Risk-based approach for structural design of ice-strengthened vessels navigating in the Baltic Sea

Abstract

Risk-based approaches have been widely used in the offshore sector and currently, these methods are increasingly used in naval architecture. However, risk-based approaches are not applied to determine structural scantlings of ice-strengthened hull areas for vessels that navigate in the ice-covered sea areas.

The aim of this thesis is to present a risk-based approach for the local design of the ship's ice-strengthened areas. This study is limited to ships navigating in the Baltic Sea. Based on that, the lifetime ice-induced loads are determined according on the long-term full-scale measurements conducted on board IA Super ice class chemical tanker MS Kemira in the Baltic Sea. The required structural scantlings are obtained by utilizing plastic design methods. The second moment reliability analysis is used to estimate the frequency of occurrence of the defined limit states.

The required scantling obtained by utilizing the risk-based approach are compared with the requirements of Finnish-Swedish ice class rules, ice class rules of the Russian Register and IACS Polar rules. Furthermore, the frequency of ice-induced damages to the ice strengthened shell structure of MS Kemira has been estimated and the results are compared to available damage statistics.

The gained results show that there is a modest variation between rule requirements for ice-strengthened shell plating thickness and obtained thickness determined based on measured ice-induced loads and define limit states. In contrast to that, the strength of the frames determined by the rule requirements seems to be somewhat insufficient. However, this finding is partly supported by the gathered damage statistics and therefore, this issue requires further research, e.g. by analysing the collapse of the frame in further detail.