Hydraulic and earth engineering – Marine structure or fabrication thereof – With pivotal connection between work deck and base
Patent
1988-03-03
1990-01-16
Taylor, Dennis L.
Hydraulic and earth engineering
Marine structure or fabrication thereof
With pivotal connection between work deck and base
405195, 405203, 166350, E02B 1702
Patent
active
048939650
DESCRIPTION:
BRIEF SUMMARY
DESCRIPTION
This invention relates to a system for offshore operations, having particular application to oil and gas production from sub-sea wells.
It is known to use a buoyant steel column attached to the seabed to support oil and gas production risers extending from a sub-sea well to a surface vessel. The column is pivotally connected to a base attached to the seabed and remains substantially vertical by virtue of its buoyancy. The pivotal connection permits the column to rock in order to accommodate sea motion and the motion of the surface vessel. It is also known to connect the top of such a column by a rigid yoke to a surface vessel in such a way that the surface vessel is free to move in response to the waves and weathervane about the column so as to assume the line of least resistance to the prevailing weather conditions. It is also known to moor the vessel by this means.
It has recently been proposed to connect such a column by a hinged A-frame to a semisubmersible oil drilling and production platform moored by a plurality of catenary chains or wires. The A-frame is mounted at its feet on the semisubmersible by two horizontally spaced hinges allowing only vertical angular movement of the A-frame relative to the semisubmersible. The A-frame is connected to the column by a single joint located at the apex of the A-frame, the joint allowing relative rotation of column and A-frame about all three orthogonal axes. A problem with this arrangement is that the transverse forces applied by the column to the A-frame cause large transverse shear forces in the A-frame and large hinge forces at the hinge connection to the semisubmersible. These forces require a heavy A-frame which is difficult to disconnect safely in severe weather conditions.
With a view to overcoming this problem the present invention provides from first aspect an offshore system comprising a control column for extending upwardly from the seabed, a semisubmersible structure, and a bridge between the column and the structure, the bridge being connected to the semisubmersible structure by a joint arrangement which permits rotation of the bridge relative to the structure about both a substantially vertical axis and a substantially horizontal axis. Preferably the joint arrangement also permits relative rotation of the bridge and the semisubmersible structure about the longitudinal axis of the bridge. Thus, the loads applied through the bridge can be purely axial.
The bridge can consequently be much lighter than the A-frame arrangement previously proposed and leads to much simplified connection to the semisubmersible. The requirement to be able to connect the bridge to the semisubmersible offshore and to disconnect the bridge from the semisubmersible in emergency can be more easily achieved than with the bridge arrangements previously proposed.
Preferably the bridge is connected to the semisubmersible structure by a releasable hook arrangement. Conveniently the bridge is provided with generally horizontal pin, and the semisubmersible structure is provided with a hook arrangement with receptacles in which the pin is releasably received for pivotal movement. Preferably the pin is released from the hook arrangement by downward movement upon release of a hydraulic catch, so that the bridge can be manoeuvred by means of a cable from a winch on the semisubmersible.
In order to produce a convenient arrangement for the equipment located on the top of the column and to be able to stow the bridge on the column, the bridge is preferably connected to the column by a joint located at the edge of the column and not on its vertical axis. The problem with this arrangement is that axial forces induced in the bridge by differences in environmental loadings applied to the semisubmersible and the column result in moments being produced in the column when the compass heading at the bridge is such that the axis of the bridge does not pass through the axis of the column. Such moments would produce unacceptably high stresses and strains on prior art pivotal connections
REFERENCES:
patent: 3435571 (1969-04-01), Oltermann et al.
patent: 3766582 (1973-10-01), Lloyd et al.
patent: 4026119 (1977-05-01), Dotti
patent: 4142820 (1979-03-01), Tuson
patent: 4470723 (1984-09-01), Michel et al.
patent: 4529334 (1985-07-01), Ortloff
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