Extreme Value Estimation of Mooring Loads Based on Station-Keeping Trials in Ice

Abstract

The purpose of this work is to perform an extreme value estimation of the mooring loads associated with station-keeping of a ship operating in ice. In general, the design of mooring lines is based on estimation of the extreme loading caused by environmental conditions within the relevant area. In March 2017, station-keeping trials (SKT) in drifting ice were performed as part of a project headed by Statoil in the Bay of Bothnia. The objective was to investigate the characteristics of the mooring loads for the supply vessel Magne Viking for different types of physical ice management schemes. Tor Viking was employed as an ice breaker as part of the physical ice management systems. The ice conditions (i.e. the ice drift velocity and the ice thickness) during the trials were monitored by using Ice Profiling Sensors (IPSs). Different patterns of ice-breaking manoeuvers were investigated as part of the physical ice management systems, such as square updrift, round circle, circle updrift and linear updrift pattern were studied as part of the field experiments. The peak values of the mooring loads for the supply vessel are determined by using the min peak prominence method. For the purpose of extreme value prediction, the peak over threshold method and block maxima method for a specific time window are applied to estimate the mooring loads that correspond to specific probabilities of exceedance (or equivalently: return periods). These loads can then be compared to the design loads that are being specified by relevant international standards.