Ice impact loads on azimuthing thrusters - small-scale impact tests and analysis

Azimuthing thrusters designed to operate in ice-covered waters have to sustain impact-type loads from ice blocks. These impacts can happen at high speeds when ships are traveling in brash ice channels. The response force seen by the thruster is determined by the dynamics of the thruster structure together with the shape and magnitude of the contact force impulse. In some cases, the response force can be larger than the contact force and the ratio between these
two is called the dynamic magnification factor (DMF). In tests described in this study the DMF varied from 0.5 to 1.4.

This paper presents results from small-scale tests with real Baltic Sea ice. Two different kind of tests were carried out. In the first tests, contact force was studied with a pendulum-type impact device. The test setup in these tests was very rigid to eliminate dynamic effects. In the second tests at the VTT towing tank, floating sea ice blocks and model-scale thruster was used for studying dynamics of the impact case. In addition, uniaxial compression tests of sea ice
were carried out in a standardized method to assess the ice compressive strength.

Pendulum-type impact tests are compared to other similar tests. Dynamic model is developed for the second set of tests to better understand the dynamics of the ice impact on thrusters. Further development of the model for impact cases can make the design of ice classed azimuthing thrusters more optimized and safe to use.