Safe speeds of navigation in ice as criteria of operational risk

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Safe speeds of navigation in ice as criteria of operational risk

Tunik, A.

Safety of ships operating in ice-choked waters depends, among other factors, on operational aspects (first of all, speed) and on the structural capability of the ship. Generally, ice class is the primary but rather vague criterion characterizing ship's operational limits and her structural strength. A better assessment of ship's safety and operational limits can be obtained from safe speed analysis for the ship in question. Solving a structural response problem with regard to the speed of ship/ice impact can yield the ship's speed limits in given ice conditions and corresponding to the chosen strength criteria. The main ice-related parameters and ship/ice impact scenario characteristics are treated statistically. This in conjunction with a wide range of structural strength criteria (including plastic deformations up to the ultimate plastic state of local side structures) made it possible to provide a probabilistic interpretation of the safe speed, e.g. via risk levels of ice-induced damage versus ship speed in ice. The paper outlines the concept of safe speed definition, describes the nature and the model of speed-dependent ship/ice impact loading, presents a range of damage criteria up to the ultimate load-bearing capacity in plastic stage, explains the probability-based approach and shows the role of safe speed analysis for different ice classes.