Transient simulation of the propulsion machinery system operating in ice – Modeling approach
Introduction
Models for steady-state or stationary analysis, commonly used for traditional propulsion machinery systems for open water conditions, are not sufficient for investigating events like the start-up of the diesel engine, random misfiring, clutch engagement, or transient load variations. Dynamics of such models are simplified since the propulsion machinery responses are considered to be slow. However, it is very common to use models developed for forced torsional vibration analysis in transient simulation. The combination of the transient simulation, clutch engagement, and bond graph (BG) method, used for modeling of the propulsion machinery system, can be found in Engja and Bunes, 2000, Bruun et al., 2005.
During ship navigation in harsh environments, e.g. a cold Arctic environment, it is expected that the propulsion machinery will be exposed to additional transient loads e.g. ice-related loads. These transient loads affect the propulsion machinery system through the propeller, which rotates in the flow of water affected by the ship wake and ice piece wake. The term ice piece wake, introduced by Veitch (1995), quantifies the disturbance of the flow field in the presence of submerged ice pieces, and it is illustrated in Fig. 1. In some cases, submerged ice pieces can collide with the propeller blades.
The classification societies (i.e. DNV, 2016; FSICR, 2010; IACS, 2007) describe the torque component of the ice-propeller load as a sequence of ice impacts that should be applied on top of the mean hydrodynamic torque from open water conditions. The rule-based ice-propeller load is used by several studies for steady-state or transient simulation of the propulsion machinery response (e.g. Dahler et al., 2010, Batrak et al., 2012, Batrak et al., 2014, Abel and Schreiber, 2013, Abel and Schreiber, 2014, Polić et al., 2013, Persson, 2014). However, the influence of the propulsion machinery model on the response is not discussed in the previous studies. Furthermore, numerical discretization of the propulsion machinery system is not covered within the design rules.
This paper aims to identify the influence of the propeller shaft sub-model and engine sub-model on the propeller shaft response and propose a minimum level of discretization. The propeller shaft is discretized using finite-lump and finite-mode techniques as a system of lumps and modes, respectively. The engine sub-model, in particular mechanical elements in the cylinder unit, is discretized either as a simple single equivalent mass or as a complex rigid dynamic system of moving crank mechanism masses. The simulation model is implemented in 20-sim software v.4.4 using BG method.
Section snippets
Propulsion machinery system
The propulsion machinery system is a coupled system that includes a diesel engine or electrical motor, transmission line, and propeller. The engine drives the propeller to generate a directional thrust force that moves the ship across open or ice-covered water (see Fig. 1). Power is transmitted from the diesel engine to the propeller through a mechanical transmission line. The main elements of such a mechanical transmission are the propeller shaft and propeller shaft couplings. It may include
Propulsion machinery modeling
Based on the overall layout of the propulsion machinery system, illustrated in Fig. 1, convenient sub-systems are identified: FPP, propeller shaft, flexible coupling, and diesel engine. The BG method is used to describe the energy propagation between and within sub-systems as illustrated in Fig. 2.
Simulation results
The dynamic response of the propulsion machinery system and sub-models is obtained using the Backward Euler (BE) solver, a simple implicit numerical method that solves ODEs using a fixed time step. Sub-models are modeled in 20-sim software v4.4, and all parameters used in the sub-models are listed in Appendix A.
Summary and discussion
The bond graph method is presented as a generalized method for propulsion machinery system modeling. A detailed description of implementation of the BG theory for each sub-model of the propulsion machinery is given.
The propeller shaft sub-model is modeled as a system of finite-lumps or finite-modes. The sensitivity of both discretization techniques to impact load is tested by comparing the propeller shaft angular velocity response. As a result, it is noticed that a minimum of 10 lumps are
Future work
The BG propulsion machinery model presented here forms a basis for the evaluation of the propeller shaft response under the transient propeller torque load. However, all sub-elements beside propeller and crank shaft, and crank mechanism should be further developed for evaluation of the propulsion machinery system performance in the transient state (i.e. non-linear behavior of the flexible coupling, more accurate PID control parameters, fuel injection system and rate of heat release in the
Acknowledgment
This research is funded through the Norwegian Research Council Project no. 194529.
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