The offshore industry has spent the last several years developing semisubmersible platforms capable of supporting both drilling and production activities. The production trees are located on top of the top tensioned risers (TTRs) on a Dry Tree Semisubmersible. A key challenge in the design of these vessels is to reduce the heave motion as much as possible to enable the use of state-of-the-art riser tensioners. A model test campaign was executed as part of the developmental program. The primary objective of this campaign was to improve the accuracy of the numerical tools to be used in the design process.
Riser tensioners are typically hydro-pneumatic devices, with a nonlinear tension-stroke relationship. A riser tensioner was developed at model scale which had a similar nonlinear behavior to the prototype. Examining the effect of this tensioner on the global motions was an additional objective of this test campaign. The techniques used to model this nonlinear spring is described, and its effect on global motions investigated.
A key challenge in model testing platforms intended for ultra-deep water (e.g. greater than 7000 ft) is the modeling of the mooring and riser system. The premise for the design of the model mooring system is 1.) maintain as best as possible the force-offset relationship of the mooring lines and 2.) be able to describe the model test configuration in the numerical tools to be used for global design. The near taut behavior of the prototype mooring system is modeled using heavy chain and a high-catenary mooring line.