Field of the invention

The invention concerns marine vessels having one or more legs which are connected to the vessel's hull, and where the leg or legs are movable with respect to the hull such that the hull may be elevated above the water surface. Such vessels are commonly referred to as "jack-up vessels". More specifically, the invention relates to a footing for a support leg for a marine structure as set out in the preamble of claim 1 , and to methods of operating the invented footing. Background of the invention

The state of the art includes US 3,183,676 (Tourneau), which describes a jack-up vessel having three height-adjustable legs slidably connected to a hull. Each leg is moved with respect to the hull via a rack-and-pinion system, driven by powerful motors. Hydraulic actuation is also common. Thus, the legs can be moved downwards, into contact with the seabed and elevate the hull above the water surface. The hull can then be used as a stable work platform to perform a number of tasks such as oil well drilling, maintenance, repairs, etc. Jack-up vessels are widely used in the offshore oil and gas industry.

One advantage of the jack-up vessel is its mobility. The hull is buoyant, so that the vessel may be floated (often self-propelled) to the desired work location when the legs are retracted. When the vessel has reached its desired location, the legs are extended into supporting contact with the seabed, in order to elevate and support the hull. The lower end of each leg is typically provided with an enlarged foot or so- called "spud-can". This lower end portion is designed to engage and if necessary penetrate into the seabed. One such example is illustrated in figure 3 in the above mentioned US 3,183,676. The seabed is oftentimes of a soft material such as mud, silt and clay, into which the spud-can will penetrate. Penetration depths of 5 to 10 metres are common. There exist various spud-can designs and shapes, for example having a shallow, conical underside. A common problem with mobile jack-up vessels that are installed on a soft seabed is that a considerable force is required to overcome the suction pressures around and on the spud-can when the leg is being pulled out of the seabed. The required leg pull-out force depends on factors such as soil composition, spud-can shape, spud- can embedment depth, and extraction rate. The pull-out forces must be kept below certain values, determined by stability criteria for the vessel.

The state of the art also includes US 4,761 ,096 (Lin), which describes a system by means of which the problem of high pull-out forces may be mitigated. A water- jetting system is connected to a spud-can, in which water under high pressure may be fed through several nozzles underneath and on top of the spud-can. This reduces pull-out resistance by minimizing the suction and friction resistance of the seafloor soils. Such high-pressure jetting systems are typically driven by pumping systems which need to be of high grade materials in order to operate in the corrosive environments caused by seawater. The pumping systems are consequently costly and require frequent maintenance.

The present applicant has devised and embodied this invention in order to overcome certain shortcomings of the prior art and to obtain further advantages.

Summary of the invention

The invention is set forth and characterized in the main claim, while the dependent claims describe other characteristics of the invention.

It is thus provided a footing for a support leg for a marine 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 with one or more orifices to the outside of the base, characterized by a housing having an internal chamber which at a first end is fluidly connected to the cavity; and valve means arranged in the chamber and operable between an open position and a closed position to allow fluid flow into the chamber.

In one embodiment, the footing comprises a fluid line connected to an inlet to the chamber and configured for selectively feeding fluid under pressure into the chamber. The fluid may 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 outside 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 members having biasing means that are arranged to bias the valve members in a closed state of the valve assembly. The biasing means comprises in one embodiment one or more buoyancy elements that are arranged to keep the valve members into a closed state of the valve assembly when the footing is immersed in water. The valve assembly may comprise one or more butterfly valves. The valve means comprises in one embodiment a valve arranged to allow flow of seawater surrounding the housing into the chamber. In one embodiment, the housing comprises a pipe which extends upwards from the base and the opening is arranged a vertical distance from the cavity. The base is in one embodiment a spud-can and the orifices are arranged on the underside of the spud-can. The orifices are in one embodiment arranged in an annular groove.

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

It is also provided a method of installing a jack-up platform which is floating in a water surface and having one or more legs that are extendible to a seabed, and each leg comprising a footing according to the invention, characterized by lowering the legs until the base has assumed supportive contact on or a distance below the seabed while keeping the valve means in a closed state, and continuing moving the legs with respect to the platform until the platform is supported by the legs at a distance above the water surface.

It is also provided a method of removing a jack-up platform which is supported a distance above a water surface via one or more legs that are in supportive contact with the seabed via a footing according to the invention, characterized by

(i) lowering the platform to the water surface by moving the legs with respect to the platform; (ii) continuing moving the legs upwards and opening the valve means such that water is allowed to flow into the chamber and further into the cavity and exit the base via the orifices. In one embodiment of the method of removing the jack-up platform, the valve assembly is opened due to the suction force being generated in the chamber by the upward motion of the leg. In one embodiment of the method, an amount of water is injected into the chamber via a fluid line before step ii, whereby water is forced out of the orifices and through any mud, silt, or similar surrounding the base. It 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 footing according to the invention.

