Challenges with the World’s First Deep Water Reeled Clad Pipe Project: Ultrasonic Inspection and Fracture Assessment

摘要

An increasing demand for transport of corrosive constituents in the subsea oil and gas sector is creating a growing demand for high strength pipelines with enhanced corrosion resistance. Such as, hot rolled bonded (HRB) clad pipes, which are made of corrosion resistant alloy metallurgically bonded to the backing carbon steel. Reel-lay is a highly effective installation method, as an alternative to S-lay and J-lay, for installation of subsea pipelines up to 18” in diameter. However, reeled pipelines are subjected to plastic straining during installation, which may have an effect on weld defect acceptance criteria. The maximum acceptable defect sizes are determined using Engineering Critical Assessment (ECA), which typically requires that the weld metal is stronger than the parent metal in terms of a stress-strain curve. However, overmatch cannot be consistently achieved for clad pipelines with nickel-based alloy girth welds, especially at elevated temperatures. Moreover, the results of tensile tests show that the backing steel of HRB clad pipe, in the strained and aged condition, may have no strain hardening. This impacts on the defect acceptance criteria and the choice of an ECA methodology for reeled clad pipelines subject to in-service lateral buckling where loads are in excess of yield. Due to the inhomogeneous nature of clad pipelines with anisotropic girth welds, inspection using conventional Automated Ultrasonic Testing (AUT) methods is problematic. Therefore, the angle beam compression wave combined with creep wave technique is applied instead. This paper gives an insight into challenges with the AUT qualification for examining austenitic girth welds in clad pipes with the ferritic backing steel. In addition, the ECA methodology, including testing and numerical approach, adopted during work on the reeled deep water clad pipelines, subject to plastic straining during lateral buckling, is discussed.