Fatigue life assessment of ship propulsion shafting due to torsional and bending vibration

In accordance with the Paris Agreement, the International Maritime Organization (IMO) mandates that carbon dioxide emissions be decreased from 2008 levels by at least 40% by 2030 and 70% by 2050. To meet these requirements, there are a number of measures, the most common of which are slow steaming and engine de-rating. As an example of a slow steaming measure, a low- speed two-stroke engine with a direct-coupled, fixed-pitch propeller arrangement is considered. Although it is common to study torsional vibrations as the cause of fatigue, bending vibrations due to sailing in rough seas were also considered in this analysis. The ship propulsion shaft is modeled with 3D beam FE elements, including torsion and bending. The torsional excitation of the engine during normal firing is defined by tangential pressure harmonics. The first two harmonics of the exciting lateral forces and moments, as well as the torsional moment, were taken from the propeller. Fig. 1 shows the FEM model of the propulsion shaft and the engine torsional excitation torques. The barred speed range for this propulsion system is between 40 and 51 rpm. After calculating vibration torques and lateral forces, a detailed 3D FEM simulation of the inter-mediate shaft is carried out. Low cycle fatigue life is assessed due to torsional vibrations, and high cycle fatigue life due to coupled torsional-lateral vibrations. The fatigue life was compared for the conditions MCR, 80% MCR, and the slow steaming condition of 70% MCR. The calculated vibrational stresses were additionally checked according to the guidelines of the International Classification Society (IACS).