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Wall fusion of buried pipelines can occur if the driving voltage and available current is sufficiently high. For example, it can occur from direct lightning strikes to earth, close to the pipelines' routing. However, not all factors for defining the energy discharge needed to fuse the pipe wall are thoroughly investigated or understood. This paper enriches the ongoing research activities in an attempt to understand such detrimental damages. In particular, the paper formulates its research contents around a documented wall fusion incident on an underground gas transmission system, due to a nearby direct lightning strike. The investigation embraces field, laboratory, and modeling activities to provide insights on the damaging mechanism. Of particular importance in understanding this mechanism is the influence of soil ionization. To this end, a model is developed to investigate the soil-ionized flow of the lightning discharge current-into the pipeline's metal wall, through existing coating defects-thus revealing its detrimental effect. 35 2 763 773

The practical experience derived from the field performance of IJs or flanges of the natural gas pipeline system of Greece is briefly presented. The defective insulation incidents on buried IJs were effectively mitigated by a proper surge protection scheme that has been applied. However, after a time period of a seemingly reliable operation, extending over a decade, two buried underground monolithic IJs lost their insulation properties despite being protected by properly installed ISGs. On another IJ the ISG malfunctioned whereas the IJ largely maintained its insulating properties. The causes of these failures are investigated and possible explanations are provided. The ageing and degradation of ISGs and the degradation of the dielectric strength of the joints is also discussed in the paper. 1 8