Brief description of the drawings

These and other characteristics of the invention will be clear from the following description of a preferential form of embodiment, given as a non-restrictive example, with reference to the attached drawings wherein:

Figure 1 is a side view of a jack-up leg having the footing according to the invention;

Figure 2 is an enlargement of the section marked "A" in figure 1 , illustrating the footing at the bottom 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 section marked "B" in figure 3; illustrating the top of the jack-up leg;

Figure 5 is a perspective view of the housing according to the invention;

Figure 6 is a schematic side view of the housing illustrated in figure 5;

Figure 7 is an enlargement of the section marked "C" in figure 6; illustrating an upper portion of the housing;

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 having buoyancy elements;

Figure 1 1 is a schematic top view of the base according to the invention;

Figure 12 is a sectional drawing along the section line A-A in figure 1 1 ;

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

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

Detailed description of a preferential embodiment

Referring initially to figures 1 , 2 and 14, the footing according to the invention is in the illustrated embodiment mounted at the bottom of a jack-up leg 4. Such jack-up leg is commonly known in the art and comprises three pipes 8a interconnected in a triangular configuration via bracing 8c. Holes 8b provide for interaction with the jack-up mechanism (not shown) on a platform 2. The leg 4 is connected at its lower end to spud-can 6, see e.g. figures 2 and 1 1. Figure 14 illustrates a situation where the spud-cans are embedded in soft mud and/silt on the seabed B. The spud-can 6 comprises a number of orifices, or nozzles, 31 arranged on the lower side, three of which are arranged in a circumferential groove 30 (see figures 10 and 13). The orifices 3 1 are connected via internal channels to a cavity 29. A housing in the shape of a pipe 14 is connected to the upper side of the spud-can 6 such that the interior chamber 32 of the pipe is in fluid connection with the cavity 29 via the lower pipe end 21. The pipe 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, thus dividing the pipe into a lower pipe part 14a and an upper pipe part 14b. The height h is dimensioned according to the intended depth on which the leg is intended to be used, for example around 20 metres. The pipe 14 is structurally connected to the leg via support plates 16. Referring now to figures 8 and 9, the valve assembly comprises in the illustrated embodiment three butterfly valves 24 in a housing 23 and connected to the flange plate 22. Referring additionally to figure 10, the butterfly valves are 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 flange ring 25 and are configured such that they are open when subjected to a force in the downward direction (cf. arrow marked "D" in figure 10), i.e. allowing an influx of water through the opening 20 and into the chamber 32.

In a preferred embodiment the valve flaps 26 are furnished with buoyancy elements 28 (shown schematically in figure 10), thus biasing each valve to a closed state when the footing is immersed in water and there is no significant pressure differential across the valve.

A fluid infusion pipe 10 is connected to the housing via an opening 1 1 , allowing fluid to be fed into the chamber 32. The infusion pipe 1 1 extends up along the leg 4, having 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 may be a liquid (e.g. seawater) or a gas (e.g. air).

In use, for example during installation, the leg is extended into the water, towards the seabed, and the submerged cavity 29 and 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 orifices 31) will be equal to the ambient pressure. Thus, as the spud-can is lowered into the mud or silt on the seabed, this equal pressure will prevent mud or silt from entering the cavity 29 and chamber 32. When the platform is to be moved, and the legs are to be retracted from the seabed, the upward movement of the leg will generate a suction force in the cavity and chamber, thus sucking the butterfly valves open. As the leg continues its upward movement, water will flow into the opening 20, through the pipe 14, and out of the orifices 31.

In the event that the spud-can is firmly embedded in the seabed and difficult to move due to suction forces between the spud-can and the soft seabed, the process may be initiated by injecting a fluid under pressure (e.g. water of air from a topsides pump, not shown) into the chamber 32 via the pipe 10 and opening 1 1. Thus the injected fluid generates a higher pressure in the chamber that in the water surrounding the pipe 14, the result being that the fluid(s) (water or water and air) inside the chamber 32 is forced out through the orifices 31 and thus jetting away adjacent mud or silt. The chamber may advantageously be furnished with a pressure sensor (not shown), such that the operator will know when the pressure in the chamber is starting to drop, thus giving an indication that the spud-can may be elevated out of the seabed.

As mentioned above, the pipe 14 height h is conveniently dimensioned according to the intended depth on which the leg is intended to be used, thus ensuring that the opening 20 is submerged when the leg is installed on the seabed. However, under certain circumstances it may be necessary or convenient to install the leg in shallower waters, i.e. in 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, illustrating one such valve). The valve may be opened when the leg is to be pulled out from the seabed, allowing surrounding seawater to flow through the chamber, as described above. In the event that the fluid infusion pipe is used, the valve 33 is kept in a closed state until there is a sufficient overpressure in the chamber.

Although the invention is described with reference to a pipe connected to the upper side of a spud-can, it should be understood that the housing may have shapes other than that of a pipe, and may be an integral part of the spud-can.

Although the invention has been explained in relation to a truss jack-up leg, the skilled person will understand that the invention is applicable also on other types of legs. Although the invention has been described in relation to butterfly valves having buoyant closing means, it should be understood that the invention is equally applicable with other biasing means. Although not illustrated, it should be understood that the valve assembly may be operated by hydraulics, pneumatics, magnetic lock, and/or mechanical means.