Fitness-for-service assessment and failure analysis of AISI 304 demineralized-water (DM) pipeline weld crack

Abstract

Fitness-for-service assessment (FFS) is employed in oil and gas refiners to assess the structural integrity of an in-service pressure vessels and storage tanks that may contain a flaw or damage. API 579/ASME FFS-1 standard provides guidance for conducting FFS assessments using methodologies specifically prepared for pressurized equipment. In the present investigation, FFS assessment (level 3, based on the Failure Assessment Diagram (FAD)) carried out on a 304 demineralized water stainless steel pipeline containing a crack at weldment eleven months before the failure is explained in detail. Despite the FFS assessment and acceptance of the pipeline, failure occurred eleven months after the FFS assessment. The distribution of the residual thermal stress introduced during welding was estimated by means of Finite Element Analysis (FEA) using Abaqus software. Further investigation revealed that the operating condition of the DM pipeline deviated from the date of FFS assessment and the failed pipeline had been under vibration. Defects such as inclusions and porosities and irregular shape of weld bead were observed in weld joint showing lack of proper quality control (QC) during welding and installation of the pipeline. The heat affected zone (HAZ) region was found to be sensitized (perhaps due to the improper welding procedure during installation) and severe precipitation of chromium carbides was observed on the grain boundaries. Further investigation showed the transgranular nature of the crack propagated perpendicular to the weld metal as a result of the dynamic loading caused by steam hammering effect leading to the vibration-induced fatigue. The Scanning Electron Microscopy (SEM) of the fracture surfaces showed that the fracture surfaces consist mostly of fatigue striations and quasi-cleavage fracture.