Some aspects of the analysis of offshore structures

Abstract

In this thesis, a study is made of the effect of random wave forces on self-supporting steel and concrete oil drilling platforms. Various methods of estimating the forces on the structure, and various ways of idealising both the forces and the structure itself, are compared, the - objective being a realistic and safe design.

The sea is here represented by a wave amplitude spectrum, from which spectra for the forces on the structure are derived using a linear wave theory, in two ways. Firstly, using the well-known Morison equation, which requires experimental drag and inertia coefficients; and secondly by considering wave diffraction from the structural members. A quantitative comparison is made of the two methods. Using the diffraction theory, it is possible to gauge the effect of sheltering - i.e. the effect on the forces on one member due to the presence of another.

The principal structures considered here are idealised as plane framed structures (though the theory is applicable for structures with, say, plate elements also), and in this connection wave forces on inclined frame members are considered. This is particularly useful for steel structures. A comparison is made between the results obtained by evaluating the forces 'consistently' and by 'lumping' them at element nodal points.

In addition, a comparison is made of solution methods which ignore certain cross-correlation terms in the equations of motion for the response with one that includes such terms, in an attempt to show that a fuller analysis is no more difficult, and is likely to be safer, than the more approximate methods.

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