A computer system has been developed for structural calculation of mobile offshore units under dynamic loadings resulting from the motions of the platform and wave forces. It uses a space frame or Finite Element modification of the structure. A number of sea spectra are selected to represent the sea in the operation area. The program determines, without manual intervention, statistical values of motions of the unit and of stresses at a selected number of locations.

Computed movements of different platforms are compared with model tests. Computed stresses are compared with measurements at sea made on the SEDCO 135 "Sea Quest" platform. The correlation between theory and measurements is good.


Semi-submersible platforms have been operating for only a short period in the very hard weather conditions of the North Sea. Though most of them were subjected to severe storms and withstood them perfectly well, on most units modifications and reinforcements have been made during the first years of operation. The fact is, of course, that we still lack experience for this sort of "ship" which has a somewhat complicated structure, when the situation is compared to that of conventional ships. The naval architect has most of the time difficulties in defining the most severe loading cases for the scantling of the structure he is studying.

During the last few years, Classification Societies have issued first provisional rules, then regulations, that can be applied to mobile platforms, but these rules are by far less precise than the regulations existing for conventional ships. For example, one cannot yet find, formulae directly giving the scantling of a bracing. This is mostly because of lack of experience, but also because of the complexity and singularity of the structures which may be very different from one to the other.

In order to check the behavior and the reliability of these platforms Subjected to different sea conditions, Classification Societies have developed their own computer programs, each unit being calculated as a separate case. We are presenting here Bureau VERITAS' method and point of view for the analysis and checking of these rather complicated but interesting structures, which are embodied into an integrated program we have developed. See fig. 1.

2.1 Basis

One possible assumption, in the design of semi submersibles, is to consider the mobile unit as a Fixed platform and subject the structure to the wave loads induced by the hundred year wave, for example, using one of the well known wave theories, such as Skye Beria?s 5th order, or others. This, of course, may appear as not such a bad approximation, since the main purpose of a semi-submersible is to have very small movements, so that as a First step, it could be considered as fixed.

In fact, for the most-oaten encountered waves periods (smaller than 6 seconds), these structures have really very small movements.

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