Mobile Drilling Platforms
Authors: Macy Robert H. 
Published: December 2007

Types of modern drilling platforms are evaluated, including jack-up, column stabilized and such special types as single hull, catamaran, workover and wireline units. Some specific design procedures are given, including determination of dimensions, weights, stability characteristics and stresses. The relation of motion to stability is discussed and the advantages of model tests are mentioned. In Appendices A and B, an new method for calculation of righting moments by optical projection through a model is given as well as a convenient method for estimating wave forces.

Introduction

This paper discusses certain design aspects of the latest types of mobile drilling platforms. Earlier types of platforms, including types which are still being built as well as those which are no longer in favor, have been discussed in the literature. An attempt is made to give some of the basic design criteria as well as to make a comparison of advantage and disadvantages.

Jack-Up Platforms

General Features

Jack-up platforms consist of a buoyant hull and three or more legs. To put such a platform in service, the legs are lowered to the bottom and jack pressure applied until a sufficient degree of bottom support is obtained, after which continued jacking will raise the hull to the desired clearance above the water line. The hull then serves as the drilling platform and is complete and self-contained in all respects.

The legs may be individual in operation without any connection between them. Usually this type of leg will penetrate into the bottom to a fairly considerable depth. Buoyancy tanks or even enlarged footings may be located at the bottom of these legs. On the other hand, the legs may be connected at the bottom by a hull or mat. The bottom mat, while it does involve a substantial amount of additional steel, is very desirable on soft bottoms and has the advantage of making the bottom of the legs fixed-ended, thus reducing the bending moment in the legs to one-half of the value for individual free-ended legs.

Determination of the air gap or distance from the water surface to the bottom of the upper hull is of major importance. Selecting this value too low could result in disaster. Having it too high will be very costly. One means of determining the over-all height is to take the designed mean low water level and add to it the following components:

anticipated bottom settlement, storm and astronomical tide, crest height of wave above still water level, and a margin of 10 to 15 ft.

The crest height of the wave above still water level may be estimated in the absence of definite oceanographic data at 55 per cent of the designed wave height. A typical example of this determination might be as follows:

Mean low water 100 ft Anticipated settlement 4 ft Storm and astronomical tide 8 ft Crest height of 40-ft wave 22 ft Margin 10 ftTotal 144 ft

In addition to the air gap margin, additional margin on the length of the legs should be provided in the event of greater settlement or the possibility of waves exceeding the design criteria. The legs may be of tubular or truss-type construction, according to the type of jacks to be used and the operating water depth. Tubular legs involve less labor cost per ton of steel but on large platforms they become too heavy and present too much projected area against waves. To keep the weight and projected area as small as possible with either type of legs, the use of alloy steel is indicated. For shallow water depths, the tubular type seems most economical; for great water depths, the truss type is best. The change-over point is probably somewhere near a height of 200 ft from sea bottom to the lower side of the hull. Most platforms of the jack-up type have the legs arranged to operate vertically in guides. In very deep water. to obtain a satisfactory resistance to overturning, this means that the legs sometimes must be spaced far apart. As a result. the bottom mat (if any) must then be quite wide, as must the upper platform. To overcome this, LeTourneau has provided hinges on the guides at the hull which permit the legs to tilt out to a fairly considerable angle. This permits a broad base at the bottom while keeping the upper platform at reasonable dimensions.

JPT

P. 1069ˆ