OTEC Cold Water Pipe Global Dynamic Design for Ship-Shaped Vessels

Abstract

Ocean Thermal Energy Conversion (OTEC) technology has been considered as a renewable power generation for the tropical oceans where a thermal gradient from subsea to surface are higher than 20°C since 1980. In 2009, the OTEC technical readiness report has identified that semi-submersible, ship-shaped vessel and spar are most feasible to OTEC application. All three are technically mature and well-established floating facilities and have been widely manufactured and operated in offshore industry all over the world. A pilot OTEC development, led by Lockheed Martin (LM) Industry Team, has configured a semi-submersible floating platform. As an alternative design, SBM is developing OTEC designs based on converted ships. Ship shapes provide good access to facilities for practical operation and maintenance activities. Our study focused on demonstrating the feasibility of constructing and installing a 4 meter outer diameter Cold Water Pipe (CWP) based on conventional land-based manufacture of Fiberglass Reinforced Plastics (FRP) followed by installation with SBM marine equipment. Based on insights gained from this exercise, we will continue to develop the installation methods for larger diameter CWPs. The CWP is a key design challenge for OTEC since it must be strong enough to withstand the forces and motions while being light enough to be installed with available marine equipment. This paper focuses on the cold water pipe global dynamic performance hosted by a converted ship for a 10MW OTEC plantship offshore Hawaii. The offshore Hawaii location was selected for purposes of comparison rather than the existence of any specific prospective projects. The CWP is connected to the vessel via a sealed gimbal device that allows the CWP’s angular motions to be decoupled from the vessel. The fundamental understanding of CWP vibrations is discussed. The CWP global dynamic responses to extreme storms and operational fatigue environments are presented. Vortex Induced Vibration (VIV) and other design issues are discussed. The key global design considerations of CWP for the ship-shaped vessel are identified and summarized.