Experimental Investigation on Combined “Dent and Gouge” Defects on Vintage Steel Transmission Pipelines

Abstract

Mechanical Damage on pipelines is the most frequent in service failure cause, at least in Europe. The category of Mechanical Damage actually covers on one hand rock dents, on the other hand, combined “Dent and Gouge” defects resulting from an impact by a ground moving machinery. The latter are complex defects in terms of geometry and mechanical behavior. Improved understanding of toughness reduction in these defects and their vicinity, especially in the case of vintage pipelines, and its impact on burst pressure and remaining fatigue lifetime is crucial in order to ensure a high safety level at the best economic conditions for transmission pipelines. PRCI and DOT/PHMSA supported a large experimental program to investigate the burst and fatigue strength of such defects in modern pipes as well as in vintage pipes by producing very detailed reference experimental data in order to support improvement of current burst and fatigue strength models for these defects. This paper focuses on experimental results data for two vintage pipes removed from service, one 26″ in diameter, X52 grade (from US) with high carbon content and sulfur content involving very low toughness and another 24″ in diameter, X63 grade (from Europe) characterized by alternated pearlite and ferrite bands. In each pipe, two different types of combined “Dent and Gouge” defects were created, “shallow dent with shallow gouge” and “severe dent with moderate gouge”. Each defect was reproduced three times: • One for destructive metallurgical investigation; • A second one for highly instrumented burst test; • A third one for highly instrumented fatigue test. The defect profile was monitored during pressure increase until burst, and a camera recorded pictures at each fatigue cycle to detect crack initiation at the gouge bottom and its further propagation. The different features of the experimental work and results described in this paper are: • Material characterization; • Metallurgical and residual stress investigation on combined dent and gouge defects; • Burst tests conditions and results; • Fatigue tests conditions and results. • Comparison with results from modern pipes containing the same types of defects. All this experimental data is made available for an in-depth understanding of the “Dent and Gouge” type defects leading to improved models, as well as reference data for validating these improved models for burst and fatigue strength of these defects.