Design and Testing of a Fatigue-Resistant High Voltage Power Cable for OTEC Plants
Authors: Roberts Kenneth P. , Garrity Thomas F. , Traut Richard T. 
Published: October 2008

In order to exploit the electric power to shore OTEC option, high voltage risercables are required to span the distance between the floating OTEC plants andfixed cables on the ocean bottom, at depths of 4000 feet or more. The risercables will be subjected to constant motions arising from both the motions ofthe OTEC plant and from driving forces directly on the cable, such as oceanwaves and currents. The fatigue resistance needed to withstand the cablemotions over the OTEC system design life, along with other unprecedentedrequirements on the OTEC riser cables, requires an extension of currentsubmarine cable technology.

Two prototype cable designs were selected from a field of candidates. The5e twodesigns were then subjected to computer analyses of cable motions and forces, followed by a laboratory test program on component materials and cable models. Finally, tests simulating the typical life history of the proposed OTEC risersare to be performed with full scale prototypes.

The focus of the development and testing effort is centered on three cablecomponents: the hermetic sheath, the external armoring and the electricalinsulation. Although fatigue resistance is the prime concern, the design andtesting will include consideration of such environmental challegnes ashydrostatic pressure, corrosion, biofouling and fishbite.

INTRODUCTION

Ocean Thermal Energy Conversion (OTEC) plants now being developed offer thepromise of significant energy from renewable sources before the turn of thecentury. 1 Potential OTEC plant locations, requiring a differencebetween surface and bottom waters of about 20°C, are located in a band from 20°North Latitude to 20° South Latitude. Suitable sites are found within fivesmiles off-shore several U.S. islands, including Hawaii, Guam, and Puerto Rico, while such sites are at least 150 to 200 miles from U.S. mainland in the caseof Florida or the Gulf Coast states.

One OTEC plant has already been put into operation experimentally off Hawaii. The Department of Energy (DOE) is funding the engineering development ofbaseline designs for OTEC plants, and will soon be seeking from prospectiveowners cost-sharing proposals for the construction and operation ofdemonstration OTEC plants in the range of 10 to 40 MW electrical capacity. Ultimately, commercial plants are envisioned in the 100 to 400 MW range.

The electrical energy from OTEC plants may be transmitted to shore in twomanners: via electrical cables on the ocean bottom, or by making energyintensive products, such as aluminum, ammonia, or hydrogen, and shipping themto shore for further processing. The transmission of power to shore via bottomcables is considered a reasonable extension of present technology.2. "Riser cables", which carry the power from the floating OTECplant to interface with the transmission cables on the ocean bottom, presentunique challenges. 4, 5,6,7 Simplex Wire & Cable Company isunder a 4-year contract to DOE to develop performance criteria for a family ofriser cables to serve OTEC plants in their various configurations.