GB2314576A

GB2314576A - Offshore platform assembly

Info

Publication number: GB2314576A
Application number: GB9713679A
Authority: GB
Inventors: John William Waddell
Original assignee: Kvaerner Oil and Gas Ltd
Current assignee: Kvaerner Oil and Gas Ltd
Priority date: 1996-06-27
Filing date: 1997-06-27
Publication date: 1998-01-07
Anticipated expiration: 2017-06-27
Also published as: GB2314576B; GB9713679D0

Abstract

A method of assembly for a gravity based offshore platform including a base slab 30 to rest on the seabed with a slender leg or legs 31 upstanding therefrom to support a deck 32 above the wave effected zone; the method comprising the steps of setting the base slab 30 on the seabed, locating a ballastable raft 36 with lifting means upstanding therefrom around the leg or between the legs 31, moving the deck 32 to a position over the lifting means, deballasting the raft 36 so that the raft rises and the lifting means raises the deck to a required position above the wave effected zone, fixing the deck to the leg or legs 31 in that position, and reballasting the raft 36 so that it descends to rest on the base slab 30, so to form a part of a gravity base for the plafform during the working life of the platform.

Description

OFFSHORE PLATFORM ASSEMBLY The invention relates to a method of assembly for an offshore platform, and to a substructure for use in that method.

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 plafforms that a heavy concrete substructure has been formed by the casting of a base in a dry dock. The base has then been floated out of the dry dock, and concrete columns have been 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 substructure/deck 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). Recently, 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 method of assembly for a gravity based offshore platform including a base slab to rest on the seabed with a slender leg or legs upstanding therefrom to support a deck above the wave effected zone; the method comprising the steps of setting the base slab on the sea bed, locating a ballastable raft with lifting means upstanding therefrom around the leg or between the legs, moving the deck to a position over the lifting means, deballasting the raft so that the raft rises and the lifting means raises the deck to a required position above the wave effected zone, fixing the deck to the leg or legs in that position, and reballasting the raft so that it descends to rest on the base slab, so to form a part of a gravity base for the platform during the working life of the platform.

In one form of the invention prior to the base slab being set on the seabed, the ballastable raft is constructed on top of the base slab.

In another form of the invention prior to the base slab being set on the seabed, the ballastable raft is constructed separately from the base slab for mating with the base slab while afloat.

The base slab may be set on the seabed with the ballastable raft.

Alternatively the ballastable raft is used in a deballasted condition to lower the base slab and so to set the base slab on the seabed, and then the raft is ballasted down onto the base slab.

Preferably the leg or legs of the platform guide the raft as it is ballasted down onto the base slab.

In one preferred form the lifting means on the ballastable raft comprises at least one hollow tower which surrounds a column forming a leg of the platform, and in which the column is used to guide vertical movements of the hollow tower.

In this form an upper part of the tower may be fixed to an upper part of the column, so to form a composite upper part of the leg, and the lower part of the tower is lowered to rest with the ballastable raft on the base slab.

In a second preferred form the lifting means on the ballastable raft comprises at least one lifting tower arranged to engage an underside of the deck.

In a third preferred form the lifting means on the ballastable raft comprise a plurality of lifting piles arranged to engage an underside of the deck.

There may be abutments outstanding from the leg or legs and corresponding abutments projecting from the lifting means, such that when the deck is in its required position, the abutments engage each other to prevent further upward movement of the ballastable raft.

It is preferred that the base slab has a plurality of upstanding legs and the ballastable raft has a plurality of columns, and the deck is moved to a position between the legs prior to being raised.

The deck may be moved to the position between the legs on a barge; or the deck may be self floating.

It is preferred that the deck incorporates vertically movable leg stubs, and the leg stubs are used to locate the deck with respect to the legs of the platform.

It is further preferred that the deck stubs have means to lock the deck at a specified vertical position.

The invention includes an offshore platform when assembled according to the method described above.

