A process modelling approach to the evaluation of ship machinery configuration alternatives of LNG carriers

Abstract

The increasing seaborne transportation of Liquefied Natural Gas (LNG) in the current volatile global market and energy supply environment puts a pressure on LNG vessels to be more efficient, environmentally friendly, and costeffective. Modern LNG carriers feature complex and tightly integrated machinery systems to convert the onboard primary energy sources to useful energy demands for propulsion, electricity and heat. Therefore, process modelling and simulation techniques combined with an integrated systems approach is required for the evaluation of different configuration alternatives of LNG carriers. In this paper, we used our in-house process modelling framework DNVGL COSSMOS to develop a generic model of an LNG carrier integrated machinery system encompassing various propulsion and energy recovery technologies. The resulting system model was then coupled with a generic operational profile description accounting for various operating modes and intended trading routes of the vessel. The integrated LNG carrier machinery process model was subsequently used for the evaluation of different technology alternatives and machinery configurations. Namely, the model was used to size the gas-fuel compression trains; assess the introduction and optimal size of an LNG reliquefaction plant; compare electric and mechanical propulsion technologies; and, assess the introduction of energy recovery technologies such as shaft generators and exhaust gas economizers. The model-based studies resulted in an improved insight of this complex integrated machinery arrangement, revealing important performance trade-offs and interrelations between the vessel's sub-systems. The results revealed high energy savings potential of 5% to 8% depending on the energy recovery options implemented, operating profile and trading route. At the same time fuel savings of about 6% were identified, improving the overall cost-effectiveness of the integrated system.