Relative Importance of Defects in Girth Welds of Linepipes

The Ormen Lange deepwater gas field is located at water depth down to 1100 meters. The irregular seabed gives severe challenges to pipeline design and verification program was launched to demonstrate adequate fatigue capacity. The research included: modern welding techniques (5G and 2G welding positions), mapping of actual welding defects, misalignment (high/low) and lack of penetration. The thick walled pipe (35mm) showed low or even compressive residual stresses at the inside. This will to some extent be “protective” to the root of the weld. The exceptions to this pattern were the repair welds and the two-sided welds. The small scale test results fell close to the full scale pipe tests when taking into account the geometrical weld distortions, loading mode, and the distribution of weld defects. The importance of the parameters influencing the fatigue capacity could be ranged as follows, most detrimental first: large crack-like defects (LOP, undercut > 1mm), hi/lo, and V-shape (radial shrinkage at the girth weld).