Effect of Electrification on the Quantitative Reliability of an Offshore Crane Winch in Terms of Drive-Induced Torque Ripples

Offshore crane winches are equipment installed on sea vessels and designed for accurate lifting and lowering of payloads at subsea levels in all conditions and are equipped with Active Heave Compensation (AHC) capabilities which keep the handled payload stable relative to the seabed. Due to logistical challenges and short maintenance windows imposed by sea conditions, maintenance of offshore crane winches in open sea is costly and time consuming, and its reliability is therefore of the highest importance. In recent times, electric drives for the actuation of the winch drum have seen a surge in popularity in favour of hydraulic drives owing to advances in control technologies of AC motors, as well as higher efficiency, lower noise and lack of an oil reservoir. In light of these changes and the importance of reliability, a study on the effect of the electrification (ie. switching from hydraulic to electric actuation) of offshore crane winches from a reliability viewpoint is necessary. In previous studies, it has been established that the different inherent properties of electric and hydraulic drives lead to torque ripples of different magnitudes and frequencies which may induce additional mechanical stresses in the drivetrain. The effects of these ripples on a system level have so far only been studied from a Noise-Vibration-Harshness (NVH) viewpoint in applications where driver comfort takes precedence in design (ex. electric vehicles, tractors). In this paper, these drive-induced ripples are simulated for common electric and hydraulic drive configurations used in offshore crane winches, and their effect on the reliability of the driven gearbox, as the most critical component in the drivetrain, is analysed. It was found that electric drives produce in general ripples of smaller magnitude (between 0.44% and 1.07% of static torque for electric compared to between 3.12% and 3.30% for hydraulic), which enhances the reliability of the driven gearbox (1.25% to 3.44% reduction in lifetime due to electric ripples, and 9.29% to 9.67% due to hydraulic ripples in comparison to an ideal torque source with no ripples). However, depending on the design and number of pumps in use, the size of the hydraulic torque ripple can have a wider range of between 1.31% and 7.69% and the subsequent reduction in lifetime compared to the ideal case ranging between 4.07% and 21.37%.