Comparison of drift force calculation methods in time domain analysis of moored bodies

This paper compares five numerical methods for calculating drift forces in time domain analysis of moored bodies. The first method is the most basic one. It uses all terms of QTF (quadratic transfer function) matrix based on double summation. It is the most correct due to no approximation but needs large memory and cpu time. The second method uses diagonal terms of QTF matrix with Newman approximation. It reduces memory but still uses double summation. When the wave spectrum is narrow banded, the third and fourth methods with more approximated single summation are applicable. Those methods are much approximated but fast due to single summation. One uses fixed local frequency and the other uses time variant local frequency in calculating QTF. The fifth method uses interpolated local frequency of the third and fourth methods. A turret moored FPSO and a spread moored DTV (deck transportation vessel) are analyzed as numerical examples and the accuracy and cpu time are compared for each method.