“High runs” of a ship in multi-chromatic seas

Abstract

When a ship operates in steep irregular waves, it can attain, in intermittent time intervals, abnormally high speed, due to waves' effect. Such events may occur in following seas and they will be called hereafter “high runs”. Investigations have been carried out for this peculiar type of ship motion, in three directions: firstly, the statistics of high runs are calculated by numerical simulation, exploring, in particular, their dependence on the wave spectrum and the sea state. Secondly, a rather neglected up to now analytical method, proposed in the 60s by Grim, for the quantification of the probability of high run occurrence, is assessed, against a direct numerical approach. Finally, the actual velocity of a ship in high run incidents is investigated. Suitable metrics are applied on the difference between characteristic velocity values associated with wave's form, and surge velocity, during high runs.