Natural gas can be brought to the consumer by a fleet of ships with compressed natural gas (CNG) technology. These ships will serve as both storage and transport vehicles discharging directly into a land based gas grid via an on/offshore discharge terminal, an offshore platform or offshore buoys.
Different proposals for transportation of gas by ship directly from the field to the consumer without the use of costly liquefaction, regasification and storage plants have been evaluated for many years. A new type of ship has been introduced having a large number of vertical pipes, designed according to enhanced pipeline design principles transporting compressed natural gas. The weight of the containment system is 50% of the weight required by conventional pressure ship design codes, making possible a large storage volume. Ships have so far been designed to transport up to 34 MSm3 gas on each voyage.
The present concept for compressed natural gas was introduced by Knutsen OAS Shipping and has been developed with assistance from Europipe GmBH and Det Norske Veritas. Economic evaluations show that the new Knutsen PNG® concept will be highly competitive as compared to pipelines and LNG transport for distances of 3,000 nautical miles. The advantage is less investment in infrastructure and greater flexibility. The potential market for CNG carriers is large as more than half of the world's known reserves are associated and stranded gas.
This paper outlines the key steps in the development process, summarizing the findings of the risk study, the cargo containment design, the qualification testing and the PNG economics for different transport scenarios.
The idea of being able to transport gas at ambient temperature and high pressure is equally old as the LNG development. The Compressed Natural Gas (CNG) option seemed especially tempting as this would allow for cost-savings in liquefaction, storage and regasification installations. Hence, the cost of land based infrastructure could be significantly reduced and also enable more flexibility in use of the ships as they would not be as dependent on a tailor made infrastructure being in place as is the case for LNG. This can also open up for offshore loading of gas direct from the gas source using established offshore loading principles.
A few decades ago the design codes available implied that the thickness and weight of the containment tanks was rather large, leaving little capacity left for carrying gas. This prevented economic solutions for CNG transport. However, important technological developments within high strength materials, welding, non destructive testing and production have taken place during the last decades mainly due to the oil companies requirements to move oil and gas into deeper water areas and over longer distances. The development of reliability based codes and standards based on first principles have been accepted and explicit risk acceptance criteria have been implicitly set.
The new pipeline standards of today are more or less based on the above technological development improving project economics without jeopardizing safety.