Ocean Thermal Energy Conversion (OTEC) technology uses the water temperature gradient between the cold deep ocean and the warm ocean surface to generate clean renewable power. This system requires one or several cold water pipes (CWPs) of large diameters to transport large amounts of deep-sea cold water to the floating platform topside. Until now, there is no stand-alone floating OTEC system in operation offshore, partly due to the associated high capital expenditure (CAPEX). In this study, a cost-effective approach for small-scale OTEC technology application is introduced by integrating one compact OTEC module directly onto a semi-submersible oil and gas (O&G) production platform. In addition to the mooring lines and risers, the CWPs would be attached to the columns of the semi-submersible platform in a way similar to that of vertically hanging risers. This study assesses whether installing four CWPs associated with a hypothetical 1 MWe OTEC module on a semi-submersible production platform would have any major effect on the hull global motions and the dynamic performance of the risers and moorings. The study investigates the feasibility of a small OTEC system installation on a semi-submersible platform and provides useful reference and guidance for OTEC technology applications on O&G facilities in general.
Global motions of a semi-submersible hull due to the installation of the OTEC module and CWPs are studied using nonlinear fully coupled time-domain analysis. The parameters of the CWPs which are investigated include length variation. The sensitivity of the hull motions is checked considering several design environmental conditions. The hull global motion and riser dynamic performance are analyzed using engineering analysis programs. The global motions from coupled analysis are applied to detailed Finite Element Analysis (FEA) models of the CWPs for strength checking. For large diameter CWPs, a slight discrepancy in the modeling and wave or current load calculation in the global coupled model and the CWP detailed FEA modeling could result in a significant error in the computation results. For this paper, the simultaneous approach is adopted by TechnipFMC in-house programs, i.e. coupling RodDyn with MLTSIM for global coupled motion analysis and stand-alone RodDyn for CWP FEA analysis. Identical CWP modeling is used in both coupled analysis and detailed FEA simulation.
From coupled analysis of the proposed semi-submersible system with CWPs, the effects of the CWPs are found to be significant yet not detrimental to the floating platform system performance as they could be mitigated in the design. Thus, installing a small pilot scale OTEC module on a semi-submersible production platform to generate around 1 MWe of electrical power is shown to be feasible.