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JapaneseEXPERIMENTAL DETERMINATION OF THE EFFECTS OF LIQUID HYDROCARBONS ON THE LONG-TERM CREEP RUPTURE PROPERTIES OF PRESSURE PIPE GRADE UNPLASTICIZED POLYAMIDE-12
James F. Mason, Akshay Ponda, Hermann van Laak, Buc Slay
# 2021 Amsterdam
The effects of liquid hydrocarbons on long term strength of pipe grade thermoplastic resins is an important consideration when designing pressure pipe for use in the oil and gas industry. There is an extensive body of work going back several decades descri bing these effects for pipe grade polyethylenes, and all the relevant pressure pipe design standards specify a fluid service factor of 0.5 to derate for loss of strength in liquid hydrocarbon service. There is a long history of safe and reliable use of pol yethylene pressure pipes using those conservative design standards.
Since polyamide-12 (PA12) pressure pipe has become approved for use in natural gas distribution pipelines, interest in its use in liquid hydrocarbon service for oil and gas gathering has accelerated for applications outside the normal pressure and temperatureuse range of polyethylene. The typical range of interest for PA12 pipe in oil and gas production applications is 40° C to 80° C (140° F to 176° F), using pipes up to 10-inch (250 mm) diameter at pressures to 350 psig (24 MPa). Pipe design is by the same equations as used for polyethylene pipe, so the fluid service factor for PA12 pipe is needed to complete the design and calculate wall thickness.
The practical aspect of doing elevated temperature, long-term hydrostatic strength testing using liquid hydrocarbons as the pressurizing medium is complex. A method and apparatus were developed to determine the effects of any liquid service environment at elevated temperatures on the long-term creep characteristics of tensile bars. Injection molded tensile bars were conditioned to saturation in water or in liquid hydrocarbons, then subjected to long term tensile loading to rupture in the same fluid. The stress regression analysis method of ASTM D2837 was used to analyze the data which extended beyond 6000 hours in both environments at 60°C (140° F). Comparing the long-term strength in each environment permits calculation of a service factor for use in hydrocarbon service. This paper describes the experimental equipment, test protocol, stress regression analytical method, and the experimentally-derived service factor for PA12 in liquid hydrocarbon service.
