WO1988008807A1

WO1988008807A1 - Arrangement in connection with a submerged storage tank

Info

Publication number: WO1988008807A1
Application number: PCT/NO1988/000038
Authority: WO
Inventors: Alf B. Hallingstad; Ivar Sandem
Original assignee: Aker Engineering A/S; Fred. Olsen & Co.
Priority date: 1987-05-12
Filing date: 1988-05-05
Publication date: 1988-11-17
Also published as: NO871964L; NO871964D0

Abstract

The present invention relates to an arrangement in connection with a submersible tank system (6; 6a; 6b), preferably for storage of oil produced from an offshore oil reservoir, said tank system being designed such that it can be used as a foundation for the mounting of one or more preferably existing platform structures (1a, 1b). This arrangement provides for a simple interconnection of existing platform structures, such that the latter may give larger room for equipment than one platform alone, said tank system increasing the storage capacity and rendering a sturdy, appropriate and constructural practical solution to the problem of assembly.

Description

ARRANGEMENT IN CONNECTION WITH A SUBMERGED STORAGE TANK

Field of the invention

The present invention relates to an arrangement in connec¬ tion with a submerged storage tank especially a storage tank for storing oil produced from an offshore oil reservoir.

The invention also relates to a method for assembling platform structures by means of such storage tanks.

Prior art

Existing platforms, for example of the semi-submersible type, are often suited for reconstruction for other pur¬ poses, for example protection of oil and gas from a field located on large sea depths. Such fields are usually located far away from an available place of bringing ashore, and it may be unprofitable to lay special conduits from the field. Consequently, according to prior art the products are loaded on board tankers which call at the platform. However, these calls may be irregular, inter alia due to the weather condi- tions, and in order to avoid interruption of the production it is necessary to establish storage tanks connected to the platform.

One type of such tanks is previously known, for example from Norwegian Patent Application No. 85.2686, wherein a semi- submersible platform structure comprises an additional unit which substantially contains storage tanks and crude oil or similar. Appropriately, the storage tanks may be added as extra pontoon-like elements.

In connection with the production of oil and gas from such fields as discussed above, and which are located on large sea depths, there may also exist a requirement for larger space for production equipment than what is achieved by one single drilling platform. An additional requirement may also exist for other functions than production, for example dril¬ ling of wells and the maintenance thereof. In view of this object the present invention is aimed at providing not only larger storage capacity, but also larger drilling and pro¬ duction capacity, and then primarily in connection with existing drilling platforms, both of the submersible type and the jack type platform.

From SE 433 733 there is known a barge structure which is used for sea transportation and the erection of a factory plant, the platform including the plant being floated during the transportation from the factory to a main transportation vessel, whereafter the barge structure later is used for the transportation of the factory plant from the main transpor¬ tation vessel to the place of erection. Thus, the prior art according to the Swedish printed and published application gives directions for a barge structure which is to be used for shorter transportation ranges, i.e. in waters wherein this short transportation can take place under safe super¬ vision.

The prior art barge or platform which supports the factory plant and parts of a beam structure, is provided with side- plates which are sufficiently high to provide an appropriate displacement, for thereby carrying the factory plant during the short sea transportation to and from the vessel. Further, the barge structure or platform is provided with inner openings which allow the inner platform to be filled with concrete on the erection place for the factory plant.

In other words , the prior art gives instructions for a plat¬ form-shaped or tank-shaped barge structure which only tempo- rarily has the function of a buoyancy body, and which at the starting point is fixedly connected to the factory plant, already on the land based place of manufacturing. In contrary to this prior art technique the present inven¬ tion is related to tanks or tank systems which are to form the foundation for the mounting of one or more platform structures which are present in deep water, i.e. for one or more existing platform structures which are to have their storage capacity expanded, and which by such a common tank system can combine their individual platform functions.

A known barge structure, for example as discussed in SE 433 733, cannot in any manner meet the. requirements for giving an existing drilling platform a larger drilling and production capacity, since the prior art barge structure, if it should be lowered down below the water surface and cons¬ titute a supporting buoyancy providing foundation, not in any way could give sufficient buoyancy, since the side plates which should give a better buoyancy, would not come into effect, it being already stated in SE 433 733 that the barge or the platform itself will not give sufficient buoy¬ ancy for the factory plant when this is in open sea.

SE 437 808 gives also instructions for a barge which corn- prices a lower part and an upper part, and which carries a process plant. The barge has sufficient displacement for effecting towing from a place of manufacturing to a place of use, the process plant or part thereof being mounted on the barge. However, on the place of use the barge has to be affixed to a support which may protrude somewhat above the sea bed, but which is not higher than allowing the barge including its already mounted process plant to be floated in above the support, when the displacement force is reduced by opening hatches in parts of the barge.

