Some aspects of the analysis of offshore structures
University of Southampton, Department of Civil Engineering,
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
guage 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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