1998

Soininen, Harri

The propeller-ice loads consist of both actual contact loads due to propeller penetration into an ice block and non-contact loads that are hydrodynamic disturbance loads generated by the presence of an ice block in the vicinity of a propeller. In this work the contact loads are studied experimentally during a milling type open propeller ice contact. A model of the ice failure process is developed based on the experiments. An effective load model is also developed in order to be able to calculate load levels. In the laboratory experiments a tool having a propeller-like profile was attached to a pendulum and impacted with an ice sheet. The global loads on the tool and the pressure distribution along the tool profile at mid-depth of the ice sheet were measured. The failure process was observed. A process model was formed. The blade leading edge opens cracks towards the groove formed by the previous blade and accordingly the face side hardly experiences any contact at all. On the back side a spall is formed and the ice is crushed within the spall. In the case of large confinement local crushing may also occur instead of spalling. The crushed ice is extruded towards both the leading edge and the trailing edge of the profile. Two-dimensional behaviour is assumed. The failure loads of solid ice are studied with slip-line theory using the Mohr-Coulomb failure criterion. The pressure distributions due to extrusion of crushed ice are studied using both viscous and granular models. The effective load of a section is considered to be the average of instantaneous loads during the process. A simplified method to calculate the effective load is formed for various contact geometry and ice strength parameters. The total load of a blade is the integrated effective loads of each section. The calculated total load is validated against some measured full-scale values. A relatively good result is achieved.