GB2268954A

GB2268954A - Flexible membrane support for jack-up rigs

Info

Publication number: GB2268954A
Application number: GB9215792A
Authority: GB
Inventors: James William Bunce; Philip John George
Original assignee: Individual
Current assignee: Individual
Priority date: 1992-07-24
Filing date: 1992-07-24
Publication date: 1994-01-26
Also published as: GB9215792D0

Abstract

A seabed support system far jack-up rigs comprises one or more flexible membranes 3 laid on the seabed between the rig legs beneath the spud cans 2. Said membranes 3 displace into the seabed under the action of the leg loads and help to disperse said leg loads over a large area and so reduce the leg penetration depth. The membranes may comprise cables running in the direction of maximum tensile stress, interwoven with textile material. <IMAGE>

Description

SEA BED SUPPORTS FOR JACK-UP RIGS Jack-up rigs as used in offshore drilling and construction operations frequently sit on very soft sea beds, and the support legs have to penetrate a long way into the ground to develop sufficient bearing capacity under the feet (termed 'spud cans') to support the weight of the rig.

A number of problems occur in these circumstances. Main concerns are: - large surface craters are formed in the sea bed which may disturb existing structure foundations, or impede installation of new structures; - leg extraction may be exceedingly difficult and time consuming, thus delaying the rig from moving off station.

In addition, it sometimes happens that thin layers of hard soil are encountered during leg penetration which fail unexpectedly, so the spud cans suddenly punch through to the underlying soft soil out of control. This situation is very dangerous and may result in loss of the rig.

The present invention relates to a method of controlling the penetration of jack-up rig legs in soft soils, and of reducing the risk of punch through, by improvising a sea bed support system to spread the leg loads over a large bearing area.

Thus, according to this invention, one or more flexible membranes (reinforced as necessary) are laid on the sea bed beneath the spud cans between the legs of the rig. As the spud cans bear onto the membranes they deflect downwards into the sea bed and compress the underlying soil. The resulting upwards acting bearing pressures over the membrane surfaces are resisted by tensile stresses in the membranes - like surface tension effects in a liquid - which in turn help to support the leg forces and thus reduce the depth of penetration.

, , invention will now be described by way of example only and with reference to Figures 1 and 2 of the accompanying drawings which show general views of a jack-up rig and an embodiment of the sea bed support system.

Figure 1 shows an elevation of the jack-up rig 1 in its operating configuration.

Figure 2 shows a plan view on the spud cans 2 of the rig.

Referring to Figures 1 and 2, the support system comprises one or more flexible membranes 3 laid on the sea bed beneath the spud cans 2. Said membranes 3 displace into the sea bed under the action of the leg loads as shown in Figure 1, and become subject to tensile stresses due to the ground bearing pressures acting upwards over the membrane surfaces.

These actions of said membranes help to disperse said leg loads over a large area of the sea bed, and so reduce the leg penetration depth and the risk of accidental punch through.

Said membranes may be constructed from any sufficiently strong durable material. By way of example only, membranes may be made out of cables 4 running in the direction of maximum tensile stress with interwoven textile strops 5 to support the ground bearing pressures between the lines of cables, as shown in Figure 2.

Said membranes may be of any desired shape and may be made up out of smaller patches joined together to form a large area.

Said membranes may be deployed on the sea bed and held in place by weights or anchors before the rig comes on station, or may be deployed by the rig as part of the leg lowering operation.

If necessary, said membranes may be attached to the rig before lowering to the sea bed by any suitable joining means, for example by shackles attached to padeyes fixed to the spud cans.

Claims

.. CLAIMS

1. A seabed support system for jack-up rigs comprising one or more flexible membranes laid on the seabed between the rig legs beneath the spud cans such that tensile stresses developed in said membranes disperse said leg loads over a large area of the seabed.

2. A sea bed support system for jack-up rigs as claimed in Claim 1 wherein membranes are made out of cables running in the direction of maximum tensile stress with interwoven textile strops to support the ground bearing pressures between lines of cables.

3. A sea bed support system for jack-up rigs as claimed in Claims 1 and 2 wherein membranes are made up of small patches joined together to form a larger area.

4. A sea bed support system for jack-up rigs as claimed in Claims 1 to 3 wherein membranes are deployed by the rig as part of the leg lowering operation.

5. A sea bed support system for jack-up rigs as claimed in Claims 1 to 3 wherein membranes are held in place on the seabed by anchors before the rig comes on station.

6. A sea bed support system for jack-up rigs as claimed in Claims 1 to 4 wherein membranes are attached to the rig by suitable joining means such as shackles connected to padeyes on the spud cans.

7. A seabed support system for jack-up rigs substantially as described herein with reference to Figures 1 - 2 of the accompanying drawing.