GB2333117A

GB2333117A - Offshore platform

Info

Publication number: GB2333117A
Application number: GB9822973A
Authority: GB
Inventors: John William Waddell
Original assignee: Kvaerner Oil and Gas Ltd
Current assignee: Kvaerner Oil and Gas Ltd
Priority date: 1997-10-21
Filing date: 1998-10-21
Publication date: 1999-07-14
Anticipated expiration: 2018-10-21
Also published as: GB2333117B; GB9822973D0

Abstract

A substructure for an offshore platform, the substructure having a generally flat base slab 10 to rest on the seabed with its undersurface in contact with the seabed, legs 11 upstanding from the base slab and configured to sustain predominantly vertical loads, and towers 18 upstanding from the base slab and configured to sustain primarily lateral loads, the relative positions of the legs and towers on the base slab being such that at least some of the towers are closer to the periphery of the base slab than are at least some of the legs. The substructure forms a lower part of an offshore platform when a deck 12 is set on the legs 11 and connected to the towers 18.

Description

OFFSHORE PLATFORM The invention relates to a substructure for an offshore platform, to an offshore platform incorporating such a substructure, to a method of forming that platform; to a drilling arrangement for use on an offshore plafform; and to provisions for oil storage on an offshore platform.

The invention is concemed with a gravity based offshore platform, having a large integrated deck containing e.g. drilling, process, utilities and accommodation.

It has been a feature of such platforms that a heavy concrete substructure is formed by the casting of a base in a dry dock. The base is then floated out of the dry dock, and concrete columns are slip formed up from the base in deep water. While the substructure is being formed, a deck may be constructed nearby, and completed as far as possible on shore or at shore. When the substructure is complete it is ballasted down at a sheltered inshore site, so that the deck can be floated over it The substructure is then deballasted to raise the deck above the wave effected zone for movement from the sheltered inshore site to its intended offshore location.

It is a requirement of this technique that the buoyancy of the substructure must be sufficient to carry the deck to its intended location. Thus for large decks, very substantial buoyancy must be built into the base. The need for the combined substructureldeck assembly always to have a positive metacentric height (i.e. its Centre of Buoyancy above its Centre of Gravity), has led to the practice of ballasting the base with iron pyrites or other high density material. This calls for the incorporation of still further buoyancy, to support the weight of the ballast The requirements for excess buoyancy and positive metacentric height during movement to the intended location have led to the design of heavy concrete substructures. These cannot be optimised for all phases of their lives - specifically for both transportation and in-situ phases.

As an alternative to floating a deck out to the intended location on top of its substructure, it is common to set a substructure (e.g. a concrete "Condeep" type structure or a steel jacket) on the seabed, and then to install an integrated deck by direct lift. This can be an effective method for the installation of small and medium sized decks. However, for large decks - say over 12,000T - there is currently no cranage capable of carrying out a direct lift For very large decks, float over installation methods have been proposed (e.g. in U.K. patent specifications No's 2,021,665 and 2,165,187). Some float over installations have been implemented in relatively calm conditions (e.g. offshore West Africa, and in Malaysian waters as reported on pages 27 to 29 of Offshore Engineer dated October 1996).

However, in waters with more severe seastates, the need to raise the deck from a transportation vessel to install it above the wave effected zone could create difficulties. These difficulties might be acute if a high lift was required.

Thus there is a need for a system whereby a large deck can be installed safely upon a gravity based substructure, at a sufficient height to leave a satisfactory air gap above the highest expected wave for the intended location.

The invention provides a substructure for an offshore platform, and having a generally flat base slab to rest on the seabed with its undersurface in contact with the seabed, at least three legs upstanding from the base slab and configured to sustain predominantly vertical loads, and at least three towers upstanding from the base slab and configured to sustain primarily lateral loads, the relative positions of the legs and towers on the base slab being such that at least some of the towers are closer to the periphery of the base slab than are at least some of the legs.

It is preferred that the legs of the substructure are of insufficient height to pierce the water surface at the intended offshore location for the plafform.

The legs may be located in receptacles comprising blind holes set in the base slab.

Altematively, the legs may be located in receptacles comprising sleeves supported off intemal edges of the base slab.

It is also preferred that, when installed, the towers are of sufficient height to pierce the water surface at the intended offshore location for the platform.

It is further preferred that at least one of the towers is fabricated as separate parts for assembly one on top of another.

It is still further preferred that at least the upper part of the at least one tower is of lattice construction.

