POWER ACROSS COOK STRAIT

Press, Volume XCV, Issue 28217, 4 March 1957, Page 8

POWER ACROSS COOK STRAIT

“Cable Thoroughly Practicable” (From Our Own Reporter) WELLINGTON, March 3. A power cable across Cook Strait is regarded as thoroughly practicable by the British company engaged by the Government to report on the practicability and approximate cost of installing submarine power cables. The company, British Insulated Callender’s Cables Ltd., recommends that transmission of power should be by direct current at a voltage of about 250kv. The estimated cost of the cable installation for a transmission of 600 megawatts is £2,984.000 for the direct current system, compared with £11,964,000 for an alternative alternating current system. For the transmission of 1200 megawatts a direct current system shows ever greater economic advantages.

When he released the company’s report today, the Minister in charge of the State Hydro-electric Department (Mr W. S. Goosman) emphasised that it dealt only with the actual cable crossing. The complete transmission system from Benmore, in the South Island, to Wellington, for 600,000 kilowatts, showed that the cost by the direct current system would be about £16,000,000, compared with nearly £26,000,000 on the alternating current system.

Estimated time for the manufacture and laying of three direct current cables is given by the company as 13 months, but it says that before manufacturing could be commenced there would be a great deal of detailed preparatory work, and proving tests would be needed.

“It has been shown that the bottom of Cook Strait in the area concerned is reasonably flat, and a high proportion is free from rock.” the report says. “There are several suitable routes. The recommended route for the first part of the project lies from Oteranga Bay in the North Island, to Fighting Bay in the South, a route length of approximately 25.5 miles. “The recommended cable is the single-core, high pressure, preimpregnated gas-filled cable made in continuous lengths and having special armourings to withstand the particular conditions of the seabed under Cook Strait.. “The cable should be laid by a fully-equipped cable ship. “In the event of damage to the cables, practical methods of repairs are available.” Seabed Investigated

Describing its investigations, made with the assistance of the State Hydro-electric Department, the Navy and the Scientific and Industrial Research Department, the company says that the Cook Strait survey was to determine the depth of water, nature of the seabed, conditions affecting design and laying of cable, and suitable landing sites and safe routes between them.

It was found that there are no precipitous cliffs or crevices, and the maximum gradient does not exceed one in five. There is a zone approximately seven miles wide, where the depth does not exceed 900 feet.

On the South Island side, the bottom is soft and sandy right to the shore. As the North Island is aproached the proportion of fine material decreases, but there are channels through the rock outcrops with smooth bottoms. As there are no sewage or industrial effluent outfalls near any of the landing points, the company expects no chemical activity, and has no evidence of attack on submarine cables by marine organisms.

Earthquake fault lines were studied. One inactive fault line would be crossed, but even in the unlikely event of fault movement the conditions would be no more onerous than those which cables will have to stand in the normal course of cable laying.

After reviewing types of cable available, the report says of the recommended type that it can be manufactured in continuous lengths without joints. If punctured or cut under water the ingress of water is limited by the gas pressure. It will withstand water pressures corresponding to the depth of the chosen route, and it is capable of being handled readily in transit and /in laying on the site.

Two Cables and One Spare To transmit 600,000 kilowatts by direct current would require two cables and one spare, whereas alternating current transmission would need 12 cables and two spares. The firm recommends that a trial length of cable be laid in a selected area to provide about two years’ experience of the worst conditions before main manufacturing begins. The trial length could be manufactured and laid within 18 months. None of the difficulties involved in laying the cables is regarded as insuperable. Qualified engineers experienced in the class of work are available, the report says, and there are suitable ships with fully experienced crews

which can be chartered for the laying operation. It is estimated that the whole laying operation for one cable should take about 17 hours. “There is no reason to suppose that the projected cables will not operate with the same reliability which is expected for land cables,” the report continues. Failure Unlikely “The possibility of a failure is extremely unlikely. If it does occur, it will in ’ all probability be confined to the shallow water where the repair is easier.” The telegraph ship Matai, suitably modified, equipment which made it suitable for repairs in conjunction with other small craft for depths up to 50 fathoms. Beyond that one of the several big cable-laying ships stationed in the Pacific would be required. Giving comparative manufacturing and laying times, the company says that manufacture of a direct current cable would take 9i months and laying 3fc months, compared with 43 months and 7 months for the alternating current system. * The costs, estimated at December, 1956, prices for cable systems, for the total 1200 megawats would be £5,163,0Q0 for the direct current and £24,264,000 for the alternating current system. Like the report on the Wairakei geothermal power scheme, the report on the cable crossing had been obtained for assistance in formulating a complete plan for providing electric power for New Zealand, the Minister said.