Effect of Different Propulsion Systems on CNG Ships Fleet composition and Economic Effectiveness

The compressed natural gas (CNG) transport is becoming nowadays an attractive solution not only for stranded gas shipping where current technologies like liquefied natural gas (LNG) and pipelines are not economically competitive, but also for long-enough distance transport of large volumes of natural gas, where LNG still represents the most economical solution. In fact, recent studies on pressure vessels (PV) materials allow to significantly reduce the ship displacement in case of PVs Type 3 and Type 4 when compared to existing prototype designs with PVs Type1 and Type2, also leading to completely different hull forms. Since CNG ship design is a new research field for Naval Architecture, the concept design phase assumes an utmost importance with respect to design of well-known ship types,not only for the main parameters determination and performances assessment, but also for the study of completely different solutions for some subsystems. To this purpose,the present paper presents a technical-economic comparison between two different kinds of propulsion systems: conventional diesel drive propulsion with propellers and a complete diesel electric solution with pods. The differences between the two solutions are influencing both internal layout and ship dimensions, leading to two completely different ship concepts. These have been compared to determine the most cost effective solution not only for the single CNG ship but mostly for the fleet composition in a given transport scenario.