INTECSEA, a global company within the Worley Parsons Group, is featuring its new semisubmersible design at the Deep Offshore Technology (DOT) conference this week. The company says the new design enables dry tree applications in harsh environments.
The Free-Hanging Solid Ballast (FHSB) Semi is an innovative new semisubmersible design that is suitable for dry tree applications due to its low heave response, comparable to that offered by spars. The new design resembles a conventional semisubmersible, with an added free-hanging, solid ballast tank placed deep below the keel of the semisubmersible hull and connected to the hull through four groups of chains. The solid ballast tank (SBT) acts as a motion damper that significantly improves the heave motion response. The company says that the chain connection between the semisub hull and the SBT facilitates transportation and installation, and solves fatigue issues that are often associated with the use of rigid connections.
Company engineers presented two papers about the new design in June, at the OMAE 2013 conference in Nantes, France.
INTECSEA says that the advantage of top tensioned risers (TTRs) is that they allow direct vertical access to production wells. However, due to tensioner stroke limitation, the use of TTRs requires a floating system that provides minimal heave response. Both spars and tension leg platforms (TLPs) have been recognized by their favorable response and therefore their suitability to support TTRs.
But in water depths beyond 1500m (5,000 ft), the company says that TLP tendon design becomes increasingly challenging, leaving spars as the only feasible solution in ultra-deepwater. However, the single column configuration of spars can support limited deck area, and poses restrictions on the maximum payload. Spars also require hull upending and topside integration through heavy lifts offshore. These operations bring their own cost and risk disadvantages.
Conventional semisubmersibles provide significantly larger deck area, are less sensitive to payload and do not require offshore integration. However, due to their large heave response, semisubmersibles are not suitable for dry tree applications.
INTECSEA says that the FHSB Semi maintains the advantages of conventional semisubmersibles, and offers favorable motion characteristics of spars to enable the use of TTRs.
Image courtesy of INTECSEA.
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