GB2241011A

GB2241011A - Offshore platform system

Info

Publication number: GB2241011A
Application number: GB9003408A
Authority: GB
Inventors: James William Bunce
Original assignee: Individual
Current assignee: Individual
Priority date: 1990-02-15
Filing date: 1990-02-15
Publication date: 1991-08-21
Also published as: GB9003408D0

Abstract

An offshore platform consists of a superstructure 1 supported on columns 2 embedded in a foundation unit on the sea floor. The superstructure 1 is supported in a lowered position on the foundation unit during construction and sea transport. The foundation unit consists of an upper section 4 and a lower section 5. The lower section 5 provides sufficient buoyancy for flotation of the entire platform assembly during sea transport. The upper section 4 can float under the weight of the superstructure 1 as the lower section 5 is sunk to the sea floor. Cables 7 passing over sheaves 8 raise the superstructure 1 to the top of the columns 2 under the action of the upper section 4 acting as a counterweight as if is sunk to its final position on top of the lower section 5. A winch 10 provides a means of controlling the descent rate of the upper section of the foundation unit. Alternately (Fig 9) the upper section is ballasted to have a small positive buoyancy and is hauled into position on the lower section by a hawser (11) connected to a tug (13). <IMAGE>

Description

SPECIFICATION Offshore Platform System This invention relates to an offshore platform system.

Conventional fixed offshore platforms are constructed in component units which are transported out to sea on barges. g substructure unit is fixed to the sea floor by means of driven or drilled piles and the superstructure units are lifted into position on the substructure unit by means of a crane ship.

In such constructions, the costs of the sea transport and installation operations including barge, and tug rentals, pile driving, crane ship charter etc., are a very significant proportion of the total cost of the platform.

In particular, for small platforms in relatively shallow waters the sea transport and installation costs are often comparable to the total structure fabrication and equipment purchase cost.

Profitable exploitation of marginal oil and gas fields calls for cost: savings in all areas of the development and there thus exists a strong incentive to economist on the sea transport and installation costs by reducing or eliminating the need for transport barges, pile driving and crane ships.

The same requirement to reduce costs exists for offshore platforms constructed for defence, communications and observation purposes The present invention relates to an offshore platform that is cheaper to construct, sea transport and install than a conventional offshore platform since the entire platform assembly can be towed out to sea as a single self-floating unit without a requirement for transport barges and it can be installed by means of simple ballasting operations without assistance from a crane ship and without a requirement for piling.

Thus, according to the present invention there is provided an offshore platform consisting of a superstructure, elongate columns and a foundation unit; in which the superstructure is capable of movement relative to the elongate columns, so that both during construction and during sea transportation the superstructure can be supported in a lowered position on top of the foundation unit, and in final service it can be supported on top of the elongate coluins above the se.

surface in which the Foundatiarl unit comprises one or more upper sections and one or more lower sections, such lower section(s) providing sufficient buoyancy for flotation of the entire platform during sea transportation; in which the elongate columns have their lower ends embedded in the lower section(s) of the foundation unit; and in which during installation, the upper sections of the foundation unit are floatable under the weight of the superstructure as the lower section(s) of the foundation unit are sunk to the sea floor taking the elongate columns downwards with them, and the superstructure and the upper sections incorporate hoisting means whereby the superstructure is raised upwards to the top of the elongate columns, as the upper section(s) of the foundation acting as counterweights(s), sink downwards onto the top of the foundation unit lower section(s).

Means of controlling the descent rate of said upper section(s) of the foundation are provided so as to avoid shock loading on the superstructure as it is raised to the top of the elongate columns.

Means of finally conneftino together the superstructure and the columns and the foundation unit sections and the columns are provided The invention will now be described by way of example only and with reference to Figs. 1 to 9 of the accompanying drawings which show, on elevation, general views of the platform: FIGURE 1 shows the platform in its final service configuration FIGURE 2 shows the platform in a dry dock or similar building yard in its as-built configuration.

FIGURE 3 shows the platform in its sea transport configuration FIGURES 4 to 7 show the platform at various stages of the installation operation.

FIGURES 8 and 9 show alternative means of controlling the descent rate of the upper section(s) of the foundation.

Referring to Fig. 1 the platform comprises a 5U perstructure unit 1 supported above the wave crest level on elongate culuins 2 embedded in a gravity type foundation unit 3 on the sea floor.

While i t is preferred that the elongate columns are ve t: j cal orie or more may be inc] ned to the horizontal The columns may be connected together by bracing (not shown). The superstructure and elongate columns are preferably made from metal for lightness. The foundation unit is preferably constructed in. reinforced and prestressed concrete for cheapness and to give the necessary stability in final service. The foundation unit may be used for storage purposes.

In the as-built configuration shown in Fig. 2, the superstructure unit 1 is supported in a lowered position on top of the upper section 4 of the foundation unit.

This low level position of the superstructure unit 1 allows construction operations to proceed near ground level without extensive high level craneage requirements.

It also serves to restrict the height of the centre-ofgravity of the platform assembly during sea transport.

Referring to Fig. 3, during sea transport to the offshore installation site the entire platform assembly floats on the lower section 5 of the foundation unit.

FIGURE 4 shows the start of the installation operation where the lower section 5 of the foundation unit is ballasted until it begins to sink to the sea floor. The upper section 4 of the foundation unit remains floating at the sea surface supportina the superstructure unit 1 above the water level. Ns the lower section 5 of the foundation unit sinks the elongate columns 2 travel downwards with it.

