This paper presents a novel method to determine experimentally both mean and time-varying drift forces_ This method is applied to measure the low frequency forces in cross-sea conditions. A brief review is given of the theory for determining the second order forces acting on a moored vessel in directional seas. Whereas in previous experimental work only mean drift forces could be determined, the present study shows that time-varying drift forces can be measured with reasonable accuracy. Surge and sway forces and yaw moments are measured for a 200,000 DYT tanker subjected to cross waves in 82.5 m water depth. Comparison between measured and theoretical values is reasonable.
A vessel moored in seas with directional spreading will be subjected to mean and low frequency wave induced drift forces. The low frequency behavior can have an important influence on the design of the mooring system. In a mooring analysis, one typically considers long-crested seas, for which an extensive amount of model test results and validated analytical tools exist. Relatively little work has been carried out with respect to drift forces in short-crested seas. So far, experimental work has been limited mostly to cross seas, i.e., the combination of two longcrested wave systems coming from different directions. Aside from facilitating the interpretation of test results, cross seas do occur quite frequently in a typical ocean environment. Pinkster (1985) derived expressions for the second order drift forces in directional seas and performed tests to measure the mean drift forces in regular cross waves with equal frequencies and also in irregular cross seas. Experiments with a spring-moored vessel in cross waves with equal frequencies were reported by Krokstad (1990), who also investigated response and wave drift damping effects in those conditions.