This chapter outlines work in progress at the University of Strathclyde on a co-ordinated research programme dealing with the stability of semi-submersibles. It begins with a brief review of the present situation before explaining why research is needed This is followed by a statement of the research objectives, an explanation of the method of approach and a description of the research programme Consideration is also given to management, initial work and future developments


Ever since the focus of offshore hydrocarbon exploration and exploitation shifted from the relatively shallow and calm waters of the Gulf of Mexico to the more hazardous ones of the North Sea In the early 1970s, one of the major challenges for the operators has been to find the most effective floating structures for carrying out various tasks under North Sea conditions Many consideration must be met before a satisfactory offshore vehicle is designed, and practical experience has shown that the deal combination of the various parameters of a particular offshore structure will never actually be achieved For example, a conventional drillship has desirable mobility characteristics but is less effective as a working platform because of its streamlined form.

Operational experience has shown that one of the most efficient vehicles for North Sea operations is the semi-submersible, in particular a design with twin pontoons, connected to the working deck by four to eight vertical columns. Semi-submersibles have built up a good record of working reliably in hazardous conditions. However, as attempts are made to get even more ‘value’ from them - for example, by increasing the deck load-carrying capacity - questions inevitably arise over how to quantify the effects of proposed changes on the safety and stability of semi-submersibles. In this respect studies must also be made that will take into account the lessons learnt from two recent major disasters, those involving the ‘Alexander Kielland’ in 1980 and the ‘Ocean Ranger’ m 1982

Safety in its broadest sense is a very complex subject, but from the point of new of semi-submersible performance it is stability which requires the major focus of attention. The importance of this aspect is fully recognised but the factors involved in semi-submersible stability are very closely related and cannot be readily translated into rules and regulations that wll make possible efficient performance along with a sound level of safety However, if cost-effective exploration and exploitation of offshore hydrocarbon resources are to continue, the most rational criteria for semi-submersible stability do need to be developed and applied.

With this background in mind it was felt that a useful contribution could be made by the University of Strathclyde because research teams have been studying the stability of ships since 1973 and more recently have investigated the motion responses of a semi-submersible listed through damage, incorrect ballasting or other causes.

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