Hydraulic and earth engineering – Marine structure or fabrication thereof – Floatable to site and supported by marine floor
Reexamination Certificate
1997-11-13
2001-08-21
Bagnell, David (Department: 3673)
Hydraulic and earth engineering
Marine structure or fabrication thereof
Floatable to site and supported by marine floor
C405S207000, C405S203000, C114S264000
Reexamination Certificate
active
06276876
ABSTRACT:
The present invention relates to an apparatus and method for providing an offshore drilling operation for oil and gas in a sea particularly, though not exclusively, a shallow inland sea or lake.
By the term “shallow” we means that the sea depth varies between about 2 m and 10 m across the expanse of water. Such seas can have severe wave conditions. A typical such inland sea is the Caspian Sea.
While we have used the term “sea” throughout the specification it should be understood that the term “sea” also includes freshwater lakes and indeed any expanse of inland water.
It is known to explore for and produce oil and gas from shallow inland seas and lakes.
The normal methods involve the use of either a so-called swamp barge, alternatively a so-called jack-up rig, a tender assist drilling unit or the use of a platform based rig transported to a fixed platform and erected there to drill wells.
A swamp barge is a large flat bottomed vessel with a large surface area to draught ratio. As its name suggests it is mainly used in swampy areas where it is floated out to site with the drilling rig on board located on a raised deck. Drilling is conducted with the barge ballasted so that its bottom rests on the sea-bed. Operations are limited to water depths capable of floating the barge but not too deep to affect the raised deck.
A jack-up rig is a floating unit on which is mounted the drilling rig. The platform has retractable extending legs. With the legs retracted the platform is floated out on the surface of the sea to the site where drilling is to take place. When in position, the legs are extended to jack the platform up above the highest wave height expected and the legs are then temporarily fixed in position. As the legs must not foul the sea floor during the period when the platform is floated out to site, the minimum water depth for operations is restricted.
In general jack-up rigs are unsuitable for very shallow water.
A tender assist drilling unit is a barge supporting a drilling derrick and associated drilling equipment. The derrick is transferred onto a fixed platform to permit drilling supported by the services on the barge. This method is not used in very shallow water and is used in conjunction with permanent fixed platforms.
Wells may be drilled by the above methods in shallow water but each method is limited on the range of water depths accommodated.
Drilling may also be conducted on a fixed platform using a platform based rig erected on the platform for the purpose. The platform however must be large to accommodate the rig and is a permanent structure placed prior to the start of drilling which may be of uncertain outcome.
It is therefore an object of the present invention to provide an apparatus and method for providing an offshore drilling operation which overcomes the disadvantages of the above described prior art techniques.
According therefore to one aspect of the present invention, we provide apparatus for providing an offshore drilling operation in a sea, the apparatus comprising a drilling derrick support structure having a hollow cavity to enable the structure to be floated to the site where drilling is to take place and being adapted to receive ballast into the cavity in such a way and in a sufficient quantity as to cause the structure to sink base first and rest upon the sea-bed with the deck clear of the surface of the sea, a drilling derrick for mounting on the deck of the structure and a tender barge adapted for transport to the site in water depths down to 1½ m and to provide services and equipment to the drilling derrick.
According to a second aspect of the present invention in a method for providing an offshore drilling operation in a sea, the method comprises floating a buoyant drilling derrick support structure to the site where drilling is to take place, the structure having a base to rest on the sea floor and a deck spaced from the base sufficiently to support the derrick above sea-level when the base is resting on the sea-bed, ballasting the structure sufficiently to cause it to sink so that its base rests on the sea floor and its deck is clear of the surface of the sea, floating a drilling derrick out to the structure, positioning the derrick on the deck of the structure and floating a tender barge out to the site in water depths down to 1½ m to provide services and equipment to the drilling derrick.
Preferably the derrick support structure is a concrete gravity based structure and suitably the structure has a plurality of well conductors extending vertically through the structure to permit the passage of drill strings and production tubes therethrough.
The barge may be adapted for transport in water depths between 1½ m and 15 m.
The barge may have a so-called “key” slot which is a recess or niche in the bow of the vessel for receiving and engaging the derrick support structure during operations.
Suitably the barge has an inner casing of steel (as conventional) and an outer shell of a concrete based material. Such a shell protects the barge from the severe frosts and ice which occur in some areas of the world during winter, e.g. that area of Russia including the Caspian Sea.
Preferably at least one support barge is provided to link disengagably with the tender barge and suitably two such barges are provided.
Conveniently an accommodation barge is provided to link disengageably with the tender barge.
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Bone David
Heywood Christopher
Hollis Andrew Patrick
Bagnell David
BG plc
Holt William H.
Lagman Frederick L.
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