Design of the Truss Spars for the Nansen/Boomvang Field Development
Authors: Bangs A.S. , Miettinen J.A. , Silvola I. , Mikkola T.P.J. , Beattie S.M. 
Published: October 2008

The Truss Spar is a new floating platform concept that is rapidly becoming one of the preferred systems for developing deepwater Gulf of Mexico fields. The Truss Spar provides a stable, cost-effective platform for dry-tree completions over a wide range of water depths. The Truss Spar is an extension of technology developed on previous "Classic" Spar projects, but has several specific advantages. These advantages were exploited to provide a fit-for-purpose platform solution for the Nansen/Boomvang Field Development. This paper describes the design of the Truss Spars for the Nansen/Boomvang Field Development. The paper reviews the Truss Spar concept, past development work, and comparison with the Classic Spar concept. The paper describes the specific Nansen/Boomvang project requirements and design criteria and how they influenced the platform configuration. The paper includes summaries of analysis results for global motions, mooring design, and hull structural design. The paper also includes a description of unique design features, including heave plates, SCR pulltubes, hard tank-to-truss connections, and toptensioned riser guides. These novel features, and other technology proven on the Nansen/Boomvang project, arereadily transferable to other Truss Spars now under design.

Introduction

The Nansen and Boomvang oil and gas fields are located in the deep waters the Western Gulf of Mexico, about 115 nautical miles south of Galveston, Texas. In February, 2000, Kerr-McGee and partners Ocean Energy and Enterprise Oil selected Spars International Inc. (SII) to provide two Truss Spars to develop the fields simultaneously, using a single project team. Additional background information is described in a paper from this same session (Thibodeaux, et al, 2002).

The Truss Spars for Nansen and Boomvang are a "first-ofa-kind" design. The Truss Spar provides a stable, costeffective platform for dry-tree completions over a wide range of water depths. The Truss spar is an extension of technology developed on previous "Classic" Spar projects, but with several specific advantages. These advantages were exploited to provide a fit-for-purpose platform solution for the Nansen/Boomvang Field Development.

SII formed a technical team using resources from parent companies J. Ray McDermott and CSO-Aker Maritime to develop the project-specific Truss Spar configuration and to perform on-going global analysis and detailed mooring engineering. CSO-Aker Rauma Offshore, with subcontractor PI Rauma, carried out detailed engineering of the Truss Spar hull, while J. Ray McDermott Engineering performed detailed engineering for the offshore installation.

Comparison of the Truss Spar and the Classic Spar

All Spar hulls built to date have three principal sections:the hard tank, which provides most of the vessel's buoyancy;the keel tank (or soft tank) which provides buoyancy for the wet tow and a receptacle for fixed ballast; andthe midsection, which connects the hard tank and keel tank. (Figure 1).

Previous Spars (including Neptune, Genesis and Hoover-Diana) were designed with a cylindrical, ringstiffened midsection, and with a similar cylindrical keel tank.