TECHNICAL FIELD The invention relates to the field of semisubmersible platforms. BACKGROUND

Offshore drilling and production in water depth greater than around 150-200 meters usually requires that operations be carried out from a floating vessel, as fixed structures are not practical. Initially, mono-hull ships were used but these were found to have significant heave, pitch and yaw motions in large waves, and the industry needed more stable drilling and production platforms. In response to this need, semisubmersible platforms were developed. Referring to Figure 1 herein, a semi-submersible platform 1 is provided with ballasted, watertight pontoons 2 located below the ocean surface and wave action. The pontoons 2 provide buoyancy. An operating deck 3 is attached to the pontoons using supports 4. The supports 4 allow the operating deck 3 to be located high above the sea level. As the pontoons 2 are located below the wave action, the operating deck 3 remains very stable and can be kept at a height well above that of any expected waves.

In order to keep the platform 1 in the desired position, the pontoon 2 is anchored to the sea bed 5 using mooring lines that run through fairleads 6. This allows the platform to remain in the desired position and prevents it from drifting off.

Because the pontoons 2 are submerged at a deep draft, the semi-submersible platform 1 is less affected by wave loadings than a normal ship. However, as the semisubmersible platform 1 has a small water-plane area, it is sensitive to load changes, and therefore must be carefully trimmed to maintain stability.

Referring to Figure 2 herein, which shows a plan view of the pontoons and support, a typical semisubmersible platform has four pontoons 7, 8, 9, 10 forming a square, and four supports 1 1 , 12, 13, 14 extending between the pontoons and the operating deck. Each support is located at a corner of a square formed by the pontoons and connects to the operating deck. Each support has the same cross-sectional area, and each pontoon has the same cross-sectional area.

The topside centre of gravity (COG) of the semisubmersible platform is usually not in the centre of the square formed by the pontoons, but is located towards one side of the operating deck, towards the process area, as this is where the bulk of the weight of the operating deck is located. However, the centre of buoyancy (COB) of the semisubmersible platform is located in the geometric centre of the platform. In order to compensate for this, the semisubmersible platform needs to be trimmed. This is achieved by using a greater volume of ballast water in the pontoon 10 furthest away from the COG than in the pontoon 8 closest to the COG in order to compensate for the offset COG. This technique is known, and is described in documents such as US 3,160,135 and US 4,538,939. As a result of using more ballast, the pontoons are submerged at a greater depth than would otherwise be required for balancing the production area weight on the platform. A lower displacement in the water means that the supports need to be longer. This is not optimal from cost and weight point of view, as there is a direct link between displacement and steel weight.

Note that semisubmersible platforms can be used in several roles. They are often used for stable platforms for offshore drilling for hydrocarbons. However, semisubmersible platforms are also used to carry cranes for offshore construction, as offshore support vessels, and as offshore production platforms.

SUMMARY

It has been realised that adjusting ballast is not the most efficient way to ensure that the Centre of Gravity aligns more closely with the Centre of Buoyancy of a submersible platform. According to a first aspect, there is provided a semisubmersible platform. The semisubmersible platform is provided with an operating deck and a hull structure for supporting the operating deck. The hull structure has a first end and an opposing second end, and comprises at least a first pontoon located substantially at the first end, and a second pontoon located substantially at the second end. The first pontoon is connected to the operating deck by a first plurality of supports, and the second pontoon is connected to the operating deck by a second plurality of supports. The first pontoon has a greater volume than the second pontoon, and at least one support of the first plurality of supports has a greater horizontal cross-section area than a corresponding support of the second plurality of supports.

In an optional embodiment, the centre of gravity of the platform is located closer to the first end than the second end. A pontoon located at the first end has a larger volume than a pontoon located at the second end, thereby ensuring that a centre of buoyancy of the semisubmersible platform is located closer to the first end than the second end.

An optional way to achieve the larger pontoon volume is to ensure that the pontoon located at the first end has a larger width than the pontoon located at the second end.

In this case, a support located at the first end optionally has a larger width than the support located at the second end.

As an option, at least one pontoon and one of its associated supports are supported by at least one common structural element. This ensures that loads are uniformly distributed.

As an option, the common structural element is used as a frame of the pontoon and a side shell of the support.

According to an optional embodiment, any of the first and second pontoons has a substantially rectangular cross-sectional area.

As an option, each support of any of the first and second plurality of supports has a substantially rectangular cross-sectional area.