Preferably, at least one of the towers incorporates conductor guides, so that drilling of oil/gas wells can be effected through the guides.

In one preferred form there are at least three columns integral with and upstanding from the base slab near to lateral extremities of the base slab, and the columns are adapted to support the towers.

In this form it is preferred that at least one of the columns is upwardly open (and so is free flooding when the top of the column is below the water surface).

It is further preferred that the base slab is rectangular in plan, and has edge beams between four comer columns and in which two opposed edge beams are substantially deeper than the two orthogonal edge beams.

The invention also provides an offshore platform comprising a substructure as claimed in any one of the preceding claims in combination with a deck, in which the deck is connected to the legs so that the legs carry the greater part of the weight of the deck, and in which the deck is connected to the towers so that the towers carry the greater part of any environmental loads (e.g. from wind and waves) and any live loads (e.g. arising from movement of a drilling rig or rigs) on the platform.

It is preferred that the deck incorporates jacking systems whereby the deck can be connected to and raised upon the legs to a position above the highest wave expected at the offshore location of the plafform.

It is further preferred that the deck has leg stubs which can be lowered onto and mated with the legs of the substructure, and upon which the deck can be raised to its position above the highest expected wave.

Preferably a part of the deck is sized to fit between the towers, and the deck and the towers have connections which can be brought into engagement in the manner of a wedge.

It is preferred that the deck carries at least one drilling rig, and the arrangement is such that the drilling rig can be located over at least one of the towers to drill through conductor guides within that tower, and in which at least that tower is capable of reacting drilling loads.

It is further preferred that skid rails for the drilling rig can be extended to overlie the top of the at least one tower.

According to a feature of the invention it is preferred that the deck comprises a buoyant hull capable of supporting the weight of a substantial part of the platform during floatation to its intended location; and in which the hull has provision for the storage of a substantial quantity of oil within the deck.

The invention also provides a method of forming an offshore platform as described above and comprising a substructure and a deck, in which the substructure is set on the seabed, and in which (either before or after the substructure has been set on the seabed) the deck is moved to a position over the base of the substructure, and the deck is then raised on the legs to a height above the highest wave expected at the location of the platform.

In one form the deck is self buoyant, and, after the substructure has been set on the seabed, the deck is moved over the legs of the substructure using only its own self buoyancy.

It is preferred that leg stubs are canied on the deck while the deck is moved to the position over the legs of the substructure, the leg stubs are lowered and mated with the tops of the legs of the substructure, and then the deck is jacked up on the leg stubs.

In another fomi the base slab is formed in a dry-dock, the dry-dock is flooded and the substructure is floated out; and in which the deck is formed separately from the base slab, and is moved over the base slab.

In this last mentioned form it is preferred that the deck is guided into its position over the base slab by at least two of the towers.

It is also preferred that the deck carries legs to be set in or on the base.

It is further preferred that the base slab of the substructure is submerged so that the upper surface of the base slab is sufficiently far beneath the surface of the water, and the deck is floated over the base slab with legs upstanding therefrom and aligned with receptacles in the base slab; and then the legs are lowered into the receptacles and the deck is raised on the legs.

It is still further preferred that the base slab is submerged to an intermediate depth at a sheltered deepwater location, and lowest portions of the legs are lowered into the receptacles; and then upper portions of the legs are added onto the tops of the lowest portions of the legs before the deck is raised.

At a sheltered deepwater location, upper portions of the towers may be added to parts of the towers already positioned on the base slab.

The buoyancy of the deck may be used to support the substructure while the base slab is being lowered to rest on the seabed.

Three specific embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a plan view of a substructure for an offshore plafform; Figure 2 is a view on the line AA in Figure 1; Figure 3 is a view on the line BB in Figure 1; Figure 4 is a plan view looking onto a deck carcass of a platform induding the substructure illustrated in Figures 1 to 3; Figure 5 is a side elevation of the arrangement shown in Figure 4; Figure 6 is a plan view looking onto the deck of a completed plafform induding the substructure illustrated in Figures 1 to 3; Figure 7 is a side elevation of the completed platform shown in Figure 6; Figure 8 is a detail plan to an enlarged scale of a comer of the platform; Figures 9 to 12 show an installation sequence for second offshore platform; Figure 13 is a plan view on the base of the platform shown in Figures 9 to 12; Figure 14 is a plan view on the deck of a third offshore platform; Figure 15 is a side elevation of that platform; Figure 16 is a plan view on the base of a substructure for that platform; and Figures 17 to 29 show an installation sequence for the plafform illustrated in figures 14 to 16.

