A Novel Passive Riser Tensioning System for a Dry Tree Spread Moored Barge

Dry tree risers on floating systems are presently supported either by TLPs or SPARs, both of which have limited payload capacity and no storage. A spread-moored barge having dry trees, storage and integrated drilling facilities has been developed. Model tests were performed for the riser tensioning system used on such a barge for offshore West Africa conditions. The risers are attached to a rectangular platform in a moon pool of the barge. This platform is supported by means of an articulated Rocker Arm System (RAS) located on the vessel main deck. The RAS has built-in counter weights hinged on pedestals fixed to the barge deck. In this arrangement gravity is used to tension the risers and when the barge heaves, the counterweights in the rocker arm system essentially decouple the barge vertical motion from the riser platform. This system provides a heave-restrained platform for dry trees with minimal dynamic loading of the risers. The model tests confirmed the feasibility of the new design. The paper describes the basic components of the gravity tensioning system. The model test set up and results from the model test are shown. Comparison of model test results with a fully coupled analytic model consisting of the barge and articulated tensioning system is also presented.