NO20111491A1 - A foot for the support leg of a maritime structure, and a method for installing and moving an upright platform - Google Patents

Abstract

A support leg foot (4) for a maritime structure (2) comprises a base (6) connected to the lower end of the leg and configured for supportive interaction with a seabed (B) and having a cavity (29). ) fluidly connected to one or more openings (31) to the outside of the base. A housing (14) having an inner chamber (32) which at a first end is fluidly connected to the cavity (29) and at a second end has a fluid opening (20) to the outside of the housing. A valve assembly (12) provided in the chamber is operable between an open position and a closed position to control fluid flow through the opening.

Description

A footing for a maritime structure's outrigger, and a method of installing

and move a jack-up platform

Scope of the invention

The invention relates to maritime vessels which have one or more legs which are connected to the vessel's hull, and where the leg or legs are movable relative to the hull so that the hull can be raised above the water surface. Such vessels are usually referred to as "jack-up vessels". More specifically, the invention relates to a foot for a support leg for a maritime structure as stated in the preamble of claim 1, and methods for operating the invented foot.

Background for the invention

Today's technical status includes US 3,183,676 (Tourneau), which describes a jack-up vessel with three height-adjustable legs slidably connected to a hull. Each leg is moved relative to the hull via a rack and pinion system powered by powerful motors. Hydraulic actuation is also common. In this way, the legs can be moved downwards, to contact the seabed and raise the hull above the water surface. The hull can then be used as a stable work platform for carrying out a number of tasks such as oil well drilling, maintenance, repairs, etc. There is extensive use of jack-up platforms in the oil and gas industry.

An advantage of the jack-up vessel is its mobility. The hull floats, so that a vessel can be floated (often with its own propellers) to the desired work location when the legs are extended. When the vessel has reached its desired location, the legs are extended until contact with the seabed to raise and support the hull. The lower end of each leg is normally designed with an enlarged foot or so-called "spud-can". This lower end section is designed to engage with, and if necessary, penetrate the seabed. Such an example is illustrated in Figure 3 in the above-mentioned US 3,183,676. The seabed is often of a soft material such as silt, mud and clay, into which the spud-can will penetrate. Penetration depths of between 5 and 10 meters are common. There are various designs and forms of spud cans, for example with a hollow, conical underside.

A common problem with mobile jack-up platforms installed on a soft seabed is that a significant force is required to overcome the suction pressures around and on the spud can when the leg is pulled from the seabed. The required leg pullout force depends on factors such as soil composition, the shape of the spud-can, the spud-can's immersion depth, and the speed of pullout. The extraction forces must be kept below given values, determined by stability criteria for the vessel.

The prior art also includes US 4,761,096 (Lin), which describes a system by which the problem of high pull-out forces can be mitigated. A water jet system is connected to a spud-can, in which water under high pressure can be fed through a plurality of nozzles below and on top of the spud-can. This reduces pullout resistance by minimizing the suction and frictional resistance of the seabed soil.

Such high-pressure jet systems are usually powered by pump systems that must be made of high-quality materials in order to operate in the corrosive environment caused by seawater. The consequence is that the pump systems are expensive and that they often need maintenance.

The applicant has conceived and designed this invention to overcome specific disadvantages of the prior art and to achieve additional advantages.

Summary of the invention

The invention is stated and characterized in the main claim, while the independent claims describe other characteristics of the invention.

Thus, there is provided a footing for a support leg for a maritime structure, the footing comprising a base connected to the lower end of the leg and configured for supportive interaction with a seabed and having a cavity fluidly connected to one or more outlet openings to the outside of the base, characterized by housing with an inner chamber which is fluidly connected to the cavity at a first end; and valve means disposed in the chamber and operable between an open position and a closed position to permit fluid flow into the chamber.

In one embodiment, the foot includes a fluid line connected to an inlet to the chamber and is configured to selectively feed fluid under pressure into the chamber. The fluid can be a liquid (such as water) or a gas (such as air).

