Ice-structure impact contact load calculation with dynamic model and simplified load formula

Shipping in arctic and in ice conditions is increasing. Modern propulsion systems often utilize azimuthing thrusters in their various forms. When operating azimuthing thruster equipped ships in ice conditions, the thruster will interact with ice. One such interaction type is an impact-type contact of an ice block and the thruster. The contact causes certain loading to the thruster. The contact load problem can be considered as an ice impact to a steel structure. In a study related to developing the Finnish-Swedish ice class rules, the impact forces due to an ice contact to a steel structure are dealt with a simple theoretical model and an experimental approach is used for the model validation. In this paper, the principle of the dynamic load model, its validation and suggestion for a simplified load estimation formula is presented. For an azimuthing thruster operated in ice conditions, an ice block impact contact load can be solved effectively with a simple three-mass dynamic-system model. The contact region is assumed to be a hemisphere indenting into ice, representing the azimuthing thruster hull or the propeller hub. Model considers ship speed, ship mass, thruster mass, ice mass and connections between these. The dynamic model was verified with available full scale data and with two sets of experiments with ice from the Baltic Sea to give good estimate of the contact loads in impacts. The impact speed considered in these cases was below 5 m/s. The suggested simplified formulation is based on the dynamic contact load model results. The dynamic contact load model was run with different initial values. This gives indication how different parameters effect the contact load. The most significant variables were identified and only they are included in the suggested simplified approach.