The invention also provides a substructure for a gravity based offshore platform, comprising a base slab to rest on the seabed with a slender leg or legs upstanding therefrom, and a ballastable raft arranged to rest on the base slab and so to form part of a gravity base when the raft is fully ballasted, in which the ballastable raft has lifting means upstanding therefrom around the leg or between the legs, such that deballasting the raft can lift a deck supported on the lifting means to a required position above the wave effected zone.

In one preferred form the lifting means on the ballastable raft comprises at least one hollow tower which surrounds a column forming a leg of the platform.

In this form there may be diametrically opposed vertical strips of low friction material between the exterior of the column and the interior of the hollow tower.

In a second preferred form the lifting means on the ballastable raft comprises at least one lifting tower arranged when in use to engage an underside of a deck.

In a third preferred form the lifting means on the ballastable raft comprise a plurality of lifting piles arranged when in use to engage an underside of a deck.

Four specific embodiments of methods according to the invention (and variants thereof) will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a diagrammatic side view of a gravity based offshore platform; Figure 2 is a section on line II - II in Figure 1; Figures 3 to 6 are sectional views on the line VI - VI in Figure 1, showing stages in the assembly of the platform; Figure 7 is a diagrammatic side view of a platform assembled in accordance with a second method according to the invention; Figure 8 is a side elevation of the substructure of the platform shown in Figure 7 at floatout; Figure 9 is a plan of the substructure shown in Figure 8; Figures 10 to 13 illustrate stages in the assembly of the platform shown in Figures 7 to 9; Figure 14 is a diagrammatic side view of a platform assembled in accordance with a third method according to the invention; Figure 15 is a plan of the deck of the platform shown in Figure 14; Figure 16 is a plan of the base of the platform shown in Figure 14; Figures 17 to 24 illustrate stages in the assembly of the platform shown in Figs 14 to 16; Figure 25 is a diagrammatic side view of a platform during assembly in accordance with a fourth method according to the invention; Figure 26 is a side view on the arrow X in Figure 25; and Figure 27 is a sectional plan view on the line W in Figure 25.

Figure 1 shows a gravity based offshore platform in its completed form.

The platform has a base slab 10, four legs 11 and a deck 12. The deck 12 is shown as a block for illustrative purposes. (In practice the deck could have drilling equipment, process and utilities, and extensive pipework.) The seabed is designated 14, and the sea surface is designated 15. The sea is illustrated as being calm; however, the deck 12 is shown as being raised to a sufficient height above the sea surface to allow the highest expected wave to pass below the deck, leaving a satisfactory air gap between the crest of the wave and the bottom of the deck. Thus, to leave a satisfactory air gap for most offshore locations, the deck needs to be raised to a significant height above the sea surface.

Following the invention, a ballastable raft 16 rests on an upper surface of base slab 10.

The legs 11 of the completed platform are formed of hollow columns 17 upstanding from the base slab 10, and hollow towers 18 upstanding from the ballastable raft 16. The columns 17 pass through apertures in the ballastable raft, and the towers 18 surround the columns concentrically. In the sectional views of Figures 3 to 6, the base slab, ballastable raft, columns and towers (together comprising a substructure for the platform) are represented with single lines for clarity.

Various combinations of materials can be used to form the substructure. Substructures for gravity based plafforms can be formed on steel or concrete bases, with steel columns on steel bases, and steel or concrete columns on concrete bases. In the present substructure, the towers 18 could be concrete or steel cylinders, or could be formed of steel lattice work similar to the legs of a jack-up platform. Concrete towers would be integral with a ballastable raft made of concrete, but cylindrical steel towers or lattice towers could be constructed as separate items. (Figures 25 to 27 show a hybrid gravity based platform, with a concrete base slab and a 'jacket' type support framework constructed of steel tubulars.) Details of the construction - not shown in the drawings - might include the provision of a separate column on the base slab 10 which would not be surrounded by a tower. This separate column could serve to take appurtenances (risers, caissons and J-tubes) through the wave effected zone. Adjacent legs could carry conductor framing between them, so that oil/gas wells would be supported between the seabed and facilities on the deck. The conductor framing would conveniently be located at one end of the platform.

