TLP and detachable derrick vessel

• US 5,542,783 A
• Filed: 12/14/1994
• Issued: 08/06/1996
• Est. Priority Date: 12/14/1994
• Status: Expired due to Term

• Current Assignee: SBM Atlantia Incorporated (SBM Offshore NV)
• Original Assignee: Imodco Inc.
• Inventors: Pollack, Jack
• Primary Examiner(s): Graysay, Tamara L.
• Assistant Examiner(s): LAGMAN, FREDERICK LYNDON
• Application Number:
  US08/355,609
• Time in Patent Office: 601 Days
• Field of Search: 405/223.1, 405/224, 405/209, 166/350, 166/358, 166/359, 166/367, 166/352
• US Class Current: 405/223.1
• CPC Class Codes: B63B 1/107 Semi-submersibles; Small waterline area multiple hull vessels and the like, e.g. SWATH
• CPC Class Codes: B63B 21/502 by means of tension legs
• CPC Class Codes: B63B 35/003 for transporting very large loads, e.g. offshore structure modules construction methods for artificial islands mounted on piles E02B17/00; for transporting marine vessels B63B35/40; docks B63C1/00
• CPC Class Codes: B63B 77/00 Transporting or installing offshore structures on site using buoyancy forces, e.g. using semi-submersible barges, ballasting the structure or transporting of oil-and-gas platforms

Abstract

An offshore production system of the type that includes a TLP (tension leg platform) (12) and a derrick vessel (14) which moves to the TLP platform (16) whenever the derrick is required. The vessel carries fastener assemblies (131) that rigidly fix the vessel to the platform so they move vertically and horizontally as a single unit, which avoids any need to separately anchor the vessel and which facilitates operation of the derrick in more adverse weather. The vessel preferably has thruster equipment (88) which not only allows it to self-propel itself to the platform, but which also allows the vessel to propel itself and the platform sidewardly, to avoid drift of the platform during drilling. The vessel includes a vessel deck (70) which lies above the platform and a pair of vessel sides (72, 74) with pontoons (76, 78), that lie on opposite sides of the platform. After the vessel moves to a position around the platform and is fixed to the platform, the buoyancy of the vessel is increased to add tension to the TLP tendons.

Claims

1. An offshore production system comprising a TLP which includes a platform that has first and second opposite sides and that is constructed to float near the sea surface and which has a platform deck, wherein said platform has a plurality of tendon holders constructed to hold tendons that extend down to the sea floor to anchor the platform and a plurality of riser holders constructed to hold risers that extend down to the sea floor to carry fluid, and which also includes a service vessel which has first and second opposite sides and which has a vessel deck and a derrick on said vessel deck wherein said vessel is constructed to move to a position astride said platform wherein said vessel deck and said derrick thereon lie over said platform deck with a portion of said first side of said vessel lying beyond said first side of said platform and a portion of said second side of said vessel lying beyond said second side of said platform, characterized by:

a plurality of fastener assemblies coupled to said platform and said vessel, said fastener assemblies being constructed to fix said first sides of said platform and said vessel together and to fix said second sides of said platform and said vessel together;

said platform and said vessel being sufficiently rigidly fixed at each of their sides against relative vertical movement that said vessel and said platform move vertically together in waves.

2. The system described in claim 1 wherein:

said vessel is free of anchoring to said sea floor except through said platform.

3. The system described in claim 1 wherein:

said vessel has a pair of pontoons for lying on opposite sides of said platform, and wherein;

said pontoons of said vessel have floodable chambers and said vessel has water and air flowing equipment that is operable when said vessel is fixed to said platform, to flow air into said chambers to pump out water and thereby increase the buoyancy of said vessel and increase tendon tension.

4. The system described in claim 1 including:

a plurality of platform devices, said platform and said platform devices each anchored to the sea floor;

said platform and said plurality of platform devices all being substantially identical so said vessel can move to and fix itself to any one of said platform devices in addition to said platform.

5. An offshore production system comprising:

a TLP which includes a platform floating near the sea surface and a plurality of tendon holders for holding tendons that can extend down from said platform to the sea floor;

a service vessel which has a hull, a deck, and a derrick mounted on said deck;

a plurality of fastener assemblies that rigidly fix said platform to said vessel so they move together both vertically and horizontally in waves, with said derrick lying over said platform.

6. A method for use with a TLP that includes a platform floating at the sea surface and that also includes a plurality of tendons extending from the platform down to the sea floor, where a derrick is not mounted on said TLP but where a derrick is mounted on a vessel that has opposite sides and a space between said sides, comprising:

moving said vessel with said derrick thereon to a position astride said platform;

rigidly fixing said vessel to said platform, so they move in unison in both vertical and horizontal directions.

7. The method described in claim 6 wherein said platform has a top and has a width, and said vessel includes a pair of sides with pontoons, said sides being spaced apart by more than said platform width, and said vessel includes a vessel deck supported on said sides, with said derrick mounted on said deck and with said pontoons having chambers which can hold air and/or water, including:

with said vessel fixed to said platform, withdrawing water from at least portions of said pontoon chambers to increase tendon tension.

8. An offshore production system lying in a sea that has a sea floor, comprising:

a TLP which includes a platform that has first and second opposite sides and that lies in said sea, and a plurality of tendons and at least one riser all extending substantially vertically from substantially said sea floor to said platform;

a service vessel which has first and second opposite sides, said vessel having a deck and a derrick on said deck, said vessel positioned astride said platform with said deck lying over said platform, with said vessel first side having a portion lying beyond said platform first side, and said vessel second side having a portion lying beyond said platform second side;

a plurality of fasteners, each connecting said vessel to said platform and preventing relative vertical movement of said vessel to said platform.