The TurkStream pipeline system is currently being constructed across the Black Sea for supply of natural gas from the Russian coast near Anapa to landfall on the Turkish coastline. The pipeline design is extremely challenging from a technical perspective with water depths along most of the route exceeding 2200m, difficult seabed conditions with steep slopes and geohazard risks, a potentially aggressive and corrosive subsea environment, and high gas deliveries requiring a large pipeline diameter.
The combination of diameter, wall thickness, and quality requirements for the TurkStream linepipe is at the limit of leading mills around the world, and outside their normal production envelope. In order to demonstrate that the manufactured linepipe will meet target reliability levels, a material development program was completed, which included a major testing program involving testing prototype TurkStream pipes manufactured by both the UOE and JCOE methods of linepipe fabrication. The results from this test program formed the basis for a probabilistic verification of the pipeline wall thickness.
This paper presents the reliability-based verification of the wall thickness for the TurkStream subsea pipelines. The collapse limit state was identified as the governing mode of failure, and was defined by a collapse prediction formula developed specifically for the TurkStream pipelines. The model was verified by full-scale tests from the test program, and then used to define the failure surface of the collapse envelope due to external pressure and bending.
The probabilistic analyses resulted in reliability levels that exceed design code requirements. In addition, these analyses highlighted the need for certain criteria to be met during coated linepipe production. These requirements are summarized in this paper.