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Farbod, Faegheh

University of British Columbia

Liquefied natural gas (LNG) is increasingly being used as fuel in ships to decrease air pollution. Although materials intended to be in contact with this cryogenic fluid (-163 °C) can safely bear the thermal stresses, the surrounding structure is made of structural steel. It is known that structural steels undergo ductile to brittle transition at low temperatures, and cracks could form in case of large LNG spills. However, there is a significant gap in scientific knowledge on the effect of small spills of LNG on the shipbuilding steels, where the material returns to room temperature after each spill; the focus of this investigation is, therefore, placed upon potential cases with repetitive small and localized LNG spills. In this study, liquid nitrogen (LN₂) was used instead of LNG since its release is not harmful to the environment and has a lower boiling temperature than LNG. A series of experimental investigations were carried out on E36 shipbuilding steel, a material commonly used for building ships that may be deployed in cold regions of temperatures reaching as low as -40 °C. Specimens were exposed to up to 150 cryocycles, after which tensile and hardness tests were performed, in addition to X-ray diffraction (XRD) and metallography investigations. Moreover, finite element analysis was performed with Abaqus on four different plate geometries with a thickness of 3, 6, and 18 mm. Non-linear material properties were used in the analyses. Out of a few models or varying complexity, the axisymmetric model was found to have good ratio between accuracy and computational cost. It was shown that material could experience high stresses, close to the tensile strength of the material; however, the experiments did not show any change in the microstructure of the material, and no crack formation was observed. XRD analysis showed the existence of residual stress in the material after cryocyclic tests; however, the trend of stress buildup was inconclusive. The results generally show that E36 steel without pre-existing cracks is safe when subjected to moderate number of LNG spills, but that needs to be re-evaluated for larger number of spills or for more complex situations.

Text

2022-05-18

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/81596

Mechanical Engineering

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/