Construction, installation, and emplacement of the deck for the platform will now be described with reference to Figures 3 to 6.

The base slab 10 is constructed in a dry dock as a typical gravity base e.g. for a Condeep type substructure. When the upper surface of the base slab 10 is finished, the ballastable raft 16 is formed on top of (and separate from) the base slab, and the columns 17 are built through the ballastable raft 16, with a small clearance therebetween. (Altematively, the ballastable raft can be constructed separately from and then floated over the base slab, as shown in Fig 8.) The base slab 10, partly formed columns 17 and ballastable raft 16 are floated out of the dry dock together at a stage of construction dependant on the depth of the dry dock. The partially completed substructure is then moved to a sheltered deepwater construction site. Slip forming builds up the columns 17, and the towers 18 surrounding those columns. The base slab 10 descends towards the seabed of the deepwater construction site as slip forming proceeds. Construction of the towers 18 follows the columns 17 upwards, and eventually the towers stand slightly proud of the columns. There may be diametrically opposed vertical strips of low friction material between the exteriors of the columns and the interiors of the towers.

When construction is complete, the substructure is generally as shown in Figure 3, but floating with a minimal remaining freeboard.

The substructure is then deballasted, and floated to the intended offshore location for the platform. At this location it is ballasted down so that it sinks to rest securely on the seabed.

This situation is shown in Figure 3. In the method described by way of example, the base slab 10 and ballastable raft 16 are sunk together. However, since the base slab and the ballastable raft are independently buoyant, the raft may be deballasted to provide substantial buoyancy, so that it can lower the base slab to the seabed, thereafter being reballasted and sunk down on to the base slab guided by the columns.

The deck 12 may be constructed contemporaneously with construction of the substructure. Figure 4 shows a situation in which the deck 12 (carried on a transportation barge 19) has been floated to a position over the legs 11.

The deck is supported on top of barge 19 by I beams 19A, and there are supports 19B extending out from the sides of the deck to overlie the tops of the towers 18.

Following the invention the raft 16 is deballasted to raise the deck 12' to a position in which its bottom surface would be clear of the highest expected wave. Corresponding abutments on the columns and towers limit upward moment of the deck. The barge 19 can then be removed, leaving the situation as shown in Figure 5. In a variant of the invention (not shown) two individual rafts could be arranged to emplace two deck elements separately.

At this stage the deck 12 is fixed to the tops of the towers 18, and upper portions of the towers 18 are fixed to the tops of the columns 17. The towers 18 are then split below the fixation between their upper portions and the columns, so that the raft and the lower portions of the towers can be ballasted down to rest on the base 10. This is the in-situ condition for the working life of the platform, and is shown in Figure 6.

The method described above by way of example has the advantages of allowing an inherentiy safe method of installing the deck, and of using the means of installation (i.e. the ballastable raft 16) as an additional component of a gravity base to stabilise the completed platform against overturning moments created by design waves. The ballastable raft, by obviating the need for additional buoyancy to lift a pre-commissioned deck to its intended location, and also by adding to the mass of the base slab for in-situ stability, serves two purposes at different phases in the life of the substructure. if the ballastable raft were eventually to be used to dismantle the platform as part of a decommissioning procedure, then that raft would serve three purposes.

The concept can be used with a self floating deck. An example of a second method according to the invention, and including such a deck, is shown in Figures 7 to 13.

Figure 7 shows second gravity based offshore platform in its completed form.

The platform has a base slab 20, four legs 21, and a deck 22. A ballastable raft 26 fits in a reduced draft slot across the base slab 20. The substructure - comprising base slab 20, legs 21 and ballastable raft 26 - is configured as shown in Figures 8 and 9. In this case lifting towers comprising steel grillage 28 have been fixed to the upper surface of the ballastable raft 26. The lifting towers extend across the raft, and are arranged to fit between the legs 21.

