LNG Tank - Revisiting Overfilling and Overpressurisation Hazards
Dennis M. H. Ngai, Venkatesh S.
This paper discusses the improvement of safety in LNG tanks designs over years, the safeguards available in modern full containment LNG tank design against the ‘credible’ hazards including overfilling and overpressurisation and the potential shortfall in the design. A frequency assessment has then been performed to quantify the likelihood of a full containment LNG tank failure scenario and compared with the generic failure frequency values as given in some of the failure rate literatures.

Since the Cleveland East Ohio Gas Explosion in 1944, the onshore LNG tanks’ design and fabrication has evolved significantly including the remarkable development of full containment tank design, the use of nickel alloy steel, etc. Following the technology advancement, there has not been any major accident or major safety or security concerns in modern LNG plants in the past 50 years. And therefore, despite the fact that huge quantity of highly flammable LNG is stored, LNG facilities are often described to have an “exceptional safety record”.

However, LNG tanks still present a significant level of risk which may be regarded as far from negligible. Based on a Fault Free Analysis (FTA) of the potential scenarios that could occur in a typical LNG receiving / production facility, LNG tank overfilling and overpressurisation are considered to be two major scenarios presenting significant risks. The analysis has taken into account the typical safety measures provided in the design. Notwithstanding this, the tank overfilling scenario is considered credible due to the frequent loading and unloading operations in a receiving terminal and near continuous filling operation in a production plant. The main causes for tank overfilling have been identified as human error during loading/filling operation and level control element malfunction. With regard to the LNG tank overpressurisation scenario, the main contributing cause is found to be ‘hot rundown’ in the event of an upset in LNG liquefaction section. Overall, the analysis has shown that the frequency of an LNG tank failure due to these scenarios could be higher than those suggested in the literature for a typical full containment tank. While the design codes provide some design guidelines for LNG tank design in relation to these potential hazard scenarios, the application of these guidelines may sometimes vary in actual practice. This paper has also reviewed some of the key LNG tank design aspects relating to safety and suggested improvements for further risk mitigation.

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