Incidents of loss of position for semi-sumersible vessels have been found to be caused by wave-drift loads. For a semi, the wave-drift loads can be significantly larger than predicted by conventional methods based on potential theory, due to viscous loads on the columns. In addition, the wave-drift load may increase due to the presence of current.

A DP system will give much stronger damping than a mooring system of similar restoring stiffness. This will alter the characteristic of the vessel's low frequency motion, which will tend to be exponentially distributed. A consequence of this is that the extreme wave-driven vessel excursions will be large in comparison with the average motion.

A marine operation can only be carried out provided the critical variables of response stay below given limits with a sufficiently large probability. To estimate probabilities of limit non-exceedance, simulation of 100 hours of vessel motion is carried out for a number of sea states. Weibull distributions are then fitted to the response data and further used for extreme value calculation. Although the processes of wave frequency (1st order, WF) motion and the low frequency (2nd-order, LF) wave-induced motion are totally different, Weibull distributions could successfully be fitted to the total (LF+WF) motion. The fitted Weibull distributions were close to the exponential distribution, which they should for a strongly damped vessel.

From the Weibull distributions quantiles of vessel motion could be calculated for given probabilities of limit non-exceedance. Examples of how this information can be used in interrupt criteria for marine operations are given.

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