DNV GL Principle Consultant Dr Chris O’Connor discuss the unique combination of properties that has allowed PE100 to globally revolutionise low pressure gas pipeline design.
by Dr Chris O’Connor, Principal Consultant – Pipelines Technical Advisory UK and West Africa, DNV GL – Oil & Gas, Crewe, UK
Materials choice can create a step-change in the way pipelines are designed and constructed, and can potentially replace traditional pipe engineering processes. This has been the case with the PE pipe grade known as PE100. While assuring improved integrity and efficient maintenance, good installation management techniques continue to be required.
Over the last 50 years, polyethylene (PE) materials have evolved with advances in polymer science resulting in PE100 grades. Using a unique combination of properties, PE100 has revolutionised low pressure gas pipeline design on a global basis and has driven the replacement of traditional pipe engineering materials.
Though it has been used extensively in operating gas systems for decades, it has recently been the material of choice for gas gathering mega-projects. PE100’s track record for safety, reliability and cost-effectiveness has been achieved through a long legacy of historical development and exacting gas industry requirements.
Today, highly engineered bimodal PE100 now provides an exceptional balance of strength, stiffness, toughness, durability and crack resistance consistent with the demands of installation, long-term gas containment, ground loading and the service environment.
Human factors warning
Despite their properties, PE models are only as good as the methods used for installation methods and the impact that human factors can place on risk to gas systems. Engineering teams and technicians must have a competent understanding of material behaviour, failure modes and product qualification. This is a concern for gas distribution where premature failure can have catastrophic effects.
To reduce this risk, there is a growing need for a management top-down vision and strategy, supporting organisational governance throughout the pipeline lifecycle and ensuring adequate levels of monitoring, audit and inspection. There are good and bad examples of practices but also recommended preventative actions to mitigate incidences of failure.
A key threat to PE100 pipeline integrity is poor fusion jointing. Joints are a known weak point in any engineering system; axial or bending stresses caused by thermal expansion or contraction, or ground movement, will increase the risk of failure of joints if installation is substandard.
Testing of joints is crucial and DNV GL, the technical authority to the oil and gas industry, has more than 40 years of PE pipeline research experience, providing technical advisory services and well as gas engineering operational experience.
Creating necessary assurances
If the three main types of PE100 fusion joint geometry – butt weld, socket joint electrofusion and saddle joint electrofusion – are to be successful, their installation methods should be strictly controlled in terms of parameters and conditions.
The fusion process involves heating PE, until the material reaches its crystalline melt point at which it becomes a visco-elastic melt. In this melt state, under the action of pressure, the long chain like molecules of PE can uncoil, disentangle and slide over each other (shear flow) as shown in Figure 1. For a safe gas supply, joints must be totally reliable, which can present difficulties when coupled with low cost labour working in wet, muddy or dry and dusty excavation environments.
Another difficulty is that reliable inspection of PE pipe joints using non-destructive testing (NDT) has proven to be difficult, since radiography and ultrasonic surveillance cannot reliably detect all key issues that are known to affect PE joint quality.
Furthermore, techniques have not yet proved sufficiently reliable or cost-effective for field implementation.
Back to basics approaches to in-field quality assurance and process control have been considered fundamental, underpinned by training to maintain competent levels of workmanship and investment by operators. This commitment to mastering the simple steps, before recommending any complexity, must come from industry leaders.
Range of failure modes
In PE pipelines, failures can be caused by factors including material defects, pressure, applied stress or contamination.
Stress crack growth (SCG) is a phenomenon in PE materials whereby slow growing cracks can occur due to the presence of stress in the material. It is widely recognised that the durability of PE pressure pipe is dependent upon its resistance to inhibit the initiation and slow growth of cracks.
This failure mechanism can also occur in all types of fusion joints. Early research of PE pipes established that SCG was critical in three major material failure modes for PE pipe, identified in internal hydrostatic pressure testing. These are ductile failure, brittle failure and brittle chemical failure as shown in Figure 2.
