SENT Testing Approach to Assess the Girth Weld Heat Affected Zone Toughness in Deepwater Risers

摘要

Girth weld heat affected zone (HAZ) toughness is an important factor for the design, manufacture and installation of deepwater risers. Adequate toughness is required to ensure that risers can be fabricated to defect acceptance tolerances that can be reliably verified by appropriate pipeline non-destructive evaluation (NDE) techniques such as AUT. This paper describes the approach taken to verify the toughness of girth welds in thick-walled metallurgically bonded X65 pipe (33mm wall thickness by 12 inch diameter and 27mm wall thickness by 10 inch diameter, both with 3mm of alloy 625 internal cladding) for the manufacture of Single Leg Hybrid Risers (SLHR) in 2000m water depth. Pipeline blowdown conditions imposed a minimum design temperature (MDT) of -20°C. The initial approach with weld HAZ toughness evaluation was to adopt a conventional fracture mechanics approach using the standard, deeply notched bend specimen (SENB) to verify that a target HAZ fracture toughness of 0.4mm CTOD at MDT could be achieved. However, it was recognized that riser pipe fracture toughness might be better represented by lower-constraint single edge notch tension (SENT) specimen. During weld evaluation, the clad pipe was found to be susceptible to strain age embrittlement in the subcritical HAZ. This caused lower toughness than required when using the SENB specimen. Analysis to verify whether SENT test results could be safely employed to describe the HAZ toughness is described. Anomalies encountered in the determination of CTOD and J-integral in SENT tests are identified as an area for further study.