GB1578003A

GB1578003A - Buoyancy assemblies for offshore platforms

Info

Publication number: GB1578003A
Application number: GB1855478A
Authority: GB
Original assignee: HEEREMA ENG SERVICE UK Ltd
Current assignee: HEEREMA ENG SERVICE UK Ltd
Priority date: 1978-05-09
Filing date: 1978-05-09
Publication date: 1980-10-29

Description

(54) IMPROVEMENTS IN OR RELATING TO BUOYANCY ASSEMBLIES FOR OFFSHORE PLATFORMS (71) We, HEEREMA ENGINEER ING SERVICE (U.K.) LIMITED, a British Company, of Henrietta House, 9 Henrietta Place, London, WIM 9AG., do hereby declare the invention, for which we pray that a patent may be granted to us and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to buoyancy assemblies for offshore platforms and, more particularly, to buoyancy assemblies which are temporarily attached to the legs of off-shore platforms either to enable the platform to be floated out on its side from the construction yard to the location where it is to be used, or to provide buoyancy for a platform when it is launched from a barge at the location at sea where it is to be installed to enable it to be maoeuvred into position.

The invention provides a buoyancy assembly for mounting on a leg of an off-shore platform comprising an elongate hollow tubular buoyancy tank, a first bracket which is adapted to engage the tank at or adjacent one end of the tank which, in use, is below water level when the platform is installed upright, the tank being releasable from the first bracket by movement in a direction longitudinally of the tank and a second bracket which is located at or adjacent the other end of the tank and, in use, is above the water level when the platform is installed upright, the second bracket being severable.

Preferably the first bracket comprises a sleeve which, in use, is welded to a leg of an off-shore platform and through which a lower part of the buoyancy tank extends for connection thereof to said leg of an offshore platform.

More specifically the sleeve may have a conical flange along the side thereof adjacent the second bracket to guide the end of the tank into the sleeve, the tank having a conical encircling ring to engage the conical flange and locate the tank with respect to the bracket.

In any of the above arrangements, the second bracket may comprise an arm welded to the tank and adapted, in use, to be welded to the leg of the platform to connect the upper end of the tank thereto and to be subsequently cut through when it is desired to release the tank from the leg.

The second bracket may be V-shaped, the apex of the V being connectable to the leg of the platform, and the end of one of the arms of the V being welded to said buoyancy tank, and the end of the other arm of the V being welded to a second buoyancy tank similar in construction to the firstmentioned buoyancy tank and disposed side-by-side therewith.

The following is a description of a specific embodiment of the invention, reference being made to the accompanying drawings in which: Figure 1 is an elevation view of part of a leg of an off-shore platform with a buoyancy tank attached: Figure 2 is a section on the line 2-2 of Figure 1 and showing the tank only; Figure 3 is a section on the line 3-3 of Figure 1; Figure 4 is a section on the line 4-4 of Figure 1; Figure 5 is a diagrammatic plan view of the leg of the platform with two buoyancy tanks spaced around the leg.

Off-shore platforms for use in the exploitation of oil wells beneath the sea bed usually comprise a main platform supported above sea level on a number of legs which are fixed by piles driven into the sea bed.

Such platforms are normally built with the pltform lying on one side at a coastal site and are towed to the site where the platform is to be installed either on a separate flotation craft or are floated by buoyancy tanks attached to the legs of the platform.

When the platform is at the required location, the buoyancy tanks of the raft or attached to the legs of the platform are flooded in a controlled- manner so that the platform is turned upright and placed on the sea bed at the required location all in one operation. The present invention is concerned with platforms which have buoyancy tanks secured to the legs of the platform either to provide the necessary buoyancy for the platform as it is floated out to its required site or, in the case where the platform is to be floated out on a barge to the location at sea where it is to be installed, for manoeuvring the platform to the required position.

Referring now to Figure 1 of the drawings, there is shown a leg 10 of an off-shore platform having an elongate generally cylindrical buoyancy tank indicated at 11 secured thereto.

