The NMIWAVE computer program is based on linear wave diffraction theory, and calculates first-order wave forces and response motions of ships and offshore structures. NMIWAVE has now been extended to compute mean and slowly-varying components of the second-order wave force. This and a companion report describe the underlying theory and experimental validation of the new computer model. Results are compared with published data and with other prediction formulae currently in use. Criteria for use of this computer model are also discussed.

Comparisons were made with two simple and widely-used approximate formulae. Havelock’s formula proved to be a convenient means of extrapolating NMIWAVE results to high frequencies, but is inadequate at lower frequencies. Newman’s formula, based on long-wave slender-body theory, proved to have a very limited range of validity, and was most unsatisfactory elsewhere.

The new programs were validated in three stages. First, they were checked against each other. Two alternative sets of programs were developed, based on so-called near-field and far-field methods. Results from these two methods agreed well with each other. The near-field results, however, proved sensitive to the way in which the vessel’s waterline was modelled.

Results from both sets of programs were then compared with existing published data, and found to agree well. Finally the programs were validated against experiments, undertaken specially for that purpose at NMI. Theoretical and experimental forces were found to agree excellently in regular wave conditions, but slightly less well in irregular seas.

The present report concentrates on mean drift forces. A comparison report discusses slowly-varying forces and low-frequency response motions.