This paper examines the modelling of wind gust spectra over the sea, and the resulting dynamic responses of a floating production platform in both calm water and waves

Measurements at the West Sole platform were analysed to provide the mean wind profile, turbulence intensities, correlations between measurements at different levels and over a period of time, coherence and phase differences between measurements at different levels. Special attention was given to analysing the low-frequency wind speed spectrum. A modified form of the Kaimal spectrum was found to fit all the selected data

A model of a typical semi-submersible floating production platform was tested in wind and waves Motions in wind and waves could generally be predicted independently, and linearly superimposed Numerical simulations of the model's response in unsteady wind were compared with the measured data The two sets of results generally agreed well though the dynamic surge was over predicted

There were significant differences between force coefficients obtained from measurements in steady and unsteady flow, and when the unsteady flow data were analysed In different ways The simulated responses were, moreover, sensitive to the values of the force coefficients Steady-flow values were used in the numerical simulations


Compliant offshore platforms may respond at low frequencies in both wind and waves The mechanisms of second-order wave forcing have been described elsewhere, and numerical prediction techniques have been developed (Pinkster, 1979, Standing and Dacunha, 1982)

There has also been growing industrial interest In the response of compliant structures to wind gust loading BMT performed a computer simulation study (Rowe et al, 1984) on the response of a semi-submersible floating production system This study showed that dynamic wind loading could cause large motions at the natural response periods, particularly In pitch Large amplitude wind-induced motions may reduce under-deck clearances, Increase riser stresses and mooring loads, and otherwise affect the safety or operation of the platform

There are a number of sources of uncertainty in predicting the response of vessels to low-frequency wind gust loading There are, firstly, uncertainties associated with the form of the wind spectrum Itself Different spectral formulae vary widely at low frequencies, and earlier work had found evidence that there might be more energy in the wind at low frequencies over the sea than had previously been thought (Eidsvik, 1985, Wills, 1986a)

Other uncertainties In predicting the motion of a compliant platform In waves and a gusting wind arise from non-linearities In the mechanisms of loading and response BMT's numerical model incorporates non-linearities arising from wind loading, buoyancy and mooring forces, but neglects wave loads and the interaction between waves and wind Pitch-dependent wind loading coefficients, required by the simulation, would normally be obtained from a wind tunnel experiment designed to measure mean forces and moments on a fixed model On the basis of current formulae for wind spectra, this simple approach predicts rather large amplitudes of motion, in reality motions may be attenuated by wave action through time-varying stiffness, additional damping and other factors

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