Optimisation of Compressed Natural Gas Marine Transportation with Composite-Material Containers

Abstract

Abstract Marine transportation of natural gas is critical for its monetization. Pipelines and liquefied natural gas (LNG) are the established technologies for offshore natural gas transportation, with compressed natural gas (CNG) recently proposed as an economically preferable alternative under certain conditions. In previous work, we have delineated areas in a transportation-distance/gas-volume diagram, where each of the three transportation means mentioned above is economically most attractive. In general, once offshore pipelines are excluded because of water depth and sea-floor terrain limitations, CNG transportation is considered attractive compared to LNG for relatively small volumes and short distances. Until recently, the preferred material for proposed CNG containers has been metal, for reasons of simplicity, robustness, and development cost. However, metal CNG containers have a number of disadvantages, the most obvious of which is that the container itself may be several times heavier than the contained gas, and, consequently, such gas may be only a small fraction of the cargo carried by a loaded CNG ship. Composite containers are an alternative to metal, with a number of advantages. First, composites are significantly lighter than metal. Second, composite containers can reach considerably higher containment pressures (hence capacity), while still maintaining lower weight compared to metal. Finally, composite containers are much less prone to corrosion, and as a result can contain gas that has not been fully treated. These facts make composite containers an attractive proposition and greatly increase the overall attractiveness of using CNG to monetize stranded gas. We present here a thorough study using both metal and composite containers and optimized operating conditions, such as pressure and temperature, to explore the applicability of CNG for marine stranded gas transportation. A new map of the economic attractiveness of LNG and CNG as a function of transportation distance and gas volume is presented, showing a considerably expanded area of CNG preference over LNG.