Motions of the SSSV "Uncle John" were measured while the vessel was operating in the North Sea. Low-frequency components of these motions were correlated with a theoretical model which combined low-frequency wind gust loads and second-order wave forces, calculated under inviscid potential-flow assumptions. The model represented response characteristics of the full-scale vessel, including its heave, roll and pitch damping, and the dynamic positioning control system with thruster force limits. The numerical model often underestimated the vessel's response motions, and this was probably due to a combination of : viscous drag forces, which were not represented at all in the theoretical model; inaccuracy in modelling features of the thruster and control systems, in particular delays introduced by thruster action and filtering; differences between wind forces acting on the actual vessel and those calculated by its "wind feed forward" system; low-frequency swell waves, which could not be measured reliably; uncertainties in the amount of damping present and numerical modelling uncertainties.
Mathematical and physical models are used during the design of offshore structures to estimate wave loads and wave-induced motions. It is comparatively rare, however, to undertake detailed full-scale measurements of vessel motions after the vessel has been commissioned, and to correlate these measurements with numerical predictions and with model test results. The adequacy of the design procedures and assumptions is therefore largely unknown, and improvements which could in principle come from operational measurements are not made. Two earlier investigations into full-scale and predicted motions of semi-submersible platforms were reported by Olsen and Verlo (1976) and by Mørch, Kaasen and Rudi (1983). The first of these papers was concerned with first-order, wave-frequency motions and structural stresses; the second with transient motions and damping. Both papers reported reasonable agreement between the measured and simulated results.