The Belly Spar - Design and Verification of an Ultradeepwater Solution

Abstract The Spar platform has developed into a well functioning solution for Gulf ofMexico environment. Considering use of this solution in the North Atlantic, themetocean conditions differ by long period swell and fatigue induced by normaloperational seas. In order to meet these challenges, it is desirable to consider a classic Sparthat is more fatigue redundant than a truss, but the swell requires highnatural periods, to avoid parametric heave-pitch resonance. A new version of the Spar in response to these requirements is the Belly Spar. It can be considered as a classic Spar with a Belly; starting below the wavesurface and extending down to the hard tank depth. A concrete Spar concept withreduced waterline diameter has also been developed by Aker Solutions for arcticapplication. This concept had the dual benefit of increasing the natural periodin heave as well as reducing the ice load from sea ice The concept has been developed for a field in the Norwegian Sea, in water depthof 1,200m (4,000ft). The hydrodynamic analyses show excellent performance, however contain assumptions on damping. The design has been by model testing ofthe design in wave and current combination representing 10,000yr events, asshown by results and correlations in the paper. The design opens up new areas for the Spar platform, with good motions that canaccommodate steel catenary and top tensioned risers. As for previous Sparconcepts, the application is in deepwater and ultradeepwater. Introduction The use of the Spar structure for oil and gas production was conceived byEdward Horton in the 80ies, ref. 1. As the first project was realized, Aker gotinvolved in the design and delivery, ref. 2. With subsequent projects cameexperiences and new design variants were developed, the truss Spar was the"next generation" and the cell Spar has been another variant. The use of the Spar has been limited to the Gulf of Mexico (except for one inMalaysia), with certain metocean conditions, mostly mild weather, butoccasionally a loop current or hurricane event. During a hurricane, theplatform is shut down and the personnel are evacuated. When considering applications in other parts of the world, the local conditionsneeds to be considered, which may yield changes to the design. Previousevaluations in use in regions with swell have observed challenges with motionresponse. The option for reducing the water plane area by a reduced topdiameter was implemented already in 1976 for the Brent Spar loading buoy andthe influence on the motions thoroughly documented by Haslum, ref. 3. The truss of a truss spar has been seen as a means of reducing the weight ofthe midsection, between the hardtank and the ballast. However, the trussconcentrates the load path into critical components without redundancy that puthigh requirements on the fatigue design. An alternative means for reducing theweight is to reduce the diameter, relative to the hard tank.