WO1987000138A1

WO1987000138A1 - Arrangement in a semisubmersible platform structure

Info

Publication number: WO1987000138A1
Application number: PCT/NO1986/000046
Authority: WO
Inventors: Per Chr. SANDNA^ES
Original assignee: Aker Engineering A/S
Priority date: 1985-07-03
Filing date: 1986-07-01
Publication date: 1987-01-15
Also published as: GB2188291B; NO852686L; GB2188291A; NO165631B; GB8702258D0

Abstract

A semisubmersible drilling platform has two longitudinal pontoons (1) from which columns (2, 3) extend upwards and support a deck (4). In order to provide a cheap production platform which may permit exploitation of marginal oil fields and concurrently provide alternative employment for such a drilling platform, it is provided with a supplementary unit (6) comprising storage tanks (7) for crude oil. The supplementary unit has an internal access tunnel (10) communicating with the deck (4) through an access shaft (11). The supplementary unit is built as an independent unit having sloptanks (8) and pump rooms (9). The supplementary unit (6) is attached to the pontoons (1) where these have bulkheads, by means of brackets (12). These brackets may simply be burnt through if the platform structure again is to be converted back to a drilling platform, or one may remove only the transverse portions of the supplementary unit and let the longitudinal portions remain in order to give the drilling platform increased carrying capacity.

Description

ARRANGEMENT IN A SEMISUBMERSIBLE PLATFORM STRUCTURE

The present invention relates to an arragment in a semisubmersible platform structure of the type comprising at least two longitudinal pontoons and columns extending upwards from these and supporting a deck.

Such platform structures are primarily intended for exploration drilling for oil and gas and gradually a considerable number has been built. However, the demand for such platforms varies as the exploration finishes in the various fields and before new exploration areas are opened. Therefore, from time to time there may be diffi¬ culties in finding employment for the existing fleet of semisubmersible drilling platforms at profitable rates. Offshore exploration drilling in further new areas has led to discoveries of smaller oil and gas fields where production by means of permanent platform structures of the conventional types is not economically feasible. Furthermore, such platform structures become dispropor- tionately expensive for water depths over about 150 m, while exploration drilling today takes place at depths down to 300 m or more.

The object of the present invention is to satisfy the demand for alternate employment of drilling platforms of the above type and for production platforms that may make marginal oil fields exploitable.

This is obtained according to the invention by a platform structure of the above type, where the charac¬ teristic features are that it further comprises at least one supplementary unit having storage tanks for crude oil. By means of such a supplementary unit the platform structure may be used as an effektive production platform. Normally the crude oil produced will be transferred more or less directly to a tanker moored to the platform, but during changing of tankers and when the weather conditions do not permit a tanker to be moored to the platform, the production may nevertheless proceed continuously by storing the crude oil temporarily in the storage tanks of the supplementary unit.

According to an advantageous feature of the inven¬ tion, the supplementary unit is installed between the pontoons of the platform structure. Here the supplemen¬ tary unit will be relatively protected and may additio- 5 nally form a structural element increasing the strength and possibly also the carrying capacity of the platform structure.

The supplementary unit may advantageously be in the form of an annular body of quadrangular shape. The supplementary unit may thus be given an optimal size, while the opening in the middle will give room for risers etc. and prevent the hydrodynamic wave forces acting on the supplementary unit from becoming too high. By any later converting back to drilling platform it will be possible to remove only the transverse portions of the annular body. The remaining longitudinal portions of the body may serve to give the platform increased carrying capacity and payload.

__ According to the invention at least one access shaft is arranged between the supplementary unit and the deck of the platform structure. The location and dimen¬ sioning of the access shaft is preferably performed for it to provide stiffening of the supplementary unit and con-

„_ currently provide an advantageous increase of the water- line cross-section of the structure. The supplementary unit may also be provided with an access tunnel, which may be arranged interiorly or exteriorly of the supplementary unit, said access tunnel being connected to the access Λ shaft or shafts.

As one of several alternatives to the annular supplementary unit, two transverse supplementary units may be arranged, one at the front and one at the rear. These may be identical and be installed symmetrically. Further- _c more, it may be advantageous to install the supplementary unit or units so that it or they entirely lie above the lowest waterline of the platform structure. Thereby the supplementary units may be installed without docking the platform structure, but just by floating the units in place while the platform structure is in semisubmerged condition, whereupon it is raised and the supplementary unit welded in place. With the supplementary units in ° this position they will also quite effectively serve to protect the drill string and riser against drifting .objects, for instance drifting ice, when the platform is in use.

However, it will also be possible to install the

'^Q supplementary units on the bottom side of the pontoons.

In this case, the supplementary units are ballasted so that they rest at a suitable place on the bottom, or are suspended from floats, whereupon the platform structure is floated in place above them. Then the supplementary units

1 are unballasted so that they come to raise the platform structure completely out of the water, whereupon the supplementary units and the platform structure may be connected to each other, also in this case without docking being necessary. n *-^v In accordance with the invention it is also suggested to make the supplementary units independent in the sense that they comprise necessary pump rooms, slop- tanks etc. in addition to the storage tanks, with equipment and piping finished in advance. In this way the ^ necessary time for joining the units to the platform structure is held to a minimum.

