At Sea Test: Hawaii Deepwater Cable Program

ABSTRACT In order to investigate and demonstrate the technical feasibility of laying a Submarine power cable in the open ocean, in depths exceeding 6000 feet, over difficult volcanic island seabed conditions, a computer assisted control system for cable vessels was developed. The control system was successfully tested at sea in November 1989, achieving three-meter placement accuracy while laying cable along a pre-selected curved route on the seabed. INTRODUCTION A major challenge to laying a proposed power cable between the islands of Maui and Hawaii in the state of Hawaii is the 1900-meter deep Alenuihaha Channel, known for its difficult currents and waves, with an especially difficult rough bottom terrain, necessitating a very narrow and winding cable path. The United states Department of Energy sponsored the Hawaii Deep water Cable Program (HDWC) with the objective of determining the feasibility of laying and operating a Submarine power cable in these waters. After the channel bottom was surveyed by the HDWC program, the best cable route identified during the surveys was found to narrow in one area to a width of 80 m. Since three cables are required for the final power system, it was decided that an at-sea test would be necessary to determine the feasibility of laying a cable to an accuracy of ±11 m on the seabed. In addition, the rough and steeply sloping bottom terrain, leading to the possibility of unacceptable cable spans on the bottom, required the cable be laid with controlled tension in the various portions of the route. Consideration of the high tensions to be encountered during laying, with 1900 m of cable suspended from the cable ship, led to a cable design with contra helically wound armor layers. This results in a cable which cannot be coiled in a tank, as in the typical case of submarine communications cables. The power cable being considered here requires storage on a revolving reel or turntable on the laying vessel, and must be kept under some tension at all times. The HDWC program developed an integrated control system to control the laying of this cable in these special conditions, and carried out an at-sea test program, laying the cable in the actual ocean positions, and recording data for use by the designers of the future commercial cable. CABLE ROUTE SURVEY The future permanent power cable will have a wet weight of about 27 kg/m and a diameter of about 12 cm. When laid over the sea bottom in the Hawaiian Islands, the cable will encounter bottom roughness due to old lava flows, submerged shorelines and reefs, escarpments and boulder fields. The cable may be laid over obstacles or suspended between obstacles in brief spans; the possibility of damage due to bending over obstacles, or fatigue caused over time by vortex-induced oscillations induced by tidal currents, must be avoided