Performance Evaluation of High Quality HFW Linepipe by a Series of Full-Scale Pipe Tests

Newly-developed high quality high frequency electric resistance welded (HFW) linepipes have recently been used in pipelines in reel-lay applications and low temperature service environments because of their excellent low temperature weld toughness and cost effectiveness. In order to clarify the safety performance of these HFW linepipes, a series of full-scale tests including a hydrostatic burst test at low temperature, fatigue test and tension test under high internal pressure was conducted, together with small-scale tests such as impact energy and standard fracture toughness tests, which are generally used in mill production and pre-qualification tests. The Charpy transition curve of the developed HFW pipe occurred at a temperature much lower than −45°C. Based on the low-temperature hydrostatic burst test with a notched weld seam at −45°C, the weld of the HFW linepipe presented the same level of leak-before-break (LBB) behavior, as observed in UOE pipes. A full-pipe fatigue test of HFW pipes with repeated internal pressurizing was conducted. The fatigue strength of the developed HFW linepipe shows better performance than butt weld joints because of the smoothness at its weld portion, which is achieved by mechanical grinding of the weld reinforcement. Full-scale pipe tensile tests of girth welded joints were performed with an artificial surface notch at the heat affected zone in the girth weld. The influence of internal pressure was clearly observed in these tests. Based on the above-mentioned full-scale tests, the safety performance of high quality HFW linepipe is discussed in comparison with the mechanical properties in the small-scale tests such as the Charpy and standard fracture toughness tests, especially when the notch was placed in the welded seam.