As shown in Figures 1 to 3, a substructure for an offshore platform has a base slab 10 and four legs 11 upstanding from that base slab. The base slab is designed to rest on seabed 14 and to support a deck for the platform (to be described later) above a sea surface 15.

The base slab 10 is of cellular construction. It has some open top cells 1 0A and some closed top cells 10B.

The legs 11 are hollow and cylindrical, and are based on four of the closed top cells 10B away from the extremities of the base slab. (The substructure as described so far could be for a conventional "Condeep" type gravity based plafform.) As shown particularly in Figures 2 and 3, the legs 11 do not extend upwards as far as the sea surface 15.

In addition to the legs 11, the substructure has four comer towers 18. These towers extend upwards from the base slab 10 to positions above the sea surface 15. The towers 18 are of open lattice construction, and are based on four of the open top cells 10A at the comer extremities of the base slab 10.

The substructure shown in Figures 1 to 3 may be self installing. That is, it may have sufficient water plane area in its legs 11 and towers 18 to sink in a stable attitude from a floating condition into a fixed position on the seabed. The sinking can be achieved in a controlled flooding operation.

Figures 4 and 5 illustrate how a deck 12 is installed on the substructure shown in Figures 1 to 3. The deck 12 is self buoyant, and may have the cross-section of an Afro max tanker. The deck has leg stubs 11A suspended in jack housings 11 B. The leg stubs 11A are positioned to be lowered onto the tops of the legs 11. (The deck and the positions of the leg stubs during transit are shown in chain dotted lines.) The deck 12 is floated into position between the lattice towers 18. The legs stubs 11A are lowered onto the legs 11. The deck is then jacked up to above the level of the highest wave expected at the location where the platform is installed. (Conventional jack up equipment is used.) The corners of the deck are connected to the tops of the towers 18.

The completed platform, with a diagrammatic representation of an operational deck, is shown in Figures 6 and 7.

The platform has two drilling derricks 21 and 22, with a piperack 23 between them. Drilling bulks are stored at 24, and first stage processing facilities are represented as the block 25. At the end of the deck away from the derricks 21 and 22 there is a helideck 26 and living quarters 27.

Additional equipment is housed within the "hull" portion of the deck.

The deck is dimensioned to ft between the towers 18, and the drilling denicks are arranged so that they can be skidded outboard onto the tops of two of the towers. Within these towers there are 4 x 5 arrays of conductor guides, so that drilling can be effected through the towers 18 and the open top cells 1 0A of the base slab 10.

Figure 8 shows how one drilling denick can reach over a lattice tower 18 to drill an array of twenty oil/gas wells. The drilling derrick is shown in its position furthest from the central area of the deck 12. The leg chord can, if appropriate, be extended inboard 28 to form a continuation of the skid rail.

Figures 9 to 12 show an installation sequence for a second offshore platform. In this case a deck 52 is being installed on a substructure having a base slab 50, legs 51 and lattice towers 58.

The substructure in these Figures is slightly different from that shown in the previous Figures.

In Figure 9 the deck 52 has been floated into a position between two open lattice towers 58.

In Figure 10 leg stubs 51A are lowered from jack housings on the floating deck and mated witty the tops of legs 51.

In Figure 11 the deck 52 is raised up on the leg stubs 51A by jacking into a position in which its under surface is above the level of the highest wave expected at the location of the plafform.

Finally, in Figure 12, connections 59 are made between the four lattice towers 58 and the deck 52.

Figure 13 shows a plan of the base slab 50 for the plafform for which the assembly sequence was shown in Figures 9 to 12. The base slab 50 is square. However, other base slab configurations can be used e.g. triangular.

Figures 14 to 16 show a general arrangement of a third offshore platform which comprises a Gravity Base Structure (GBS) with a deck installed by jacking.

The GBS comprises a concrete base slab 110 with four steel lattice towers 118 at the comers. These lattice towers are used to support conductors, appurtances, etc, and as the main load path for environmental loads and deck live loads.

The deck 112 is a barge-like plated structure that is floated between the GBS lattice towers 118 at an inshore location. The substructure/deck combination is floated to site using the buoyancy of the deck for support. Connectivity between the GBS and deck during floatation phases is achieved through four large diameter piles (or legs) 111 which pass through sleeves in the deck and are fixed into receptacles (pile sleeves) in the base slab.

