ABSTRACT

For a major deepwater pipeline project in the Black Sea, Buckle Arrestors will be deployed to prevent catastrophic buckle propagation in the event of collapse. Buckle Arrestor Assemblies (BAAs) will be installed using the S-lay pipeline installation method, then introducing cyclic plastic strain on the BAAs' girth welds during their passage over stinger's rollers. Fracture during installation is one of the potential failure modes for the girth weld. A material testing and assessment program has been launched at Centro Sviluppo Materiali (Italy) aimed at evaluating the impact of in-field strain sequence on a defected girth weld. The program was articulated in the evaluation of toughness by using single-specimen method with compliance technique on large thickness SENT samples. Then cyclic tearing sequence has been applied on Segment specimens with increased daylight length, aimed at reproducing the real pipe remote strain conditions by small-to-medium scale testing. Accompanying the testing program, a series of ECA calculations has been performed to investigate the robustness of the segment testing methodology used to evaluate the resistance of flawed pipes when subjected to tearing plus cycling loading scenario. As a main conclusion, the segment with increased daylight methodology has been found robust. It has been confirmed by comparison of experimental results and ECA pipe solutions provided by both BS 7910 and API 579.

INTRODUCTION

South Stream Transport BV (SSTTBV) is developing a major gas transmission system comprising up to four (4) pipeline strings to be installed in water depths up to 2200 m. The full system will have a massive capacity to transport 63 billion cubic metres (bcm) of natural gas per annum, over a distance of more than 900 km through the Black Sea. The pipeline outside diameter (D) will be 32-inch and its wall thickness (t) 39 mm. The material grade of the line pipe is DNV SAWL 450 SFDU and, depending on the supplier, 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. In order to prevent catastrophic propagation of a buckle in the unlikely event of pipeline collapse, inline structures (Buckle Arrestors, BAs) are deployed at certain spatial intervals when the water depth exceeds that equivalent to the buckle propagation pressure for the pipeline. The BAs were designed and sized according to offshore design standard DNV-OS-F101 (2010). The BAA consists of three parts, i.e. one machined thick (BA) section girth welded between two pup pieces manufactured from line pipe sections of nominal dimensions. Material grade is the same for the BA section and the adjacent pup pieces, i.e. DNV SAWL 450 SFDU. However, BA and pup pieces were subjected to different thermal history. Fig. 1 schematically presents the inline Buckle Arrestor Assembly (BAA) that will be deployed in the pipeline project.

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