The tank 11 has a heavy gauge steel wall which is re-enforced internally by annular T-section ribs 12 encircling the tank at locations spaced apart along the tank. The upper end of the tank is closed by an integral end wall 13 and spaced a short distance down the tank from the upper end wall there is a sleeve 14 mounted coaxially within the tank and having outwardly flared conical ends 15. The sleeve 14 serves as a pile guide during fixing of the platform in position as described later. Two further similar sleeves 16 are mounted at spaced locations down the tank coaxially with the tank. Each further sleeve 16 has outwardly flared conical ends 17 and also has a heavy rubber diaphragm 18 extending across the sleeve to form the tank 11 into separate chambers 19, 20 and 21. The lowermost end 22 of the tank is closed by a similar rubber diaphragm 23.Separate air supplies and valve controls are provided for filling the respective chambers with air and flooding the chambers as required when tilting the platform into its position on the site.

The general sea level when the platform is located in situ is indicated at 24 and the tank 11 is connected to the leg 10 of the platform towards its lower end below sea level by a sleeve 25 through which the tank extends secured by lugs 26 to the leg. The upper end of the sleeve 25 has an outwardly flared frusto conical seat 27 in which a frusto conical ring 28 encircling the tank 11 engages. Adjacent the upper end of the tank above sea level. the tank is secured to the leg 10 by a V-shaped bracket 29 as best seen in Figure 4 of the drawings. The apex of the bracket are welded to the leg 11 and the two arms of the bracket is welded to the leg 11 and the two arms of the bracket are welded to two buoyancy tanks 11 spaced around the leg. Figure 5 of the drawings shows the disposition of the two tanks around the leg 10.It will of course be understood that more tanks could be mounted around each leg to provide the buoyancy required.

After the tanks have been flooded and the platform installed in the upright position on the sea bed at the required location the upper part of each tank 11 is cut through above the sleeve 14 along a line of cut indicated at 30 on Figure 1. The tank can then provide a guide for a pile to be driven down the leg of the tank and through further spaced pile guides on the leg into the sea bed. The sleeves 14 and 16 within the tank serve to guide the nose of the pile through the tank, the nose being indicated diagrammatically partway down the tank at 31. The nose punches its own hole through the rubber diaphragms 18 and 23 as it passes down the tank.

Once the piles have been driven into place and the legs firmly anchored in their final positions to the piles, the tanks 11 can be removed from the legs. This is simply effected by cutting through the brackets 29 adjacent the upper ends of the legs and then lifting the tanks 11 out of the sleeves 25.

Thus no underwater cutting operation is required and the pile guides provided by the tanks are removed as well. The tanks can be re-used on further platforms provided that the piles to be driven are of a similar size.

WHAT WE CLAIM IS: 1. A buoyancy assembly for mounting on a leg of an off-shore platform comprising an elongate hollow tubular buoyancy tank, a first bracket which is adapted to engage the tank at or adjacent one end of the tank which, in use, is below water level when the platform is installed upright, the tank being releasable from the first bracket by movement in a direction longitudinally of the tank and a second bracket which is located at or adjacent the other end of the tank and, in use, is above the water level when the platform is installed upright, the second bracket being severable.

2. An assembly as claimed in claim 1 wherein the first bracket comprises a sleeve which. in use, is welded to a leg of an off-shore platform and through which a lower part of the buoyancy tank extends for connection thereof to said leg of an offshore platform.

3. An assembly as claimed in claim 2 wherein the sleeve has a conical flange along the side thereof adjacent the second bracket to guide the end of the tank into the sleeve and the tank has a conical encircling ring to engage the conical flange and locate the tank with respect to the bracket.

4. As assembly as claimed in any of the preceding claims wherein the second bracket comprises an arm welded to the tank and adapted, in use. to be welded to the leg of