The base slab 20 is ballasted down so that it floats with the upper surface of the reduced draft slot some distance below sea level. In this state the ballastable raft can be floated over the base slab (as shown end on in Figure 8). It takes up a position with its ends overhanging the edges of the base slab (as shown in plan in Figure 9).

The base slab 20 is then deballasted, and the substructure is floated out to its intended location. At this location the base slab 20, legs 21 and ballastable raft 26 are sunk to the seabed (Figure 10).

A self floating deck 22 is moved to a position over the legs 21 (Figure 11). Leg stubs 21A on the self floating deck are then lowered into contact with the tops of the legs 21. When the leg stubs 21A have located the deck accurately, the raft 26 is deballasted so that the steel grillage 28 contacts the underside of the deck 22 as shown in Figure 12. The leg stubs 21A provide fixity between the deck 22 and the legs 21 before lifting proceeds.

The steel grillage 28 is of such a height that further deballasting of the raft 26 can lift the deck 22 up to its final intended position (Figure 13). The deck is fixed in that position; and raft 26 is then ballasted down to take up its permanent position (shown in broken lines) on top of the base slab 20 as part of the substructure. In this position the weight of the ballastable raft contributes to the gravity base of the completed platform.

Figure 14 shows a third gravity based offshore platform in its completed form.

The platform has a base slab 30, four legs 31 and a deck 32. The arrangement of the deck is shown in Figure 15, and the configuration of the base slab is shown in Figure 16.

Following the invention, the buoyancy of a ballastable raft 36 is used to lift the deck 32.

The ballastable raft achieves this in a practical and robust manner. A key operation is the transfer of deck load from transportation barge (39) to the ballastable raft 36. This involves mating two moving bodies (even though the movement of one body, the ballastable raft 36, is likely to be very restricted). In order to provide a supplementary load transfer mechanism, the concept may include an intermediate stage whereby the deck load is transferred from the barge 39 to the legs 31 via a jacking system, as described below. This does not detract from the generality of the invention.

For discussion purposes, the example shown in Figures 14 to 24 assumes a concrete gravity based platform in 100m water depth supporting a deck weight (dry) of around 10,000 tonnes. The platform is configured as a four legged structure with the legs 31 spaced 45m to 50m apart, at least in one direction, to allow access for the deck transportation barge 39. This tends to dictate a shallow deck 32 with large plan dimensions. Notionally, the deck dimensions would be around 80m x 45m in plan with a height of 10m to 12m.

The leg spacing and water depth have a significant influence on the plan area of the base slab 30, which would be at least 80m x 80m, depending on soil strength. The base slab would be 15m to 20m high to satisfy transportation floatation requirements, and advantageously to provide oil storage capacity in the region of 500,000 bbls. The ballastable raft 36, used to raise the deck 39, would be sized to constitute a lifting capacity in the region of 15,000 tonnes to lift the 10,000 tonne deck with an adequate contingency. This implies a ballastable raft with a displacement of around 30,000 tonnes.

A configuration that matches the functional requirements described above is shown in Figure 14. The substructure includes the base slab 30 and the movable ballastable raft 36.

Both base slab and ballastable raft are independently self floating with conventional multicell design. The base slab and raft are mated inshore by deballasting the base slab, and floating the ballastable raft over the base slab. The ballastable raft is barge shaped and sized to fit between the legs of the base slab. The base slab 30 has a reduced draft slot which allows the ballastable raft 36 to be mated without completely submerging the base slab 30.

The raft 36 is able to move vertically by deballasting or ballasting. During lifting the ballastable raft is always fully submerged, to maximise lift capacity, which leads to a requirement for a load transfer device between the top of the ballastable raft and the underside of the deck 32. This is formed by tubular lifting members 38 which are fixed into the top of the ballastable raft via transition steelwork designed to spread the lifting load. These tubular members 38 extend the full height of the legs 31 and are guided like foundation piles.

