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July, 1955

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Some months ago the National Coal Board announced their intention to build drilling towers for offshore drilling in the Firth of Forth and off the coast of Durham and Northumberland Fig. 2. The first drilling tower Fig. 1. was built at the small old harbour of St. Davids on the Fife coast about a mile and a half below the Forth Bridge, for positioning 1 miles seaward of the new Seafield Colliery, and this was effected on May 22.

The design of this tower was confided to Messrs. Maunsell, Posford and Pavry, consulting engineers, whose work in connection with wartime sea forts is well known, and the main contractors were the Cleveland Bridge and Engineering Co. Ltd. The steelwork for the base girders, pontoons and similar equipment, was fabricated in their Darlington works and transferred to the yard of James A. White & Co., Ltd., shipbreakers at St. Davids, where they were assembled with the tubular steelwork supplied by Tubewrights Limited. The drilling contractors are the Foraky Boring and Shaft Sinking Co. Ltd., and the first hole is expected to be to 1,800 ft. deep and to be completed in about nine months; a position has been selected for a second hole about 2¾ miles offshore if the first is successful. It should be noted that the whole area has been explored by geophysical methods and one unexpected discovery has been the absence of a glacial drift channel in the middle of the Forth.

The drilling rig is identical with that used on land except that a 24 in. tube 145 ft. long is slung from the tower deck to protect the rods and is sealed into the strata to prevent the entry of sea water to the borehole. Welded to the end is a case-hardened shoe to bite down into the sea bed. When the tube is lowered it will be guyed to make sure that it goes down without any deviation from the vertical and when in position it will be braced to the main members of the tower at a point between the high and low water marks. As no sea bottom can be guaranteed dead level and the tower may be slightly tilted this tube is adjustable for a degree or two of verticality. The tower can be used to a depth of 120 ft. at low tide-the expected depth in the Forth is about 60 ft.-and is built to remain steady in gales up to 80 miles per hour and waves 30 ft. from crest to trough. It weighs about 490 tons and stands 185 ft. above the underside of the base girder frame. This is a heavy steel box-girder construction, X-shaped in plan, each leg of the X being 163 ft. long, 3 ft. wide, and 7 ft. deep. The legs are hollow and can be used like submarine ballast tanks, i.e. they may be flooded or pumped dry according as it is desired to float or submerge the tower base. Above them rises the tubular steel casing, of the usual spider-leg appearance, and this carries a steel deck, octagonal in shape and 85 ft. from side to side. It is laid with timber, and 130 ft. above the underside of the box girder base, above which the drilling rig rises another 54 ft. On this deck, Fig. 4, is space for stacking drill rods, and 90 tons of casing; there is a spacious workshop and a two-ton mobile crane.

Between this upper and a lower deck are 24 cabins for the drillers and another for the boremaster (and an office for him). There is a galley, mess-room, recreation room, and first aid room and, like any other craft, the tower needs an adjustable accommodation ladder. Power is supplied by three 300 h.p. diesel driven 50 kW. generators, Fig. 3, for sea-water pumps and for heating, lighting, and signalling. Fresh water is obtained entirely by distillation, the distilling unit yielding 42 gallons per hour which is aerated for all domestic purposes, and sufficient food is stored to keep the tower self supporting for at least a week. There are hot and cold baths for each four cabins and a radio set in the recreation room. One of the single rooms in the accommodation quarters of the tower can be seen in Fig. 5.

Crompton Parkinson Limited were responsible for the supply of the 50 kW. d.c. generators and a switchboard (via Crossley Bros.), the 45 h.p. d.c. motors (via Woodfield Hoist Co.) for the two hoists on each of the two pontoons and the 6 kW. d.c. generators (via Pelapone Engine Co.) being one for each pontoon for lighting purposes and auxiliary power.

