Effects of hydrodynamic coefficients on the global motion characteristics of spar structures

Abstract

A number of studies have already discussed the influence of various nonlinear effects on the response of a spar and the variation of the results with different approaches. In this work, the effects of the values of the inertia and drag coefficients used in Morison's equation on the response of a spar are studied in order to assess the sensitivity of the results to the assumed values of C[] and C[]. Two different programs, NDASPAR and SPARTCOM, developed independently at the Offshore Technology Research Center (OTRC), have been used for the study of the dynamic response due to excitation by two bichromatic waves, denoted as BC5 and BC8. Comparisons have been made between the experimental results obtained at the wave basin of the OTRC, results published originally by the respective authors of the two programs and the results obtained in this work. The quantities selected for comparison were mean offsets and maximum and minimum responses from the computed time histories of the surge and pitch motions, as well as the peaks of the frequency amplitude spectra (FAS) of these motions, representing the frequency content of the motions and allowing us to distinguish between linear and nonlinear components. The studies show that both programs give results which are in reasonable agreement with the experimental data as far as maximum (and minimum) response values are concerned and also show some significant differences in their frequency content and the amplitudes of the nonlinear peaks in the FAS of the motions. It was found that SPARTCOM reproduces the mean offset values much better, whereas the available version of NDASPAR does not account for mean drift effects. The study also shows that the value of the C[] primarily affects the amplitudes of the peaks at the natural frequencies while C[] affects the amplitude of the peak at the difference-frequency, as well as the natural frequencies. The two programs assume that changing the value of C[] also changes the added mass and therefore results in a change in the natural frequencies.