Design of Unplasticized Polyamide 12 Oilfield Line Pipe Based on Published Regression Curves and ASTM F3524

Abstract

The long-term strength of unplasticized polyamide 12 (PA-U12) has been characterized using standard methods for plastic pressure pipe during the qualification process for use in buried piping systems for natural gas delivery operating at low ambient temperatures, usually less than 20 to 30°C at pressures previously served only by steel pipe. The high strength characteristics of the material also made it interesting as a steel substitute for buried and above-ground industrial pressure pipe applications. So far, typical geometries are diameters up to 6 inch (162 mm) with wall thicknesses up to DR9. Assuming a design factor of 0.63 applied to the PPI TR-4 listed HDB, DR9 PA-U12 pipes have a maximum pressure at 23°C of 496 psig (3.4 MPa), although design pressures will typically be lower. These industrial systems can operate at much higher temperatures, up to ~ 50°C for above-ground installations in pipe supports or up to ~ 65°C for surface-laid oilfield pipes. Design engineers need the temperature dependent strength curves to properly design a thermoplastic pressure piping system. Therefore, the regression curves have been extended by LTHS tests up to 120°C. Corresponding test durations enabled to define the location of knees with determination of second branches. These branches are caused by hydrolytic degradation of the polymer resulting in brittleness of the polyamide after long times at elevated temperature in sufficiently wet environments or services. The first appearance of a knee is at 60° C and approximately 44 years. This paper describes various standards to which PA-U line pipe and materials must conform, and the development of standardized, temperature dependent, long-term strength reference curves for PA-U12 pressure pipe, including the transition from ductile to brittle behavior at long times and high temperatures. These curves have been standardized first in DVS and followed by ISO based on the combined LTHS data of pipes made from two different PA-U12 180 compounds and one PA-U11 180 compound. An example of a typical PA-U12 180 pipe design is presented, applying chemical resistance derating factors for oil and gas applications in the design process. Short Summary: Oilfield pipeline design engineers need temperature dependent strength curves to properly design a thermoplastic pressure piping system for use at temperatures often far above the typical range for natural gas distribution. Therefore, LTHS regression curves have been experimentally developed at temperatures up to 120°C to define the location of knees and characterization of second branches. In this paper we describe various standards to which PA-U line pipe and materials must conform, and the development of standardized, temperature dependent, long-term strength reference curves for PA-U12 pressure pipe, including the transition from ductile to brittle behavior at long times and high temperatures. These curves have been standardized in ISO and DVS and apply to both PA-U12 and PA-U11 [1,2]. For PA-U12 line pipe we also present an example for a typical pipe design, where chemical resistance derating factors are applied for oil and gas applications in the design process.