Transient torsional vibration analysis of Polar-Class propulsion shafting systems under ice-propeller interaction loads

The design of propulsion plants for Polar-Class vessels has to ensure the safety and preservation of ship operations in sea ice environments. In particular, the effects of ice-propeller interaction on shafting segments constitute a potential hazard for the integrity of the entire propulsion system. Collisions of ice blocks with propeller blades induce high torsional vibrations along shaft lines and cause the prime mover’s output torque to fluctuate abruptly. These processes also undermine ships’ propulsion efficiency and manoeuvring capability during ice breaking expeditions, as well as during ice navigation of commercial vessels and offshore operations. Consequently, the development of effective design methods dedicated to the dynamics of ship propulsion systems is fundamental to simulate the effects of ice-induced loads correctly. To this end, full-scale measurements have proven essential to support the evolution of robust design criteria and updated regulatory guidelines. The research activity presented in this doctoral thesis aims to accomplish two objectives: firstly, delivering a modelling methodology to simulate the propulsion shaft lines of Polar-Class vessels, or the analysis of the torsional dynamic response caused by ice- propeller interaction processes; secondly, providing indications about the mathematical characterization of the ice-propeller torque pattern. The research project includes full-scale measurements conducted aboard the two Canadian Coast Guard icebreakers Henry Larsen and Terry Fox. Torsional vibration data acquired from CCGS Terry Fox’s shaft lines validate an original mathematical model to simulate the dynamic torque delivered by Diesel engines. Besides, an innovative integrated measurement system is installed aboard CCGS Henry Larsen icebreaker to achieve concurrent monitoring of the shafts’ dynamic response, sea ice conditions, and propulsive performance. The experimental measures obtained in open-water navigation are employed to validate the numerical models of both vessels’ shaft lines. Conversely, the datasets of the ice-induced shaft responses are used to calculate the actual ice-propeller milling torques and correlate them to the ongoing ice conditions; this represents a unique outcome in this subject’s state-of-the-art scenario. Alterations of the main properties of the rules’ ice-propeller excitation—defined by the current design recommendations—are then tested to determine their influence on the shaft line dynamic response. Overall, the results suggest that the mathematical procedure defining the ice-propeller torque pattern, which is described in the seguidelines, might need to be reviewed