GB2102481A - A gravity structure with detachable buoyancy chambers to port an offshore work platform - Google Patents

Abstract

A gravity structure to support an offshore work platform has buoyancy chambers (4) which are made of concrete and are frusto-conical. They are detachable when the structure is in its final location and are applied against the roof (38) of the base (1) not only by temporary fixing components (15) but also by the difference in the pressure maintained during transport and submersion operations between firstly the sea in contact with the frustoconical walls (32) of these chambers and secondly an intermediate chamber (10) which communicates with the atmosphere and is provided between the roof of the base and the bottom (36) of each buoyancy chamber. Application to extracting oil from shallow seas. <IMAGE>

Description

SPECIFICATION A gravity structure with detachable buoyancy chambers to support an offshore work platform The present invention relates to a gravity structure having buoyancy chambers and for supporting an offshore work platform.

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

The main components of the base are juxtaposed compartments whose walls are made of reinforced prestressed concrete and which are equipped with cocks to allow water to be let in or removed and thus to vary the draught of the structure when floating.

It is estimated that for a structure of total height between about 100 and 200 metres, the height of such a base must be between about 35 and 80 metres so that buoyancy and stability are sufficient when the structure is assembled or moved. Indeed, in accordance with known laws of floating body equilibrium, the taller the floating base, the greater the stability of the structure. This is due to the heavy weight and the great height of the superstructure.

When the work platform is to be placed on the column(s) of known structures, the structure is towed to a sheltered assembly site, is submerged under water by progressively letting a controlled and limited quantity of water into the compartments of the base until only a few metres at the top of each column projects above the water, the platform is brought over the structure on floats and is raised by removing water from said compartments so as to make the floating structure bear the weight of the platform and thereby relieve the floats.

The disadvantage of conventional gravity structures is that they have to be built at a site where the adjacent sea is deep enough to enable the structure to be completed while it is afloat and then towed to its assembly site. Suitable depths are around 40 to 60 metres. This limits the number of coastal sites which can be used for such construction.

That is why proposals have been made to poduce low structures and to provide stability by adding buoyancy chambers which protrude from the roof of the base; the tops of these chambers are above water level while the structure is being transported.

In contrast, after final installation, the structure is submerged to a sufficient depth for it not to be subjected to the effects of swell.

However, when the final working site of the structure is in relatively shallow water, and the load at the top of the structure during transport is heavy, it appears that the height of the buoyancy chambers required to maintain stability during transport plus the height of the base, is close to or greater than the depth of the water at the final work site. in such case, the effect of the swell on the upper portions of the buoyancy chambers compromises that stability of the structure on the sea bed after final setting in position of said structure.

Preferred embodiments of the present invention allow a gravity structure to be built at sites where the water is shallow while still poviding the necessary stability both when under tow, even with a heary top load and after installation on the final work site, even if the water is relatively shallow.

The present invention povides a gravity structure with detachable buoyancy chambers to transport an offshore work platform to a final work site and then to support it on said site by resting on the sea bed, said gravity structure comprising - a horizontal concrete multicellular base which forms at least four seaied ballast compartments equipped with ballasting and deballasting systems to allow water to be brought into or removed from said compartments so as to make the structure float or to submerge it while controlling its stability constantly; - at least one column mounted on said base to support a work platform above the water when said base rests on the sea bed; and - at least three vertical closed buoyancy chambers of substantially circular cross-section, said chambers being spaced out around the edge of the base being rigidly fixed thereto and projecting above the roof thereof so as to stabilize the structure when it supports the work platform during towing and during submersion of the structure on its final work site; - wherein said gravity structure comprises a circular spacer ring around the edge of the base of each bouyancy chamber and disposed between the bottom of said chamber and the roof of the base so as to form an intermediate chamber there; - the buoyancy chamber bearing on the base via said spacer ring and freely allowing said chamber to be fixed to said base at a separation surface; - a sealing ring being disposed on said separation surface to prevent water entering the intermediate chamber from the outside;; - the bouyancy chambers tapering upwardly; - a vent pipe making the submerged intermediate chamber communicate with the outside atmosphere and keeping it at atmospheric pressure so as to apply a vertical suction force to the base of the buoyancy chamber, said force resulting from the vertical component of the pressure of the water on the inclined walls and on the ceiling of said chamber, if any; - a temporary connection system being disposed between the buoyancy chamber and the base to make the base of said chamber fast with the roof of the base during the transport and submersion operations and then to allow said chamber to be removed.

An embodiment of the invention is described by way of example with reference to the accompanying drawings, in which : Figure 1 is a vertical cross-section of a gravity structure in accordance with the invention during a first towing operation before the work platform is assembled thereon.

Figure2 is a plan of said structure.

Figure 3 is a vertical cross-section of said structure while a platform is being assembled thereon.

Figure 4 is a vertical cross-section of the structure on its final work site after being submerged and installed on the sea bed.

Figure 5 is a vertical cross-section of the structure on its final operation site after separation and removal of the buoyancy chambers.

