An Improved Method for Model Test Based Identification of Drift Coefficients and Damping for Floating Platforms

摘要

A new method for identification of wave drift coefficients and low-frequency damping from model testsrnis presented. The method is motivated by the lack of robust numerical methods for the calculation of thernviscous contribution to the mean and slowly varying wave drift forces – often the largest contribution forrnsemisubmersibles in storm sea states. The drift coefficients represent the diagonal of the differencernfrequency quadratic transfer function, and because Newman’s approximation is implied by the method,rnthese drift coefficients will give rise to both mean and slowly varying forces.rnThe new method is based on a weighted least squares solution of the equation of motion, and uses arncombination of linear and non-linear least squares methods. The parameters are identified such as tornminimize the error in the slowly varying horizontal motion over the entire measurement record.rnThe method will be demonstrated using data from physical model tests of three different semisubmersiblesrnin a large number of different sea states, including wave only, waveu0002current andrnwaveu0002currentu0002wind tests. The accuracy of the obtained results is investigated by the ability of thernidentified model to back-calculate the measured motions. In addition, a u0003standard erroru0003 on the driftrncoefficient curves is provided that shows the frequency interval where the identified drift coefficients canrnbe trusted.rnThe case studies show that the identified models are able to back-calculate the measured motions withrngood accuracy. Both viscous effects and wave-current interaction is seen to have an impact on the driftrncoefficients and damping. The quadratic damping is seen to be of much less importance than the linearrndamping in most cases. Still, it is argued that a significant portion of the linear damping comes fromrnviscous effects, and that the linear viscous damping represents a low-frequency best-fit to the actualrnall-frequency quadratic damping.