GB2088937A

GB2088937A - A Gravity Structure for Supporting an Off shore Work Deck

Info

Publication number: GB2088937A
Application number: GB8133465A
Authority: GB
Original assignee: Sea Tank Co SA
Current assignee: Sea Tank Co SA
Priority date: 1980-11-21
Filing date: 1981-11-06
Publication date: 1982-06-16
Also published as: GB2088937B; JPS57146814A; MX153987A; FR2494745B1; JPS5944448B2; NO813920L; FR2494745A1

Abstract

The height of the base (1) lies between 10 and 20 metres and is sufficiently small to allow the light draught floating structure to be towed from a port in medium-depth water and it is provided with buoyancy compartments (4) which allow it to be kept stable while the work deck (3) is being assembled and final submersion. Application to search for and produce oil out at sea. <IMAGE>

Description

SPECIFICATION A Gravity Structure for Supporting an Off-shore Work Deck The present invention relates to a gravity structure for supporting an off-shore work deck and to a method of providing it.

Background of the Invention Such a structure conventionally has a concrete base which rests on the sea bed and is topped by one or more columns intended to support a work deck in particular above an undersea oil field.

The base is mainly constituted by juxtaposing compartments whose walls are made of prestressed reinforced concrete and are fitted with cocks to allow water to be brought therein or removed therefrom and thus vary the draught of the floating structure. In a known way, it is considered by persons skilled in the art that for a total structure height lying between about 100 and 200 metres, the height of such a base must lie between about 35 and 80 metres so as to provide sufficient buoyancy and stability when the structure is being assembled or moved. Indeed, according to known laws of floating body equilibrium, the higher the base, the greater the stability of the structure.

When it is required to place the work deck on the column(s) of a known structure, the structure is towed to a sheltered assembly site, is progressively sunk into the water by progressive controlled and limited filling of the compartments of the base with water until a few metres of the top(s) of the column(s) project from the surface of the water, the work deck is then floated out over the structure which is then raised by emptying the water from the compartments so as to make the floating structure bear the weight of the work deck and release the floats.

The disadvantage 9f known structures is that they have to be constructed at a site near which the sea must be sufficiently deep, e.g. 40 to 60 metres, to allow the structure to be finished afloat and to be towed to the assembly site. This limits the number of sea shore sites which can be used for such construction.

The present invention aims to produce a gravity structure to support an off-shore work deck which has a wider option of construction sites and remains stable during its transportation afloat and during its submersion.

Summary of the Invention The invention provides a gravity structure for supporting an off-shore work deck, said structure including: a horizontal multicellular concrete base which forms at least four sealed buoyancy compartments equipped with cocks to allow water to be brought into or removed from said compartments so as to make the structure float or to submerge it while constantly controlling its stability; at least three buoyancy tanks of circular crosssection and whose axes are vertical, the buoyancy tankds being spaced out and rigidly fixed to the edge of the base and projecting above the roof thereof so as to keep the structure stable even when it supports the work deck during towing thereof and submersion thereof on its final site;; at least one column which is at least 80 metres high assembled on this base to allow a work deck to be supported above the water when the base lies on the sea bed; and wherein the height of the base is between 10 and 20 metres to float the structure in water of a depth which is likely to be found near sheltered sea-side construction sites.

Preferably, this structure has more than three buoyancy tanks whose inherent height is greater that that of the base and which are not so high that they emerge when the base finally rests on the sea bed.

Brief Description of the Drawings A non-limiting description with reference to the attached schematic figures is given hereinafter of how the invention can be put into effect. It must be understood that, without going beyond the scope of the invention, the components described and illustrated can be replaced by other components which perform the same technological functions when a component is illustrated in several figures, it bears the same reference symbol in all of them.

Figure 1 is a vertical cross-section through a structure in accordance with the invention during the first towing operation before assembly of the work deck.

Figure 2 is a plan view of the same structure.

Figure 3 is a vertical cross-section through the same structure at the time the work deck is assembled.

Figure 4 is a vertical cross-section of the structure on its final site.

Description of Preferred Embodiment The gravity structure described has a multicellular base (1) which is low relative to known gravity structures and has a greater area of horizontal cross-section. Its draught is therefore lighter and this makes it possible to construct it in a practically ordinary port installation instead of on a favourable site where the water must be deep in the immediate vicinity of the shore. The column(s) may be made of concrete, for example or may consist of steel structures in a triangular configuration.

It is necessary to be able to continuously control the submerging of such a weight structure and to keep it stable during towing and submerging and especially while the work deck (3) loaded with all its equipment is being installed on column(s) (2). Further, the permissible load at the head(s) of the column(s) must be increased to a maximum during the floating and submerging steps. That is why between four and eight frustoconical or cylindrical buoyancy tanks (4) whose axes are vertical, whose walls are thin and are made of reinforced concrete and which are closed at the top by domes or by thick slabs of reinforced concrete are constructed on the roof of the base into which they are sunk.Indeed, it has been found that installing such rigid buoyancy tanks whose height is much greater than that of the base itself and which are located on the edges of the base allows good stability in spite of the low height of the base, while sacrificing only very little of the advantage constituted by the reduction of the light draught.

However, to increase this stability, other means can be used such as buoyancy tanks which are not rigidly fixed to the base.

