GB2148203A

GB2148203A - Semi-submersible marine platform

Info

Publication number: GB2148203A
Application number: GB08328518A
Authority: GB
Inventors: Alexander George Copson
Original assignee: Central Oil & Gas Ltd
Current assignee: Central Oil & Gas Ltd
Priority date: 1983-10-25
Filing date: 1983-10-25
Publication date: 1985-05-30
Also published as: GB8328518D0

Abstract

This invention provides a semi-submersible marine platform having a deck 10 supported over horizontal buoyant base members 20 by vertical support means 23, 24, 25, each support means being connected to at least one adjacent support means by a base member 20, any or all of the support means comprising a group of at least two close but separate essentially equivalent columns 23, 24, 25 and the intercolumn distances within groups being less than the inter-group distances. Preferably the intercolumn distances within groups are about 1 column diameter. <IMAGE>

Description

SPECIFICATION Semi-submersible marine platform The present invention relates to a semi-submersible marine platform (hereinafter referred to as a rig) and provides a rig having a deck supported over horizontal buoyant base members by vertical support means, each support means being connected to at least one adjacent support means by a base member, any or all of the support means comprising a group of at least two close but separate essentially equivalent columns, and inter-column distances within groups being less than the inter-group distances.

The buoyant base members for the rig may for example be of conventional hull type or they may be in the form of a hollow annular polygon.

Preferably the base members are compartmentalised. Also the columns of the vertical support means are preferably compartmentalised and provide some buoyancy for the rig.

When the distance between columns in a group is small a high drag may be induced by the passage of water between members of the group, e.g.

when towing at sea. Therefore, it is preferable to space the columns sufficiently to avoid high drag, e.g. by providing an inter-column separation of about 1 column diameter.

In conventional rigs of the prior art with single support columns at the apices of the deck, damage to and flooding of one vertical support column will leave one deck apex completely unsupported and may easily lead to loss of stability and capsize. In contrast in a rig according to the invention complete flooding (loss of buoyancy and cut water plane areas) of one column in a group will not lead to complete loss of buoyancy and cut water plane area in that position since there will be at least one other column close-by. Thus an improved safety factor and increased damage tolerance is provided by a rig according to the invention without the need to increase the total buoyancy and cut water plane area.

Three embodiments of the invention will now be described in detail by way of example with reference to the accompanying schematic drawings, in which: Figure 1 is a plan view of the base of a rig according to the invention; Figure 2 is a plan view of the deck of the Fig.1 rig; Figure 3 is a plan view of the base of another rig according to the invention.

Figure 4 is a side elevation view of another rig according to the invention; and Figure 5 is an end elevation view of the Fig.4 rig.

With reference to Figures 1 and 2, the rig has a hollow annular base 1 which is divided into buoy-ant compartments 2. The base is essentially square in shape. Near each apex of the base are means 4 for supporting the deck 10. The deck 10 is similarly square in shape and each support 4 is attached to the deck.

In this particular embodiment each support means 4 comprises a pair of essentially equivalent vertical columns 5,6. The columns 5,6 are compartmentalised and provide some buoyancy for the rig.

Each column 5 is connected to another column 6 by a base member 2. The distance between columns of a group 5,6 is about 1 column diameter whereas the distance between adjacent groups is about 5 column diameters.

In Figure 2 the deck 10 is shown with a grid of cross-bracing members 11 to support a high deck payload e.g. equipment for floating production of oil and/or drilling.

In Figure 3 the rig has a base comprising twin hulls 20 interconnected by cross-bracing 21.

The deck outline 22 is shown by a dotted line and, in this example the deck 22 is supported at its apices by groups of three vertical columns 23,24,25. The intercolumn spacing within the groups is about 1 column diameter.

In Figures 4 and 5 another rig according to the invention has a deck 22 supported over twin hulls 20 interconnected by cross-bracing 21. In this embodiment the support means are four groups each of two vertical columns 30,31. These figures also show various equipment which may commonly be found on conventional semi-submersible marine platforms and may also be found on those according to the invention e.g. a derrick 40, helipad 41, deck crane 42, anchor chains 43 and thrusters 44.

The high redundancy safety factor of this rig may make it particularly suitable for use in exploration drilling in locations where bad weather is to be expected.

Whilst the invention has been exemplified with reference to a rig with a hollow annular base and paired columns and with reference to twin-hulled rigs with columns in groups of tow and three, other semi-submersible marine platform designs having other numbers of grouped columns are envisaged within the scope of the invention.

Whilst rigs have been exemplified having an inter column separation within groups of about 1 column diameter, greater and lesser separations are considered within the scope of the invention.

