GB2034789A - Offshore structure - Google Patents

Abstract

An offshore structure includes a plurality of tower elements (1, 4, 5, 6) which are disposed one above the other to extend from the seabed to above the maximum water level. The elements are assembled on site from the bottom upwards until the required height is reached. The structures 1, 4, 5 are of concrete and the top portion 6 is of steel. <IMAGE>

Description

SPECIFICATION Offshore structure This invention relates to offshore structures such as used, for example, in the exploitation of offshore oil and gas fields.

Concrete platforms are examples of such structures and have hitherto been used in oil and gas fields in water depths not exceeding 1 45m.

The principles governing their design have been a monolithic construction in dry and wet docks, installation of a deck and some construction equipment at a deep water inshore site, the towing to the oil field location of the completed platform, and the sinking of the completed platform to the seabed by controlled ballasting.

As oil and gas have been sought in deeper waters the cost of the platforms, in both steel and concrete, escalates. Estimates of concrete quantities required for the construction of platforms in water depths from 75m to 300m indicate that the quantities rise as the square of the depth.

Some factors now weighing against the concrete platforms and similar structures, and in favour of steel, are a lesser priority for iarge storage capacity at the oil field site owing to improved techniques and capabilities in pipelaying on the seabed, a great advance in heavy cranage capacities, in particular from selfpropelled vessels which also carry the loads to be placed, and a shorter construction time for steel platforms.

In accordance with one aspect of the present invention there is provided a method of constructing an offshore tower structure comprising prefabricating a plurality of tower elements and submerging the tower elements and fitting them together so that they rise from the sea bottom.

In accordance with another aspect of the present invention an offshore tower structure comprises a plurality of tower elements fitted together and extending upwardly from the sea bottom.

Preferably, the uppermost of the tower - elements is constructed essentially of structural metal members and, preferably also, the lowermost tower portion is constructed of concrete.

An embodiment of the invention designed for 200m water depth shows major reductions in cost and construction time compared with traditional concrete platforms. Construction and installation are feasible in a 2 year period. It provides the advantages of pre-drilling, and of removal after use to an extent which does not leave a hazard to shipping and the removable part may be re-used.

The particular embodiment is a tower constructed in a number of superimposed concrete elements which are joined at the oil field site and surmounted by a braced steel frame tower and steel deck in the upper 60m of water.

The present invention encompasses various other aspects which may be taken individually or in various combinations and the nature of these aspects will be detailed, and in some cases specifically identified, in the following description of one embodiment of the invention incorporating these aspects and illustrated in the accompanying drawing which illustrates diagrammatically the construction of this embodiment which is a drilling and extraction platform for oil.

In order to assist in an understanding of the invention' various quantitative values are given in the following description of the embodiment which is intended for use in a water depth of about 200m on a seabed having a ground bearing capacity of the order of 25 tonnes per square metre. The values given are approxjmate and are not necessarily directly related to each other.

The embodiment of the invention illustrated in the drawing is a platform having an annular base 1 which is 1 20m in outer diameter, 40m in core diameter and has a height of 20m. An optional pre-drilling template 2 with a protective curtain wall may be pre-installed on the seabed prior to the base installation and is located below a shelf 11 formed on the wall of the core of the base. The base 1 is of vertical cellular construction, the cells being designed to resist implosion at the full water depth during installation. The base is fitted with small compressed air steel spheres 3 for buoyancy regulation during its descent to the sea bed.

Socketted into the core of the base 1 is a lower tower element 4 whose annular wall is of cellular construction designed to resist implosion at the installation depth. Socketted into the lower tower element 4 is an upper tower element 5, the jointing detail being similar to that between the base and the lower tower element. The length of each element is 72m, the overlap at joints being 1 2m. Mounted on the upper element is a braced steel tower 6 effectively in a water depth of 60m, carrying a steel deck 7 the underside of which is about 23m above mean sea level.

The cellular construction for the base consists of vertical voids 3.6m in diameter at 4m centres.

The concrete between these voids can be further voided with 0.8m diameter holes. The roof of the base is finished flat.

