Commercial OTEC development has been constrained by high capital cost, lack of operational experience and perceived technical risk. Reduction in the price of oil plus environmental concerns led to a decline in offshore drilling and the premature scrapping of a number of high specification drillships. This work looks at the potential to covert a redundant drillship into a 64 MW net grazing OTEC production facility. Redundant vessels can be purchased at scrap value, which minimizes capital cost and thereby helps to improve economic feasibility while also reducing technical uncertainty.
A review was undertaken to identify a promising drillship for conversion. The selected unit is a full Dynamically Positioned fifth generation unit. Based on the hull dimensions it was possible to fit within the hull 6 × 10.7 MW net closed cycle power modules using plate heat exchangers giving a power output of 64MW.
A Computational Fluid Dynamics (CFD) analysis demonstrated the potential to use the discharged warm and cold water to propel the unit in search of highest possible surface temperatures based on satellite data and also to avoid extreme weather conditions. The energy produced is planned to be exported as anhydrous liquid ammonia to dynamically positioned ammonia dedicated shuttle tankers using hoses, similar to tandem offloads from a FPSO.
A new design of composite Cold Water Pipe (CWP) has been proposed, which would be hung off the drillship's central moonpool using a novel motion decoupling three point support. A dynamic simulation of the response of the drillship and CWP in severe quartering seas did not result in exceedance of allowable values.
The capital cost of the converted unit was estimated and an investment appraisal carried out. It was found that for areas such as the Hawaiian Islands, where energy costs are high, a converted drillship represents a commercially attractive option and also reduced technical risk.