The lowest guide is designed to act as a stop mechanism during lift to prevent the deck from being raised excessively. The ballastable raft and base slab are mated inshore to commission and test the ballasting systems including the system required for controlled ascent and descent of the raft 36. The vertical travel of the ballastable raft is around 15m, which defines an inshore water depth requirement of around 40m for commissioning and testing purposes.

The deck 32 can be configured as a conventional trussed design or with deep plated girders similar to production jack-up deck designs. Although the latter type of deck can itself be designed as a self-floater (see Figures 7 to 13), it is assumed that the deck shown in Figures 14, 15 and 18 to 24 is transported to the offshore site on a barge in a conventional way. For either deck design, connectivity between the substructure and the deck is formed of leg stubs 31A. These leg stubs are carried within the deck (32) and can be lowered to mate with the legs (31). The top of each leg stub has integral crosshead beams which can be used to lift and/or support the deck.

Construction, installation, and emplacement of the deck for the platform shown in Figure 14 will now be described with reference to Figures 17 to 24.

The base slab 30, legs 31 and ballastable raft 36 are placed on the seabed as shown in Figure 17. The deck 32 has been constructed contemporaneously with construction of the base slab 30 and legs 31. Figure 18 shows a situation in which the deck 32 (carried on a barge 39) has been floated to a position over the legs 31.

It should be noted that the barge 39 is arranged at right angles to the ballastable raft 36 so that the tubular members 38 are located behind the legs 31, and hence are protected from collision damage.

When the deck 32 is positioned above the ballastable raft 36, the leg stubs 31A are lowered from the deck to mate with the tops of the legs 31 (Figure 19). The leg stubs 31A are then temporarily latched to the legs 31 by a connection strong enough to resist marine forces transferred from the transportation barge. The temporary leg/leg stub connections can be made permanent (i.e. fixed) at the end of deck lifting operations.

In Figure 20 the deck 32 is tied off to crossheads built into the top of the leg stubs 31A.

The barge 39 is then ballasted to transfer the deck load to the leg stub crossheads (Figure 21).

After removing the barge 39, the tubular members 38 of the ballastable raft 36 are brought into contact with the underside of the deck. The raft 36 is then deballasted to begin the lifting process (Figure 22). At any stage during the lifting process, tie members between the deck and the leg stub crossheads can be used to lift and/or support the weight of the deck.

The deck is raised to its fullest extent, just above the final required level, controlled by guide stops in the legs 31 which limit the upward travel of the ballastable raft 36. The deck 32 is then locked off at the top of the crossheads, and the raft 36 is ballasted in small increments to transfer load to these crossheads (Figure 23). The raft 36 is then ballasted to achieve neutral buoyancy and is finally lowered, via winches, back onto the base slab 30 (Figure 24).

The concept is different from "conventional" high level floatover concepts in one key respect. In this concept the deck is raised by buoyancy (possibly supplemented by mechanical means) whilst conventional floatover concepts are based on lowering the deck, using gravity.

The concept was primarily conceived to exploit buoyancy as a means of lifting a deck.

The ballastable raft 36 could also serve other functions including an installation aid for substructure transportation and installation, oil storage, and possible future use for platform abandonment purposes. These potential benefits remain, but a significant enhancement is considered to be the concept of leg stubs lowered from the deck, allowing construction and transportation conventionally, at low level.

The base slab 30 and ballastable raft 36 are robust units that invoive little sophistication either in construction or during inshore/atshore mating. The use of tubular members 38 to effect the lift also represents simple and robust construction. However, these require care in achieving dimensional control to ensure that the ballastable raft can be raised freely without incurring significant stick-slip forces in the guides.

Careful construction should allow free vertical movement, and the raft 36 should be kept in a horizontal attitude whilst deballasting is underway.

The three assembly methods described so far are related to conventional gravity based concrete substructures. However, the invention could be applied to a hybrid design with a steel tower and a concrete base slab (see Figures 25 to 27).