By making the pontoons and/or the supports substantially rectangular in cross- section, common structural elements can be used more easily. The invention may be applied to any type of semisubmersible platform. Examples of semisubmersible platforms include any of a production platform, a crane platform, an offshore support vessel, and an offshore production platform. BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 illustrates schematically a side view of a known semisubmersible platform;

Figure 2 illustrates schematically a plan view of the pontoons and supports of a known semisubmersible platform;

Figure 3 is a perspective view of pontoons and supports for a semisubmersible platform according to an embodiment of the invention; Figure 4 illustrates schematically a side view of a semisubmersible platform according to an embodiment of the invention; and

Figure 5 illustrates schematically a plan view of the pontoons and supports of a semisubmersible platform according to an embodiment of the invention.

DETAILED DESCRIPTION

Referring to Figure 3, there are shown four pontoons 15, 16, 17, 18 and four supports 19, 20, 21 , 22. In use, the pontoons are submerged below the surface of water and the supports connect the pontoons to an operating deck (not shown).

To assist in describing the invention, the side of the platform at which pontoon 18 is located is referred to as the first side, and the side of the platform at which pontoon 15 is located is referred to as the second side.

It can be seen from the drawing that the pontoon 18 and the supports 21 , 21 at the first side have a greater volume than the corresponding pontoon 15 and supports 19, 20 at the second side. This is because the pontoon 18 at the first side is substantially wider than the pontoon 15 at the second side. The width of the pontoon 18 at the first side is denoted D in Figure 3. Similarly, the supports 21 , 22 at the first side are substantially wider than the supports 19, 20 at the second side (again, denoted by D), and so the supports at the first side have a greater volume than the supports at the second side.

The drawing shows the supports and pontoons as being rectangular, but it will be appreciated that they could be of any suitable shape, such as oval, substantially circular and so on.

Turning now to Figure 4, there is illustrated a side view of the platform 23. The platform has an operating deck 24, on which the first end 25 and the second end 26 are shown. Pontoons 15 and 16 are shown, along with supports 19 and 21 . The centre of gravity, owing to the weight distribution of the operating deck, is located towards the first side, as described above with known semisubmersible platforms. However, when the pontoons 15, 18 and supports 19, 21 are ballasted, the centre of buoyancy is also located towards the first side, unlike known semisubmersible platforms. This is because the buoyancy of pontoon 18 and supports 21 , 22 is greater than the buoyancy of pontoon 15 and supports 19, 20 when they are equally ballasted. This ensures that the centre of buoyancy moves towards the first side.

Figure 5 shows a plan view of the pontoon and supports. In this example, the supports 19, 20, 21 , 22 are rectangular or square. The supports 21 , 22 on the first side 25 have a larger cross-sectional area than the supports on the second side 26. Similarly, pontoon 18 has a greater volume than pontoon 15 owing to having a greater width. Moving the centre of buoyancy towards the centre of gravity by altering the size of the pontoon and supports means that the larger pontoons have more buoyancy without ballast, and so the pontoons and supports do not need to be ballasted individually. This is in contrast to known semisubmersible platforms, in which the pontoons and supports must be carefully ballasted to compensate for the difference between the centre of gravity and the centre of buoyancy. This has the advantage that the pontoons need not be displaced as deeply underwater, which reduces material costs of a semisubmersible platform.

The semisubmersible platform with rectangular columns and matching pontoons enables more optimal hull design by moving the centre of buoyancy (COB) towards the centre of gravity, without compromising the structural strength and stress flow, as corresponding pontoon and column side-shells are located in the same planes and have structural backing in the nodes. Nodes are the connections between pontoons and columns. Furthermore, moving the centre of buoyancy by altering the dimensions of the pontoons and supports, rather than individually ballasting them, ensures that the structural strength and stress flows in the platform are not compromised.

Referring to Figure 5, support 21 is rectangular and has one cross-section dimension D that corresponds with the width of pontoon 18, and the other cross-sections dimension d as pontoon 17. By ensuring that the side-shells of the supports correspond to the frames of the pontoons, the strength of the pontoons and supports is improved, allowing for a more efficient use of construction materials.

This is illustrated in Figure 3, in which a support element 18a is shown. Of course, the structure will include many support elements that are not shown in order to maintain the clarity of Figure 3. Support element 18a is part of a framework that provides support to both pontoon 18 and support 21. In this example, support element 18a also provides support for pontoon 17. By using the same support element 18a to support both pontoon 18 and support 21 , the strength of the structure is improved, and costs are reduced as fewer support elements are required is they are shared between pontoons and supports.

The concept of using the same framework for the supports and the pontoons, in addition to the concept of altering the dimensions of the pontoons and supports, ensures that a more efficient semisubmersible platform design can be realised that requires less displacement in the water, and uses less construction material. By reducing the required displacement, the weight and cost of the semisubmersible platform can be minimized. It will be appreciated by the person of skill in the art that various modifications may be made to the above-described embodiments without departing from the scope of the present invention.