This paper describes the main results of a recently-completed research programme on wave drift forces. Methods for predicting both mean and low frequency components of the drift force are reviewed. A simple parameter gives some indication of the relative importance of viscous drag and wave diffraction effects. Numerical procedures, based on so-called ‘near-field’ and ‘far-field’ expressions for the drift force, are described. Calculated forces on a moored drill-ship, obtained using the NMIWAVE computer program, are compared with simple analytic solutions and with experiment.
Predicted mean forces and first-order response motions agree well with measured values in regular waves and in wave groups. Mean forces in irregular random waves agree slightly less well. Discrepancies are attributed to a mixture of numerical and experimental causes.
The paper also discusses the validity of three different methods for estimating the low-frequency force: the NMIWAVE solution, which calculates the full quadratic transfer function, and two more approximate methods. The main features of these three methods are related to the effects of wave diffraction, and of spatial gradients associated with both the first and second-order wave fields. These in turn are related to three simple parameters. General conclusions based on these parameters are borne out by numerical and experimental data. The importance of the second-order wave is discussed. It is difficult to simulate this component adequately either numerically or experimentally.
