Hull resistance and air-injected ballast free system performance for liquified natural gas ship

Abstract

Ballast water discharge may introduce and transport unwanted marine organisms to the discharging area. The Marine Environment Protection Committee (MEPC) of the International Maritime Organization (IMO) considers ballast water hazardous due to possibly have a negative impact on the receiving ecosystems. Many researchers have investigated possible solutions for the management of ballast water to minimize the risks, including the ballast-free system. However, the application of the ballast-free system has created a new issue on the hull resistance. Many techniques have been developed in order to reduce the frictional resistance of ship navigation. The need to have an advanced Liquefied Natural Gas (LNG) ship with environmentally friendly and low fuel consumption has brought to the application of an LNG ship with an air-injection ballast-free system. This research aims to determine the effect of resistance on the LNG ship which has been fitted with ballast-free system and to evaluate how the air-injected pressured bubbles reduce ship resistance and improve the system performance. Firstly, the total hull resistance of the LNG model with a onesided system was determined by simulation using ANSYS CFX and validated by laboratory experiment. Secondly, the resistance of the two-sided system was generated using ANSYS CFX. The experiment and simulation were limited to Froude number Fr=0.17 to Fr=0.22 at the ballast draft. The total resistance in the model’s scale was extrapolated to the ship’s scale according to the International Towing Tank Conference (ITTC – 1957) equations. From the extrapolated result, the LNG hull with the ballastfree system has increased the total bare hull resistance by 7.58% and 23.71% for onesided and two-sided systems, respectively. The increment of resistance is due to the additional wetted surface area of the ballast tanks and pipes. Meanwhile, the 0.5 bar air injection shows the optimum resistance reduction compared to the other air injection pressures. The 0.5 bar air injection has reduced the total bare hull resistance by 20.17% and 24.67% for one-sided and two-sided systems, respectively. The reduction of resistance from the two systems is due to more area on the hull’s bottom surface has been surrounded by air bubbles. Thus, these findings can be a guideline for the estimation of power calculation and future improvement from the current works.