Stratification and rollover in liquefied natural gas storage tanks

During October 1993 a rollover occurred in a British Gas liquefied natural gas storage tank. The pressure in the tank rose rapidly and the two process relief valves on the tank lifted. The emergency relief valves subsequently lifted and natural gas vented from the tank for two hours. Approximately 150 tonnes of gas were vented to the atmosphere. The tank had been filled during July as part of the commissioning run of a modified liquefaction plant. The modified plant produced a product which was up to 13 kg cm(-1) (3%) lighter than the heel of the tank. This was sufficient to cause a stratification in the tank. The tank stood for 68 days after which the rollover occurred.