In other words, it is also here talking about a barge-like structure which in advance is fixedly mounted on a struc¬ ture part, but SE 437 808 does not give any instructions for any novel manner of arranging two or more platforms at sea in connection with each other, let alone gives any instruc¬ tions for an arrangement which provides not only sufficient storage capacity by the connection of two or more platforms, and which otherewise provides a sturdy, appropriate and structurally practical manner of joining said platforms, at the same time as the storage capacity is expanded so to say unlimitedly.

SE 438 300 and SE 442 736 relates to upwardly erected stabilizing elements to be used together with platform structures.

GB 2 080 860 relates to the mounting of pillars on a deck which rests on a floating support. However, this is an opposite process of what is suggested in accordance with the present invention, wherein the tank system is mounted below a finished semi-submersible platform structure, or several such platform structures which subsequent to the assembly provide an "expanded" oil platform.

Brief discussion of the invention

Thus, the present invention gives instructions for a new manner of arranging two or more platforms in connection with each other, such an arrangement bringing forth not only sufficient storage capacity by the joining of two or more platforms, but also providing a sturdy, appropriate and constructionally practical manner of joining said platforms, as well as providing a storage capacity which is expanded so to say unlimitedly. Necessary stability is arranged by means of stabilizing columns.

The above discussed problems are resolved by an arrangement in connection with a submerged tank of the type as stated in the preamble, the tank according to the present invention being characterized in that the thank or the tank system is brought together with one or more, preferably existing, offshore platform structures which constitute the basement for said platform structure(s) . In other words, it is aimed for constructing a tank of sufficient size and having a shape which is adapted to the geometry of the platforms, such that the latters as a whole may be positioned on the tank which thereby constitutes the foundation of the platform or the platform groups. The tank or the tank items can be shaped in such a manner that oil conduits, drilling strings, etc. can be guided to the sea bed from the platform decks, and can for example take the form of parallel tank units or a larger tank provided as a frame.

In other words, the present invention gives instructions for a tank system which is brought together with one or more platform structures in deep sea for the interconnection of one or more platforms, for thereby providing an "expanded" oil platform as a final product, comprising both submers¬ ible and jack platforms, all of which is arranged for utili¬ zation in open sea.

The method according to the invention is to the fact that the tank system is provided apart from the platform struc¬ ture or the platform structures, and that the tank system is brought together with one or more platform structures in deep water for interconnection to said one or several plat- forms .

The columns used in connection with the present invention have a function beyond stabilization during the rig opera¬ tion, i.e. to provide stability and draft control of the storage tank when this is submerged to allow the existing platforms to be floated thereabove, for subsequently - by deballasting the columns - to be brought in contact with the attachment means between the tank and the platforms.

Sufficient buoyancy for the tank or the tanks is necessary in connection with the assembly of the existing platform, i.e. as this is further defined in the method claims. Brief description of the drawings

Practical implementations of the invention will no be further described with reference to the appended drawings.

Figure 1 is a diagrammatical front view of a first embodi¬ ment of an arrangement according to the invention.

Figure 2 is a top view of the embodiment illustrated in Figure 1.

Figure 3 is a section as seen substantially in the direc¬ tion of the arrows III-III in Figure 1, in which some parts are partly cut away.

Figure 4 is a section similar to Figure 3, but illustrates a variant of the arrangement according to the invention.

Discussion of embodiments

In Figures 1, 2 and 3 which illustrate front view, top view and a horizontal section of a first embodiment of the arrangement according to the invention, la designates a first platform structure, whereas lb designates a platform structure number two. The two platform structures, la and lb, resepectively, each comprises two longitudinally extend¬ ing pontoons 1 including upwardly therefrom extending columns 2 and 3 which support their own deck structure, 4A and 4B, respectively.

The two platform structures la and lb are arranged in^" con¬ nection with each other, there in the area of the pontoons 1 and preferably therebelow being arranged a tank 6 which is constructed such that it constitutes the foundation for the assembly of said two platform structures.

In the embodiment illustrated in Figures 1-3, the tank 6 takes the form of a frame, the inner area of the frame rendering space for oil conduits, drilling strings, etc. extending between the sea bed and the platforms la and lb, respectively.

The assembly of the tank 6 and the two platform structures la and lb may appropriately be effected by submerging the tank 6 below the platform structures la and lb, whereafter the platforms la and lb are floated in place in relation to the position to be taken on the frame 6. Thereafter, the tank 6 can be raised such that the actual joining between the tank 6 and the platforms la and lb can be effected, but also the joining between the two platform decks 4A and 4B, which otherwise are equipped with devices as know per se for utilization in drilling and/or production.

It is to be understood that the tank 6 which is designed as a frame and especially adapted to the geometry or geometries of the platform to be mounted on the tank 6, due considera- tion being attended to the best possible bracing and attach¬ ment as regards arrangement and system. For example, the bracing can be arranged such that simple demounting of the platform structures la and lb is possible at a later stage.

