GB2182375A - Method of constructing an offshore structure - Google Patents

Abstract

In constructing and installing a tripod supported on a foundation on the sea bed, the foundation (15, 16, 17) is fabricated, installed and hydraulically pre-loaded, and three positively buoyant tubular concrete columns (10, 11, 12) are fabricated and floated to the site. There each column is ballasted to reduce its positive buoyancy, has a cable attached thereto and is pulled down at one end towards the foundation structure (Fig. 5). The heads of the columns (15) are then connected together temporarily above sea level and all three columns are then pulled down together onto their bearing positions on the foundation structure (Figs. 6, 7), whereafter the columns (10, 11, 12) are joined together permanently below the water level. The topside structure comprising a deck (26) and a supporting structure (28) is then added to the tops of the columns. A non load bearing shaft (30) protects drill strings etc. <IMAGE>

Description

SPECIFICATION Segmental offshore structure The invention relates to offshore structures and methods of constructing them.

There are at present two quite different approaches to offshore construction; the pilesupported tubular steel structure and the concrete gravity structure. The former is cheapest in terms of basic construction cost but requires a considerable amount of offshore work. The latter is more expensive in structural terms due to the vast amount of additional material needed, and requires very deep water for its construction; but the offshore work has been greatly reduced.

The aim of the present invention is to provide a structure, and a method of installing the structure, which can compete successfully with existing structures, especially when the water gets deeper. It is a tubular structure, but much simpler in concept than conventional tubular structures. There are no welded node points and the tubular members are so large that they can be made of concrete, which in many countries is preferable.

It is simple to fabricate, several existing construction sites can be used, and the offshore installation work and assembly work does not require expensive specialised marine plant and will not depend upon exceptionally good weather conditions.

The structure is supported by direct bearing on the seabed as with gravity structures, but because of the vastly reduced structural volume it attracts much smaller wave forces, and due to the special preloading technique which has recently been established smaller bearing areas are required; furthermore it can be applied to a greater range of soil conditions than the much heavier conventional gravity structures. It does not require very deep water for construction or towing to site, so that a number of exisiting construction sites would be suitable not only in Norway but in several other countries-even for structures aimed for water depths greater than 300 m, which are being considered at present.

According to the present invention, there is provided a method of constructing an offshore installation of the kind comprising a tripod structure supported on a foundation resting on the sea bed, said method comprising fabricating, installing and hydraulically preloading and preconsolidating a separate foundation structure; fabricating three positively buoyant tubular concrete columns and floating these columns to the site; ballasting the columns to reduce their positive-buoyancy; attaching a cable to each column and pulling one end of the column down towards the foundation structure; temporarily connecting the heads of the columns together above sea level; pulling the columns down together onto their bearing positions on the foundation structure; and permanently joining the columns below the water level.

Suitably, the foundation structure comprises three pressure-resistant rafts each provided with a skirt adapted to penetrate into the subsoil when the structure is sunk onto the sea bed. Preferably, the foundation structure is hydraulically pre-loaded by suction which may also remove unsuitable material from beneath the rafts.

It may be necessary to adjust the sea bed level to provide the desired degree of flatness and/or stability for the foundation, and this is suitably done either by introducing sand or similar foundation material beneath the respective raft or removing unsuitable material. Small differences in level can be accommodated by an adjustment in the lengths of the main columns. This adjustment can take place inshore without causing delay to the construction programme.

Advantageously, the foundation structure includes footings for the columns, and each footing incorporates a cable which can be fed through the interior of the respective column to enable the column to be stressed down onto the foundation. Preferably, each column is ballasted to the required positive buoyancy before it is drawn down onto the foundation to ensure that this operation does not exceed the design moment for the column.

The heads of the columns are advantageously provided with temporary access shafts so that it is possible to get inside the hollow column heads in atmospheric pressure after the columns have been joined and pulled below water so that the joints between the column heads can be made permanent. The column heads are suitably pulled together by winches mounted on the column heads; rubber fenders may guard against impact forces when contact is being made. Shear keys engage automatically in the joints and flexible seals around the edges make the joints watertight. After the joints have been temporarily prestressed the external winch connections are dismantled. The flexible seals can accommodate the movements in the joints which take place when the columns are pulled down to the foundation structure.When the three columns have been fixed to the foundation structure, the column heads are suitably connected permanently from the inside in the same manner as the joining of immersed tunnels. The temporary access shafts are then removed and the completed tripod stands as a finalised foundation structure ready to receive a complete super-structure and topside installation which is preferably floated in position on temporary buoyancy. The column heads when connected form a hollow box structure acting as a foundation raft for the topside structure which can rest on it as a gravity structure.

