Analysis of the effect of fast transients in LNG process equipment

Some years ago a root cause analysis was made of a large oil spill caused by a leak in an oil offloading system. It appeared that the leak was caused by a large pressure pulse initiated by the sudden closure of the bow loading valve at the tanker, called water hammer. The magnitude of the initial pulse was largely amplified by reflections at several diameter changes. Similar phenomena can occur in (offshore) LNG transfer systems with even more serious consequences. In addition to the water hammer "column separation" may cause even larger pressure pulses. This occurs when, due to a passing water hammer wave, the pressure locally drops to the vapor pressure and the liquid starts to boil. As the LNG in a transfer system is close to its boiling conditions, this phenomenon is likely to occur. In order to evaluate to what extent water hammer and column separation can endanger the integrity of the system a computer model has been made of three generic ship-to-ship transfer systems. The peak pressures during an emergency release of the flexible transfer hose at full flow have been calculated. The sensitivity for length of the hose, closure time and wave propagation speed has been evaluated. In spite of the modeling uncertainties the results show that water hammer and column separation can cause shock loads that are an order of magnitude larger than the static loads. Therefore it is recommended to perform an analysis as part of the design and to optimize the layout of the system. It is suggested to include this as a requirement in the applicable design codes. Also it is recommended to test components such as flexible hoses on dynamic loads caused by water hammer.