Compressed Natural Gas (CNG) marine transportation is poised to be a viable solution to bring natural gas supplies to new markets or provide a solution for monetizing stranded gas. However, transporting CNG can be dangerous when there is a leakage of from cracks. The leak may cause a change in the material properties of the pressure vessel containing the gas, hence the crack will propagate and resulting in catastrophic failure. In this project, a numerical simulation is created to simulate gas leaking through cracks and investigate the effect it has on the vessel wall due to Joule-Thomson effect. This report will present to the reader how the numerical simulate is created, and the results from the simulation using experiment conditions. Comparison of simulated and experiment results are also given in this report. Bachelor of Engineering (Mechanical Engineering)