Most of LNG carriers have the boil-off gas problem which takes place during storage, loading or discharging and the shipвЂ™s voyage. Therefore, the purpose of this chapter is to investigate the various LNG carrier propulsion systems, taking into account the latest technology progress and innovation in this field. This results in the insufficient BOG to fuel the propulsion plant and leads to the development of alternative more fuel efficient propulsion systems.ДЄfter an exhaustive review of works related to propulsion systems of LNG carriers, an extensive variety of systems installed on board has been found, ranging from turbines to internal combustion engines with endless variants. The boil off rate is significantly reduced due to the improvement in LNG tank insulation. In order to comply with the stricter regulation, the propulsion plant has to improve its emission performance and fuel efficiency. The NOx emission limits and the Energy Efficiency Design Index (EEDI) are getting more and more stringent. So this change requires a more flexible and efficient propulsion system to accommodate various operating profiles. However, the proportion of short-term contracts and even spot cargoes has increased substantially since 2000. Traditionally, LNG was almost exclusively traded under inflexible long-term contracts and the vessels are operating on fixed sailing routes. The driving factors for the development of the propulsion system come from the following three aspects :Д¬hange of trade pattern. LNGC fleet and order book by propulsion type (as of May 2017). So they become a popular propulsion system choice for LNG carriers. The two-stroke dual fuel engines can offer substantial efficiency advantages over both the DFDE and steam turbines.

After 2010, two-stroke dual fuel technology has made a breakthrough and has been applied to LNG carriers, including both the high pressure and low pressure gas injection concept. In 2004, the four-stroke dual fuel engine broke the domination of the steam turbine and started to be used on LNG carriers as a part of dual fuel diesel electric propulsion system.

However, in order to improve operational efficiency, reduce engine room size and increase cargo capacity, a number of alternative propulsion options have been developed in the industry. Because it offers dual fuel burning capability, low maintenance cost and high reliability. Steam turbine has been the dominating propulsion plant used on LNG carriers since 1960. The development history of the propulsion system for LNG carriers is presented in Figure 1. So far there is no standard propulsion system that applicable to all types of LNG carriers. Steam turbine domination in the LNG carrier sector has been gradually broken.

Since 2004 many LNG carrier projects with propulsion other than steam turbine have been under construction, such as dual-fuel diesel electric propulsion and two-stroke diesel engine propulsion with reliquefaction plant. Influencing factors including economic consideration, environmental regulation, as well as safety issues made a profound impact on the technology developments implemented on LNG carrier propulsion systems. The LNG shipping industry had been tremendously cautious in choosing the propulsion system, and the steam turbine had been practically an exclusive option for LNG carriers over the last several decades. Furthermore, comparisons among different propulsion system were also carried out and related evaluation was presented. In this context, this chapter investigated the main characteristics of these propulsion options in terms of BOG treatment, fuel consumption, emission standards compliance, and plant reliability. Since propulsion system significantly influenced the shipвЂ™s capital, emission regulation compliance and navigation safety, the selection of a proper propulsion option with technical feasibility and economic viability for LNG carriers is currently a major concern from the shipping industry and thus must be comprehensively assessed. There are various proposed propulsion solutions for LNG carriers, ranging from the conventional steam turbine and dual fuel diesel electric propulsion, until more innovative ideas such as slow speed dual fuel diesel engine, combined gas turbine electric & steam system, and hybrid propulsion based on steam turbine and gas engine. The LNG carriers are undergoing a period of rapid and profound change, with much larger size ships and novel propulsion systems emerging for fulfilling the market trends of LNG shipping industry.

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