Correlation Between Hydrolysis and the Ultimate Mechanical Properties of Polyamides in Offshore Conditions

Knowledge of mechanical properties of polymers used in the field of very technical applications is crucial for guaranteeing the safety of those applications. This is particularly true for pressure sheaths of offshore flexible pipes. For polyamide 11 (PA-11), which has been used for 30 years in the field of oil & gas applications, the decrease of its mechanical properties due to hydrolysis can be monitored by the decrease of the molecular weight, or Corrected Inherent Viscosity (CIV). According to API 17TR2 a design limit of CIV = 1.2 has been established for the use of PA-11 in dynamic applications. This design limit includes a safety factor, related to the kinetics of the molecular weight decrease of the PA-11, between CIV = 1.2 and the end of life criteria (CIV = 1.05). We show, in this paper, that this kinetic shall be established with tests enabling to precisely determine the near-equilibrium CIV value. This supposes sufficiently long-term ageing experiments at high temperatures. As a consequence, for any new polyamide, the time to reach the CIV of 1.2 can’t be considered as a sufficient design factor. Moreover, the safety factor for a new polyamide should be carefully estimated. Therefore, the complete characterization of the mechanical behaviour should be done in relation with the ageing time and temperature. Only at this condition, one can expect to be able to establish a new and correct end of life criterion for a new polyamide based on mechanics. In a second time, this criterion could be linked to some physico-chemical parameters. We point out that the maximum elongation evolution of the material with the molecular weight couldn’t be considered as sufficiently precise to conclude about the mechanical resistance of the aged material for flexible pipe application. We thus present in this paper a new mechanical protocol.