Figure 25 shows a fourth gravity based offshore platform during installation of its deck.

The platform has a base slab 40, and a steel lattice (or "jacket") support framework with four corner legs 41. The platform has a deck 42. In this illustration the steel framing of the deck carcass is shown.

Installation of the deck 42 is achieved by using a ballastable raft 46. The raft 46 has the planform of a hollow square and surrounds the legs 42. The planform is best seen in Figure 27. The ballastable raft has vertically aligned tubular lifting members 48 resting on its upper surface.

For installation the deck 42, supported on a barge 49, is floated between upper ends of the legs 41 (see Figure 26). By deballasting the raft 46, the tubular lifting members 48 can raise the deck to its final position above the wave effected zone.

Such a design would still rely on gravity foundation principles, perhaps supplemented by bucket type skirts. This design could provide competition to lift installed plafforms with jacket and deck weights that typically exceed 2,000 tonnes and tend to have high installation costs (because of crane vessel scarcity).

For cases other than with a self floating deck, the deck will be supported on a grillage, with the underside of the deck some 4m above the barge. The minimum size of barge required is governed by stability considerations: for a deck of around 10,000t a 36.6m (120') wide barge will provide the required range of positive stability.

The transfer of deck weight onto the substructure, whether it is achieved by support cables from above the deck via the crosshead beams or by lifting from the ballastable raft at underdeck level, is likely to be influenced by heave motions. Both methods of lifting the deck involve deballasting the barge as quickly as possible, which is no different from a high level floatover. What differs conceptually between the two approaches is that a conventional fioatover depends solely on barge deballasting whilst the present concept involves deck raising (on a raft), as well as deck barge deballasting.

Claims

1. A method of assembly for a gravity based offshore platform including a base slab to rest on the seabed with a slender leg or legs upstanding therefrom to support a deck above the wave effected zone; the method comprising the steps of setting the base slab on the sea bed, locating a ballastable raft with lifting means upstanding therefrom around the leg or between the legs, moving the deck to a position over the lifting means, deballasting the raft so that the raft rises and the lifting means raises the deck to a required position above the wave effected zone, fixing the deck to the leg or legs in that position, and reballasting the raft so that it descends to rest on the base slab, so to form a part of a gravity base for the platform during the working life of the platform.

2. A method as claimed in Claim 1 in which, prior to the base slab being set on the seabed, the ballastable raft is constructed on top of the base slab.

3. A method as claimed in Claim 1 in which, prior to the base slab being set on the seabed, the ballastable raft is constructed separately from the base slab for mating with the base slab while afloat.

4. A method as claimed in any one of Claims 1 to 3 in which, when the base slab is set on the seabed, the ballastable raft is set with the base slab.

5. A method as claimed in any one of Claims 1 to 3 in which the ballastable raft is used in a deballasted condition to lower the base slab and so to set the base slab on the seabed, and then the raft is ballasted down onto the base slab.

6. A method as claimed in Claim 5 in which the leg or legs of the platform guide the raft as it is ballasted down onto the base slab.

7. A method as claimed in any one of Claims 1 to 6 in which the lifting means on the ballastable raft comprises at least one hollow tower which surrounds a column forming a leg of the platform, and in which the column is used to guide vertical movements of the hollow tower.

8. A method as claimed in Claim 7 in which an upper part of the tower is fixed to an upper part of the column, so to form a composite upper part of the leg, and the lower part of the tower is lowered to rest with the ballastable raft on the base slab.

9. A method as claimed in any one of Claims 1 to 6 in which the lifting means on the ballastable raft comprises at least one lifting tower arranged to engage an underside of the deck.

10. A method as claimed in any one of Claims 1 to 6 in which the lifting means on the ballastable raft comprise a plurality of lifting piles arranged to engage an underside of the deck.

11. A method as claimed in any one of the preceding claims in which there are abutments outstanding from the leg or legs and corresponding abutments projecting from the lifting means, such that when the deck is in its required position, the abutments engage each other to prevent further upward movement of the ballastable raft.

