Williams Field Services (WFS) use of the classic Spar hull form for the deepwater field development projects is the first classic Spar hull since ExxonMobil installed the Diana/Hoover Spar in 2000. Since that time, 14 Truss Spars and one Cell Spar have been installed. Learnings from those Spars along with increases in the metocean condition since 2000, significant increases in topside payload during detailed design, support of the five initial SCRs on a porch at the keel plus the decision to fabricate the hull in a US graving dock provided new challenges for the Naval Architects from design through to hull installation.

The paper discusses the key drivers, constraints and criteria that had to be reconciled into a floating system with acceptable global motions, acceptable horizontal trim and robust characteristics for changes in topside payload and CGs. Examples of these sometimes conflicting design requirements include: the increased hull freeboard which was driven by the new metocean condition which then led to a higher topside VCG, a 30% increase in the topside's maximum operating payload between the end of FEED and midway into final design yet the depth and length of the graving dock dictated that these changes had to be accommodated without increasing either the diameter or draft of the hull. This paper presents the challenging results of the model tests and their impacts on the design of the mooring system as well as the very close tolerances in trim and stability for the tow out of the graving dock and then to site.

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