As part of the final stages of the design and construction of an LNG terminal, AFAA were commissioned to perform a surge analysis to assess survivability of the pipework against surge pressures caused by a very rare scenario of valve stem failure. The closure of a particular valve in less than one second would have resulted in a pressure wave travelling throughout the installation, causing large transient forces and triggering the activation of non-return valves.
AFAA modified the original pipe stress model which had been created in the pipework stress analysis software. Surge pressures for the valve stem failure case were determined from a CFD analysis; the pressure-time history generated was converted to a force time history at the installation elbows and applied to the model.
The primary aim of the analysis was to provide data for the assessment of various fittings and supports in the pipework against valve stem failure case.
ASME B31.3 provides allowable stress criteria for occasional loads which are loads that occur infrequently every year. An event frequency for a valve stem failure leading to a major incident has been estimated to be of the order of 1×10-8 per annum. Thus the case was considered to be a very low frequency event outside the scope of B31.3, and therefore code compliance checks normally required would have been inappropriate.
Component fitness for purpose was assessed using 3D FE models against criteria taking into consideration both brittle and ductile fracture. Support Assessments Several support types were present in the installation, where peak support loads during the surge were found to exceed the manufacture’s recommended limits, 3D models of the supports were created and analysed using the forces and moments generated from the pipe stress analysis.
Based on the finite element modelling and analysis results, modifications were determined for each of the overloaded supports where required.