In one embodiment, the chamber has at a second end a fluid opening to the exterior of the housing, and the valve means comprises a valve assembly arranged to control fluid flow through the opening.

In one embodiment, the valve assembly comprises one or more valve elements having biasing means arranged to bias the valve elements in a closed position at the valve assembly. The biasing means comprise in one embodiment one or more buoyancy elements which are arranged to hold the valve elements in a closed position at the valve assembly when the foot is submerged in water. The valve assembly may comprise one or more butterfly valves. The valve means comprise in one embodiment a valve arranged to allow the inflow of surrounding seawater into the chamber.

In one embodiment, the housing comprises a tube, which extends upwards from the base and the opening is arranged at a vertical distance from the cavity.

The base is in one embodiment a spud-can and the outlet openings are arranged on the underside of the spud-can. In one embodiment, the outlet openings are arranged in an annular groove.

In one embodiment of the invention, the leg is slidably connected to the maritime structure and the maritime structure is a floating jack-up platform.

There is also provided a method for installing a jack-up platform which floats in a water surface and has one or more legs which are extendable to a seabed, and each leg comprises a foot according to the invention, characterized by lowering the legs until the base has assumed supporting contact on , or at a distance below, the seabed while maintaining the valve means in a closed position, and continuing to move the legs relative to the platform until the platform is supported by the legs at a distance above the surface of the water.

There is also provided a method for moving a jackable platform which is supported at a distance above the water surface via one or more legs which are in supporting contact with the seabed via a foot according to the invention, characterized by (i) lowering the platform to the sea surface by move the legs relative to the platform; (ii) continuing to move the legs upwards and opening the valve means so that water is allowed to flow into the chamber and further into the cavity and exit the base via the outlet openings.

In one embodiment of the method of moving the jack-up platform, the valve assembly opens due to the suction force generated in the chamber by the upward movement of the leg. In one embodiment of the method, a quantity of water is injected into the chamber via a fluid line prior to step ii, whereby water is forced out of the outlet opening and through any sludge, mud or the like surrounding the base.

There is also provided a jack-up platform comprising a deck structure and a plurality of jack-up legs, characterized in that each leg comprises a foot according to the invention.

Brief description of the drawings

These and other characteristics of the invention will become clear from the following description and a preferred embodiment, given as a non-restrictive example, with reference to the attached drawings where: Figure 1 shows a side view of a jack-up leg with the foot according to the invention; Figure 2 is an enlargement of the area marked "A" in Figure 1, and illustrates the foot at the base of the jack-up leg; Figure 3 is another side view of the jack-up leg illustrated in Figure 1; Figure 4 is an enlargement of the area marked "B" in Figure 3; and illustrates the top of the jack-up leg; Figure 5 is a perspective view of the house according to the invention; Figure 6 is a schematic side view of the house illustrated in Figure 5; Figure 7 is an enlargement of the area marked "C" in Figure 6; and illustrates an upper part of the house; Figures 8 and 9 are perspective views of a valve assembly for use in connection with the housing; Figure 10 is a schematic side view of a butterfly valve which has buoyancy elements; Figure 11 is a schematic top view of the base according to the invention; Figure 12 is a sectional drawing of along the section line A-A in Figure 11;

Figure 13 is a perspective view illustrating the underside of the base; and

Figure 14 is a schematic sketch of a maritime structure supported by jack-up legs having the base and housing according to the invention.

