Shaft Fatigue Considerations for Two-Stroke Direct-Coupled CPP Propulsion Systems

The problem an engineer faces today when commencing a torsional vibration analysis of a diesel engine propulsion plant is the ever-increasing complexity of these plants. One aspect of the greater complexity is the absolute increase in calculations that are required due to the use of PTI and PTO gears with attached generators, multiple geared diesel engine arrangements and direct-coupled two-stroke engines with a CPP system. In order to predict the behavior of any physical systems, a model suitable for mathematical analysis is of great importance. The requirement of the model is it has to be able to predict the behavior of the system with sufficient accuracy. With the ever-ongoing development of higher output diesel engines and increased complexity in operation and design of propulsion plants have proven the need for mathematical models capable of predicting behaviors at non-resonant conditions. These calculations must include the effect of damping, vibratory torques in reduction gears and elastic couplings, and heat losses in elastic couplings and torsional dampers. For certain installations it must also include more detailed analysis of fatigue limits based on fatigue theory such as low cycle fatigue, high cycle fatigue, torque reversal, and transient vibration criteria. The analysis and calculations must involve all expected operating parameters of the propulsion plant in question.