The invention also comprehends an offshore installation whenever constructed in accordance with the aforesaid method.

In order that the invention may be more fully understood, embodiments in accordance therewith will now be described by way of example only with reference to the drawings, which illustrate the sequence of building and positioning an offshore structure.

In the drawings, which are diagrammatic, Fig. 1 shows the completed structure; Figs. 2a and 2b are respectively a plan and elevation of the foundation structure; Figs. 3a and 3b show the sequence of lowering the foundation structure; Figs. 4a, 4b, 4c and 4d show the sequence of building the main columns; Fig. 5 shows the operation of placing a main column in position;, Fig. 6 shows an intermediate stage in the drawing together of the main columns; Fig. 7a is a partial plan view of Fig. 6 showing the column heads, and Fig. 7b is a section through x-y in Fig. 7a showing the joint between the column heads; and Figs. 8a, 8b, Sc, Sd and 8e show details of the installation and construction of the topside structure.

The Structure Figure 1 shows the completed structure.

This comprises a tripod composed essentially of three main tubular concrete columns 10, 11, 12 supported on three foundation rafts 15, 16, 17 on the seabed 18. The three rafts are tied together by three tubular concrete members 20, 21, 22 to form a single foundation structure, see Figure 2.

The three main members of the tripods are hollow concrete cylinders 10, 11, 12 tapering to a solid section at the support on the rafts.

The heads of the columns are interconnected to form a concrete support structure 25 located below water level 19. A topside structure comprising a deck structure 26 and steel columns 28 is supported on support structure 25. As will be described, the topside structure comprising deck structure 26 and columns 28 can be floated into position as a single unit after the columns 10, 11, 12 have been joined together.

With just three straight main members the structural concept is the simplest possible, and with the deck structure 26 supported on straight steel columns 28 tied together at their footings resting on a concrete foundation 25 as a gravity structure, the entire assembly is probably as simple as it can possibly be. The structure is provided with a central non-load bearing shaft 30 for the protection of drill strings etc.

It is however not the structural concept, but the method of construction and offshore installation which is most important and makes this proposal so different from existing structures.

It is a segmental structure constructed in five separate parts, which are assembled into one structure at the offshore site, and the foundation is preloaded and consolidated before the payload is applied.

The Foundation Structure The foundation structure 14, Figures 2a and 2b, comprises three foundation rafts 15, 16, 1 7 tied together by three circular concrete tubes 20, 21, 22. Each raft is designed to spread the load from one main column to the seabed and they are tied together to help each other resist the horizontal forces acting on the structure as a whole.

The foundation structure is designed to be positively buoyant and capable of standing up to great external hydrostatic pressure, so that it can be towed as a complete unit from the inshore fabrication site to the offshore destination and there be lowered to the seabed without being crushed by the external water pressure. This operation is shown in Figures 3a and 3b.

The sinking of the foundation structure to the seabed is in principle the same operation as the sinking of an immersed tunnel element into a predredged trench. Barges 32, 33 (the third barge is not shown) carry winches 34 which are connected by cables 35 to the foundation structure 14. When the structure 14 has been towed out to the site, it is bai- lasted down to negative buoyancy and lowered onto the seabed by winches 34 (Fig. 3a).

The footings for the raft may be piled but preferably the underside of each raft 15,16,17 is provided with a downwardly-depending skirt 38 which penetrates into the seabed (Fig 3b).

Suction hoses 39 connected to the underside of the rafts extend to the barges on the surface and submersible pumps 40 are used to reduce the water pressure under the rafts to a specified level, thus pre-loading the foundation to the design value. Vertical drains 41 may be needed to speed up the consolidation of the subsoil and a suitable rig 42 for pushing or washing down the vertical drains may be provided on each raft.

The anchor cables 44 for the main legs are permanently connected to the rafts and these are kept on the surface whilst the foundation structure is positioned and the footings consolidated.

The design of the rafts will be similar to the lower part of conventional gravity structure s-only in plan area smaller and shallower in depth, and the skirts 38 underneath (which may be of steel or concrete) will be similar to the skirts on a number of existing gravity structures.

The size of the rafts and the length and strength of the skirts will be calculated to provide enough carrying capacity to support the design loads from the structure; but it may occur that the ground conditions are such that excessive and unacceptable settlements would take place even with extremely large rafts and very deep skirts. To deal with this contingency and in general to make the foundation more versatile, economic and reliable, most if not all likely settlements will be eliminated before the structure is placed on the foundation 14.

This is achieved by the hydraulic preloading of the foundation, and there will be plenty of time to maintain the preloading until most consolidation settlements have taken place, since the foundation structure 14 can be ready for placing on the seabed well in advance of the rest of the structure and the topside facilities. Large scale experiments in the North Sea have demonstrated that this approach is feasible.