Ductile failure mode results in yielding and reflects a material’s propensity to undergo largescale, irreversible ‘plastic’ deformation when under stress. The mechanism results in localised expansion of the wall section and final rupture of the deformed zone, seen in Figure 3.
Brittle failure is associated with creep, creep rupture and SCG. Creep is a plastics time dependant, non-reversible deformation that occurs when exposed to a constant tensile stress.
Creep rupture is the terminal event of creep and is a measure of the time that a material under a constant applied tensile load takes to fail. Creep rupture can be accelerated by temperature, stress concentrations, fatigue and chemical environment.
Brittle chemical failure is related to degradation and embrittlement of the plastic due to thermooxidation with time. Early research showed that it was necessary to take account of the long-term failure mode of PE to ensure the safe operation of gas distribution networks.
Rapid crack propagation (RCP) of PE pipes is a less known phenomena and is initiated at defects within the pipeline by sudden mechanical shock, such as a high velocity impact from excavation equipment or a pipeline pressure pulse. Once initiated, ruptures can travel at high speed (100–300 ms-1) along the length of the pipe over significant distances.
This failure mode has largely been eliminated through robust qualification testing of pipe materials to assure their resistance to this failure mode.
DNV GL’s Spadeadam Research and Testing Site in Cumbria, UK – shown in Figure 4 – undertakes analysis of the phenomenon of RCP and provides verification and certification services for global manufacturers of PE and other non-metallic materials, pipe manufacturers and PE pipeline operators.
Of paramount importance was a high degree of long-term strength to resist creep rupture for sustained low pressure loading of 50 years and more.
Resistance to stress cracking was also critical to inhibit crack growth from notch type damage (scores, scratches and gouges) developed during transportation and installation, and point loading due to rock and root impingement.
Hence, an important design factor for PE pipes became commonly known as environmental stress cracking resistance (ESCR), which led to the development of medium density PE or MDPE.
Contaminations can be a hindrance in achieving a good bond between two surfaces to be jointed. These can compromise fusion integrity and particles can act as stress concentration sites, the precursors to SCG.
Considering that the jointing of PE pipes is normally carried out under field conditions where contamination from the surroundings is a continual threat, great care in joint preparation is necessary.
The types of contamination may include:
- gross contamination such as mud, soil or tar deposits on the pipe surface
- slight contamination such as dust, grease or oily deposits from contaminated cloths or unclean hands touching the surfaces to be joined
- oxidation of the surfaces due to exposure to the air, extended weathering of the surfaces due to prolonged exposure to ultraviolet light, surface water or moisture.
For all fusion jointing techniques, adequate preventative measures should be taken to protect the joint from contamination and the external environments prior to fusion. These measures should include the mandatory use of ground sheets and tents.
Furthermore, end caps must be fitted to pipes to prevent ingress of water and reduce wind chill effects, which can cause a fluctuation in fusion temperature.
Fusion quality assurance
The ‘Achilles heel’ of PE100 pipeline systems is at the point of installation. With the lack of mature NDT techniques, in-field quality assurance is the only solution.
This approach should be supported by comprehensive installer training and technology, which helps reduce procedural defects, while providing a means of auditing the fusion process. Some of the practices adopted include automatic butt fusion, bead inspection, band back testing and destructive testing.
In terms of destructive testing, operators can test joint samples for overall quality and give customer assurances on the quality of the pipeline being laid. Cut samples can be taken from the weld and subjected to tensile tests. Any failures must be ductile rather than brittle and pass criteria to PE80 tensile strength > 15 MPa, PE100 tensile strength > 20 MPa.
PE pipelines are susceptible to failure modes systematic of defects built into the pipeline due to poor installation during the construction phase.
A combination of poor training, poor compliance with codes of practice, lack of robust in-field quality assurance and spot check auditing methodology are key factors contributing to poor pipeline construction and, ultimately, premature joint failure.
Based on DNV GL’s experience, front end investment and a further cultural shift to combat these issues would provide real cost benefits in terms of safe, maintenance-free PE100 pipelines for more than 100 years.
This article was featured in the Fall edition of Pipelines International. To view the magazine on your PC, Mac, tablet, or mobile device, click here.
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