For the better understanding of the invention it shall be described more closely with reference to the exemplifying embodiments shown in the appended drawings.

30 Fig. 1 shows a vertical cross-section through a platform structure according to the invention.

Fig. 2 shows a plan view of the platform in Fig. 1, seen generally along the line II - II in Fig. 1.

Fig. 3 shows a plan view of an alternative embodi-

35 ment of the platform structure according to the invention.

Fig. 4 shows a further alternative embodiment of the platform structure.

The platform structure shown in Figs. 1 and 2 com- prises two longitudinal pontoons 1 and columns 2 and 3 ex¬ tending upwards from the pontoons and supporting a deck structure 4. The pontoons contain a number of tanks 5 for e.g. ballast and fuel oil. Between the pontoons 1 a supplementary unit 6 is arranged, the unit having the general form of an annular body of quadrangular shape. The supplementary unit comprises a number of storage tanks 7 for e.g. crude oil, and in addition has sloptanks 8 and pump rooms 9. An access tunnel 10 is arranged inside the supplementary unit, said access tunnel communicating with two access shafts 11 leading to the deck 4.

The supplementary unit is connected to the pontoons 1 by means of brackets 12, which are welded in places where the supplementary unit and the pontoons have internal bulkheads. If the platform structure later is to be converted back to a pure drilling platform, the brackets 12 may be burned through so that the entire supplementary unit can be removed, or the longitudinal portions of the supplementary unit may remain in place, while the transverse portions at the front and rear are removed. The remaning portions of the supplementary unit will thus entail an upgrading of the load carrying capacity of the platform. Fig. 3 shows an alternative embodiment where two supplementary units 13 and 14 are arranged between the pontoons 1. These units are identical and are arranged symmetrically, one at the front and the other at the rear of the platform structure. The supplementary units also in this case are subdivided into storage tanks 15 and are connected to the deck by means of an access shaft 16.

In the exemplifying embodiment in Fig. 4, supple¬ mentary units 17 and 18 are arranged alongside the pontoons 1, which have had their bow portion modified, where pump room 19, sloptank 20 and access shaft 21 are placed.

It will be understood that the invention is not restricted to the exemplifying embodiments shown, but that it may be modified and varied in a number of ways within the scope and spirit of the invention. Thus, the trans¬ verse portions of the supplementary unit may be placed closer to the center of the platform in order to give less pitching motion. Furthermore, it may be advantageous to arrange additional columns, e.g. between the supplementary unit and the deck, in order to provide a larger moment of inertia and waterline area, while the heave response of the platform is maintained near the original value. Such further columns may also be utilized as storage tanks.

Claims

C l a i m s

1. Arrangement in a semisubmersible platform structure having at least two longitudinal pontoons (1), from which columns (2, 3) extend upwards and support a deck (4), c h a r a c t e r i z e d in that it further comprises at least one supplementary unit (6, 13, 14, 17, 18) com¬ prising storage tanks (7, 15) for crude oil or the like.

2. Arrangement according to claim 1, c h a r a c t e r i z e d in that the supplementary unit (6, 13, 14) is installed between said pontoons (1).

3. Arrangement according to claim 1 or 2, c h a r a c t e r i z e d in that at least one access shaft (11, 16, 21) is arranged between the supplementary unit (6, 13, 14, 17) and said deck (4).

4. Arrangement according to claim 3, c h a r a c t e r i z e d in that an access tunnel (10) is arranged interior or exterior of the supplementary unit, said access tunnel (10) communicating with said at least one access shaft (11).

5. Arrangement according to a preceding claim, c h a r a c t e r i z e d in that the supplementary unit (6) is formed generally as an annular body of quadrangular shape.

6. Arrangement according to one of the claims 1 - 4, c h a r a c t e r i z e d in that two supplementary units (13, 14) are provided, one (13) at the front and one (14) at the rear of the platform structure.

7. Arrangement according to a preceding claim, c h a r a c t e r i z e d in that at least one supple¬ mentary unit (6, 13, 14) is installed so that it at least partially lies above the lowest waterline of the platform structure .

8. Arrangement according to claim 7, c h a r a c t e r i z e d in that said at least one

° supplementary unit (6, 13, 14) is installed resting on the pontoons (1) .

9. Arrangement according to one of the claims 1 - 6, c h a r a c t e r i z e d in that at least one of the 0 supplementary units (13, 14) is installed on the bottom side of the pontoons (1).

10. Arrangement according to a preceding claim, c h a r a c t e r i z e d in that the supplementary 5 unit (6, 13, 14, 17, 18) is manufactured as an independent unit comprising pump room (9, 19) and sloptank (8, 20).

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