12. A method as claimed in any one of the preceding claims in which the base slab has a plurality of upstanding legs and the ballastable raft has a plurality of columns, and the deck is moved to a position between the legs prior to being raised.

13. A method as claimed in Claim 12 in which the deck is moved to the position between the legs on a barge.

14. A method as claimed in Claim 12 in which the deck is self floating.

15. A method as claimed in Claim 14 in which the deck incorporates vertically movable leg stubs, and the leg stubs are used to locate the deck with respect to the legs of the platform.

16. A method as claimed in Claim 15 in which the deck stubs have means to lock the deck at a specified vertical position.

17. A method of assembly substantially as hereinbefore described with reference to Figures 1 to 6, 7 to 13, 14 to 24 or 25 to 27 of the accompanying drawings.

18. An offshore platform when assembled according to the method of any one of the preceding claims.

19. A substructure for a gravity based offshore platform, comprising a base slab to rest on the seabed with a slender leg or legs upstanding therefrom, and a ballastable raft arranged to rest on the base slab and so to form part of a gravity base when the raft is fully ballasted, in which the ballastable raft has lifting means upstanding therefrom around the leg or between the legs, such that deballasting the raft can lift a deck supported on the lifting means to a required position above the wave effected zone.

20. A substructure as claimed in Claim 19 in which the lifting means on the ballastable raft comprises at least one hollow tower which surrounds a column forming a leg of the platform.

21. A substructure as claimed in Claim 20 in which there are diametrically opposed vertical strips of low friction material between the exterior of the column and the interior of the hollow tower.

22. A substructure as claimed in Claim 19 in which the lifting means on the ballastable raft comprises at least one lifting tower arranged when in use to engage an underside of a deck.

23. A substructure as claimed in Claim 19 in which the lifting means on the ballastable raft comprise a plurality of lifting piles arranged when in use to engage an underside of a deck.

24. A substructure substantially as hereinbefore described with reference to and as shown in Figures 1 to 6, or Figures 7 to 13, or Figures 14 to 24, or Figures 25 to 27 of the accompanying drawings.

25. An offshore platform including a substructure according to any one of Claims 19 to 24 and a deck set on that substructure by a method according to any one of Claims 1 to 17.

26. An offshore platform substantially as hereinbefore described with reference to and as shown in Figures 1 and 6, or Figure 7, or Figures 14 to 16 and 24 of the accompanying drawings.

Amendments to the claims have been filed as follows 1. A method of assembly for a gravity based offshore platform including a base slab to rest on the seabed with a slender leg or legs upstanding therefrom to support a deck above the wave effected zone; the method comprising the steps of setting the base slab on the sea bed, locating a ballastable raft with lifting means upstanding therefrom around the leg or legs or between the legs, floating the deck to a position over the lifting means, deballasting the raft so that the raft rises and the lifting means raises the deck from a sea level position to a required position above the wave effected zone, fixing the deck to the leg or legs in that position, and reballasting the raft so that it descends to rest on the base slab, so to form a part of a gravity base for the platform during the working life of the platform.

12. A method as claimed in any one of the preceding claims in which the base slab has a plurality of upstanding legs and the ballastable raft has a plurality of columns, and the deck is floated to a position between the legs prior to being raised.

13. A method as claimed in Claim 12 in which the deck is floated to the position between the legs on a barge.

14. A method as claimed in Claim 12 in which the deck is self floating.

19. A substructure for a gravity based offshore platform, comprising a base slab to rest on the seabed with a slender leg or legs upstanding therefrom a deck( the configuration of the legs or legs beiong such that a deck can be floated over or between the leg or legs), and a ballastable raft arranged to rest on the base slab and so to form part of a gravity base when the raft is fully ballasted, in which the ballastable raft has lifting means upstanding therefrom around the leg or between the legs, such that deballasting the raft can lift a floating deck supported on the lifting means from a sea level position to a required position above the wave effected zone.