These piles (or legs) are termed XJack-Up Piles" (JUP's) since the deck is raised on these JUP's by conventional jacking systems reacted against the base slab. After raising the deck it is locked to the JUP's which thereby carry the initial deck dead weight. The deck is subsequently connected to the lattice towers, which serve to react with environmental loads and deck live loads.

At the end of field life, the platform can be removed by reversing the above installation sequence. It is conceivable that the deck/JUP combination can subsequently be re-used as a conventional drilling unit.

In the following description, typical dimensions are given by way of example for a platform to stand in a water depth of about 100m in the North Sea.

The concrete base slab 110 has four comer columns 116, each 30m in height, which form supports for the steel lattice towers 118. Spanning between these columns are four edge beams 50m long by 15m wide. The dimensions of the edge beams are sized to give adequate buoyancy and to permit the entry of the deck during the inshore mating phase. Hence two beams are 16m high, whilst the two orthogonal beams are 30m high. Inside the 50m x 50m central core of the base slab there are four closed cell structures that house the sleeves that support the JUP's. The rest of the inner core of the base slab is left open since this is the least effective area in resisting applied loads. All of these base elements are of closed cellular construction designed to provide buoyancy for floatout and subsequent installation phases.

Additional cells, some open-topped and some closed, are located around the periphery of the base slab for increased resistance to in-place loads and to reduce the floatout draft.

These cells are 7.5m wide and with sufficient height (around 12.5m) to ensure that the open cells are not overtopped during floatout from the graving dock. The open cells allow solid ballast to be added either for marine stability purposes or to increase on-bottom weight should uplift loads be calculated for peak stonn loading.

The lattice towers 118 have sufficient strength to transfer environmentally induced loads and deck live loads through the deck-tower connections. Two of the towers are designed to support conductors. Preliminary dimensions are 15m x 15m in plan with a height of around 110m. Skid rails may be provided on the towers used for drilling. These could be made continuous with similar skid rails along the deck structure.

The deck 112 is of simple plated construction with dimensions around 90m x 45m in plan and around 9m in height. The 45m deck width allows two drilling rigs to be located side-byside, prior to being skidded over the adjacent lattice columns. The 90m deck length is a consequence of space requirements for process and utility equipment. The 9m deck depth is governed by displacement requirements to support the topsides equipment and substructure during floatation phases.

The installation sequence is more fully described below with reference to the accompanying Figures 17 to 29.

Figure 17 shows the completed substructure in a graving dock. The four lattice towers 118 are ideally installed on the columns 116 in the dock, but could be installed whilst the base slab 110 is floating. The plan dimensions are largely sized by the requirement to minimise floatout draft and hence clear the dock sill. With the provision of 2m deep skirts for in-place scour protection and a nominal 0.5m clearance over the sill, the present design requires a draft of around 13m, which is available in at least one UK and one Norwegian facility. Draft reductions could be achieved by using temporary floatation aids or by part plating elements in the central core of the base slab.

Figures 18 and 19 show the addition of water ballast, first into the open cells and then into the closed cells, probably in the peripheral cells in the first instance. The end of the initial ballasting operation corresponds with the overtopping of the open cells (Figure 19) which reduces the substructure buoyancy by the capacity of the open cells.

In Figures 20 and 21 the ballasting is continued to lower the base slab to the 22m depth required to allow access for the deck. The closed cells are sized such that there is adequate reserve buoyancy available to allow subsequent deck mating and JUP lowering operations.

In Figure 22 the deck 112 is sailed between the towers 118 and in Figure 23 the four lower sections of the JUP's, installed within the deck structure, are lowered and grouted (or mechanically fixed) into the substructure sleeves. After these grouting/fixing operations (Figure 24) the buoyancy of the deck can be used to lower the substructure via controlled flooding to reach neutral buoyancy and then for further descent. Winches, strandjacks, mechanical stops or other clamping tools can be used between the deck and the JUP's to provide stability and prevent uncontrolled descent of the substructure.

Figure 25 shows the substructure lowered further to allow access by a 1,000 Tonne lift size crane barge to install the upper sections (111A) of the JUP's. This requires a water depth of around 60m, which is available adjacent to graving docks in Scotland and Norway. This lowering operation also brings the JUP splice to near deck level for easy welding access.

Relative motions between deck and substructure can be restrained by fendering and by stabilising arms 117 fitted to the top of the deck 112 and designed as siiding guides against the lattice towers 118. These devices will supplement the relative motion control inherent in the close tolerance clearance between the JUP's and deck sleeves.

