ON THE STATISTICS OF ICE LOADS ON SHIP HULL IN THE BALTIC

Probabilistic models are developed for ice loads in specific ice conditions and for long term ice loads encountered by shell structures of ships in the Baltic Sea. The models developed are aimed to give results that can be used in the development of design codes for ice-strengthened ships. The developed methods are verified by extensive full scale measurements onboard IB Sisu, MS Arcturus, and MS Kemira. Also, a database of damage statistics is gathered and analysed to evaluate, for verification purposes, the extreme load level at the midship and aftship areas. The physical background for the ice loads is studied through idealisation of ship-ice impacts. The idealisation covers the ice edge failure process, ice conditions and ship's ability to move in ice. The ice edge failure is assumed to initiate by crushing and the crushing continues until the ice edge breaks either due to bending or shearing failure. These failure events determine the peak values for the ice loads. The physical parameters included in the idealised ice edge failure models are defined as random variables. The main random variables are level ice thickness, thickness of the consolidated layer in a ridged ice field, ice flexural strength, and ice crushing pressure. Four point bending field tests are done to evaluate the statistical properties of ice flexural strength. The statistical characteristics of the ice crushing pressure is determined by modelling the crushing as a compound Poisson process. For a long term statistical approach, a semi-empirical approach is developed for estimation of long term ice loads on various parts of a ship. The empirical database required in the developed approach is gathered by four year measurements onboard MS Kemira. The effect of ship hull shape and ice breaking capability are included based on the calculated relationships applying the idealised model of ship-ice impacts. An equivalent level ice thickness of a ridged ice field is determined to include the increase of load level caused by the consolidated ridged ice on various sea areas. The long term approach developed is verified by four year measurements onboard MS Arcturus.