Mitigation of soil plug hazards requires early detection and so requires the plug 20 to be monitored during installation of a suction caisson 10. Current monitoring systems, as shown in Figure 1, comprise an echo sounder 34 projecting acoustic pulses 36 downwardly through the vent hole 26 to the top of the plug 20. However, by measuring plug heave only in a narrow central area of the plug 20 directly beneath the vent hole 26, this will produce unrepresentative plug heave measurements if the top face of the plug 20 departs from a horizontal plane. For example, the top face of the plug 20 may form a funnel shape as water is sucked out from the inside. Against this background, the invention resides in a suction caisson that comprises a suction chamber and at least two plug level sensors, such as echo sounders or sonars, mutually spaced across the suction chamber. For example, the plug level sensors may be spaced about a central longitudinal axis of a skirt that surrounds the suction chamber. The plug level sensors can be distributed radially and / or circumferentially within the suction chamber. The plug level sensors may be supported by a suction pipe that communicates with a vent hole of the suction caisson. The vent hole can, for example, penetrate a top plate of the suction caisson. The plug level sensors may be aligned with respective suction inlets of the suction pipe. The suction pipe may extend across the suction chamber, for example in a radial direction. The inventive concept embraces a corresponding method of monitoring plug heave within a suction caisson. The method comprises measuring a level of the plug at two or more locations mutually spaced across a suction chamber of the suction caisson, for example spaced about a central longitudinal axis of a skirt that surrounds the suction chamber. Sensing emissions can be projected toward the plug from respective locations along a suction pipe that communicates with a vent hole of the suction caisson. The sensing emissions can be projected through, or otherwise in alignment with, respective suction inlets of the suction pipe. In embodiments of the invention, a suction caisson comprises a suction chamber and at least two plug level sensors mutually spaced across the suction chamber, for example about a central longitudinal axis of a skirt that surrounds the suction chamber. The plug level sensors are supported by a suction pipe that communicates with a vent hole of the suction caisson and can be aligned with respective suction inlets of the suction pipe. This sensor arrangement enables a method of monitoring plug heave, the method comprising measuring a level of the plug by projecting sensing emissions, such as acoustic pulses, toward the plug at two or more locations mutually spaced across the suction chamber. The plug heave monitoring system of the invention is easy to fabricate and provides reliable plug heave measurement during installation of a suction caisson for marine applications such as offshore wind. To put the invention into context, reference has already been made to Figure 1 of the accompanying drawings, which is a schematic sectional side view of a suction caisson comprising a plug heave monitoring system of the prior art. In order that the invention may be more readily understood, reference will now be made, by way of example, to the remaining drawings in which: Figure 2 is a schematic sectional side view of a suction caisson comprising a plug heave monitoring system of the invention; Figure 3 is a schematic top plan view of a suction pipe of the suction caisson shown in Figure 3; and Figure 4 corresponds to Figure 3 but shows a variant of the suction pipe. In the suction caisson 10 of Figures 2 and 3, in which like numerals are used for like features, the pump 28 communicates via the vent hole 26 with a suction pipe 38 fabricated beneath the top plate 22 disposed within the suction chamber 24. In this example, the suction pipe 38 extends radially across the internal diameter of the skirt 16. The suction pipe 38 has multiple downwardly-facing suction inlets 40 distributed along its length and hence distributed radially across the internal diameter of the skirt 16 within the suction chamber 24. Each suction inlet 40 is fitted with a respective plug level sensor, exemplified here by echo sounders 34 that project acoustic pulses 36 downwardly to the top of the plug 20. Suction inlets 40 and echo sounders 34 lie on both sides of the central longitudinal axis 18, hence sensing the level or heave of the plug 18 at multiple locations across the width of the plug 18 during a suction operation. This applies even if the vent hole 26 is offset laterally from the central longitudinal axis 18 as shown here. It will be apparent that, in this example, the echo sounders 34 are distributed radially or diametrically within the suction chamber 24. Figure 4 shows a variant in which the suction pipe 38 comprises arms 42 extending radially from the central longitudinal axis 18 in a star-like configuration. Thus, the echo sounders 34 are distributed radially and circumferentially within the suction chamber 24. As shown here, the suction inlets 40 and the echo sounders 34 need not be equispaced about the central longitudinal axis 18. The arrangement shown here also exemplifies how the suction pipe 38 need not abut the skirt 16 or, therefore, extend across the full width of the suction chamber 24. Many other variations are possible within the inventive concept. For example, the suction pipe 38 could spiral about the central longitudinal axis 18 or about the vent hole 26. Plug level sensors such as echo sounders 32 could be fitted to a structure other than a suction pipe 38, such as to the underside of the top plate 22. Echo sounders 32 or other plug level sensors may be distributed around the central longitudinal axis 18 in a similar way to the arrangements shown in Figures 2 to 4 and described above. A suction pipe 38 could therefore be omitted but if a suction pipe 38 is present, it should be positioned not to interfere with the pulses emitted from the echo sounders 32. Plug level sensing could be performed non-acoustically, for example using electromagnetic sensing emissions instead of acoustic pulses or other acoustic sensing emissions.

Claims

1. A suction caisson comprising a suction chamber and at least two plug level sensors mutually spaced across the suction chamber.

2. The suction caisson of Claim 1, wherein the plug level sensors are spaced about a central longitudinal axis of a skirt that surrounds the suction chamber.

3. The suction caisson of Claim 1 or Claim 2, wherein the plug level sensors are distributed radially within the suction chamber.

4. The suction caisson of any preceding claim, wherein the plug level sensors are distributed circumferentially within the suction chamber.

5. The suction caisson of any preceding claim, comprising a top plate penetrated by a vent hole and having a suction pipe that communicates with the vent hole and supports the plug level sensors.

6. The suction caisson of Claim 5, wherein the plug level sensors are aligned with respective suction inlets of the suction pipe.

7. The suction caisson of Claim 5 or Claim 6, wherein the suction pipe extends across the suction chamber.

8. The suction caisson of Claim 7, wherein the suction pipe extends radially across the suction chamber.

9. A method of monitoring plug heave within a suction caisson, the method comprising measuring a level of the plug at two or more locations mutually spaced across a suction chamber of the suction caisson.

10. The method of Claim 9, comprising measuring the level of the plug at locations spaced about a central longitudinal axis of a skirt that surrounds the suction chamber.

11. The method of Claim 9 or Claim 10, comprising projecting sensing emissions toward the plug from respective locations along a suction pipe that communicates with a vent hole of the suction caisson.5 12. The method of Claim 11, comprising projecting the sensing emissions throughrespective suction inlets of the suction pipe.A

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