Ship Propulsion Hydrodynamics in Waves

Abstract

The shipping industry is paramount for global economic growth by enabling the trading of enormous volumes of goods across the world. However, maritime transport is a huge and growing source of greenhouse gas emissions. Consequently, the shipping industry is required to speed up its environmental transition towards a zero-carbon emissions fleet. Alternative marine fuels, in combination with ship optimization in realistic operating conditions, could be a solution to reduce the marine ship emissions drastically. The emissions of harmful gases and particulates from the engine increase when the ship operates in waves. This phenomenon is particularity problematic for lean-burn natural gas engines because of the increased amount of unburnt methane emitted. The solution to this problem requires studying the interaction between the ship hydrodynamics and the engine dynamics. For this purpose, a coupled engine-shaft-propeller model capable of predicting its performance in waves needs to be developed. At the same time, evaluating the ship propulsion system performance in realistic operating conditions is essential to estimate the installed power of the main engine and to optimize the ship voyage. The purpose of the present work is to investigate the interaction between propeller loads and engine response of a ship sailing in realistic operating conditions. First, an investigation was carried out to determine the propeller model necessary to estimate the propulsive forces in waves. Second, a coupled propeller-engine model was built to evaluate how the environmental effects influence the ship propulsion system performance in terms of propulsive forces and unburnt methane released in theatmosphere. Third, the effect of waves on the propulsive coefficients was studied by conducting numerical simulations and model experiments. The traditional method applied to compute the propeller performance in waves, knownas the quasi-steady approach, was adequate to estimate the propulsive forces in realistic operating conditions. The simulations performed with the coupled engine-propeller model proved that neglecting time-varying wake field, ship motions,and propeller close-to-or-breaking water effects would lead to a poor prediction of the propulsive forces in waves. The coupled engine-propeller model allowed determining that the amount of unburnt methane released in the atmosphere considerably increases when the ship operates in waves. The investigation conducted on the propulsive coefficients showed that the effective wake fraction depends on both the propeller loading and the motions of the ship. An inverse non-linear correlation between the thrust deduction fraction and the propeller loading was observed. A small influence of the ship motions on the thrust deduction fraction was noticed. The propulsive efficiency was mainly affected by the variation of the open-water efficiency caused by the propeller loading. Therefore, using the propeller open-water curves or performing overload self-propulsion model-scale experiments in calm water would provide a sufficiently accurate estimation of the time-averaged propulsive efficiency in waves for the considered case studies. The results of the PhD project are useful to investigate the performance of marine propulsion systems in realistic operating conditions. The techniques and tools employed in the current study can be directly applied in the ship propulsion optimization process to include the effect of waves. The work conducted in this research also constitutes a step towards the implementation of the liquefied-natural gas as a marine fuel.