Compressed Natural Gas (CNG) is an emerging marine transportation technology for natural gas. Knutsen OAS Shipping’s Pressurized Natural Gas (PNG®) System is one of several concepts. PNG® provides sufficient pipeline gas storage volume by pressurizing at ambient temperature without any additional gas treatment. One ship classification society published rules and two other developed guidelines for the approval and classification of this new technology. The idea is already more than some decades old, but today in applying state of the art pipeline technology instead of traditional pressure vessel codes, this technology has become economically viable. The main difference to a pipeline is that the cylinder’s main design criterion is not burst but fatigue due to the frequent charging and discharging process in the transportation chain. For PNG, EUROPIPE supplies assembled cargo tank cylinders. Several successful hydraulic full scale fatigue tests have been carried out in the accordance with the requirements in Det Norske Veritas rules for CNG Carriers. The result was always that a fatigue crack penetrated the cylinder wall creating a leak without break in excess of the required lifetime limits. This local failure mode enables gas detection and safe pressure relief of an effected cargo tank cylinder in operation on a ship. The integrity of a cargo tank cylinder under gas pressure in presence of a gas leak was not investigated during these tests. Due to the leak, gas escapes from the cylinder and the pressure decay leads to a temperature drop due to the Joule-Thomson Effect. It should be determined that a fatigue crack of certain maximum size does not become unstable due to the temperature drop and deterioration of the toughness properties of the material which would lead to rupture of the cylinder. The leak before failure diagrams available to evaluate a critical crack length for a given load were verified in a real environment with decreasing temperature of the escaping gas and the cylinder material. The temperature measurements were used to verify models to determine the temperature profile of the gas and the cylinder material resulting from the Joule-Thomson Effect during blow down. Also the impact of the escaping gas on a cylinder placed adjacent to the leaking cylinder at the same distance as in a ship was measured. This test is not only of interest for CNG cylinders onboard ships, but the results are also of high interest for pipelines transporting natural gas under higher pressure.
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Full Scale Gas Leak Test at a Large Diameter X-80 DSAW Pipe
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Reepmeyer, O, Lothe, P, Valsgard, S, Erdelen-Peppler, M, & Knauf, G. "Full Scale Gas Leak Test at a Large Diameter X-80 DSAW Pipe." Proceedings of the 2006 International Pipeline Conference. Volume 3: Materials and Joining; Pipeline Automation and Measurement; Risk and Reliability, Parts A and B. Calgary, Alberta, Canada. September 25–29, 2006. pp. 1-8. ASME. https://doi.org/10.1115/IPC2006-10005
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