Evaluation of Environmental Effects of Propulsionsystems for a PSV
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- Institutt for marin teknikk 
An increased focus on the environment in the offshore industry has required several new restrictions relating to the emission of greenhouse gases (GHG), and most agrees that additional measures must be implemented to ensure a reduction of GHG. Stricter emission regulation ensures that the industry must take action in order to meet the new regulations and be in the forefront in regards of environmental thinking. Regulations for the marine industry are given by IMO and MARPOL. To date, there is no regulation of CO2 emissions, but there are indications that this will be implemented in the future. The goal of this study is to evaluate the effect of running the main engines in a propulsion system with variable speed compared to constant speed in a diesel/gas-electric propulsion system for a PSV. The study includes fuel consumption, CO2, CH4 and NOx emissions to air. The study also evaluates the possibility to make use of multiple smaller diesel engines rather than the current standard mid-sized diesel engines in a propulsion system.The model made as a part of the assignment in this thesis, is based on consumption and emission data provided by the engine manufacturers. Total fuel consumption and emissions for all the different systems are evaluated up against a given operational profile for the vessel. The systems that are to be evaluated in the study is are:? Diesel electric consisting of four CAT 3516C engines? Diesel electric consisting of 10 CAT C18 engines? Diesel electric consisting of six CAT C32 engines? Gas electric consisting of four RR C26:33L9 engines? Dual-fuel electric consisting of four Wärtsilä 34DF enginesThe results show significant differences in consumption and emissions for the different systems. In regards to fuel consumption and global warming potential, the gas solution when operated with variable speed on the engines seems like the best solution. However, not much separated the three top solutions, and the two diesel solutions when operated with variable speed (CAT C3516C and CAT C18) comes out just as good or even better then the gas solution when this system is operated with constant speed, which is the case for gas solutions installed today. What is interesting to see is that every solution has a great potential of reducing consumption and emissions if they are operated with variable speed rather than constant speed on the main engines. Even the dual-fuel solution, which comes out as the least favourable solution when it is operated with constant speed, have a great potential for reducing consumption and emission when the engines are operated with variable speed. However for this system the NOx emission is increased when variable rpm is applied.In regards of electric efficiency in a diesel/gas-electric system, it looks out to be a significant difference when the engines are operated with variable speed rather than constant speed, and the overall efficiency is improved with variable speed. This is due to better efficiency of the generators, and that the transformer is no longer necessary between the main engines and propellers due to the AC-DC-AC conversions. It is also seen that the efficiency varies with the engine load in both cases. However the overall efficiency for systems utilizing variable, varies less with the engine load and are higher in the low load area for the engines.The study and model shows that when just looking at the environmental effect, and when all economics and operational aspects are put to side, the solution that consists of gas engines operated with variable rpm is the most environmentally friendly solution. However due to challenges such as limited availability of gas, which makes it a bit risky to build pure gas operated vessels, ship-owners might want a solution that ensure availability of the vessel regardless of vessels location. This is one of many reasons why we see the dual-fuel engines installed on todays ?gas operated vessels?. The study indicates that by installing diesel engines and operated them with variable speed, these systems would be better for the environment then the dual-fuel engines.