Elsevier

Ocean Engineering

Volume 124, 15 September 2016, Pages 437-449
Ocean Engineering

Transient simulation of the propulsion machinery system operating in ice – Modeling approach

https://doi.org/10.1016/j.oceaneng.2016.07.011Get rights and content

Highlights

  • Bond graph method is introduced as a generalized method for modeling of the propulsion machinery system exposed to transient ice-related torque load.

  • Sensitivity of the propeller shaft response to a different number of propeller shaft lumps and modes is reported.

  • Sensitivity of the produced power and torque in the diesel engine to a different discretization of the crank mechanism is reported.

  • The proportional-integral-derivative (PID) controller is used to guide the combustion process inside the cylinders.

  • The most suitable discretization format of the propeller shaft and crank mechanism is recommended for the presented propulsion machinery system, considering the sensitivity of the propeller shaft response and computational time.

Abstract

The propulsion machinery system is one of the major concerns in the design of ships that operate in ice-covered waters. Numerical models and simulations are important tools for a better understanding of the system subjected to the transient ice-related loads. Design loads and design criteria applied in the typical numerical simulations are described by classification societies. However, the rules do not cover the level and type of discretization of the propulsion machinery system is not covered. Therefore, in this paper we study the sensitivity of the system response to: 1) finite-lump vs. finite-mode propeller shaft sub-models; 2) the number of propeller shaft lumps and modes; 3) simplified vs. complex sub-model of the diesel engine. The propulsion machinery system is modeled using the bond graph method. Dynamic torque and angular velocity responses of all elements of the propulsion machinery system are simulated. Simulation results show that the selection of sub-model discretization and fidelity are important parameters for the accuracy of the dynamic response and natural frequencies. Furthermore, energy flow and computational time are used as criteria to identify the most suitable discretization and fidelity of the sub-models in question.

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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