Thermal Stresses and Crack Propagation in PVDF Pressure Barriers Submitted to High Temperature Variations

During the use of flexible pipes, important thermal variations can occur leading to thermal stresses. As a result two aspects were identified, in the past, due to thermal stress cycling [1,2] : – pull out of the pressure barrier, – crack growth of the pressure sheath. Pull out of the PVDF pressure sheath has been stopped by the design of new end connections [3]. Nevertheless, in order to get more knowledge of thermal stresses, Technip-Coflexip has developed a theoretical approach which is able to calculate whatever the plasticizer content, the initial temperature, the final temperature and the temperature variation, the thermal stresses for flexible pipes using PVDF materials. Comparisons between theoretical predictions and full scales test results show very good accordance and allow confirming that the Technip-Coflexip designs of end-connections are suitable for the offshore industry. Crack growth through the pressure sheath has been experienced only during accelerated testing at SINTEF and has not so far been seen in risers in operations. Crack growth through PVDF pressure sheath can be fully predicted by a model developed by Technip-Coflexip. The two major conclusions of the work performed are: – failure experienced at SINTEF are only linked to rough hand-machining and very severe test conditions, – failure can not occur in normal operation due to the fact that thermal stresses generated, linked with a good surface machining, are below the threshold of crack propagation.