Detailed description of a preferred embodiment

By first referring to Figures 1, 2 and 14, the foot according to the invention in the embodiment shown is mounted at the bottom of a jack-up leg 4. Such a jack-up leg is well known in the field and comprises three tubes 8a connected in a triangular configuration via bracing 8c. Hole 8b enables interaction with the jacking-up mechanism (not shown) on a platform 2. The leg 4 is connected at its lower end to a spud-can 6, see for example figures 2 and 11. Figure 14 shows a situation where the spud-cans are lowered into soft mud and/or mud on the seabed B. The spud can 6 comprises a number of outlet openings, or nozzles, 31 arranged on the lower side, three of which are arranged in a circumferential groove 30 (see figures 10 and 13). The outlet openings 31 are connected via internal channels to a cavity 29. A tube-shaped housing 14 is connected to the upper side of the spud-can 6 so that the inner chamber 32 of the tube is in fluid communication with the cavity 29 via the lower tube end 21 The tube 14 extends upwards from the spud can and has an opening 20 at its upper end. At a height h above the spud-can, a valve assembly 12 is arranged in the pipe via a flange plate 22, in order to divide the pipe in this way into a lower pipe part 14a and an upper pipe part 14b. The height h is dimensioned according to the planned depth where the leg is intended to be used, for example around 20 metres. The tube 14 is structurally connected to the leg via support plates 16.

With reference to Figures 8 and 9, the valve assembly in the embodiment shown comprises three butterfly valves 24 in a housing 23 and connected to the flange plate 22. Referring additionally to Figure 10, the butterfly valve is of the two-flap type where each of the valve flaps 26 is rotatably supported by a hinge 27. The butterfly valves are arranged to close against a flanged ring 25 and are configured so that they are open when subjected to a force in a downward direction (cf. arrow marked "D" in Figure 10), i.e. they allows an inflow of water through the opening 20 into the chamber 32.

In a preferred embodiment, the valve flaps 26 are provided with buoyancy elements (shown schematically in figure 10), in order in this way to bias each valve to a closed position when the foot is immersed in water and there is no significant pressure difference across the valve.

A fluid infusion tube 10 is connected to the housing via an opening 11, and allows fluid to be fed into the chamber 32. The fluid infusion tube 11 extends up along the leg 4, and has connection points and valves 18 at regular intervals for easy connection to a source of pressurized fluid ( not shown) on the platform. The pressurized fluid can be a liquid (for example seawater) or a gas (for example air).

During use, for example during installation, the leg is extended into the water, towards the seabed, and the sunken cavity 29 and the chamber 32 will fill with water. The connection valves 18 are closed. The submerged buoyancy elements 28 will cause the valve assembly 12 to close and the pressure in the chamber and cavity (which is open to the surrounding water via the outlet openings 31) will be equal to the ambient pressure. In this way, this equal pressure, when the spud-can is lowered into the mud or mud on the seabed, will prevent mud or mud from entering the cavity 29 and the chamber 32.

When the platform is to be moved, and the legs are pulled up from the seabed, the upward movement of the leg will generate a suction force in the cavity and chamber, in this way sucking the butterfly valves open. As the leg continues its upward movement, water will flow into opening 20, through tube 14, and out of outlet openings 31.

In the case where the spud-can is solidly submerged in the seabed and difficult to move due to the suction forces between the spud-can and the soft seabed, the process can be initiated by injecting a fluid under pressure (for example water or air from an overhead pump, not shown) into the chamber 32 via the pipe 10 and the opening 11. In this way, the injected fluid generates a higher pressure in the chamber than in the water surrounding the pipe 14, and the result is that the fluid(s) (water or water and air) inside the the chamber 32 is pushed out through the outlet openings 31 and thus wash away nearby sludge or mud. Preferably, the chamber can be equipped with a pressure sensor (not shown), so that the operator knows when the pressure in the chamber begins to drop, and in this way to get an indication that the spud-can can be raised from the seabed.

As mentioned above, the height h of the pipe is dimensioned in relation to the depth where the leg is intended to be used, so it is certain that the opening 20 is submerged when the leg is installed on the seabed. Likewise, under certain circumstances it may be necessary or convenient to install the leg in shallower water, i.e. at depths less than h; whereby the opening 20 will be above water. Therefore, one or more optional valves 33 are installed on the pipe, in a lower part of the pipe (see figures 2, 6, 12 which show one such valve). The valve can be opened when the leg is to be extracted from the seabed, to allow surrounding seawater to flow through the chamber, as described above. In the event that the fluid infusion tube is used, the valve 33 is held in a closed position until there is a sufficient excess pressure in the chamber.

