The polymeric barrier is one of the key components in a flexible pipe, the sound function of which is essential for the containment of the transported medium, ensuring no leakage to the environment which could result in undesired consequences. According to API 17J the barrier design must be able to sustain certain static and dynamic strain conditions however; the actual design or the fatigue assessment of the barrier is not covered within the standard. Since the barrier is subjected to the same dynamic loading as the pipe the durability and integrity of the barrier is a key issue that needs to be addressed during barrier design for dynamic risers. This paper discusses a fatigue life assessment procedure for a barrier made of copolymer Grade 60512 PVDF.
A flexible pipe barrier is manufactured by continuous extrusion of polymer onto a metallic carcass. The carcass has a spiral structure with an irregular outer profile. As such, the extruded polymer on this irregular surface inevitably gives non-uniform thickness and geometric anomalies where the polymer has flowed into gaps in the carcass. During pipe loading such anomalies act as stress concentrations and become critical locations for fatigue crack formation. The evaluation of the effect of the barrier profile shape on the fatigue durability of the barrier is therefore an essential requirement, in particular, in cases where the barrier consists of a single extruded polymer layer.
Within the procedure outlined in this paper, the fatigue assessment of the barrier is made using the local plastic strain behaviour. The maximum stress concentration factor and the acceptable profile for the extruded barrier are selected to provide adequate safety margins for the project specific loading conditions. A procedure has been proposed to predict the local plastic strain of the barrier using global service loading data.