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May, 1961

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For two and a half years a crew of 16 men has lived and worked aboard the 230-foot-high offshore drilling tower of the National Coal Board, probing the rich coal seams beneath the sea bed off the Durham coast.

The drilling crew spend maximum periods of three weeks on the tower, which is fully equipped with dining room, private quarters, bathrooms, restlounge, television, radio, power supply and radiotelephone for shore communication, as well as workshops and stores for drilling apparatus and sample cores.

Though tenders carrying the tower’s food and other supplies occasionally bring stocks of fresh water, the crew’s water supply is normally drawn from the sea, and water for drinking, washing, cooking and enginecooling is distilled and purified on the tower.

The tower’s distillation plant will convert 100 gallons of sea water into 80 of fresh water in an hour, and this is adequate for all normal fresh water requirements aboard. The plant, supplied by Buckley and Taylor Limited, Oldham, comprises an evaporator equipped with immersion heater, a compressor, a steam chest, a heat-exchanger, centrifugal pumps and electric motors. Incoming sea water is pre-heated in the heat-exchanger (which cools the distillate), then enters the evaporator at a temperature near to boiling point. Steam and water rise in the evaporator tubes and an arrangement of baffles removes the water particles, allowing the steam to pass into the suction pipe of the compressor. Any excess feed water spills into an overflow pipe, to be carried away to waste.

During compression the temperature is raised and the steam is returned to the chest, from which the distillate flows under gravity to a pump which lifts it to the fresh water storage tanks. The remaining concentrated brine is discharged from the plant during the frothing action of the boiling feed.

A fine-mesh strainer incorporated in the suction feed pipe can be cleaned without disturbing the flow.

To conserve heat the items of equipment which operate at above the ambient temperature are insulated and covered with aluminium alloy sheeting. Hot water for baths, showers, washbasins and the galley is obtained from five 12-gallon tanks fitted with Creda thermostatically-controlled immersion heaters, and these tanks obtain their water supply directly from the fresh-water storage.

The Durham drill-rig is the second and larger of two constructed by the N.C.B. When designing the first, which operated off the Scottish coast from 1955 to 1960 and is now closed down, engineers realised that the ideal pumps for coping with the suction head — 60 to 80 feet from sea level to the working deck — would be submersible electric centrifugal units of the Flygt range. But only d.c. power was available and the encapsulated motor of a Flygt pump is an a.c. one. Conventional centrifugal pumps, designed for working at a normal head of 25 feet, would have produced maintenance difficulties, as would the use of compressed air, and it was decided to fit the Scottish tower with hydro-ejector pumps.

However, after two years of operating the Scottish rig in the Firth of Forth, when the Durham tower was planned it was decided, in the interests of simplicity and efficiency, to install motor-generators converting to a.c. current, so that a Flygt submersible pump could be used. This was embodied in the specification, and the water supply on the Durham tower is delivered by a Flygt B80L fully submersible electric pump, a three-inch-discharge five-h.p. unit, capable of delivering 250 gallons per minute against a 100-foot head.

The water obtained from the distillation plant is quite tasteless, and must be aerated to make it palatable for drinking. The equipment for this consists simply of a vessel inserted in the gravity feed pipe from the main storage tank and divided by a weir into two compartments, one of which contains "Norbit" carbon. Water enters at the bottom of this compartment, passes up through the carbon and flows over the weir into the other compartment, from which the draw-off connection to the galley is taken.