Although the invention has been described with reference to a tube connected to the upper side of a spud-can, it should be understood that the housing may have other forms than tubes and that it may be an integral part of the spud-can.

Although the invention has been described in relation to a jackable leg of a truss, the person skilled in the art will understand that the invention can also be used on other types of legs.

Although the invention has been described in relation to butterfly valves with liquid closing means, it should be understood that the invention is equally suitable together with other biasing means. Although not shown, it should be understood that the valve assembly can be operated by hydraulic, pneumatic, magnetic lock, and/or mechanical means.

Claims (16)

1. Footing for a support leg (4) for a maritime structure (2), the footing comprising a base (6) which is connected to the lower end of the leg and which is configured for supportive interaction with a seabed (B) and which has a cavity (29) fluidly connected with one or more outlet openings (31) to the outside of the base, characterized by a housing (14) with an inner chamber (32) which is fluidly connected to the cavity (29) at a first end; and valve means (12; 33) arranged in the chamber and operable between an open position and a closed position to allow fluid flow into the chamber (32). 2. The foot according to claim 1, further comprising a fluid line (10) connected to an inlet (11) to the chamber (32) and configured for selective feeding of fluid under pressure into the chamber. 3. The foot according to claim 1 or claim 2, in which the chamber (32) at a second end has a fluid opening (20) to the outside of the housing, and that the valve means comprise a valve assembly (12) arranged to control fluid flow through the opening (20). 4. The foot according to claim 3, the valve assembly (12) further comprises one or more valve elements (26) which have biasing means (28) which are arranged to bias the valve elements in a closed position at the valve assembly. 5. The foot according to claim 4, the biasing means comprise one or more buoyancy elements (28) which are arranged to keep the valve elements in a closed position of the valve assembly when the foot is submerged in water. 6. The foot according to any of the preceding claims, wherein the valve assembly (12) comprises one or more butterfly valves. 7. The foot according to any one of the preceding claims, wherein the valve means comprises a valve (33) arranged to allow the inflow of seawater surrounding the housing into the chamber (32). 8. The foot according to any one of the preceding claims, in which the housing comprises a tube (14) which extends upwards from the base (6) and the opening (20) is arranged at a vertical distance (h) from the cavity (29). 9. The foot according to any one of the preceding claims, where the base (6) is a spud-can and the outlet openings (31) are arranged on the underside of the spud-can. 10. The foot according to claim 9, in which the outlet openings (31) are arranged in an annular groove. 11. The foot according to any one of the preceding claims, in which the leg (4) is slidably connected to the maritime structure (2) and the maritime structure is a floating jack-up platform. 12. Method for installing a jack-up platform (2), which floats in a water surface (S) and has one or more legs (4) extendable to a seabed (B), each leg comprising a foot according to any of claims 1-11, characterized by lowering the legs until the base (6) has assumed supporting contact on or at a distance below the seabed while the valve means are held in a closed position, and continuing to move the legs relative to the platform until the platform is supported by the legs at a distance ( d) above the water surface. 13. Method for moving a jackable platform (2) which is supported at a distance (d) above the water surface (S) via one or more legs (4) which are in supporting contact with the seabed (B) via a foot according to which preferably from claims 1-11, characterized by (i) lowering the platform to the sea surface by moving the legs relative to the platform; (ii) continuing to move the legs upwards and opening the valve means so that water is allowed to flow into the chamber (32) and further into the cavity (29) and exit the base (6) via the outlet openings (31). 14. The method according to claim 13, wherein the valve assembly (12) is opened due to the suction force generated in the chamber (32) by the upward movement of the leg. 15. The method according to claim 13 or claim 14, where a quantity of fluid is injected into the chamber (32) via a fluid line (10) before step ii, whereby water is forced out of the outlet opening (31) and through any mud or mud that surrounds the base. 16. Jack-up platform, comprising a deck structure (2) and a plurality of jack-up legs (4), characterized in that each leg comprises a foot according to any one of claims 1-11.