Improving the Motions of a Semi by the Addition of Heave Plates

Similar to the manner in which heave plates provide a Truss Spar with low global motions, a heave plate system can be used to significantly mitigate the motions of a conventional semi-submersible. This paper will focus on the global performance of a conventional semi-submersible that has been properly configured with heave plates. The design case is a Gulf of Mexico deepwater environment similar to that for a Spar located in 5,600 feet of water. The system has eight top-tensioned risers (TTR) supported on the top of the semi hull with tensioners, a spread mooring and a 10,000-ton operating payload. Extensive global motion analyses and comprehensive model testing have been carried out to validate the in-place behavior of such a solution. This paper presents the validation results from the analytical prediction and the model test. A semi-submersible with properly configured heave plates can be used as a viable deepwater dry-tree floating solution in such harsh environments as in the Gulf of Mexico. Because of its superior motion characteristics, this solution is also more SCR friendly and can be a better wet-tree application as compared to conventional semi-submersibles. The analytical and model test results demonstrate that the excitation of a semi-submersible hull by wind, wave and current can be adequately mitigated by the proper placement and sizing of a heave plate system. The analysis and testing also indicates that to achieve suitable motions in a cost effective manner requires incorporating fabrication and installation issues into the heave plate system. The in-place behavior of this solution has been validated and thus proving the concept provides a viable cost effective dry-tree floating solution for deepwater developments.