For local ice loads, and based on numerical simulation, this paper presents a method for calculating fatigue damage to a ship's structure as it navigates in ridged ice fields. A semi-empirical method is introduced to develop the numerical model of ship-ice interaction in level ice and consolidated layer in ice ridges. Rankine's plasticity model is applied to calculate ice loads in ridge keels. A Weibull model is useful to describe ice load peaks. The structural fatigue stress is found using structural beam theory. According to the ice thickness distribution and a proper S-N curve, fatigue damage can be estimated based on the Palmgren-Miner rule. An example of fatigue damage calculation is presented. The calculated fatigue damage in ridged ice is much greater than that in level ice because of the ice ridge effects.
Fatigue damage can be important for ships operating in the harsh environment of ice-covered waters. Damage can entail oil leakage or even catastrophic failure, threatening overall structural safety. Nevertheless, research into fatigue damage caused by ice action has not been developed well compared with wave action. To date, most studies of fatigue damage caused by ice-induced loads have been conducted using field measurements. Zhang and Bridges (2011) introduced deterministic fatigue assessment using the Ship Right FDA ICE Procedure, as proposed by Lloyd's Register, to assess fatigue damage to a ship's structure induced by ice loads. Suyuthi et al. (2013) derived closed form expressions of fatigue damage for several statistical models of stress amplitude. However, the field measurements are usually quite limited and incomplete. For that reason, it is difficult to evaluate fatigue damage correctly and to provide guidance for the design of new structural components or new ship routes. Compared to field measurements, the ice conditions and ship hull can be easily varied in a numerical simulation, which is useful to complement the lack of ice load data in some regions, or to predict the fatigue life for new structures when only ice condition data are needed. The numerical method seems promising to evaluate fatigue damage.