Figure 6 is a detail A of Figure 1 shown to a larger scale.

The gravity structure described has a multi-cellular base 1 which is less tall thank known structures and whose horizontal cross-section occupies a greater area. Its unladen draught is therefore much shallower, which makes it possible to build it in practically any port area instead of requiring a priviledged site where the water is deep in the immediate neighbourhood of the shore. Columns such as 2 may be made of concrete, for example, or may be constituted by triangular steel structures.

It is necessary to be able to continuously control the submersion of such a gravity structure and to provide for its stability during towing and submersion operations and especially during the installation of the work platform loaded with all its equipment 3 (see Figure 3) on the column(s) 2. Further, it is necessary to increase as much as possible the permissible load at the head(s) of the column(s) during the floating and submersion operations. That is why three to eight frusto-conical buoyancy chambers 4 are built on the roof of the base and fixed thereto. The buoyancy chambers have vertical axes and thin walls 32 made of reinforced concrete. They are closed at the top by ceilings 34 made of domes or thick slabs of reinforced concrete and they are closed at the bottom by reinforced concrete flooring slabs.

It has been found that equipping the gravity structure with such rigid buoyancy chambers which are very much taller than the base itself and which are situated on the edges thereof ensures good stability to be obtained despite the fact that the base is low.

Only a small part of the advantage of small unladen draft is forefetted thereby.

An intermediate chamber 10 delimited by a spacer ring 11 which extends the wall 32 of the buoyancy chamber is provided under each buoyancy chamber, between the roof 38 of the base and the slab 36 which closes the bottom of the chamber. The ballast chamber rests by means of said spacer ring on the roof of the base. The contact surfaces 12 between the buoyancy chambers 4 and the base are provided with sealing ringa 13 which prevent water from entering the intermediate chamber.

A concrete ring 14 moulded on the roof of the base 1 and made integral therewith is disposed around the spacer ring 11. It constitutes a stop which bears the horizontal forces at the base of the buoyancy chamber. Further, a temporary connection system 15 which may be either active or passive holds the buoyancy chamber 4 fast to the roof of the base 1.

The connection system 15 may be constituted e.g. by steel tie rods spaced out around the ring 11 and bearing against the inside of the ring 14.

A vent pipe 16 puts the intermediate chamber 10 into communication with the outside atmosphere to keep it at atmospheric pressure.

In the base 1, special compartments Al, A2, A3 and A4 placed directly under the control of the buoyancy chambers 4 are isolated from the other ballast compartments B1, B2, B3 and B4 and the roofs of these special compartments are provided with openings 17 which said special compartments into communication with the intermediate chambers 10.

It must also be observed that the rigid buoyancy chambers 4 can be made to withstand high outside pressure during the submersion operations relatively cheaply due to the circular shape of their horizontal cross-section. These chambers thus allow the submersion of the structure to be controlled by varying the ballast, while the compartments of the base other than the special compartments Al, A2, A3 and A4 are completely filled. This makes it possible to make the pressure inside the ballast compartments equal to that outside them, i.e. to make them communicate openly with the outside. They do not therefore have to withstand significant overpressure and can be built cheaply using crossed concrete partitions.

Each buoyancy chamber 4 has a water ballast control system by which the water level insde each chamber is controlled during the operations of transporting an submerging the gravity structure.

Each compartment A has a pipe 42 connected to an immersion module 40 equipped with remotecontrolled pumps and cocks to allow water to be pumped in and out.

Considering the above-described dispositions, submersion of the structure comprises the following steps: 1/The ballasting compartments B1, B2, B3 and B4 in the multicellular base 1 are filled with water by opening the corresponding cocks which are grouped in the submersion modules 40 installed at the bottoms of the columns; firstly, compartments B1 and B3, and then compartments B2 and B4 are filled.

At the end of said first step, the pressure inside the base and the pressure outside the base are rapidly equalised after the roof of the base has submerged; the buoyancy chambers and the column(s) maintain the buoyancy and stability of the structure while it is being towed to a sheltered maritime construction side where the work platform is to be installed on the structure.

In said configuration, the special compartments Al, A2, A3 and A4 situated adjacent the buoyancy chambers may be partially filled with water, perhaps in order to ensure that the structure is vertical (see Figure 1).

2/ After arrival at the assembly site, the buoyancy chambers 4 are partially filled with water so as to submerge the structure almost completely, to within a few metres of its top; ballasting of each chamber is controlled independently via the submersion modules so that any list of the structure can be corrected at any moment. The buoyancy chambers held on the roof of the casing by the vent pipe 16 which keeps the intermediate chamber 10 at atmospheric pressure, and any possible leakage past the sealing ring 13 can be easily compensated via the deballasting system for the special compartments Al, A2, A3 and A4 connected to the submersion module.

The work platform 3 is supported at each end by a float 6, 7, is then brought over the column(s) of the structure. By simple Archimedean thrust, controlled deballasting of the buoyancy chambers makes the structure rise and bear the load of the work platform, thus relieving the floats which supported it previously.