There may be soil conditions where the material even after the pre-consolidation is still unsuitable as a foundation, in which case the material will be removed by jet-cutting and suction-dredging and replaced if required by sand before the hydraulic pre-consolidation will be carried out.

Fabrication of the Main Columns If a convenient deep water site is available the three main columns 10, 11, 12 can be constructed together in one concreting operation using vertical slip-forming as with conventional gravity structures. If deep water is not readily available the columns can be constructed horizontally as immersed tunnel elements.

Vertical slipforming is undoubtedly the most economic way of concreting the main columns, and Figs. 4a, 4b, 4c and 4d show a method which takes full advantage of slipforming, but requires less than half the water depth needed for conventional gravity structures. The columns originate as six separate sections A, B, C, D, E and F which are manufactured in a dry dock and then floated (Fig 4a) to a floating working platform 50 (Fig.

4b). The sections are then extended into half columns by vertical slipforming from the floating working platform 50. The six sections (which are formed simultaneously) are cast by vertical slipforming on the platform, and ballasted down into the water. (Fig. 4c) once completed, they are jointed horizontally under positive buoyancy and the complete columns 10, 11, 12 are then towed out to the site complete with the column heads 25 and temporary access shafts 27 (Fig. 4d).

Installation of the Main Columns On arrival at the offshore site the lowering gear used for bringing the foundation structure down to the seabed is attached to the end of the main column. This comprises, for the column 10, a barge 60 and winch 61 coupled to a cable 62 passing around a point 63 on the raft 15 and connected at 64 to the end of the column 10. The up-ending of the column is carried out under positive control by winching the bottom end of the column down towards the foundation raft 15. Before the up-ending the positive buoyancy and the weight destribution in the column is adjusted to ensure that the moments during the up-ending will not exceed the design moments for the working conditions.

The column will be stressed down to the footing to ensure against up-lift while the own weight of the structure is too small to guarantee compression in the joint during wave action, and to that end the anchor cable (or prestressing tendon) 44 was cast into the foundation raft. It extends to the water surface, and when the end of the column is connected to the lowering gear 60, 61, 62, the anchor cable 44 is fed into the central shaft in the column. As the column is winched down towards the footing, the anchor cable is pulled up through the central shaft, Figure 5.

When the column has reached the vertical position with the heads of the column 25 still above the water surface, the anchor cable 44 is fed into a prestressing arrangement (not shown) and anchored off temporarily. The lowering gear and the pontoons can then be released.

The procedure is the same for the other two columns.

Once all three columns are in position, the three column heads 25 can be pulled together by means of winches 66 mounted on the column heads, Figures 6 and 7. The heads of the columns 25 are designed to interlock around the central non-load bearing shaft 30, and winches 66 pulling in the direction of the arrows in Fig. 7a draw the column heads together. Rubber buffers 68 guard against accidental impact forces due to wave action when the column heads are brought into contact with each other. The operation is not particularly sensitive to wave action, but hydraulic model tests will have to be carried out for each structure to establish the relation between waves and motion and determine the seastate at which it would- be safe to perform the operation.

At the time when the column heads are being pulled together the columns have not yet come to rest on the footings and the column heads are still above water, Figure 6. Shear keys (not shown) in the column heads automatically engage in corresponding recesses when the heads are brought together. At this point temporary joints 69 are made between the three column heads similar to the joints between immersed tunnel elements (Fig. 7b).

Flexible seals along the edges will make the joints watertight and make it possible to finalise the joints from the inside in the dry and atmospheric pressure maintained by the access shafts.

Once the heads of the columns are temporarily joined together, the next stage is then to jack the three columns down to their bearings on the footings 15, 16, 17. To bring the columns in contact with their bearings will not produce impacts of any consequence as long as the wave action is unable to make the anchor cables go slack. A safe seastate for this operation will also be established through hydraulic model tests.

Movements in the joints between the column heads will take place during this operation, but the flexible seals will permit these movements and keep the joints sufficiently watertight to aliow the final jointing to take place under water in the dry and atmospheric pressure.

Finalising the Structure When the three columns have come to rest on their bearings, they will be stressed down to the foundation structure 14 to counteract any tension due to wave action. The joints are then pressure grouted and the prestressing cables anchored off by grouting of an adequate length at the bottom end of the columns.

The joints between the column heads are then made rigid by grouting and prestressing from the inside of hollow heads 25, whereafter the temporary access shafts 27 can be removed and the structure is ready to receive the superstructure.

