An Innovative ECA Approach For Reeled CRA Welds And Its Validation Programme

Abstract

This paper presents an innovative defect growth ECA methodology for pipeline girth welds and its validation programme, applied specifically to reeling ECA of pipelines with under-matched strength welds. The ECA method is a tear-fatigue approach that accounts for the blunting limit in JR curves during pipe spooling and reel-lay. Fatigue crack growth may occur by low cycle high stress fatigue and by tearing, but the latter only if the crack tip opening displacement exceeds the blunting limit. Conventional ECA with BS7910 is limited because the weld's strength needs to be over-matched. Alternative industry methods for the application of FEA to under-matched strength welds are computationally more intensive than the presented innovative approach. Fatigue crack growth for low cycle high stress fatigue is calculated using Paris’ Law in the approach but, if the crack tip opening due to the tearing mechanism is less than the blunting limit then tearing growth is zero. With the innovative method, if the crack tip opening displacement exceeds the blunting limit then the tearing defect growth is included. Hence, the method is a combined tear-fatigue approach. Welded pipe strings were fabricated from pups composed of clad material; i.e. carbon backing steel pipe with a 3 mm layer of corrosion resistant alloy (CRA) on the inner circumference. Each test string was approximately 10.5m long and fabrication was from a mix of six 0.5m length pups in the central zone of each string and two longer end pups. Three girth welds included EDM notches for test purposes which simulated planar flaws. The notches were on the extreme tension fibre, as the test string gets pulled to the reel former in a reeling test rig. Full scale reeling simulations involved pulling the test strings up to 6 times to the reel former in a reeling test rig. Measurement of defect growth associated with the EDM notches was by scanning electron microscope (SEM), from specimen segments extracted from the test strings. Predictions of defect growth were by finite element models in combination with pipe-specific data that was the outcome of an associated small-scale test programme. Validation of the ECA-by-FEA approach is by a predictive best estimate study, for which there is excellent agreement between the measured values and the calculated defect growths. The ECA-by-FEA approach is conservative for project work, as shown by a high estimate study and an offset blunting limit study. Early development of the ECA approach was for small diameter CRA pipelines during the execution of the Guara-Lula project (Sriskandarajah et al, 2015). The presented full-scale tests, innovative defect growth measurement by scanning electron microscope and the FEA and defect growth calculations were full validation of the approach, with pipe strings that had outer diameter of 323.9mm.