PRINCIPLES FOR LIFE TIME ESTIMATION OF PVDF PRESSURE BARRIERS IN HIGH TEMPERATURE FLEXIBLE PIPES BASED ON FRACTURE MECHANICS

摘要

Flexible risers with PVDF pressure barriers have in the past experienced leaks due to pull out of the barrier from the end connections. The probable cause of failure was movement of the pressure barrier due to thermal stress cycling. In addition to the thermal stress was a possible influence of the self-weight of the pressure barrier itself. On top of that was stresses induced by material shrinkage due to loss of plasticiser. New end connection designs have dramatically improved the number of cycles to failure in the "mid-scale" thermal testing done by SINTEF. The failures of the new designs were due to crack propagation in the liner. A methodology to predict the failure due to crack growth is outlined in this paper. The lifetime of the pressure barrier can be estimated by using a fatigue fracture mechanics approach based on an experimentally determined fatigue crack law. The stresses due to temperature variations in the riser are used as an input to the fatigue crack model.