Heavy Wall Linepipe Girth Weld Fatigue Strength: Investigating Weld Root Performance

Abstract

The feasibility of Heavy Wall (HW) Seamless Line pipe application for high pressure/high temperature deep and ultra-deep field developments must be experimentally demonstrated to establish acceptable performance. Detailed mechanical, corrosion and fatigue qualification programs are required for validation of heavy wall line pipe and the associated girth welds for riser and flowline systems, especially those in fatigue-critical applications. Pipe manufacturers and Operators share a common interest in developing new technologies to economically and safely produce from challenging reservoirs. In OMAE 2013, Tenaris and ExxonMobil jointly presented the Phase 1 qualification effort performed on HW line pipe and associated girth weld via mechanical and full scale fatigue testing. The results confirmed the mechanical properties were acceptable and the fatigue performance of the associated girth welds was reasonable. As expected, most of the fatigue failures originated at the outer pipe surface. As such, the focus of the present work is to further examine the fatigue performance considering fatigue failure initiation at inner pipe surface. This would provide a better assessment of riser and flowline fatigue performance for applications where the inner pipe surface is subjected to degradation from potential aggressive components in produced fluids. To investigate the root girth weld fatigue performance, the adopted plan was to create an outer surface free of stress raisers/imperfections at weld cap toe which would help “push” the fatigue initiation to the root weld toe at the inner pipe surface. To achieve this, an enhanced grinding and flapping technique on the fatigue test specimen external surface was applied to remove the cap overfill and produce the desired flush profile. The same surface treatment technique was also applied on the pipe body in the proximity of the girth welds. This program utilized identical pipe and girth weld used for the Phase 1 qualification effort. The heavy wall pipe was an X65, 10 ¾″ OD × 46 mm WT. The welding procedure was a narrow-groove bevel, STT® process for the root pass and the GMAW process for hot, fill, and cap passes. Four full scale fatigue tests were conducted for this investigation, two girth welds per fatigue specimen. In the end, the enhanced surface treatment technique was successful in “pushing” the fatigue failure initiation to the inner pipe surface. This paper presents the fatigue performance results due to crack initiation at the inner pipe surface as well as post-mortem analyses of the failure surfaces.