Potential Wave Drift Force Simplified

Morison's equation has the advantage of simplicity, and has been used extensively to determine wave forces on small diameter members of offshore platforms. For larger members the inertia coefficient of the Morison equation is replaced by a modified coefficient derived from diffraction theory to calculate first order wave forces. Linear wave analysis is not sufficient for compliant structures such as the Tension Leg Platform. Sophisticated nonlinear analysis and costly computer programs are used to calculate the nonlinear terms that cause steady wave drift and slow drift oscillations. This report reexamines the modified Morison equation, and integrates the inertia term up to the wave profile to yield a nonlinear steady potential wave drift. Structure motion and hydrodynamic interaction between columns are included in the analysis. The validity of the simplified approach is verified by comparison with exact solutions. Limits of applicability of the simplified solutions are explained. The heuristic formulations developed can be used for hand analysis checks or incorporated into first order motion analysis programs.