WO1984001554A1

WO1984001554A1 - Floating, semi-submersible structure

Info

Publication number: WO1984001554A1
Application number: PCT/NO1983/000045
Authority: WO
Inventors: Einar Kjelland-Fosterud
Original assignee: Kvaerner Eng
Priority date: 1982-10-20
Filing date: 1983-10-19
Publication date: 1984-04-26
Also published as: NO823489L; EP0122273A1

Abstract

A floating, semi-submersible offshore structure comprises at least three mutually connected legs which are spaced a distance of at least twice the largest cross-sectional dimension of the legs and adapted to contain ballast at their lower ends to increase the stability of the structure, as is conventional in semi-submersible vessels. However, the structure according to the present invention has a draught in operation which is at least twice the maximum wave height in the waters for which the structure is designed.

Description

Floating, semi-submersible Structure

The invention relates to a floating, semi-submersible structure of steel and/or concrete, comprising at least three mutually connected legs which are spaced a distance of at least twice the largest cross-sectional dimension of the legs and adapted to contain ballast at their lower ends to increase the stability of the structure.

In the recovery of petroleum products offshore there are normally used stationary structures which are either secured by piles rammed into the sea bed or rest on the sea bed with their own weight. With increasing water depths the weight and thereby the costs of stationary structures increase dramatic¬ ally. Thus, a steel platform constructed for 300 m water depth in the North Sea will need 60-80.000 ton of steel for the platform itself. Additionally, the piles require large amounts of steel. The weight referred to is based on the assumption that the weight of the fully equipped deck is about 30.000 ton. However, the steel weight required for the structure does not vary much with the deck weight.

It is therefore necessary to concentrate the production equipment on a small number of very large platforms, even if the structure of the reservoir should indicate a more scattered development.

There is thus a need for a type of platform which is cheaper and which can be used on the larger water depths on fields which will be developed in the future.

For this reason large sums have been used on the develop¬ ment of floating production platforms. These are connected to the wells on the sea bed and to pipelines etc. through risers.

The most important problem on which the development has been concentrated, concerns a reduction of the movements of the platform due to waves to levels which can be tolerated by the riser system, which system must accommodate the relative

O H _ movement between the platform and the sea bed. In the North Sea the maximum wave has a height of 32 m, and its period is around 15 seconds. A ship lying abeam such large waves will move vertically almost 32 , and the movement in a horizontal direction is also substantial. No known riser system can stand such movements.

A semi-submersible vessel would under such conditions move somewhat less than half the movements of an ordinary ship's hull. Semi-submersible vessels in principle consist of one or more submerged pontoons plus a number of legs or columns which penetrate the water surface and support a deck or a superstructure above the water. The improvement of the characteristics with respect to vertical movement or heave is achieved by balancing the cross-sectional area of the columns against the volume and shape of the pontoons, as will be explained:

It is known that the pressure pulsations below a wave decrease exponentially with the distance from the surface, and the pressure pulsations are thus always larger on the upper surface of the pontoon than on the lower surface. This difference will cause a neutrally buoyant submerged pontoon to move with the surrounding water. However, if the upper surface area is reduced by suitably sized columns penetrating the water surface, the forces acting on the upper surface can be reduced to approximately the same size as on the lower surface, since the forces are determined as the product of the pressure and the area. Hence the vertical movements of the pontoon due to waves will be less than those of the surrounding water. This reduction of the vertical movement adds to the reduction due to the fact that the water at larger depth moves substantially less than the surface waves. It is necessary to avoid resonance at the wave frequences encountered, and this restricts the extent to which it is possible to make use of the tuning possibilities.

The first floating production platform which has been built, is the Tension Leg Platform for Hutton. In this plat-

•^ $£! c

OMPI iVty 1PO form the hull is shaped as a semi-submersible vessel to reduce the forces acting on the tension legs which tie the platform at a fixed distance from the sea bed. The platform substantially moves on a spherical surface with its centre in the anchorage on the bottom.

The weight of the equipped deck of the Hutton platform is 17.000 ton, and the tension in the legs totals 13.600 ton in calm sea. The supporting structure must withstand the variations in tension in a seaway, also in case one tension leg should break. The supporting structure weighs around 30.000 ton. The total displacement is thus about 60.000 ton, and the weight of the anchorages amounts to another 8.000 ton. These figures show that the ratio of payload to total struc¬ tural weight is nearly as poor as for a stationary platform if the water depth is 300 m.

If it were possible to replace the tension legs of the Hutton platform by conventional anchors, most of the 13.600 ton used in tension could in theory be used for pay- load. Additionally, the strains on the supporting structure due to the tension legs would be eliminated, so that the supporting structure could be more lightly built. Thereby, the ratio of payload to structural weight could be nearly doubled.

This reasoning, which does not take into account practical problems like stability, shows that by using tension leg platforms a high price is paid in terms of steel weight in order to reduce movements due to waves.

