Full-scale fatigue and burst testing of girth-welded steel pipe under combined internal pressure and bending has consequences for pressure safety margins

The strategic role of hydrocarbon transmission pipelines is unquestionable today. This role can be interpreted within a country, but also between neighbouring countries, or within larger regions, continents and between continents. An incident on such a pipeline can therefore cause disruption both within a country and beyond, in wider regions. The critical elements of high-pressure gas transmission pipelines are often the girth welds. The aim of the article is to introduce our full-scale pipeline fatigue and burst tests applying cyclic internal pressure and superposed external bending stress on girth welds. A unique test system was developed, and 4-m-long pipeline sections containing girth welds with no unacceptable imperfections detected by NDT and with artificial flaws were tested. The artificial notches were located in the heat-affected zone (HAZ) of the girth welds and along the longitudinal axis of the pipe, crossing the girth welds. One hundred thousand fatigue cycles were applied, the internal pressure was varied between 60 and 100% of the maximum allowable operating pressure (MAOP), and the bending stress was 2–8 times the axial stress from the MAOP. The tests were evaluated using internal pressure versus time and burst pressure values. Based on characteristic pressure values, “simple” and reassessed safety factors have been calculated to evaluate and compare the risk of defects in pipeline sections and girth welds.