PNG ? the natural solution

Norwegian shipping company Knutsen OAS has launched a new gas carrier design. The ship, if built, is intended to get natural gas from its point of origin at sea to processing facilities. The design tackles the challenge of profitably exploiting marginal gas fields, where the use of conventional gas transportation methods (e.g. LNG, pipeline) proves uneconomic. It does this by storing the gas at pressure in specially-constructed bottles. These are based on the gas industry?s standard 42 inch diameter pipe. Knutsen OAS?s design is similar to the Coselle system (The Motor Ship, September 2001), where gas is stored in standard 6.6 inch diameter pipe coiled into a carousel, in that it challenges the idea that using large pressure bottles for carrying natural gas is too expensive. However the Knutsen OAS design has earmarked a pressure of 250 bar for gas containment. This compares to the Coselle system?s proposed maximum of 200 bar. The Norwegian shipowner hopes to build a fleet of these pressurised natural gas (PNG) bowloaders. It believes the ships could also find employment supplementing conventional gas transportation methods on larger fields. The proposed prototype vessel for the Knutsen system is an offshore loading version. It can load gas products, including LPG. Operating as a floating production unit, it would load gas through a small processing plant into a large reservoir of PNG tanks. This reservoir contains a large number of interconnected vertical cylinders made out of 42 inch diameter pipe. The stack would stand 36m high. Knutsen also suggests the vessel could be used for terminal to terminal operations. It is claimed that the vessel can transport about half the quantity of gas (in terms of energy) of a similar-sized traditional LNG carrier. With an overall length of 275m and a beam of 59m, the projected design will have a carrying capacity of 40,000t, or between 950 and 1,200 million cubic feet (27-34 million cubic metres). This compares to the 7,700t or 330 million cubic feet (9.7 million cubic metres) of the prototype Coselle design. The design uses existing approved components, so there are no technical obstacles to prevent the building of a bottle gas tanker. A safety assessment performed by DNV indicates better or equal safety compared to conventional LNG transport. A DNV spokesman confirmed to The Motor Ship that it has been working with Knutsen for more than a year on the PNG carrier design. He said the fundamental approach for the concept was a combination of state-of-the-art practices from the offshore and shipping industries. The scope of work carried out by DNV included the provision of technical support in the development of the Knutsen ship, safety assessment of major critical issues, identification of critical failure modes and cylinder design. “We think the project is feasible” said the spokesman. He adds that, like the Coselle system, a number of large oil and energy companies have been in contact with both the class society and Knutsen to explore the potential of the system. At the moment no approach has been made to any shipyard. The only cost estimates The Motor Ship could obtain were that the cost of the PNG vessel would be “somewhat higher” than a 138,000m3 LNG carrier (which at today?s best prices is around $170 million). Although the vessel carries less cargo than a LNG tanker, its advantage lies in not requiring LNG liquefaction systems or a degasification plant.