Hydraulic and earth engineering – Marine structure or fabrication thereof – With anchoring of structure to marine floor
Reexamination Certificate
1997-11-14
2001-03-06
Bagnell, David (Department: 3672)
Hydraulic and earth engineering
Marine structure or fabrication thereof
With anchoring of structure to marine floor
C405S195100
Reexamination Certificate
active
06196768
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a heave resistant, deepwater platform supporting structure known as a “spar.” More particularly, the present invention relates to reducing the susceptibility of spars to vortex induced vibrations (“VIV”).
Efforts to economically develop offshore oil and gas fields in ever deeper water create many unique engineering challenges. One of these challenges is providing a suitable surface accessible structure. Spars provide a promising answer for meeting these challenges. Spar designs provide a heave resistant, floating structure characterized by an elongated, vertically disposed hull. Most often this hull is cylindrical, buoyant at the top and with ballast at the base. The hull is anchored to the ocean floor through risers, tethers, and/or mooring lines.
Though resistant to heave, spars are not immune from the rigors of the offshore environment. The typical single column profile of the hull is particularly susceptible to VIV problems in the presence of a passing current. These currents cause vortexes to shed from the sides of the hull, inducing vibrations that can hinder normal drilling and/or production operations and lead to the failure of the anchoring members or other critical structural elements.
Helical strakes and shrouds have been used or proposed for such applications to reduce vortex induced vibrations. Strakes and shrouds can be made to be effective regardless of the orientation of the current to the marine element. But shrouds and shakes materially increase the drag on such large marine elements.
Thus, there is a clear need for a low drag, VIV reducing system suitable for deployment in protecting the hull of a spar type offshore structure.
SUMMARY OF THE INVENTION
Toward providing these and other advantages, the present invention is an improved spar for offshore hydrocarbon recovery operations having a vertically oriented elongated floating hull with a buoyant upper section and a ballasted lower section and an anchoring system connecting the hull to the ocean floor. The hull of the spar is provided with a vertically oriented, rotatable fairing whereby low drag VIV suppression protects the spar.
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Allen Donald Wayne
Henning Dean Leroy
Bagnell David
Lagman Frederick L.
Shell Oil Company
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