Deployment and Operation of the 50KW Mini-OTEC Plant

Abstract

ABSTRACT In 1979, the world's first successful at-sea closed cycle OI'EC plant was moored offshore of Kona, Hawaii and produced the first net useful electricity from the solar heat resource stored in ocean surface waters. The unique mooring combined the Ocean Thermal Energy Conversion (OI'EC) mId water pipe and mooring" tension leg in one polyethylene pipe. Operation of the electrical generating process was successful in waves up to 15-20 feet. Introduction The State of Hawaii, Lockheed Missiles and Space Company, and the Dillingham Corporation agreed in lq78 to design and build a demonstration Ocean Thermal Energy Conversion (OI'EC) plant to be installed on a floating platform. By August 1978 a Navy barge had been obtained. The actual instruction of the plant took place between December 1978 and June 1979, and the barge was placed on station in July 1979. On August 3, 1979, Hawaii's Governor George Ariyoshi was able to announce the world's first successful generation of net useful OTEC power from a closed cycle plant operating at sea. The plant continued operating successfully until November 1979, when available funds had been depleted, and at that time the Mini-OTEC returned triumphantly to Honolulu Harbor. MOORING DESIGN REQUIREMENTS The underlying requirements for Mini-OTEC were to supply 3200 GPM of 40°F. water from 2200 feet to a surface barge and to moor this surface barge, a 103 × 34 ft. rake-ended hull, in currents which were known to exceed two knots. Winds of 30 knots and waves of 25 foot significant height were assumed to be acting in the same direction with the ocean current. A system was required which mold be deployed within a weather window of only one day. The sea bed at the required mooring site (a State-sponsored, environmentally blessed research area) is a steep rocky volcanic slope with very rough topography. There are essentially no bottom sediments that would allow an anchor to "grab". The seabed slopes so -steeply that there was concern that an anchor might simply slide down the underwater hill, which is merely a continuation of the shoulder of the volcano above. Research on candidate materials for the Mini-OTEC cold water pipe (CWP) had led to consideration of fiberglass, polyethylene, polyvinylchloride, rubber and polypropylene. Polyethylene was attractive as a CWP material due to its very smooth interior for flow characteristics and low susceptible to befouling. In addition, a fusion welding process was available that provided reliable joints. The polyethylene is very flexible and rugged and is not brittle even at low temperatures. After selecting ultra high molecular weight polyethylene pipe as the CWP material, and in looking at the polyethylene properties, it was soon realized that the breaking strength of the pipe was quite high and a most attractive solution was to use the cold water pipe itself as the mooring line. This was considered to be a rather daring step at the time and much discussion ensued.