Computer Analysis of Offshore Drilling Platforms
- Robert M. Meith (Chevron Oil Co.) | Albert B. Gooch (Chevron Oil Co.)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- September 1966
- Document Type
- Journal Paper
- 1,056 - 1,062
- 1966. Society of Petroleum Engineers
- 1.10 Drilling Equipment, 1.6 Drilling Operations, 4.5 Offshore Facilities and Subsea Systems, 4.1.9 Tanks and storage systems
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- 203 since 2007
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This paper discusses some of the major design problems related to an offshore drilling structure and the series of computer programs developed by the Chevron Oil Co. to aid in the solution of these problems.
The engineer concerned with the design of an offshore platform is faced with two semi-conflicting goals: (1) to design a safe platform to protect lives and the increasing investment required in these multi-million dollar platforms, and (2) to design a less costly structure to keep unit production costs competitive. To most advantageously meet both these goals the engineer must utilize the best tools available. Adoption of known engineering science for use on the digital computer is one of the major tools that has been (and is still being) developed in recent years. It has enabled the engineer to provide more rigorous solutions to many of the problems with which he is faced. The computer can play an important role as a tool to aid the engineer in the design of an offshore platform.
Structural Design Problems
To more easily see how the computer can aid the engineer in the design of an offshore platform, it is necessary to have some understanding of the structural problems involved. Some major problems encountered are the high degree of indeterminacy, the variety of imposed live and dead loads, foundations and secondary stress effects. The degree of indeterminacy of a platform is usually no greater than that found in the design of a building or bridge and is not, therefore, peculiar to the offshore oil industry; but the variety of loadings for these platforms is one of the most severe encountered in any structural design.
Loads Loads imposed upon a platform differ with the operating criteria of a company and with the geography of the location. An example of the effect of differing operating criteria would be where, for the same location, one company would use a self-contained platform (all operations carried on from the platform) and another uses a tender-operated platform (some operations handled by a moored vessel). An example of the geographic effect would be the large ice loads imposed upon a platform in Cook Inlet, Alaska. For the Gulf of Mexico, a variety of loads must be considered in the design of a drilling platform: 1. Static dead loads (weight of the platform, production units, engines, mud tanks and pumps, etc.). Total static dead load may range from 3,000 tons for a small platform in 40 ft of water to 10,000 tons for a large platform in 200 ft of water. 2. Installation loads, such as lifting or launching loads. Most operators in the Gulf preassemble as much of the platform in the fabrication yard as possible, with the size of the platform sub-units usually governed by the lift and reach capability of the available ocean-going construction equipment. The addition recently of three revolving 500-ton derrick and several large launch barges, in addition to the other heavy construction equipment in the Gulf, have enabled operators to install platforms made up of much larger pieces. Now it is not unusual to have 1,500- to 2.000-ton jackets (templates) launched on location with the remaining portions of the platform in 350-to 400-ton sub-assemblies, lifted into place. Therefore, each unit must be designed not only as an integral part of the platform, but also as a unit capable of being launched or lifted. 3. Dead loads that move from one location to another, such as derrick and pipe rack loads. Pipe rack and derrick combined loads may range from 300 to 1,500 tons. These loads move with the derrick as it shifts from one well position on the platform to another. 4. Live loads, such as wind and waves. This is where the design of an offshore platform in the Gulf is most significantly different from that of most any other structure. Hurricane wind loads may range from 60 to 70 lb/sq ft of average surface area and, occurring simultaneously, hurricane wave loads may be 5 to 10 times greater than the most severe wind loads. Both loads may occur, depending on the location, from any direction. Of particular significance to the designer is that the centroid of the wave forces is proportionately higher on the platform with increasing water depth (Fig. 1). 5. The varying combinations of Loads 3 and 4. The drilling program of these platforms will normally range through one or two hurricane seasons; therefore, the designer must concern himself with many combinations of wind-wave and moving dead loads, with each possibly determining the size of members in a particular section of the platform.
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