GB2231843A

GB2231843A - Platform recovery and re-use unit

Info

Publication number: GB2231843A
Application number: GB9007370A
Authority: GB
Inventors: Malcolm Lee
Original assignee: Individual
Current assignee: Individual
Priority date: 1989-04-03
Filing date: 1990-04-02
Publication date: 1990-11-28
Anticipated expiration: 2010-04-02
Also published as: GB8907432D0; AU5403490A; GB9007370D0; GB2231843B; WO1990012158A1

Abstract

A platform recovery and re-use unit as illustrated in Figs. 1, 2 and 3 comprising a series of buoyant cellular caissons (2) designed to be assembled as one complete caisson encircling a platform jacket (4) below the surface of the water and with provision for connection (5) of the unit to the jacket legs. Provision is made within the caissons (2) for moorings (1) and DP systems to control the lateral movement of the caisson with pumps and flood compartments to ballast or deballast the connected platform after controlled cutting or separation of the jacket legs. The equipment within the caisson is powered electrically and controlled via an umbilical cable to a remote location on the surface. During deballasting of the caisson and initial recovery of the platform additional buoyancy and stability is provided with temporary floats or hollow structural columns (6) connected to the caisson to the water surface.

Description

PLATFORM RECOVERY AND RE-USE UNIT This invention relates to an offshore platform recovery and re-use unit.

Offshore oil and gas production platforms are mainly supported by steel jacket structures secured to the sea-bed by piles. The platform is designed to allow development of the oil or gas product over a variable and usually indeterminate number of years. The development life of the field may be extended by secondary or tertiary systems of recovery dependant upon economical studies.

Present legislation requires that at the end of the useful or economical life of the platform i.e.

when no further production can or will take place, the owner or operator must remove the platform. This would be carried out using currently available technology and equipment such as large derrick barges removing sections of the platform to be transported inshore for further break-up. Eventually the support jacket itself would be removed in sections, leaving an agreed amount above sea-bed level.

This procedure would be extremely expensive and difficult, particularly as the operation is dependant on sea conditions. Apart from the possibility of recovering some of the items of equipment it is evident that the platform would be totally written off for scrap.

According to the present invention there is provided a buoyancy system comprising cellular caissons containing ballasting and de-ballasting equipment in addition to mooring, D.P. and positioning facilities, together with an arrangement to allow connection(s) to a platform jacket structure. The caisson structures are provided with guide and connection devices to allow the caisson units to act as one individual unit in the final position. As the connection to the jacket is below the water surface, forces due to wave and storm conditions will be minimised.

A specific embodiment of the invention will now be described by way of example with reference to the accompanying drawing in which: Fig 1 illustrates in section the position of the caisson with the connection to the jacket legs and the stabilising columns attached Fig 2 shows the individual caissons in their pre-connected position and finally in their assembled position in relation to the platform jacket structure Fig 3 shows in section the completed operation with the platform and the removal and recovery system de-ballasted from the cut-away section of the jacket.

Referring to the drawings, the buoyant cellular caisson units '2' are manouevered into position using the catenary anchors '1' and the D.P. system comprising computerised guidance and positioning. In addition, locating guides and connection devices '3' provide for assembly of the caisson '2' and to form one individual unit. As assembly of the caisson will be carried out below sea level, wave and storm effect and therefore forces exerted on the jacket from the caisson will be minimised. However, the positioning and anchoring system will be designed to control excursions of the caisson and provision is made for guides to absorb dynamic impact loads.

Following assembly of the caisson buoyancy system '2' sub-sea connections '5' are made to the legs of the jacket structure '4'. At this stage the caisson is attached to the jacket with lateral forces being limited by the positioning and anchoring system and vertical forces controlled with the caisson ballasting system. All power and control systems are supplied via umbilical cables to a remote barge or vessel or to the platform itself.

Depending on the size of the platform, additional temporary buoyancy may be required to the caisson structure during the disconnection of the platform from the remainder of the jacket.

This could be in the form of vertical hollow columns '6' projecting from the caisson through the sea water surface or floats tethered from the surface to the caisson. These are removed when the combined platform/caisson unit has achieved a stable draught.

Following connection to the jacket the combined platform and recovery and re-use system is disconnected from the remainder of jacket structure by controlled cutting of the legs at the same time de-ballasting of the caisson is carried out.

The now floating platform can be towed to a new location for use as floating production system or emplacement on the sea bed as a gravity production platform.

Claims

1. An offshore platform recovery and re-use unit comprising a cellular buoyant caisson of sufficient size to provide sufficient buoyancy and stability for removal, transportation and re-use of an existing offshore platform installation.

2. An offshore platform recovery and re-use unit according to claim 1 wherein the individual caissons can be positioned around the jacket and connected together to form one complete caisson buoyancy unit.

3. An offshore platform recovery and re-use unit according to claim 2 wherein the caissons (2) include winching and mooring equipment to enable controlled positioning of the caissons for assembly and subsequent positioning, anchoring and mooring of the complete system.

4. An offshore platform recovery and re-use unit according to any of previous claims wherein the caissons include dynamic positioning in conjunction with thruster systems to further control the positioning of the caissons and the complete system.

5. An offshore platform recovery and re-use unit according to any of previous claims wherein the caissons include a ballasting and deballasting system of flood compartments and pumps to control the vertical movements of the system.

6. An offshore platform recovery and re-use unit according to claims 3, 4 and 5 wherein the power supply and control to the equipment is provided via an umbilical cable(s) from a remote location on the surface of the sea.

7. An offshore platform recovery and re-use unit as claimed in claim 6 whereby additional buoyancy can be provided in the form of hollow structural columns (6) projecting from the caisson through the water surface or floats connected from the caisson to the water surface. These requirements normally only being necessary during disconnection from the supporting jacket and takeover of the platform weight.

8. An offshore platform recovery and re-use unit according to previous claims wherein the caisson is provided with guides and connection devices to allow attachment of the caisson to the jacket structure. The connection devices are designed to provide the principal connection between the caisson and the platform.

9. An offshore platform recovery and re-use unit as previously claimed whereby the connected caisson/platform assembly can be separated from the lower jacket support structure by use of controlled cutting of the jacket legs.

10. An offshore platform recovery and re-use unit as claimed in any previous claim whereby the combined platform and connected caisson can be towed to a new location for re-use as a floating production facility or after re-emplacement on the sea-bed in shallower, water, a fixed production facility.

11. An offshore platform recovery and re-use unit as claimed in claims 8 and 9 whereby the platform can be transported to an inshore location for modification or repair and subsequently returned and reconnected to the base of the jacket; the caisson buoyancy unit then being removed in the reverse of the sequence as previously described.