Evaluation of aging process of PA11 from offshore flexible pipe pressure barrier

Abstract

The research presented proposes a new methodology for measuring degradation of polyamide 11 (PA11), as an alternative to the classic criterion established by the American Petroleum Institute in API17TR2. PA11 flexible pipes may be subjected to different degradation conditions, being hydrolysis one of the main aging mechanisms. Aged PA11 pressure barriers coming from flexible pipelines were sliced into inner and outer parts, having a pristine PA11 as a reference. Rheological measurements were performed on air and inert (N2) atmospheres. Mark-Houwink Equation was used to correlate zero-shear viscosity with molecular weight (Mw) and corrected inherent viscosity (CIV). PA11 showed a shear thinning behavior, with higher zero-shear values with the increase in CIV. Cross and power law models were adopted for apparent viscosity behavior prediction purposes with over R2> 90 %. The shifting of the crossover points of elastic (G′) and loss modulus (G″) showed a correlation with molecular weight and molecular weight distribution (MWD). Based on Cole–Cole, real (η′) and imaginary (η″) components of complex viscosity (η*) were investigated to determine the viscoelastic behavior of PA11, showing an overall prevalence of elastic behavior. Scanning electron microscopy (SEM) confirmed a more brittle behavior in the regions presenting lower CIV values. The average VIC values by rheology for inert and oxidative atmosphere internal samples were 1.28 and 1.14 dL/g, respectively. These values are 75.3 % and 67.1 % lower than the virgin sample VIC value, respectively. The average VIC values by rheology for external samples in an inert and oxidizing atmosphere were 1.54 and 1.38 dL/g, respectively. These values are 90.6 % and 81.2 % lower than the virgin sample, respectively.