Consequently, designers search for other and less costly ways of reducing movements in a seaway to magnitudes which can be tolerated by a riser system.

A well known design is the Shell Spar which is installed on the Brent field. This is a narrow, deep concrete cylinder having a smaller diameter close to the surface of the sea than lower down. The cylinder is anchored in a conventional manner. It is designed as a storage and loading structure with only small deck weight. The structure is kept upright by means of

_ OMPI ballast at the lower end. The large draught reduces heave motions, and the reduced waterplane area results in small horizontal motions as well. However, it appears that it has not been necessary fully to exploit the principles of semi- submersible structures. The simple geometrical shape of the structure will provide very little hydrodynamic damping, so that when set in motion, the structure will continue to oscillate vertically for a long period. This is a disad¬ vantage of this design. Another disadvantage if the structure is used as a production platform, is that it would require large amounts of ballast to achieve stability with heavy deck loads.

Another design is the Shell Semi-Spar. This is a semi- submersible platform having a tuned volume ratio between the columns and a cylindrical pontoon. Ballast in six legs con¬ tribute to the stability. These legs will be lowered after the platform has been towed into position. Before this is done, the structure has a very small draught and can be con¬ structed at shallow water yards. Since the pontoon is positioned at a relatively large depth, the heave motion is limited to approximately 1/3 of the wave height. For this reason the platform can probably be used in recovering oil in the North Sea, but not for gas production, which places heavier demands on the riser system.

A further design is the articulated tower. This structure is kept in an upright position by buoyancy elements and ballast, and the tower is secured to the seabed by an universal joint. The amount of ballast is chosen so that the joint is hardly stressed in calm sea. The structure can thus be regarded as a floating platform which is anchored in the joint. Articulated towers are built as loading towers or flare stacks. If designed as a production unit, they are made from concrete or a combination of concrete and steel, or as a shell or framework structure. The dimensions can be optimized as for semi-submersible platforms so as to reduce the loads imposed on the universal joint. A disadvantage is that the joint is difficult to repair because of its size and position. An articulated tower platform for 38.000 ton pay- load and 300 m water depth will have a steel weight of

32.000 ton in addition to the universal joint and the foun-

3 dation. It will have buoyancy elements of about 100.000 placed at a depth of 100 m, and the ballast will be approxi¬ mately 32.000 ton. The ratio between payload and steel weight will be almost the same as for the semi-submersible platform in spite of the added buoyancy provided by the ballast. The reason is that a deep framework having a moderate width is a more effective way of resisting wave forces.

Most floating platform structures are intended to be built and towed in a fully equipped state in shallow waters, since most yards are situated in estuaries. For structures intended for use on the Norwegian continental shelf, this is an unnecessary limitation, since there are sheltered deep- water building sites in the Norwegian fjords with access to the deep sea through deep draught towing paths. This fact has been exploited in the Condeep designs for gravity concrete structures. By utilizing this geographical advantage also for floating platforms, fewer restrictions are put on the design, and weight and cost savings can be achieved.

The main purpose of the invention is to develop a floating structure in which the riser system is subjected to significantly smaller loads than in previously known structures of floating production platforms, thus making deep water production of both oil and gas possible. Another object is to avoid the complicated,, heavy and costly tension leg systems used in tension leg platform designs, to avoid the depth limit and the vulnerable joint of the articulated tower and to avoid the risk of capsizing after damages because of the poor stability of conventional semi-submersible platforms. According to the present invention, the draught of the structure in operation is at least twice the maximum wave height in the waters for which the structure is designed. In the North Sea the maximum wave height is 32 m, and hence the platform shall have a draught of at least 64 m. However, it is preferred to make the draught at least 128 m, and a large platform can have a draught of approximately 200 m.

At this depth the surface waves will be almost fully dampened, and the heave motions of the platform in storm waves will be very small. The part of the riser system which takes care of the vertical motions can be correspondingly simplified.

In view of the advantages which may be obtained by pro¬ viding a semi-submersible vessel having several legs with a draught according to the invention, it might have been ex¬ pected that suggestions in this direction had been made previously, if the concept had been obvious or near at hand. However, it is believed that Shell Semi-Spar represents the structure closest to the present invention, and not even this structure has a draught which comes close to that prescribed according to the invention. In order to find structures having such a draught one has to consider structures which do not have the mutually connected legs of a semi-submersible vessel, but which are instead constructed as a narrow deep concrete cylinder (Shell Spar) .

Accordingly, the invention can also be regarded as a combination of a semi-submersible vessel (which is stable because of the spreading of the waterplane area over a large area and the positioning of ballast at a substantial distance from the vertical centre axis) and a deep cylinder of the Spar type. No indication of such a combination can be seen to have been suggested or constructed.