Seabed risers 8 are in horizontal tubes within the thickness of the base floor passing between the main vertical voids. The risers pass into selected vertical voids and project above the base around its inner perimeter. Extensions to the risers from this level are not shown and are external to the tower and attached to it.

The base 1 rests on narrow shoulders 9 beneath which project short steel skirts 10 dividing the base area into a number of areas into which grout can be pumped to ensure uniform contact between base 1 and sea bed.

The lower tower element 4 has an outside diameter of 39m, its wall thickness being 4m. The wall is voided with 3.6 diameter vertical cavities at 4m centres, further voids being provided by 0.8m cavities between the larger cavities. The core of the tower element is open at both ends.

The upper tower element 5 is constructed in the same way as the lower tower element 4 with an outer diameter of 30m and an inner diameter of 22m.

The steel tower 6 is constructed in a known manner in tubes fabricated from steel plate. The legs of the tower are located in sockets formed in the upper tower element 5 which might be the upper portions of selected voids in the element walls. The steel tower indicated in Fig. 3 has eight vertical legs typically 2,m in diameter with a wall thickness of 50mm. Horizontal bracing rings formed of 1 .6m diameter tubes are located at 30m intervals. Diagonal crossbracing connects the legs at 1 Om spacing. Conductor guide frames are incorporated at the levels of horizontal bracing.

The lower element 4 sits immediately above.a shelf 1 2 formed on the wall of the base core above the shelf 11. The space between the tower element 4 the shelf 12 and the base core wall is filled (after assembly) with concrete, the walls in this zone having projections to ensure that the structural joint transmits the applied bending moments. Initially, the lower tower element 4 rests on three pads 1 3 slightly above the shelf 12, which lie at slightly different levels although in one plane. The lower end surface of the tower element 4 is cast in a plane lying at a slight angle to the plane normal to the tower axis. Thus, by changing the orientation of the tower element 4 relative to the base 1 a correction in verticality may be made at the joint.The joint between the upper tower element 5 and the lower tower element 4 is of similar design based on a shelf 1 6 and may have similar adjustment capability in accordance with this aspect of the present invention.

The base and tower elements are designed to be lowered to the seabed with the addition of water ballast in their voids to give slight negative buoyancy and to have hydrostatic stability during this process.

Drilling conductors 14 are preferably arranged in two rings - 20 on the outer ring and 10 on the inner ring. Such an arrangement permits the adjustment of verticality of the tower elements described above to be carried out by orientating the tower elements at 180 increments. However, other arrangements of conductors are possible.

The base element 1 is constructed in a dry dock. The dock may be formed by excavating a basin at a suitable location near the shore line, a bund between excavation and sea being removed after completion of the construction and after the flooding of the basin. In order to reduce the draught of the base element and thereby keep the dock depth to a minimum, the core of the element may be temporarily sealed with a concrete plate 1 5 having a thickness of 1.2 metres. This plate is constructed first and during floatation it is pressed upwards by hydrostatic pressure against the shelf 11 formed on the wall of the base core.On completion of its construction the base is floated out to a suitable location where the depth of water allows the core to be flooded thus relieving the hydrostatic pressure on the temporary plate 1 5 allowing it to be lowered to the seabed where it may be regarded as expendable. Should a number of platforms be required the plate can be constructed in steel having buoyancy compartments so that it can be re-floated and reused. The principle of this plate - or plug as it is sometimes known - employed as a temporary measure to reduce the draught is the subject of British Patent No. 1 499 506.

The tower elements 4, 5 are constructed in dry dock which may be the same dock as used for the base element 1 or at à different dock, if preferred, at a completely different construction site. Again, temporary plates or plugs may be used to reduce the draught of the tower elements. These press on shelves 1 7 constructed on the inner walls of the towers. It should be noted that whereas in construction of the base the plug is not essential, it can play a substantial part in the construction of the tower elements 4, 5, particularly in the later stage of construction of these elements. The towers partially constructed to a height of 35 metres in the dry dock and floated out to an inshore site having a water depth of 35 metres where they are anchored.Construction is completed whilst they are floating, water ballast being added to the core to ensure stability during this period. On completion of their construction they are towed toward the oil field location and on reaching a water depth of 70 metres their cores are completely flooded and the plugs thereby jettisoned to the seabed. Their floating draughts thereafter would be of the order of 60 metres.