It is to be understood that the tank 6 may be equipped with ballasting systems, for example comprising pure or combined tanks. Appropriately, such a ballasting system may be con¬ structed in such a manner that it at any time keeps the total structure floating within fixed water line levels.

In order to achieve a favourable anti-sway stability which is approximately equal in all directions, there may in addition to the columns 2 and 3 be built further columns 7, 8 especially on the sides where the stability could be least. These additional columns, or the original columns 2 and 3, may constitute the foundation for positioning equip¬ ment, serve as access trunk to the tank 6, or constitute a platform for exporting oil and gas. It is further to be understood that in addition to the standard equipment which might be on the platform structure la and lb prior to the joining thereof, the assembled plat¬ form structures may be wholly or partly equipped for dril- ling, completion, processing and export of oil and gas, as well as for maintaining wells.

It may be appropriate to devide the tank in several units, which is visualized in the embodiment illustrated in Figure 4. Here, the submerged tank is composed of two separate units 6A and 6B, and in the assembly these units may indi¬ vidually be brought below the platforms which are to be joined.

In connection with the assembly of the preferably existing platform structures it will also here be used a method which is to the fact that apart from one or more platform struc¬ tures la, lb there is provided a tank system 6; 6A, 6B which is lowered sufficiently down below the platform structures la, lb in deep sea for the floating thereinto of the plat¬ form structures to their joint position, whereafter the tank system 6 6A, 6B together with the platform structures la, lb are raised to a suitable level for being brought together.

Appropriately, the tank system may thereafter be raised so high together with the platform structures la, lb that the connection between the tank system and the platform struc¬ tures may take place above the water level.

Claims

P a t e n t c l a i m s

1. Arrangement in connection with a submerged tank, especially for storing oil produced from an offshore oil reservoir, c h a r a c t e r i z e d i n the tank or the tank system (6; 6A, 6B) is brought together with one or more, preferably existing, offshore platform structures and constitutes the foundation for said platform structure(s) (la, lb).

2. Arrangement as claimed in claim 1, c h a r a c t e r i z e d i n that the tank or the tank system (6; 6A, 6B) which comprises bracing elements, devices and connections corresponding to the geometry or geometries of the platform(s) which are mounted on the tank (6; 6A, 6B), is possibly provided for simple demounting of said platform structures (la, lb).

3. Arrangement as claimed in claim 1 or 2, the arrangement comprising tanks which either can be filled with ballast water or with oil, or partially with ballast water and oil, c h a r a c t e r i z e d i n that the tanks or tank system houses so much ballast that the overall structure floats within fixed water line levels.

4. Arrangement as claimed in any of the claims 1-3, c h a r a c t e r i z e d i n that the tank or the tank system (6; 6A, 6B) comprises columns (2, 3, 7, 8) extending above the water surface and rendering to the arrangement an anti-sway stability which is approximately equal in all directions.

5. Arrangement as claimed in claim 1 or 4, c h a r a c t e r i z e d i n that the columns (7, 8) constitute a foundation for positioning equipment, are provided with access trunk to the tank or the tank system (6; 6A, 6B) and comprise storage volume for export of oil and gas .

6. Arrangement as claimed in any of the claims 1-5, c h a r a c t e r i z e d i n that the assembled plat¬ form structure(s) (la, lb) comprise equipment for drilling, completion, processing and export of oil and gas, as well as maintenance of wells, i.e. an "expanded" drilling platform.

7. Arrangement as claimed in any of the claims 1-6, c h a r a c t e r i z e d i n that the tank or tank system (6; 6A, 6B) has a dimension and a buoyancy capacity which during the assembly phase with the platform struc¬ tures (la, lb) allow for the submergence of the tank system below said platforms, and a subsequent raising with suffici¬ ent buoyancy for lifting the platform structures completely above the water surface.

8. Arrangement as claimed in claim 1, c h a r a c t e r i z e d i n that it comprises a tank system consisting of at least two separate units (6a, 6b).

9. Method for assembling preferably existing platform structures, c h a r a c t e r i z e d i n that apart from one or more platform structures (la, lb) there is pro¬ vided a tank system (6: 6a, 6b) which is submerged suffici¬ ently down below the platform structures (la, lb) in deep sea for the floating of said platform structures to their position of assembly, whereafter the tank system (6; 6a, 6b) together with the platform structures (la, lb) are raised to an appropriate level for the joining thereof.

10. Method as claimed in claim 9, c h a r a c t e r i z e d i n that the tank system (6; 6a, 6b) is raised so high together with the platform struc¬ tures (la, lb) that the joining between the tank system and the platform structures may take place above the water surface.