Since the structure is shaped as a floating tower, it will be possible to provide bearings or anchorages supporting the riser or to protect it by passing it through a column

OMP- part way or all the way down to the maximum draught of the platform. From this level and down to the sea bed the riser will not be subjected to wave forces. Additionally, the current in these depths usually flows at less than half the speed of the surface current. The free length of the riser is reduced by a length corresponding to the draught of the platform, and also this fact contributes to a reduction of the forces on the riser. Since the wave forces have a resulting force attacking near the surface, the wave induced rotations of the platform will be centered close to the maximum draught. Thereby, the waves will induce only small horizontal motions of the free length of the riser,^* and small changes in angle between the riser and the platform.

By attaching the anchoring lines close to the maximum, draught of the platform, the influence of the wave forces on the anchoring system will be reduced, and it becomes possible to use tighter anchoring lines, thus reducing the lateral displacement of the platform due to wind and current. Also the horizontal movements of the riser system are reduced.

If desirable, for instance to provide a larger oil storage volume, the volume of the lower part of the structure may be increased, thereby providing an effect similar to the pontoons. This may upset the balance required by the semi- submersible theory between the volumes of the columns and the horizontally enlarged parts or pontoons, and hence increase the heave amplitudes. To restore the required balance it may be useful to streamline the volumes of the horizontally enlarged parts so that less water is set in motion.

The platform can be constructed in several ways. A pre¬ ferred alternative is to build the supporting or main struc¬ ture and the deck separately and to connect these parts at an inshore deep water site on.the route to the site at which the platform is to be used.

The main structure can be built as a conventional steel jacket platform, launched, upended and lowered by filling

O PI e/ty. . 1PO with water until only a few meters of at least one leg pene¬ trates the water surface. Alternatively, it can be built like a condeep, the lower part being built in a dry dock, floated out, and lowered progressively into the water by water filling while the legs are built. A third method is to build the large cylindrical legs separately and connect these with the bracing while floating horizontally.

The fully equipped deck can be self buoyant or carried on a barge. After placing the deck over the main structure, water will be pumped out of the main structure, which then lifts the deck clear of the water or the barge. The com¬ pleted platform can then be towed to its destination.

If no deepwater assembly site is available the main structure can be towed out horizontally and upended near its final operating site. Equipment in the form of modules can then be lifted on board as for a jacket.

A platform according to the invention is diagrammatically shown in the drawing. The platform constitutes a floating structure of steel and/or concrete consisting of legs or columns 1 having horizontal and possibly diagonal bracings 2 and 3, respectively. The platform may be terminated at a large draught by one or more pontoons 4. The geometry of the pontoons can be tuned to the crosssectional area of the legs in the same manner as for other semi-submersible vessels. The upper part 5 of the legs is filled with air to a depth providing an adequate buoyancy. If required, a part of the platform can be shaped as a tube 6 between the deck and a point well below the surface. This tube can be used for pro¬ tecting the riser 7 from wave forces. Riser guides and an arrangement for accommodating angular motions between the riser and the platform, if desired, can be mounted within the platform structure. The platform will usually depend on fixed ballast 8 to be stable. If an oil storage volume is required, the lower part of the columns and the pontoons will be kept full of oil 9 and water 10 in varying ratios. Trim-

O PI ^S-VATl ming tanks 11 in the legs are used for compensation of weight changes. These tanks will be filled with gas/oil or air/ water in controlled ratios. If required, the volume of the trimming tanks can be used as oil separators to save the weight of deck mounted separators.

The platform is anchored with anchor lines 12. If the fairleads for these anchor lines are mounted far down on the legs, the anchor lines may be significantly shortened com¬ pared with a mounting of the fairleads higher up. Also the dynamic forces in the anchor lines will be reduced, and the horizontal motions for the riser will be smaller. On the other hand the heel angles induced by current, wind and wavedrift will increase. If required, this heel may be offset by asymmetrical filling of the trimming tanks or the storage volume.

Because of the width/depth ratio and the positioning of the centre of gravity due to the ballast, the platform cannot capsize even if one of the legs should lose its buoyancy totally. The security can be increased using known methods such as dividing the buoyancy elements by watertight partitions, giving extra attention to collision zones, and designing the deck with sufficient buoyancy to keep it floating even if the main structure should lose a major part of its buoyancy.

Although the invention has been discussed above with reference to a production platform, a structure according to the invention can be useful for several other uses in which an articulated tower would otherwise be employed.

Claims

P a t e n t C l a i m s

1. A floating semi-submersible structure of steel and/or concrete comprising at least three mutually connected legs (1) which are spaced a distance of at least twice the largest cross-sectional dimension of the legs and adapted to contain ballast at their lower ends to increase the stability of the structure, c h a r a c t e r i z e d i n that the draught of the structure in operation is at least twice the maximum wave height in the waters for which the structure is designed.

2. A structure according to claim 1, c h a r a c t e ¬ r i z e d i n that the draught of the structure is at least four times the maximum wave height.

3. A structure according to claim 1 or 2, c h a r a c ¬ t e r i z e d i n that the legs (1) are connected by bracings (2, 3), pontoons (4) and/or shearwalls, the volume of which is adjusted to the total volume so as to minimize the heave motions.