Dr. Z. Jimmy Zhou, Dr. Dane Chang
Papers # 2014 Chicago
External flaw could be introduced into pipe wall during the transportation, installation and service processes of polyethylene pipe. The performance of non-flawed pipe is well addressed by both the standard stress extrapolation methods specified in ASTM D 2837 and ISO 9080. However, there are very limited data for extrapolating the performance of flawed pipe. In our previous paper presented at PP XVII, we reported the ligament stress regression results of notched PE100/PE4710 pipe. In this paper, we will report the ligament stress regression evaluation of notched PE80/PE3608 pipe. Hydrostatic test was conducted at various temperatures and stresses on notched PE80/PE3608 black pipe specimens with various notch depths. Hoop stresses were normalized to ligament stresses. It was found that the hydrostatic testing data points of ligament stress versus failure time essentially fell on to the same master curves regardless of the notch depths. Ligament stress regression was conducted by the Rate Process Method, the 3- parameter extrapolation equation, specified in ISO 9080 and ASTM D 2837. The extrapolated 100-year LTHS values exceeded the maximum hydrostatic design stresses at 23°C for 800 psi and 60°C for 400 psi if used as PE3608. The ligament stress regression data offered a new approach to estimate the maximum wall thickness reduction, i.e. maximum flaw size. The standard stress regression in accordance to ISO 9080 and ASTM D 2837 ensures the lifetime of non-flawed pipe for at least 100 years design life under standard conditions. The ligament stress regression of flawed PE80/PE3608 pipe indicated that it can still have 100+ year lifetime if the flaw sizes are below the validated maximum wall thickness reduction limit. This is especially important for trenchless installation where pipe scratching is very difficult to avoid.