Simulation of manoeuvring safety of ships in adverse weather conditions when subject to limited power imposed by EEDI improvement by engine derating

Abstract

Motions of a ship in adverse weather conditions results in increased effects on roll, pitch and heave motions to non-negligible values that would increase the speed drop to a disproportionate level, likelihood of contact with seabed in shallow waters, propeller emergence, passenger discomfort. Predicting the influence of adverse weather on the safety of a ship if the engine is derated for the purpose of improving the Energy Efficiency Design Index (EEDI) has been a hot topic since its recommendation by the Marine Environment Protection Committee (MEPC) of the International Maritime Organisation (IMO). A lot of research is being done towards developing an acceptable method of predicting the impact of derating a ship’s engine. Presently, most researches are based on predicting the power loss and speed drop. This research proposes a method for determining power level to which a ship’s engine can be derated such that safe operations is sustained at defined environmental conditions. Normally, the tests to predict the manoeuvring capabilities of a ship at the design stage are usually achieved through detailed captive, free running physical model tests in the laboratory (or designated basin) or by the use of Computational Fluid Dynamics techniques. Thus to predict the suggested dynamic characteristics at different levels of power in adverse weather condition will require numerous modelling attempts, which is a very expensive and time consuming process. Developing Eshipman is an attempt to demonstrate that it is possible to practically minimise the time and financial demands of carrying out these predictions with minimal error but acceptable accuracy. In order to test the idea, the physical dimensions of a specimen ship and non-dimensional hydrodynamic derivatives of the ship’s hull, rudder and propeller, from the experimental results from published article (see section Chapter 9 – Appendix A for the data) were used to simulate the motion of the ship in calm weather and the results were compared with the experimental plots. The program that simulates the environmental conditions were merged with the calm water code, thereby creating in a program that simulates the motion of a ship in adverse weather condition. EShipman was used to simulate the motion of the ship in calm weather; the results of the turning circle trajectories with its associated roll effects in calm waters, and zigzag motion simulation were compared with the experimental results. This thesis also shows a zigzag motion plot which was labelled to show how the logic for writing the algorithm was produced. In the present state of manoeuvring studies, the windage area of the ship was given (even by the class societies and researchers alike) as a constant value or some formula that does not take the effect of roll motion on the windage area into account. A formula for obtaining the approximate lateral windage area was modified to obtain a new formula which incorporates the effect of instantaneous roll angle so as to obtain a more indicative result. The simulation of the vessel with low metacentric height in ballast to perform a turning circle motion with the rudder at 35 degree angle, showed a stable roll angle of 10 degrees which implies a 53% increase in windage area on one side of the ship. If this happens to be the windward area, it will result in an overturning moment which may lead to a capsize in a combined wind and wave effect. Applying this correction to the windage area formula for modelling the motion of a ship in a wind only (initially 270 degrees) condition shows an increased roll angle and some drift motion in the form of reduced tactical diameter. Details is in section 5.1. A turning circle motions simulation using a subject ship showed an unacceptable drop in speed (i.e. below 4 knots being minimum navigational speed provided, IMO (2017) due to reduced rudder inflow velocity which disproportionately reduces the rudder performance) when power was reduced to 65% (a region of maximum efficiency). It also shows how the roll angle does reduce with a reduction in ship power and additional simulations were done to show the speed drop for different sea states (Beaufort number) under the influence of wind and wave. This was compared with the semi-empirical work of Kwon (2008) and the empirical work of Kim et al. (2017). It also demonstrates how the heave and pitch response increases with reducing power and then starts dropping from 65% power. These motions, indicates that reducing the maximum continuous rating by 35% or more in order to improve the efficiency, will make the ship unsafe when faced with the defined environmental condition or higher. Also, the modular principle proposed by the Mathematical Modelling Group of Japan (MMG), was tested and it was derived that with a 9% increase in rudder area, the 65% MCR simulated turning circle tactical diameter in calm water was reduced to the calm water 100% MCR turning circle tactical diameter. It was demonstrated that increasing the rudder area improved the ship’s dynamic performance in adverse weather condition. EEDI calculations were done for 100%MCR and 80%MCR to demonstrate that the efficiency actually improves with reduced power. Furthermore, some explanation highlighted the fact that just reducing the power alone will not always improve the efficiency due to the specific fuel oil consumption characteristics which has a minimum point at 70% MCR. Equations were formulated which fits the specific fuel oil consumption curve to make for ease of application. On the overall outlook, this research does propose a methodology that can be easily used to evaluate how derating the engine of a ship (for the purpose of improving its EEDI) will influence its manoeuvring safety in adverse weather condition.