Abstract
Selected results from previous model basin studies on slow drift motions of moored semis in irregular waves are reviewed. Valuable experiences have been achieved, which indicate the conditions when conventional diffraction analysis can be used and when they need to be corrected. In high sea states, significant viscous drift forces need to be taken into account and calibration to model tests is important. Furthermore, when currents are added to the waves, the drift forces will normally be increased, and corrections with validations are needed. At the same time, the current will also increase the motion damping.For further investigation into this topic, results selected from a new set of systematic experiments are investigated and compared to a numerical model including viscous drift effects. In this paper, only an initial analysis is made of a limited set of the data. The results show that in waves only, slow-drift surge measurements in severe irregular sea states can be reasonably well reproduced by the numerical model using horizontal drag coefficients Cd=1.15 on both the columns and the pontoons, except for the upper zones of the columns where Cd=2.0 has been used. With current included, the measurements confirm that drift motions are increased, especially in longer wave periods where viscous wave-current interaction effects contribute together with potential flow effects.
