Ships – Floating platform
Reexamination Certificate
1997-12-23
2001-05-08
Avila, Stephen (Department: 3617)
Ships
Floating platform
Reexamination Certificate
active
06227137
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 drag and 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 strakes 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
The present invention is a spar platform having a deck supported by a buoyant tank assembly having a first buoyant section connected to the deck, a second buoyant section disposed beneath the first buoyant section; and a buoyant section spacing structure connecting the first and second buoyant sections in manner providing a horizontally extending vertical gap therebetween. A counterweight is connected to the buoyant tank assembly through a counterweight spacing structure.
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J. A. van Santen and K. de Werk, “On the Typical Qualities of SPAR Type Structures for Initial or Permanent Field Development,” OTC 2716, paper presented at the Offshore Technology Conference, Houston, Texas, May 3-6, 1976.
F. Joseph Fischer et al., “Current-Induced Oscillations of Cognac Piles During Installation—-Prediction and Measurement,” Practical Experiences with Flow-Induced Vibrations, Symposium Karlsruhe/Germany, Sep. 3-6, 1979, University of Karlsruhe, pp. 570-581.
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Allen Donald Wayne
Balint Stephen W.
Henning Dean Leroy
McMillan David Wayne
Avila Stephen
Shell Oil Company
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