Gas fuelled ships: effect of LNG plant design and ship operational profile on boil-off and gas composition

LNG utilization in ship propulsion seems to offer significative reduction in operational costs and environmental impact even in ships other than LNG carriers. However, still different technological aspects have to be in deep investigated in order to define a general LNG system design approach. In this study the installation of a B-type tank for gas propulsion in ships other than LNG carriers has been considered. LNG composition, Boil Off Gas (BOG) production and pressure variation during ship sailing have been analyzed. In particular, an analysis on a specific vessel during its trip has been carried out in order to obtain BOG production, gas composition and then its characteristics such as heating value and Methane Number. Three different gas composition and three different tank insulations have been considered. Results show high changes of gas propriety during the vessel trip, in particular significantly differences of the BOG and LNG proprieties have been highlighted. This study shows that an accurate managing of BOG and LNG flows has to be done in order to obtain a mixture more homogeneous as possible during the entire ship trip. The proposed methodology is indeed necessary to allow plant designer and ship owner to chose the right plant components and lay-out together with ship operational profile that allow to achieve the lowest cost and reduce the environmental impact.