Optimizing the bearing vertical offset of a marine propeller shaft system

Abstract

The Vertical bearing offset is vital for the safe and efficient operation of marine shafting systems. The bearing offset is made relative to the shaft and must be tested for various operational conditions to prevent overheating, whipping, vibration, and wear that can lead to costly repairs and downtime. The offset of each bearing is achieved by minute adjustment of the bearing pedestal performed during dry-docking. However, the hull load variation can cause the preset bearing offsets to be inadequate resulting in potential bearing damage. Classification Society of ABS and DNV specifies the limit of bearing reaction force, shaft slope, and bending moment to ensure safe operation. These values are usually determined using a finite element model or experimentally using costly strain gauges. A more cost-effective solution is using the stiffness matrix of the bearing shaft system to derive the influence coefficients for the bearing reaction forces (BIC), shaft slope (SIC), and bending moment (MIC). This approach is validated with the existing model of a marine shafting system of a 50 DWT oil tanker that consists of one tail and intermediate shaft supported by stern tube bearing (STB), and intermediate bearing (IB) respectively along with six bearings at the engine shaft using the finite element method. Besides, the study applies the Multi-Objective Particle Swarm Optimization (MOPSO) technique to optimize the vertical position of bearings. It uses BIC, Slope SIC, and MIC to construct the objective functions and find a Pareto-optimal solution that satisfies the requirements of ABS and DNV in terms of bearing reaction forces, bending moments, and slope at the main stern tube and intermediate bearing. The optimal bearing offsets show improved shaft bending state for the operating condition under evaluation and reduced the slope at the STB by 34%, indicating its effectiveness in solving optimization problems.