Damage and fracture of PVDF at 20°C

International audienceFlexible oil pipelines are multilayered structures used for the transport of crude oil or natural gas from the seabed to the surface in offshore oil fields. Because of severe service conditions, composite structure made of metallic and polymeric layers must be used. PVDF is a good candidate as it accommodates tensile and flexural deformations and guarantees watertightness. The present paper focuses on the mechanical properties of PVDF at 20°C. Several specimens with different geometries were tested: notched specimen and cracked specimen under tension. Fracture surfaces were examined to determine fracture mechanisms. Based on mechanical testing and microscopic observations, a modified Gurson-Tvergaard-Needleman model for semi-crystalline polymers is proposed. The model allows to represent the non-linear behavior and the cracking of polymer structures

Prediction of flexible pipe annulus composition by numerical modeling: identification of key parameters

This article describes recent improvements made in the design process of offshore flexible pipes. These improvements consider the more precise complex geometry and architecture of the flexible pipes, while considering their corrosive environment (high pressure, high temperature, acid gases (CO2, H2S), sea water, etc.) and the relevant physics. MOLDITM, the design software that was developed 20 years ago to predict flexible pipes annulus environment, has been constantly upgraded to increase its representativeness: use of chemical potential or fugacity to better describe the mass transport, improvement of the thermodynamic module to better describe the interaction between chemical species and allow to model the purging through gas release valves. Recently, a major advance has been made allowing to cope with flooded annulus flexible pipes. When the annulus is flooded, the tortuosity produced by the presence of steel wires inside the annulus can take a major importance. Therefore, a new model, named 3DIFF, has been developed to describe the 3-dimensional characteristics of the annulus and its impact on fugacity profile across the structure. Depending on permeation properties of each layer, the result is that the presence of water can produce a fugacity gradient within the annulus. This heterogeneity must be considered during the design process to be fully representative of service conditions and allow to select flexible pipes materials with confidence. Experimental devices used to generate permeation database are under constant evolution to study even more complex mechanisms such as the diffusion process in a flooded tortuosity or the compression effects of polymeric material on their permeation properties