FIGURE 5 shows the lower section 5 of the foundation unit in its final position on the sea floor. Skirts 6 around the perimeter of the foundation unit penetrate the sea floor, if required, to prevent scour and erosion and to assist in resisting sliding forces from hydrodynamic loads on the platform in final service conditions Fig.

5 also shows hoisting means, for example, in the form of cables or- chains 7 that run from the superstructure unit 1 over- sheaves 8 at the top of the elongate columns back down to the upper section 4 of the. foundation unit. FIGURE 6 shows the upper section 4 of the foundation unit as it is ballasted and begins to sink below the sea level. Tension forces in the hoist cables or chains 7 cause the superstructure unit 1 to rise under the counterweight action of the upper section 4 of the foundation unit as it sinks.

FIGURE 7 shows the platform at the end of the installation operation with the superstructure unit 1 and the foundation unit upper section 4 in their final positions.

FIGURE 8 shows one method of controlling the descent rate of the upper section(s) 4 of the foundation by means of a winch 9 in the su)er-structure, connected to the upper section(s) of the foundation by a lowering line 10. The upper section of the foundation ts water ballasted until it becornes negatively buoyant unde; the upwards pull from the superstructure hoist cables 7 so that the lowering line Jo becomes tiyht. Opejatj:on of the winch 9 will then cause the upper section of the foundation 4 to sink at the desired velocity FIGURE 9 shows an alternative method of controlling descent of the upper section of the foundation 4 by means of a hawser 11 attached to the upper section of the foundation 4 and passing through a hawespipe 12 in the lower section of the foundation 5 to a tug 13.The upper section of the foundation 4 is water ballasted until a small positive buoyancy remains under the upwards pull from the superstructure hoist cables 7 so that the hawser 11 becomes tight, pulling action from the tug 13 will then cause the upper section of the foundation 4 to sink at the desired velocity Connections between the superstructur e unit 1 and the columns 2 and between the columns 2 and the foundation unit sections 4, 5 are made and the hoist means 7 and sheaves 8 can be removed If necessary, cement grout can be in jected between the foundation unit sections to ensure good contact.

Similarly, cement grout can be injected below the foundation in t to ensL're floor contact with the sea floor The platform can be removed from the installation site by reversing the operations described above

Claims

CLAIMS 1. An offshore platform consisting of a superstructure, elongate columns and a foundation unit; in which the superstructure is capable of movement relative to the elongate columns, so that both during construction and during sea transportation the superstructure can be supported in a lowered position on top of the foundation unit, and in final service it can be supported on top of the elongate columns above the sea surface; in which the foundation unit comprises one or more upper sections and one or more lower sections, such lower section(s) providing sufficient buoyancy for flotation of the entire platform during sea transportation; in which the elongate columns have their lower ends embedded in the lower section(s) of the foundation unit; and in which during installation, the upper section(s) of the foundation unit are floatable under the weight of the superstructure as the lower section(s) of the foundation unit are sunk to the sea floor taking the elongate columns downwards with them, and the superstructure and upper section(s) incorporate hoisting means whereby the superstructure is raised upwards to the top of the elongate columns, as the upper section(s) of the foundation acting as counterweight(s), sink downwards onto the top of the foundation unit lower section(s); in which means of controlling the descent rate of said upper section(s) of the foundation unit are provided so as to avoid shock loading on the superstructure as it is raised to the top of the elongate columns.
2. An offshore platform system according to claim 1 in which there is provided means of finally connecting together the superstructure and the elongate columns and the foundation unit sections and the elongate columns.
3. An offshore platform system according to claims 1 and 2 in which the hoisting means consist of cables or chains passing through sheave units attached to the top of the elongate columns such that the superstructure is raised upwards as the upper sections of the foundation unit sink downwards.
4. An offshore platform system according to any of claims 1 to 3 in which the elongate columns are vertical.
5. An offshore platform system according to claims 1 to 4 in which the method of controlling the descent rate of the upper section(s) of the foundation is by means of a winch in the superstructure connected to the upper section(s) of the foundation by a lowering line so that the negatively buoyant upper section(s) of the foundation can be lowered into position.
6. An offshore platform system according to claims 1 to 4 in which the method of controlling the descent rate of the upper section(s) of the foundation is by means of a hawser attached to the upper section(s) of the foundation passing through a hawespipe in the lower section of the foundation to a tug so that the positively buoyant upper section(s) of the foundation can be hauled down into position.
7. An offshore platform system according to any of claims 1 to 6 in which one or more of the elongate columns is inclined to the horizontal.
8. An offshore platform according to any of claims 1 to 7 in which the elongate columns are connected together by bracing members.
9. An offshore platform system according to any of the preceeding claims in which the superstructure is for offshore mineral extraction purposes, habitation, communications, control or observation purposes.
10. An offshore platform system according to any of the preceeding claims in which the foundation unit is used for storage purposes.
11. An offshore platform system according to any of the preceeding claims in which the foundation unit incorporates skirts or piles to assist the maintenance of stability in service conditions.
12. An offshore platform system according to any of the preceeding claims which embodies means of reversing the installation method to retrieve the platform.
13. An offshore platform system as herein before described and with reference to the accompanying drawings.
14. An array of offshore platforms according to any of claims 1 to 13.