Seawitch

Prologue

NORMALLY there are only two types of marine machines concerned with the discovery and recovery of oil from under the ocean floor. The first, mainly engaged in the discovery of oil, is a self-propelled vessel, sometimes of very considerable size. Apart from its towering drilling derrick, it is indistinguishable from any oceangoing cargo vessel; its purpose is to drill boreholes in areas where seismological and geological studies suggest oil may exist. The technical operation of this activity is highly complex, yet these vessels have achieved a remarkable level of success. However, they suffer from two major drawbacks. Although they are equipped with the most advanced and sophisticated navigational equipment, including bowthrust propellers, for them to maintain position in running seas, strong tides and winds when boring can be extremely difficult, and in really heavy weather operations have to be suspended.

For the actual drilling of oil and its recovery— principally its recovery—the so-called «jack-up system» is in almost universal use. This system has to be towed into position, and consists basically of a platform which carries the drilling rig, cranes, helipads and all essential services, including living accommodations, and is attached to the seabed by firmly anchored legs. In normal conditions it is extremely effective, but like the discovery ships it has drawbacks. It is not mobile. It has to suspend operations in even moderately heavy weather. And it can be used only in comparatively shallow water: the deepest is in the North Sea, where most of those rigs are to be found. This North Sea rig stands in about 450 feet of water, and the cost of increasing the length of those legs would be so prohibitive as to make oil recovery quite uneconomical, even though Americans have plans to construct a rig with 800-foot legs off the California Coast. There is also the unknown safety factor. Two such rigs have already been lost in the North Sea. The cause of those disasters has not been clearly evaluated, although it is suspected, obviously

Sea witch

not without basis, that there may have been design, structural or metallic faults in one or more of the legs.

And then there is the third type of oil rig— the TLP—technically, the tension leg drilling/ production platform. At the time of this story there was only one of its type in the world. The platform, the working area, was about the size of a football field—if, that is, one can imagine a triangular football field, for the platform was, in fact, an equilateral triangle. The deck was not made of steel but of a uniquely designed ferroconcrete, specially developed by a Dutch shipbuilding company. The supports for this massive platform had been designed and built in England and consisted of three enormous steel legs, each at one corner of the structure, the three being joined together by a variety of horizontal and diagonal hollow cylinders, the total combination offering such tremendous buoyancy that the working platform they supported was out of reach of even the highest waves.

From each of the bases of the three legs, three massive steel cables extended to the base of the ocean floor, where each triple set was attached to large sea-floor anchors. Powerful motors could raise or lower these cables, so that the anchors could be lowered to a depth two or three times that of most modern fixed oil derricks, which meant that this rig could operate at depths far out on the continental shelf.

The TLP had other very considerable advantages.

Its great buoyancy put the anchor cables under constant tension, and this tension practically eliminated the heaving, pitching and rolling of the platform. Thus the rig could continue operating in very severe storms, storms that would automatically stop production on any other type of derrick.

It was also virtually immune to the effects ot an undersea earthquake.

It was also mobile. It had only to up anchors and move to potentially more productive areas.

And compared to standard oil rigs, its cost of establishing position in any given spot was so negligible as to be worth no more than a passing mention.

The name of the TLP was Seawitch.

JO