Load spectra for slender offshore structures in waves and currents

Abstract

AbstractThe influence of currents on the wave induced dynamic response of offshore structures with slender members is assessed by deriving appropriate load spectra. The analysis permits the implications of the modified Morison equation to be examined, and suggests that methods of equivalent linearization commonly employed may yield unconservative results. An alternative formulation is proposed, in which the equations of motion are linear in the structural kinematics but the non‐linear dependence of drag force on fluid particle motions is retained. This leads to a load spectral density matrix expressed as a sum of terms, involving convolutions of the wave kinematics spectra. Results are obtained for a plane frame idealization of an offshore jacket structure. These demonstrate the strong influence of current, and the importance of retaining in the loading terms up to the third power of wave particle velocity.