The lowering operation is effected by controlled ballasting and with the aid of strandjacks. As the GBS descends slowly under controlled ballasting, the strandjacks simultaneously pay out wires whilst maintaining almost constant tension throughout. These strandjacks also activate the large waterplane of the floating deck to provide stability for the GBS.

In Figures 26 and 27 the plafform is towed to site and mated with a pre-installed template.

Figure 28 shows the deck being lifted with jacking systems used in conventional drilling jack-up units or, probably more economically, by strandjacks. In a conventional arrangement the deck is raised by engaging pins, around 450mm diameter, into close tolerance holes drilled in the JUP's. These holes extend the full height of the legs in conventional jack-up units, but for this concept they are only required in the top 40m section. At the end of this stage the deck is mechanically locked to the JUP's which transfer deck dead loads into the substructure.

Finally, Figure 29 shows the provision of structural connections 119 between the deck and lattice towers to provide a load path for environmental and deck live loads.

The plafform can be installed without the use of a Heavy Lift Crane Vessel (HLV). The plafform can accommodate a large number of conductors (more than 40) and two drilling rigs.

The construction cost is likely to be larger than that of a conventional jacket supported platform, but installation costs would be smaller. Overall, the cost of the present concept should be less than that of a conventional plafform when the costs of field abandonment are included.

Development of small oil fields at locations remote from existing pipelines has been inhibited by the problems of transporting oil away from the fields.

One solution has been to use a Floating Production, Storage and Offloading vessel (FPSO). Such vessels can store several days of oil production, and can discharge periodically into shuttle tankers. The shuttle tankers transport the oil to onshore refineries. This system has the disadvantage that the production facilities are mounted on a floating vessel, and so may suffer downtime because of rough weather.

Another solution has been to mount production facilities on a fixed platform, and to have a separate storage tanker permanently linked to that plafform by a pipeline. Production can then continue in all most all weathers. Several days of oil production can be held in the storage tanker, and discharged periodically into shuttle tankers for transport to onshore refineries. This system has worked well, but incurs the cost of a storage tanker permanently on station.

Yet another solution has been to construct a fixed plafform with a massive concrete base, within which there is provision for the storage of several days production. This eliminates the need for a storage tanker, but is only applicable if the plafform has a large gravity based substructure. In many locations this type of plafform is inappropriate.

Thus there is a requirement for an offshore plafform having the capacity to store oil production without the need for a massive concrete base.

A specific embodiment of a possible solution to this requirement will now be described by way of example with reference to the foregoing drawings. These drawings showed offshore plafforms having decks designated 12, 52 and 112 respectively.

The decks are described as having self buoyant hulls, for the purpose of floating out a substantial part of the respective platform to its intended offshore location. The present concept is concerned with the feature of using a major part of the watertight enclosed volume within the hull of the respective deck for the storage of oil.

Thus the hull would serve two purposes at different times during the life of the plafform.

It would serve as buoyancy during transportation; and it would serve as oil storage volume during production.

The configuration illustrated in the drawings might be modified by elongating the deck to give extra storage capacity. The aim would be for the deck to store oil produced between visits by offlake shuttle tankers.

