Some durability aspects of thermoplastics for oilfield flexible pipes

Some initial mechanical property, fatigue, high pressure (HP) methane permeation and explosive decompression data are reported for plasticized PVDF thermoplastics before and after various exposures in two fluids. The exposures were for different times and elevated temperatures, and were usually performed at 5000psi (345 bar). The fluids were a 97/3 methane/carbon-dioxide mixture saturated with water vapour, and 100% methanol, both of which induce some physical or physico-chemical changes in the material, but negligible chemical change. Thus measurements were made using reasonable simulations of fluids which could be met in oil production flexible pipe service. Effects of'extra' plasticisation by absorbed gas, deplasticisation of actual plasticizer, and ageing, are shown for some combinations. Arrhenius-type plots for HP methane permeation, allowing predictions of rates at other temperatures, and showing hydrostatic compaction effects, have been obtained: HP methane permeation rate is reduced by prior exposure to the methane/carbon-dioxide gas, but is increased in the presence of methanol.