Condition assessment of polyethylene pipeline systems

Abstract

This study is an industrial project commissioned by British Gas plc. (UK) to investigate the state of in-service naturally aged polyethylene (PE) buried pipes and to define procedures or techniques available to facilitate the condition assessment programme of PE pipelines systems. The primary goal of this study was to establish a better understanding of the ageing process in commercial piping materials and to understand its effect on long term integrity of PE pipeline systems. Eltex, Rigidex and Aldyl A are trade names of the PE gas grade resins used in this study and they represent the range of pipe materials used in the transport of potable water and gas in the UK. Sections of pipes used in this project were supplied by British Gas plc. in the form of unaged pipes and pipes which were aged in-service. Laboratory based accelerated ageing of the pipe resin samples was also carried out. The ageing regimes considered were water ageing at 23°C, air-oven ageing at 80°C, water ageing at 80 C, vacuum ageing at 80°C and in-service aged samples. Compression moulded plaques were produced as reference material. By utilising specific instrumentation and designs, several reliable procedures were developed to produce specimens directly from pipes. The feasibility of using micro-samples instead of large samples was demonstrated. A methodology was developed to retrieve disk samples using electrofusion "tapping-tee" saddles. Appropriate test specimens were designed for chemical and physical evaluations. This sampling method negates the need for excavating large sections of pipe material for the purpose of condition assessment. Reliable micro and macro-sampling test methods were developed and established as techniques for the condition assessment programme. Characterisation techniques included: (i) differential scanning calorimetry (DSC) to measure (from the same specimen) both the degree of crystallinity and the oxidation induction time (OIT), (ii) HPLC analysis was used to quantify additives concentrations, (iii) Fourier transform infrared spectroscopy was used to monitor the carbonyl index and to identify a pipe resin type from site, (iv) micro- and macro-tensile and fatigue tests to assess the changes in the mechanical properties as function of ageing. The fatigue test procedure was developed to produce brittle fracture at laboratory scale within a shorter period of time as compared to existing procedures such as the hydrostatic test. A reference data-base was created using the above identified tools and the criteria and methodology for carrying out site condition assessment inspection was compiled. The fundamental mechanisms of chemical and physical ageing were studied along with the possibility of their effect on the mechanical properties of PE pipes.