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. tanks attached to the legs of the platform. When the platform is at the required location, the buoyancy tanks of the raft or attached to the legs of the platform are flooded in a controlled- manner so that the platform is turned upright and placed on the sea bed at the required location all in one operation. The present invention is concerned with platforms which have buoyancy tanks secured to the legs of the platform either to provide the necessary buoyancy for the platform as it is floated out to its required site or, in the case where the platform is to be floated out on a barge to the location at sea where it is to be installed, for manoeuvring the platform to the required position. Referring now to Figure 1 of the drawings, there is shown a leg 10 of an off-shore platform having an elongate generally cylindrical buoyancy tank indicated at 11 secured thereto. The tank 11 has a heavy gauge steel wall which is re-enforced internally by annular T-section ribs 12 encircling the tank at locations spaced apart along the tank. The upper end of the tank is closed by an integral end wall 13 and spaced a short distance down the tank from the upper end wall there is a sleeve 14 mounted coaxially within the tank and having outwardly flared conical ends 15. The sleeve 14 serves as a pile guide during fixing of the platform in position as described later. Two further similar sleeves 16 are mounted at spaced locations down the tank coaxially with the tank. Each further sleeve 16 has outwardly flared conical ends 17 and also has a heavy rubber diaphragm 18 extending across the sleeve to form the tank 11 into separate chambers 19, 20 and 21. The lowermost end 22 of the tank is closed by a similar rubber diaphragm 23.Separate air supplies and valve controls are provided for filling the respective chambers with air and flooding the chambers as required when tilting the platform into its position on the site. The general sea level when the platform is located in situ is indicated at 24 and the tank 11 is connected to the leg 10 of the platform towards its lower end below sea level by a sleeve 25 through which the tank extends secured by lugs 26 to the leg. The upper end of the sleeve 25 has an outwardly flared frusto conical seat 27 in which a frusto conical ring 28 encircling the tank 11 engages. Adjacent the upper end of the tank above sea level. the tank is secured to the leg 10 by a V-shaped bracket 29 as best seen in Figure 4 of the drawings. The apex of the bracket are welded to the leg 11 and the two arms of the bracket is welded to the leg 11 and the two arms of the bracket are welded to two buoyancy tanks 11 spaced around the leg. Figure 5 of the drawings shows the disposition of the two tanks around the leg 10.It will of course be understood that more tanks could be mounted around each leg to provide the buoyancy required. After the tanks have been flooded and the platform installed in the upright position on the sea bed at the required location the upper part of each tank 11 is cut through above the sleeve 14 along a line of cut indicated at 30 on Figure 1. The tank can then provide a guide for a pile to be driven down the leg of the tank and through further spaced pile guides on the leg into the sea bed. The sleeves 14 and 16 within the tank serve to guide the nose of the pile through the tank, the nose being indicated diagrammatically partway down the tank at 31. The nose punches its own hole through the rubber diaphragms 18 and 23 as it passes down the tank. Once the piles have been driven into place and the legs firmly anchored in their final positions to the piles, the tanks 11 can be removed from the legs. This is simply effected by cutting through the brackets 29 adjacent the upper ends of the legs and then lifting the tanks 11 out of the sleeves 25. Thus no underwater cutting operation is required and the pile guides provided by the tanks are removed as well. The tanks can be re-used on further platforms provided that the piles to be driven are of a similar size. WHAT WE CLAIM IS:

1. A buoyancy assembly for mounting on a leg of an off-shore platform comprising an elongate hollow tubular buoyancy tank, a first bracket which is adapted to engage the tank at or adjacent one end of the tank which, in use, is below water level when the platform is installed upright, the tank being releasable from the first bracket by movement in a direction longitudinally of the tank and a second bracket which is located at or adjacent the other end of the tank and, in use, is above the water level when the platform is installed upright, the second bracket being severable.

the platform to connect the upper end of the tank thereto and to be subsequentlv cut through when it is desired to release the tank from the leg.

5. An assembly as claimed in any of claims 1 to 4, wherein the tank also provides a hollow guide through which a pile may be passed to be driven into the sea bed.

6. An assembly as claimed in claim 5, wherein a plurality of guide sleeves are mounted at spaced locations along the tank to guide a pile through the tank.

7. An assembly as claimed in claim 6. in which the tank is divided into a number of separate chambers along the length of the tank separated by resilient diaphragms which are respectively mounted in at least some of the guide sleeves.

8. An assembly as claimed in any of claims 1 to 6. wherein the tank is divided into a plurality of separate chambers along the length of the tank separated by resilient diaphragms extending across the tank.

9. An assembly as claimed in any of claims 1 to 8. wherein the said second bracket is V-shaped. the apex of the V being connectable to the leg of the platform. and the end of one of the arms of the V being welded to said buovancv tank. and the end of the other arm oithe V being welded to a second buoyancy tank similar in construction to the first-mentioned buoyancy tank and disposed side-bv-side therewith.

10. A buoyancy assembly substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

11. An off-shore platform having a plurality of buoyancy assemblies mounted thereon, substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.