Design limit exceedance probability of ice loading on propulsion shaft of a polar research vessel

Abstract

Probabilistic estimates of ice impact loading on the propulsion systems of vessels designed to operate in polar waters are necessary to assess the adequacy of current design specifications. Over the course of 13 days of operations in sea ice, high frequency inboard shaft line deformation measurements were recorded aboard the SA Agulhas II, a polar supply and research vessel, and inversion of the dataset performed to determine ice loading on the propeller. The inversion method filters out resonant vibration of the propulsion shaft around its natural frequency, and is implemented as a rapid algorithm developed for application to long time series full voyage data as well as real-time monitoring. Extreme value analysis of inferred ice-induced impact loading in active ice navigation was conducted to obtain Weibull and Gumbel distribution parameters for 1-second interval ice loading maxima. The resulting annual exceedance probability curves indicate that the loading specification used in the design of the propulsion systems for the SA Agulhas II implies a 1×10−5 to 5×10−5 probability of exceedance during a year of regular operations. These quantitative probabilistic reference values for the ice-induced impact loading exposure of a polar vessel operating in first-year and multi-year sea ice conditions represent information of key significance to propulsion design of future polar vessels.