The superstructure with all the topside installations will be completed inshore and floated out and placed as a monolithic gravity structure on top of the column heads by means of temporary buoyancy tanks as shown in Figs. 8a to 8e. Fig. 8a shows the complete topside structure 70 being floated into position above the column heads 25, now located below water level. The structure comprises a circular deck structure 26 with a central core 73, and a fabricated tubular supporting frame 28.

The topside structure is floated on a ring of semi-submersible temporary buoyancy tanks 75 which are disposed between the deck structure 26 and the supporting frame 28 (Figs. 8d and 8e). Each buoyancy tank has a triangular hull section 77 and a circular column section 78 to give stability. They are of semisubmersible design to minimize movements due to wave action.

The flotation system can be optimised to obtain the most advantageous construction programme and as little offshore work as possible.

The upper surface of column heads 25 is provided with a receiving channel 79 which forms a seating for lower tubular frame members 80. These are disposed in a hexagonal configuration (Fig. 8b) and engage the seating when the topside structure is sunk into position on top of the column heads. This final position is shown in Fig. 1.

Summary The structure is simple in concept and simple to analyse and it is also simple to construct and install offshore. It is rational in its use of material and must therefore be competitive. It can be constructed in quite shallow water if a deep water construction site is not readily available.

It is a segmental and composite construction comprising five main elements using concrete and steel where it is most appropriate.

The foundation rafts are of concrete, and the three columns are for most of their length, where wave action is not too significant, hollow concrete cylinders. The upper portions of the structure, where wave forces tend to govern the design, are of tubular steel, and the proposal permits the upper part to have all the advantages of an open steel structure without the disadvantage of having to allow for the driving of supporting piles.

It lends itself to a short construction programme. A number of suitable construction sites are in existence and the fabrication of the main components could run in parallel in more than one place.

It can be adapted to very deep water, and would in fact appear to become even more competitive as the water gets deeper. It can also be adapted to very heavy topside facilities, and the foundations can be tailored to suit a wide range of soil conditions.

The offshore work is not too dependent upon weather and the structure is not at risk should it be necessary to stop work for a while. Furthermore, the offshore work does not rely on the use of specialised and expensive floating plant making delays due to weather a double expense.

Claims (12)

1. A method of constructing an offshore installation of the kind comprising a tripod structure supported on a foundation resting on the sea bedi comprising fabricating, installing and hydraulically pre- loading and preconsolidating a separate foundation structure on the sea bed; fabricating three positively buoyant tubular concrete columns and floating these columns to the site; ballasting the columns to reduce their positive buoyancy; attaching a cable to each column and pulling one end of the column down towards the foundation structure; temporarily connecting the heads of the columns together above sea level; pulling the columns down together onto their bearing positions on the foundation structure; and permanently joining the columns below the water level.

2. A method as claimed in Claim 1, wherein the foundation structure comprises three interconnected positively-buoyant rafts which are floated out to the site and sunk onto the sea bed, and each raft is provided with a skirt adapted to penetrate into the sub-soil when the structure is sunk onto the sea bed.

3. A method as claimed in Claim 1 or Claim 2, wherein the foundation structure is hydraulically pre-loaded by suction which may also remove unsuitable material from beneath the rafts.

4. A method as claimed in any preceding claim, wherein the foundation structure includes footings for the columns, and each footing incorporates an anchor cable which is fed through the interior of the respective column, and the column is stressed down onto the footing through said anchor cable.

5. A method as claimed in Claim 4, wherein each column is ballasted to reduce its positive byouancy before it is drawn down towards the foundation structure.

6. A method as claimed in any preceding claim, wherein each column is provided with a head structure adapted to be interconnected with the other head structures to form a support platform for a topside structure, and the heads of the columns are drawn together by winches mounted on the head structures, resilient means being provided to prevent collision damage to the head structures during interconnection thereof.

7. A method as claimed in Claim 6, wherein the column heads are hollow sections and are provided with temporary access shafts which extend above sea level when the columns are supported on the foundation structure, and the column heads are joined together permanently from inside the hollow sections once the columns have been drawn down onto their bearing positions.

8. A method as claimed in Claim 6 or Claim 7, further comprising floating a temporarily buoyant topside structure into position above the support platform formed by the immersed column head structures; and sinking the topside structure into position on the support platform, said topside structure comprising a tubular framework which is received on the support platform, and a deck structure located above water level supported by said framework.

9. A method as claimed in any preceding claim, whereby the columns are cast in concrete by vertical slipforming sections of the columns which sections are subsequently assembled in a horizontal floating condition.

10. A method of constructing an offshore installation substantially as hereinbefore described with reference to the accompanying drawings.

11. An offshore installation whenever constructed according to the method as claimed in any of Claims 1 to 10.

12. An offshore installation substantially as hereinbefore described with reference to the accompanying drawings.