Failure analysis of buried piping and cold drain vessel

This paper describes the failure analysis of a cold drain vessel used to collect hydrocarbon fluids off liquefied natural gas through a network of buried piping. The analysis deals with (i) the corrosion of the buried piping and (ii) the cracking of the head plate of the vessel. The analysis comprises visual inspection, chemical and mechanical testing, and finite element thermo-mechanical stress analysis. Corrosion of the buried pipe was found to result from the combined effect of saline ground water, defective coating, ineffective cathodic protection, and stray current. Improper corrosion treatment led to stress corrosion cracking which ultimately caused complete fracture of the vessel head plate at a nozzle attachment. It is recommended to improve coating and cathodic protection, and to enhance welding quality and employ better design considerations to reduce the local stresses.