Truss semi-submersible offshore floating structure

Abstract

A semi-submersible structure. The hull includes four columns that are supported by two pontoons. The columns support the topsides and the topsides structural framing serves as horizontal framing between the columns. A truss frame is attached to the columns. The truss frame preferably includes heave plates. The truss frame extends downward below the pontoons a sufficient distance in the water such that it minimizes motions caused by environmental forces. The hull section and the truss space frame are constructed separately and assembled together at the offshore site where the structure is used for drilling and / or production.

Description

FIELD AND BACKGROUND OF INVENTION [0001] The invention is generally related to floating offshore structures and more particularly to semi-submersible floating offshore structures. [0002] The semi-submersible is a type of floating structure that has vertical columns supporting topsides and supported on large pontoons. The structure is held in position by the use of spread mooring lines that are anchored to the seafloor. The semi-submersible has a number of unique characteristics compared with other floating structures such as a spar and TLP (tension leg platform). These advantages include: The semi-submersible has good stability because of a large footprint and low center of gravity for the topsides. The hull requires lower steel tonnage. The hull can be a new build or converted from an existing drilling semi. The semi-submersible may include drilling capability. The semi-submersible can support a large number of flexible risers or SCRs (steel catenary risers) because of the space av...

AI Technical Summary

Benefits of technology

[0009] The present invention addresses the deficiencies in the known art. The hull includes four columns that are supported by two pontoons. The columns support the topsides and the topsides structural framing serves as horizontal framing between the columns. Additional braces may be added between columns and topsides framing as necessary. A truss space frame is attached to the columns. The truss space frame preferably includes heave plates and possibly a keel tank. The truss space frame extends downward below the pontoons a sufficient distance in the water column that minimizes motions caused by wind and wave forces and eliminates the deficiencies in the known art. The hull section integrated with topsides and the truss space frame are constructed separately and assembled together at the offshore site where the structure is used for drilling and / or production.

Problems solved by technology

As a result it has not been suitable for a dry tree riser arrangement.

Another problem from the large motion of the semi-submersible is that it causes fatigue in the SCRs more easily, which requires more stringent fatigue design for the SCRs and thus costs more.

For a platform in ultra deepwater with large diameter SCRs, the solutions to this problem could become technically or economically unfeasible.

However, the heave motion is still relatively large compared with spars and TLPs.

Also, the dry tree arrangement is still not feasible.

The known designs suffer several deficiencies.

The hull has to be a new build and conversion of an existing semi-submersible hull is not possible.

The extendable columns take too much deck space.

In some cases it could be as much as thirty percent of the total deck space, which is impractical from a topsides equipment layout point of view.

The column-to-deck connections are complicated.

They are hard to build, risky during installation, and difficult to maintain.

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