Claims

CLAIMS 1. A substructure for an offshore platform, and having a generally flat base slab to rest on the seabed with its undersurface in contact with the seabed, at least three legs upstanding from the base slab and configured to sustain predominantly vertical loads, and at least three towers upstanding from the base slab and configured to sustain primarily lateral loads, the relative positions of the legs and towers on the base slab being such that at least some of the towers are closer to the periphery of the base slab than are at least some of the legs.
2. A substructure as claimed in claim 1 in which, when installed, the legs of the substructure are of insufficient height to pierce the water surface at the intended offshore location for the plafform.
3. A substructure as claimed in claim 1 or claim 2 in which the legs are located in receptacles comprising blind holes set in the base slab.
4. A substructure as claimed in claim 1 or claim 2 in which the legs are located in receptacles comprising sleeves supported off intemal edges of the base slab.
5. A substructure as claimed in any one of the preceding claims in which, when installed, the towers are of sufficient height to pierce the water surface at the intended offshore location for the plafform.
6. A substructure as claimed in claim 5 in which at least one of the towers is fabricated as separate parts for assembly one on top of another.
7. A substructure as claimed in claim 6, in which at least the upper part of the at least one tower is of lattice construction.
8. A substructure as claimed in any one of the preceding claims in which at least one of the towers incorporates conductor guides, so that drilling of oil/gas wells can be effected through the guides.
9. A substructure as claimed in any one of the preceding claims in which there are at least three columns integral with and upstanding from the base slab near to lateral extremities of the base slab, and the columns are adapted to support the towers.
10. A substructure as claimed in claim 9 in which at least one of the columns is upwardly open (and so is free flooding when the top of the column is below the water surface).
11. A substructure as claimed in claim 9 or claim 10 in which the base slab is rectangular in plan, and has edge beams between four comer columns, and in which two opposed edge beams are substantially deeper than the two orthogonal edge beams.
12. A substructure substantially as hereinbefore described with reference to and as shown in Figures 1 to 3 of the accompanying drawings.
13. An offshore platform comprising a substructure as claimed in any one of the preceding claims in combination with a deck, in which the deck is connected to the legs so that the legs carry the greater part of the weight of the deck, and in which the deck is connected to the towers so that the towers carry the greater part of any environmental loads (e.g. from wind and waves) and any live loads (e.g. arising from movement of a drilling rig or rigs) on the plafform.
14. A platform as claimed in claim 13, in which the deck incorporates jacking systems whereby the deck can be connected to and raised upon the legs to a position above the highest wave expected at the offshore location of the plafform.
15. A platform as claimed in claim 14, in which the deck has leg stubs which can be lowered onto and mated with the legs of the substructure, and upon which the deck can be raised to its position above the highest expected wave.
16. A platform as claimed in any one of claims 13 to 15, in which a part of the deck is sized to fit between the towers, and the deck and the towers have connections which can be brought into engagement in the manner of a wedge.
17. A plafform as claimed in any one of claims 13 to 16 in which the deck carries at least one drilling rig, and the arrangement is such that the drilling rig can be located over at least one of the towers to drill through conductor guides within that tower, and in which at least that tower is capable of reacting drilling loads.
18. A plafform as claimed in claim 17 in which skid rails for the drilling rig can be extended to overlie the top of the at least one tower.
19. A plafform as claimed in any one of claims 13 to 18, in which the deck comprises a buoyant hull capable of supporting the weight of a substantial part of the platform during floatation to its intended location; and in which the hull has provision for the storage of a substantial quantity of oil within the deck.
20. A platform substantially as hereinbefore described with reference to and as shown in figures 6 and 7 of the accompanying drawings.
21. A method of forming an offshore platform as claimed in any one of claims 13 to 20 and comprising a substructure and a deck, in which the substructure is set on the seabed and in which (either before or after the substructure has been set on the seabed) the deck is moved to a position over the base of the substructure, and the deck is then raised on the legs to a height above the highest wave expected at the location of the plafform.
22. A method as claimed in claim 21 in which the deck is self buoyant, and, after the substructure has been set on the seabed, the deck is moved over the legs of the substructure using only its own self buoyancy.
23. A method as claimed in claim 21 or claim 22 in which leg stubs are carried on the deck while the deck is moved to the position over the legs of the substructure, the leg stubs are lowered and mated with the tops of the legs of the substructure, and then the deck is jacked up on the leg stubs.
24. A method as claimed in claim 21 in which the base slab is formed in a dry-dock, the dry dock is flooded and the substructure is floated out; and in which the deck is formed separately from the base slab, and is moved over the base slab.
25. A method as claimed in claim 24 in which the deck is guided into its position over the base slab by at least two of the towers.
26. A method as claimed in claim 24 or claim 25 in which the deck carries legs to be set in or on the base.
27. A method as claimed in claim 26 in which the base slab of the substructure is submerged so that the upper surface of the base slab is sufficiently far beneath the surface of the water, and the deck is floated over the base slab with legs upstanding therefrom and aligned with receptacles in the base slab; and then the legs are lowered into the receptacles and the deck is raised on the legs.
28. A method as claimed in claim 27 in which the base slab is submerged to an intermediate depth at a sheltered deepwater location, and lowest portions of the legs are lowered into the receptacles; and then upper portions of the legs are added onto the tops of the lowest portions of the legs before the deck is raised.
29. A method as claimed in any one of claims 24 to 28 in which, at a sheltered deepwater location, upper portions of the towers are added to parts of the towers already positioned on the base slab.
30. A method as claimed in any one of claims 24 to 29 in which the buoyancy of the deck is used to support the substructure while the base slab is being lowered to rest on the seabed.
31. A method substantially as hereinbefore described with reference to the accompanying drawings.