Safety precautions include a lifeboat, life-raft, and lifebuoys and four red side lights as well as a "masthead" light on the drill tower flashing every half second; there are, in addition, the usual tower floodlights for night drilling. There are also fog signals and recognition lights, and emergency rockets to communicate with the shore base at Kirkcaldy should the V.H.F. radio-communication equipment break down. A motor boat is in attendance to bring out stores, move personnel, and take the drill cores to the shore base for temporary storage till taken to the laboratory for inspection.

The tower

The tubular tower is the first of its kind and in many respects unique; all the members and joints were prefabricated at the Clyde works of Stewarts & Lloyds, Limited. A view looking down one of the tubular steel legs of the tower can be seen in Fig. 6. There are 90 tubular members (of which 64 are in compression and 26 in tension) and 24 multi-branch joints where from 4 to 10 tubes meet and these multiple branch joints represent a fine example of precision work in structural engineering. At the four corners where the tower is bolted to the cruciform girder base, each joint has five branches composed of 24 in., 18 in., and 15 in. tubes. The compression members are flanged and bolted to the joints while the tension members have double-jawed heel pieces which are pinned to a single-heel piece on their respective joints. The four main 24 in. members run up to the next joint, 66 ft. above. Each of these four joints has six branches, two for the 24 in. and four for the 18 in. members, which taper to 16 in. at the joints to allow the branch stubs to be joined to the large diameter tubes with good welded connections. The 24 main branch joints have special internal plate diaphragms welded in to stiffen the fittings against the loads imposed by the branch connections. The ends of all joint stubs and tube members are sealed off to prevent corrosion and were tested before sealing to pressures of 50 to 100 lb. per sq. in.

The tubular tower has 80 ft. sides at the base, tapering to 45 ft. at half height. The 40-ft. square section then extends for another 60 feet, but 20 feet from the top further members branch out to support the girders forming the base of the working deck; the deck is normally about 40 ft. above water. The first borehole is planned for a depth of 60 to 80 ft. but if greater depths are desired an extra 40 ft. square section can be built in at the top of the tower under the deck. The basic box girder is double and divided into four airtight sections which can be filled with compressed air for towage, and for that purpose the cruciform girder is shackled to the pontoons and these are crossbraced by 18 in. tubular-steel booms 94½ ft. long.

The drilling crew will be volunteer employees of the Foraky Company and will work continuously in twelve hour shifts. The personnel will include an engineer-in-charge, a boremaster, two assistant boremasters, and six drillers, besides three winchmen one cook, one fitter, and one fitter's mate.

Launching and towing

The launching programme called for departure from St. Davids at 3.35 p.m. and arrival at site by 6.30 p.m. on May 22, 1955, but these times were of course dependent on weather conditions and the contractors were doubtless well content to be only half an hour behind schedule at the site. The pontoons were already shackled in position and resting on the bottom; as the tide flowed the draught increased from 2 ft. 6 in. to 4 ft. 3 in. when waterborne and nearly 5 ft. in the course of towage. One of the sloping members is marked at intervals up to 75 ft. and it was noted that the 15 ft. mark was flush with the deck of the pontoon. Each pontoon was manned by 13 winchmen and others wearing life-jackets and they were distinguished by the names "odds" and "evens" for ease in issuing orders; Mr. Posford took charge, standing on a kind of pulpit on "odds"; the depth in the harbour was 9 ft. As soon as the pontoons were waterborne the winchmen started heaving in and by this process the huge structure was nosed out of the harbour, assisted in the later stages by a small dockyard tug. At 3.55 p.m. the three main Admiralty tugs, Flamer, Impetus and Energy, sister ships of some 1,200 h.p. picked up the hawsers and the tow commenced. The tide was running out and the 12-mile passage downstream was easily accomplished. The site was marked by a triangle of flag buoys, two on the base line, and within this triangle lay the mooring vessel Recovery. The three tugs swept round to head upstream into the triangle- Impetus in the centre hooked on to the moored Recovery, the other tugs fanned out and the tower was eased downstream to its site. It was hoped to lower the tower in half an hour but owing to an electrical fault the power for the winches was off for some time; work had to be started by hand.

Drawings and Photographs accompanying the article