Design And Selection Methodology Of Propulsion Systems For LNG Carriers:a Focus On Slow Speed Dual Fuel Engines

The increasing demand for LNG, coupled with tighter emission standards has led to a shift of LNG carrier propulsion systems design and selection from the conventional Dual Fuel (DF) Steam turbine option to other alternatives, with the latest being the 2-Stroke Dual Fuel engines.This study was aimed at designing and selecting a suitable 2-Stroke Dual Fuel propulsion system for a 147,210 m3 (referred to as the GUANGDONG PROJECT), initially fitted with a steam turbine propulsion. To do so, the factors affecting the design and selection of LNG carriers’ propulsion systems were investigated. An evaluation was further done on the designed and selected 2-stroke slow speed propulsion compared to a typical Single Fuel Diesel Mechanical and the conventional steam turbine propulsions.Chapter one gives an introductory summary of the propulsion system design problem with emphasis on the Boil off gas (BOG) handling as the main criteria for LNG carrier propulsion system selection. It concludes by highlighting the main and specific objectives of the study. A review of the pros and cons of the available LNG carrier propulsion options as well as the entire propulsion system design process is presented in chapter two, with the differences specific to the LNG carrier propulsion and propulsion system evaluation parameters noted.The rigorous methodology for design and selection process for the 147,210 m3 vessel is given in chapter three. As quick decision making tools, analytical and estimation methods have been utilized in the estimation of ship resistance, in the form of Holtrop and Mermen’s Method which was programmed in excel spreadsheet using simple if loops. As a comparison to the results of propeller open water characteristics curves obtained from the Wageningen B-series charts, a manual plot was done on excel, again using the simple if loop programming. As an unbiased comparison of propulsion options, a Comparative Life Cycle Cost (LCC) evaluation was performed with the chosen 2-Stroke slow speed propulsion, Single Fuel Diesel Mechanical and the conventional steam turbine propulsions. Chapter four tackles the design of suitable shafting system, using a high strength, high corrosion resistance material (Aqualoy 22) in accordance to ABYC standards and Lloyd’s Register shipping rules. Comprehensive torque and stress analysis was done from first principles and the vibration issues associated with the 2-stroke propulsion briefly outlined.The research revealed the benefits of the adoption of 2-stroke DF propulsion as the most suitable option for LNG carriers considering the current trend which points to increasing sizes on LNG carriers owing to the soaring global demand for LNG, a gaseous fuel poised to play a major role as the alternative fuel of choice to traditional liquid fuels in the foreseeable future, not only for marine applications but also across the entire trans- port sector.