The steel tower element 6 may be constructed at any suitable depot and towed to the oil field site in the final stage of platform installation.

Referring once again to the optional drilling template 2, preferably this is constructed in solid concrete built on a platform spanning barges or floating tanks. It may be equipped with recoverable steel buoyancy tanks to decrease its weight during installation. Its weight should preferably not be reduced permanently by using other materials or methods as its sliding stability on the sea bed is an essential requirement to protect the seabed risers and terminal equipment during the installation of the platform above them.

The base 1 is towed to the oil field location, where it is lowered by cables to the seabed in a state of slight negative buoyancy. This is achieved by ballasting the voids within the base, adjustments being made by ballast within the steel buoyancy spheres 3, the latter being controlled by compressed air. It should be noted that during submersion the displacement of the base element 1 decreases due to compression by hydrostatic forces resulting in a loss of buoyancy of several hundred tonnes. It is primarily to prevent this loss that the steel buoyancy spheres 3 are employed. After being lowered-to the seabed (and partly sitting on the pre-drilling template if this has been used), valves in the base are automatically opened to flood its voids. The skirts 10 penetrate the seabed and the base then rests on the concrete shoulders 9.The spaces between the base floor and the seabed are then filled with concrete grout to ensure uniform contact over the whole area. This is done from surface vessels through flexible grout lines which are paid out during the sinking of the base.

The lower tower element 4 is then towed out, given slight negative buoyancy by ballasting, and lowered by cable on to the shelf 12/pads 1 3 in the base core. Mating is assisted by steel guides 1 8 on the top of the base 1. Grout lines installed in the walls of the lower base element 4 are connected at the top of the element to flexible lines lowered from surface vessels. This can be done by a diver. The joint between the tower element and the base can then be concreted. The upper tower element 5 is installed on the lower tower element in like fashion.

The steel tower 6 is towed out to the site on a barge in the usual manner, launched and upended, and.adjusted in position above the upper tower element 5. It is then lowered by cables with suitable ballasting of buoyancy tanks until its legs meet with the sockets in the walls of the upper tower element where they are grouted through grout lines carried up the tower legs.

The deck and its equipment are installed by a floating-on method or by floating crane in the usual manner. Drilling conductors are installed from the deck passing through guiding sleeves provided in the steel tower and upper and lower tower elements.

The particular embodiment of the invention described above indicated the use of a base structure together with a plurality of concrete and metal tower elements. The present invention is also applicable to situations in which only one tower element and one base element are provided and to various other constructions in which the basic concept of pre-fabricating two tower elements and then locating these so as to form a tower structure rising from the seabed is incorporated. In particular, the particular designation of one element as being formed of concrete or structural metal members is not a critical characteristic of the invention.

In the case, however, in which a structural metal tower element is the uppermost tower element it is convenient to give this a total length, or height, of about 60 metres since this would place the tower structure in the region of maximum wave pressure (to which a metal structural tower is less affedted) and extends to a depth to which divers can have access for maintenance purposes.

Claims (10)

1. A method of constructing an offshore tower structure comprising prefabricating a plurality of tower elements and submerging the tower elements and fitting them together so that they rise from the sea bottom.

2. A method according to claim 1 wherein said tower elements are vertically disposed in relation to each other.

3. A method according to claim 1 or claim 2 wherein the uppermost of said elements is constructed essentially of structural metal members and extends above maximum wave height.

4. A method according to any preceding claim wherein the elements are socketed into each other.

5. A method according to any preceding claim wherein at least the lowermost tower element is constructed essentially of concrete.

6. An offshore tower structure comprising a plurality of tower elements fitted together and extending upwardly from the sea bottom.

7. A tower structure according to claim 6 wherein the tower elements are vertically disposed one above the other.

8. A tower structure according to claim 6 pr claim 7 wherein the uppermost tower element is constructed of structural metal members and the lowermost tower element is constructed of concrete.

9. A method of constructing an offshore tower structure including an inventive aspect as described herein with respect to the drawing.

10. An offshore tower structure having an inventive characteristic as described herein with reference to the accompanying drawing.