After finally fixing the work platform onto the top(s) of the column(s) controlled deballasting of the buoyancy chambers us continued to bring the structure to the draught required for towing it to its final site.

3/The third and last submersion step is carried out once the structure is correctly positioned over its final site; said submersion is affected by continuing to fill the buoyancy chambers 4. Once the base 1 is laid on the sea bed and provided its position is correct, the platform is temporarily supported by completely filling the buoyancy chambers (see Figure 4).

4/ Final location ends by removing the buoyancy chambers 4. To do this, the temporary connection systems 15 between the base and the buoyancy chambers are released, either automatically or by divers. Then the special compartments Al, A2, A3 and A4 are opened to let sea water in. This has the effect of making the pressure in the intermediate chamber 10 equal to that of the surrounding medium. The buoyancy chambers 4 are then fixed to the base 1 only by the effect of their weight which can be reduced by lowering the water level inside these chambers which are then hoisted to the surface then emptied to float naturally and be towed towards the nearest sheltered location.

5/ To seat the structure finally on the sea bed, the column(s) which support(s) the work platform out of the water is (are) then filled.

In short, the invention therefore allows an offshore gravity structure having a concrete base 1 surmounted by at least one column 2 to float stably and to have its submersion continuously controlled, the upper portion of said column supporting a work platform 3 loaded with all its equipment and installed at the top(s) of the column(s) out of the water before the final structure is towed to the final submersion site, said base having ballast compartments B1, B2, B3, B4, Al, A2, A3, A4, at least three bouyancy chambers 4 which are rigidly attached to the upper portion of the base 1 and are able to resist the outside water pressure until they are removed after final submersion of the structure.

For this purpose, while the compartment Al, A2, A3 and A4 are partially filled with water and kept at atmospheric pressure, the ballast compartment B1, B2, B3, B4 are filled with water for the structure to be towed from its work site to a sheltered maritime construction site. The buoyancy chambers 4 are then partially filled with water to sink the structure lower into the water so as to allow the work platform to be brought on floats and placed on the top of the column. Some of the water is then emptied out of these chambers to make the structure float higher in the water and thereby raise the platform 3 and relieve the floats 6, 7. These chambers are then filled with water to submerge the structure to its final depth and then they are detached from the base 1 by filling the compartments Al, A2, A3, A4. Lastly the chambers 4 are partially emptied, removed and towed away from the site.

The invention therefore makes it possible to build offshore structures for medium depths (100 to 200 metres) in dry docks on construction sites where the sea adjacent to the construction site is not exceptionally deep.

Claims (6)

1. A gravity structure with detachable buoyancy chambers, to transport an offshore work platform to a final work site and then to support it on said site by resting on the sea bed, said gravity structure com prising - a horizontal concrete multicellular base which forms at least four sealed ballast compartments equipped with ballasting and deballasting systems to allow water to be brought into or removed from said compartments so as to make the structure float or to submerge it while controlling its stability constantly; - at least one column mounted on said base to support a work platform above the water when said base rests on the sea bed; and - at least three vertical closed buoyancy chambers of substantially circular cross-section, said chambers being spaced out around the edge of the base, being rigidly fixed thereto, and projecting aove the roof thereto so as to stabilize the structure when it supports the work platform during towing and during submersion of the structure on its final work site; - wherein said gravity structure comprises a circular spacer ring around the edge of the base of each buoyancy chamber and disposed between the bottom of said chamber and the roof of the base so as to form an intermediate chamber there; - the buoyancy chamber bearing on the base via said spacer ring and freely allowing said chamber to be fixed to said base at a separation surface; - a sealing ring being disposed on said separation surface to prevent water entering the intermediate chamber from the outside;; - the buoyancy chambers tapering decreasing upwardly: - a vent pipe making the submerged intermediate chamber communicate with the outside atmosphere and keeping it at atmospheric pressure so as to apply a vertical suction force to the base of the buoyancy chamber, said force resulting from the vertical component of the pressure of the water on the inclined walls and on the ceiling of said chamber, if any; - a temporary connection system being disposed between the buoyancy chamber and the base to make the base of said chamber fast with the roof of the base during the transport and submersion operations and then to allow said chamber to be removed.

2. A gravity structure according to claim 1, wherein beneath each intermediate chamber, the base has a special compartment which is isolated from the other compartments, the roof of said special compartment having at least one opening to make said compartment communicate with the intermediate chamber above it and thus to provide equal pressure to both surfaces of said roof.

3. A gravity structure according to any preceding claim, wherein the spacer ring is partially constituted buy a downward extension of the concrete side wall of the buoyancy chamber, the sealing ring being placed under said extension.

4. A gravity structure according to claim 3, wherein said roof of the base has a lateral holding ring which surrounds the spacer ring.

5. A gravity structure according to any preceding claim, including a ballasting and deballasting system which makes it possible to ballast and deballast each buoyancy chamber and each special compartment.

6. A gravity structure substantially as herein described with reference to and as illustrated in the accompanying drawings.