Ships – Floating platform
Reexamination Certificate
1997-12-23
2001-07-24
Avila, Stephen (Department: 3617)
Ships
Floating platform
Reexamination Certificate
active
06263824
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 store. 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, a buoyant tank assembly supporting the deck, and a counterweight. A counterweight spacing structure connects the counterweight to the buoyant tank assembly. The buoyant tank assembly has a first buoyant section connected to the deck and a second buoyant section disposed beneath and axially vertically aligned with the first buoyant section. The second buoyant section has a substantially larger diameter than the first buoyant section and a buoyant section spacing structure connects the first and second buoyant sections in a manner providing a horizontally extending vertical gap therebetween.
Another aspect of the invention is a method for reducing VIV in spar platform by providing a substantially open horizontally extending vertical gap in the cylindrical buoyant tank assembly between a first buoyant section and a second buoyant section. The first and second buoyant sections are vertically aligned and are selected to combine first and second buoyant sections of substantially different diameters.
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Allen Donald Wayne
Balint Stephen W.
Cox Bobby Eugene
Ekvall Anders Gustaf Conny
Fischer, III Ferdinand Joseph
Avila Stephen
Shell Oil Company
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