It must also be noted that rigid buoyancy tanks which are preferably used within the scope of the present invention can'economically be made capable of bearing high outside overpressure during the submersion steps due to their circular horizontal cross-section. These tanks thus make it possible to control submersion of the structure by varying the ballasting while the compartments of the base are completely filled. It is thus possible to make the pressure inside them equal to the pressure outside them by making them communicate openly with the surrounding water.

Therefore, they do not have to bear appreciable overpressure and can be constructed cheaply, using thin plane intercrossed concrete walls.

Taking into account the depositions described hereinabove, submersion of the structure inludes the following steps: 1. Filling the ballast compartments (B1, B2, B3, B4) of the multiceliular base 1 with water by opening the corresponding cocks which are grouped together in a submersion module installed at the bottom of (one of) the column(s); first compartments B1 and B3, then B2 and B4 are filled. At the end of this first step, the pressure inside the base is rapidly made equal to the pressure outside it once its roof is submerged; the buoyancy tanks and the column(s) keep the structure floating stably for towing towards a sheltered sea site where the work deck is to be installed on the structure (see Figure 1).

2. On arrival at the assembly site, the buoyancy tanks are partially filled with water so as to submerge the structure almost completely, leaving only a few metres in height thereof above the surface; each tank is ballasted independently through the submersion module so as to be able to correct the balance of the structure at any time The work deck (3), supported at each end by a float (6,7), is then towed out until it is above the column(s) of the structure. Controlled unbailasting of the buoyancy tanks makes the structure rise and, by simple Archimedian thrust, bear the load of the work deck, thus relieving the floats which previously supported it.

Afterfinal fixing of the work deck to the head(s) of the column(s) controlled unballasting of the buoyancy tanks is continued to bring the structure to the draught required for towing it towards its final location.

3. The third and last submersion step is carried out once the structure is correctly positioned at its final location; the structure is submerged in such a way as to firstly complete the filling of the buoyancy tanks. During this operation, the structure is laid on the sea bed and if its position is correct, the pressure in the buoyancy tanks is rapidly made equal to the outside pressure.

To finally balance the structure lying on the sea bed, the column(s) which support(s) the work deck out of the water is (are) filled.

The present invention therefore also provides a method of providing stable floating and continuously controlled submersion of an offshore gravity structure which has a concrete base (1) above which at least one column (2) rises for its upper portion to support a work deck (3) out of water, said work deck being loaded with all its apparatus and being installed at the top of the column(s) before the structure is towed to the final submersion site; said base having buoyancy compartments (B1, B2, B3, B4) at least three buoyancy tanks (4) rigidly attached to the upper portion of the base (1 ) and being able to withstand the outside hydraulic pressure until the pressure inside them is equal thereto on final submersion of the structure, the buoyancy compartments (B1, B2, B3, B4) being filled with water while the structure is being towed from its construction site to a sheltered sea site where it is to be assembled, the buoyancy tanks (4) then being partially filled with water to sink the structure in the water so as to allow the work deck brought out on floats to be placed over the top of the column, said tanks subsequently being partially emptied of water to make the structure rise to raise the work deck (3) and to release its floats (6,7), these tanks finally being filled with water to submerge the structure permanently.

The method in accordance with the invention thus makes it possible to construct a mediumdepth (100 to 200 metres) offshore structure entirely in dry dock, at construction sites where the depth of the sea vicinity of the shore is not exceptional.

Claims

1. A gravity structure for supporting an offshore work deck, said structure including: a horizontal multicellular concrete base which forms at least four sealed buoyancy compartments equipped with cocks to allow water to be brought into or removed from said compartments so as to make the structure float or to submerge it while constantly controllng its stability; at least three buoyancy tanks of circular crosssection and whose axes are vertical, the buoyancy tanks being spaced out and rigidly fixed to the edge of the base and projecting abovethe roof thereof so as to keep the structure stable even when its supports the work deck during towing thereof and submersion thereof on its final site; at least one column which is at least 80 metres high assembled on this base to allow a work deck to be supported above the water when the base lies on the sea bed; and wherein the height of the base is between 10 and 20 metres to float the structure in water of a depth which is likely to be found near sheltered sea-side construction sites.

2. A structure according to claim 1, wherein there are more than three buoyancy tanks whose inherent height is greater than that of the base and which are not so high that they emerge when the base finally rests on the sea bed.

3. A structure according to claim 2, wherein the column is hollow, made of prestressed reinforced concrete and fitted with cocks to allow water to be brought therein.

4. A method of providing stable floating and continously controlled submersion of an off-shore weight structure which has a concrete base above which at least one column rises for its upper portion to support a work deck out of water, said work deck being loaded with all its apparatus and being installed at the top of the column(s) before the structure is towed to the final submersion site; said base having buoyancy compartments, at least three buoyancy tanks rigidly attached to the upper portion bf the base and being, able to withstand the outside hydraulic pressure until the pressure inside them is equal thereto on final submersion of the structure, the buoyancy compartments being filled with water while the structure is being towed from its building site to a sheltered sea site where it is to be assembled, the buoyancy tanks then being partially filled with water to sink the structure in the water so as to allow the work deck brought out on floats to be placed over the top of the column, said tanks subsequently being partially emptied of water to make the structure rise to raise the work deck and to release its floats, these tanks finally being filled with water to submerge the structure permanently.

5. A method according to claim 5 of making a structure float stably and of controlling its submersion, wherein the column(s) is (are) hollow and made of prestressed reinforced concrete and wherein it is (they are) filled with water when the base is located resting on its final site.

6. An off-shore gravity structure substantially as herein described with reference to and as illustrated in the accompanying drawings.