ABSTRACT

For a major deepwater pipeline project in the Black Sea, buckle arrestors will be deployed to prevent catastrophic buckle propagation in the unlikely event of collapse. If large diameter pipe is installed in deepwater using S-lay, plastic strain will develop when the pipe travels over the stinger. Presence of a relatively stiff feature will enhance straining locally. The thick buckle arrestor is girth-welded to thinner pipe sections. Consequently, strain concentration will occur in the girth weld. When traversing the stinger, tens of sequential loading-unloading cycles are introduced. The girth weld is loaded when the buckle arrestor is on top of a roller box and unloaded when located in between two roller boxes. To support assessing the impact of such load sequence on the integrity of the girth weld, a material testing program is launched. Fracture behaviour is investigated both qualitatively and quantitatively. It is essential to confirm the correct mechanism of crack growth under repetitive cycles of large strain. To establish the correct mechanism, segment tests with increased specimen "daylight" length are performed. After testing, the fracture surface is examined, measured and classified using a Scanning Electron Microscope (SEM). Fracture morphologies of ductile tearing, fatigue growth and brittle fracture can be distinguished. Ductile tearing is observed in the first loading cycle. Subsequent cycles are dominated by fatigue crack growth, which can be quantified using Paris Law.

INTRODUCTION

South Stream Transport BV (SSTTBV) is developing a major gas transmission system that comprises up to four (4) pipeline strings to be installed in water depths up to 2200 m. The system will have a massive capacity to transport 63 billion cubic metres of natural gas per year over a distance of more than 900 km through the Black Sea. The pipeline steel outside diameter (D) will be 812.8 mm (32-inch) and its wall thickness (t) 39 mm. Material grade of the linepipe is DNV SAWL 450 SFDU and, depending on the supplier, it is manufactured using either UOE or JCOE method. This project can be considered as one of the most challenging pipeline projects ever, stretching the limits of present-day industry.

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