1. A semi-submersibie marine platform having a deck supported over horizontal buoyant base members by vertical support means, each support means being connected to at least one adjacent support means by a base member, any or all of the support means comprising a group of at least two close but separate essentially equivalent columns and the intercolumn distances within groups being less than the inter-group distances.

2. A semi-submersible marine platform according to claim 1 wherein the intercolumn distances within the groups are about 1 column diameter.

3. A semi-submersible marine platform according to claim 1 or 2 wherein the base members form a hollow annular polygonal base.

4. A semi-submersible marine platform according to claim 3 wherein the support means are at or near apices of the polygon.

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Semi-submersible marine platform The present invention relates to a semi-submersible marine platform (hereinafter referred to as a rig) and provides a rig having a deck supported over horizontal buoyant base members by vertical support means, each support means being connected to at least one adjacent support means by a base member, any or all of the support means comprising a group of at least two close but separate essentially equivalent columns, and inter-column distances within groups being less than the inter-group distances. The buoyant base members for the rig may for example be of conventional hull type or they may be in the form of a hollow annular polygon. Preferably the base members are compartmentalised. Also the columns of the vertical support means are preferably compartmentalised and provide some buoyancy for the rig. When the distance between columns in a group is small a high drag may be induced by the passage of water between members of the group, e.g. when towing at sea. Therefore, it is preferable to space the columns sufficiently to avoid high drag, e.g. by providing an inter-column separation of about 1 column diameter. In conventional rigs of the prior art with single support columns at the apices of the deck, damage to and flooding of one vertical support column will leave one deck apex completely unsupported and may easily lead to loss of stability and capsize. In contrast in a rig according to the invention complete flooding (loss of buoyancy and cut water plane areas) of one column in a group will not lead to complete loss of buoyancy and cut water plane area in that position since there will be at least one other column close-by. Thus an improved safety factor and increased damage tolerance is provided by a rig according to the invention without the need to increase the total buoyancy and cut water plane area. Three embodiments of the invention will now be described in detail by way of example with reference to the accompanying schematic drawings, in which: Figure 1 is a plan view of the base of a rig according to the invention; Figure 2 is a plan view of the deck of the Fig.1 rig; Figure 3 is a plan view of the base of another rig according to the invention. Figure 4 is a side elevation view of another rig according to the invention; and Figure 5 is an end elevation view of the Fig.4 rig. With reference to Figures 1 and 2, the rig has a hollow annular base 1 which is divided into buoy-ant compartments 2. The base is essentially square in shape. Near each apex of the base are means 4 for supporting the deck 10. The deck 10 is similarly square in shape and each support 4 is attached to the deck. In this particular embodiment each support means 4 comprises a pair of essentially equivalent vertical columns 5,6. The columns 5,6 are compartmentalised and provide some buoyancy for the rig. Each column 5 is connected to another column 6 by a base member 2. The distance between columns of a group 5,6 is about 1 column diameter whereas the distance between adjacent groups is about 5 column diameters. In Figure 2 the deck 10 is shown with a grid of cross-bracing members 11 to support a high deck payload e.g. equipment for floating production of oil and/or drilling. In Figure 3 the rig has a base comprising twin hulls 20 interconnected by cross-bracing 21. The deck outline 22 is shown by a dotted line and, in this example the deck 22 is supported at its apices by groups of three vertical columns 23,24,25. The intercolumn spacing within the groups is about 1 column diameter. In Figures 4 and 5 another rig according to the invention has a deck 22 supported over twin hulls 20 interconnected by cross-bracing 21. In this embodiment the support means are four groups each of two vertical columns 30,31. These figures also show various equipment which may commonly be found on conventional semi-submersible marine platforms and may also be found on those according to the invention e.g. a derrick 40, helipad 41, deck crane 42, anchor chains 43 and thrusters 44. The high redundancy safety factor of this rig may make it particularly suitable for use in exploration drilling in locations where bad weather is to be expected. Whilst the invention has been exemplified with reference to a rig with a hollow annular base and paired columns and with reference to twin-hulled rigs with columns in groups of tow and three, other semi-submersible marine platform designs having other numbers of grouped columns are envisaged within the scope of the invention. Whilst rigs have been exemplified having an inter column separation within groups of about 1 column diameter, greater and lesser separations are considered within the scope of the invention. CLAIMS

5. A semi-submersible marine platform according to claim 1 or 2 wherein the base members include a pair of elongate buoyant hulls.

6. A semi-submersible marine platform according to any preceding claim wherein each support means comprises two or three columns.

7. A semi-submersible marine platform substantially as hereinbefore described with reference to Figures 1 and 2 of the accompanying drawings.

8. A semi-submersible marine platform substantially as hereinbefore described with reference to Figure 3 of the accompanying drawings.

9. A semi-submersible marine platform substantially as hereinbefore described with reference to Figures 4 and 5 of the accompanying drawings.