Designers of offshore installations for the North Sea and adjacent waters face a number of exceptional demands specially regarding the extreme and continuing severity of the weather and the large water depths. In response to this challenge, there has been rapid development of new design concepts and a major trend already well established is for the emergence of new types of large monolithic structure. These pose many difficult design problems and, in particular, are not generally amenable to conventional methods for estimating wave loads. For such structures, prediction methods based on diffraction theory are needed.
This paper describes experience in Ship Division of the National Physical Laboratory in the development, validation and application of a computer program written for this purpose that has been supplied to a number of computer bureaus and to Lloyds Register of Shipping and has been extensively used by the offshore industry. This work has been undertaken as part of a research program approved by the Ship and Marine Technology Requirements Board. Some earlier accounts have already been published in Refs. 1 and 2, the first being mainly concerned with the computer program development and the second with experimental validation and preliminary operational experience. The present paper recapitulates the salient results of the earlier publications and reports recent progress including some further experimental results, but emphasizing the more extensive experience since gained in practical applications of the computer program.
Before discussing this experience in detail it may be helpful to be more specific about the nature of the structures concerned. The term monolithic used above is intended in this context to refer rather loosely to a range of structure types that are very diverse in configuration. All, however, contain at least one component which is very much larger in diameter or equivalent section dimension and often quite complex in shape compared with the normal tubular components of a steel jacket. It is because of these features that diffraction theory methods of wave loading analysis are generally needed in fact for reasons that are explained in detail in Ref. 3 and will be briefly indicated in a later section of this paper. In many cases construction is primarily of concrete and diameters of the main components may sometimes range up to 100 or 200 m.
There are currently several broad classes of structure to which this description applies, and these include gravity platforms, tension leg and tension stay structures and storage tanks. At the time of writing the program has mainly been used for gravity platforms for which a substantial number of orders have now been placed. The level of demand may in fact be judged from a recent list of UK North Sea Platform Orders4 showing a count of 11 concrete gravity structures in comparison with 13 steel jackets. The definitive feature of these types of platform is that they rest on the sea bed under their own weight without piling.