Examining failure - polyethylene pipes in the utilities sector
Dr Chris O’Connor, Senior Consultant at GL Noble Denton reports on polyethylene pipe failure in the utilities sector.
Polyethylene (PE) pipes and fittings are used across the utilities sector to operate gas and water distribution systems safely, reliably and economically. They enjoy an excellent performance track record – modern PE is highly engineered and provides a good balance of strength, stiffness, toughness and durability.
The failure of PE pipe systems is typically found in older generation materials, whose properties offer limited resistance against severe environmental and operating conditions. Human error can also result in failure, often when a mistake is made in the supply chain and site construction of installed pipe systems.
The general mode of field failure reported for PE pipe is brittle, slow crack growth (SCG) through the pipe wall. Cracks can start at microscopic stressraising flaws, and these brittle mechanical failures are typically slit-type fractures that lie parallel to a pipe’s extrusion direction and the driving force for crack opening is circumferential hoop stress in the pipe wall. Typical slit-type fractures are shown below, left.
Circumferential cracks can also be initiated on the outside or inside surface of pipes as a result of secondary stresses, such as bending or impingement on the material. The premature failure of meltfused joints is also common failure, where cracking initiates at stress concentrations created by poor installation practices.
Visually, brittle cracks are typically smooth, featureless and devoid of any yielding and deformation process, as shown above right.
There are three major failure modes for PE pipe, as shown in this graph:
Ductile failure (mode I) 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 as shown here:
Failure mode II is associated with creep, creep rupture and SCG. Creep is time-dependant, nonreversible deformation as a result of exposure 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. This graph is representative of a creep rupture curve where the ductile-brittle transition signals the onset SCG:
The events that lead to creep rupture are shown in the next graph:
After crack initiation, voids develop ahead of the crack. These voids gradually merge into larger voids that are spanned by highly orientated load-bearing fibrils. This process, known as crazing, continues until a point is reached when the most highly stretched fibrils will rupture, resulting in fracture.
For PE, the tenacity of fibrils and their resistance to rupture will be highly dependent on molecular architecture, in particular molecular weight, molecular weight distribution, branching, crystallinity and tie molecules. The tie molecules are embedded in the crystallites. They transverse amorphous regions, acting as mechanical links between the crystalline domains, and play a decisive role in the resistance to fibril failure and overall mechanical properties when subjected to stress.
Mode III failure is related to degradation and embrittlement of the plastic due to thermo-oxidation with time.
In the late 1970s, workers at British Gas R&D discovered that thick wall MDPE pipes operated at a critical pressure and, below a critical temperature, could fail catastrophically by sustained, axial rapid crack propagation (RCP) (see main image, top of article).
This propagation can be initiated by either a defective butt joint, third party damage (such as a high velocity impact from excavation equipment) or a pipeline pressure pulse. Once initiated, ruptures can travel at high speed along the length of the pipe over significant distances, so long as the stored energy from the contained pressure source is sufficient to drive the crack faster than the rate at which the energy is released. When approaching the speed of sound, the crack becomes unstable, branching in a sinusoidal pattern, until it slows and stops.
Rapid crack propagation is dependent on several factors:
- Pipe diameter – as outside diameter increases, the possibility of RCP increases.
- Operating pressure – as pipeline stress/pressure increases, the possibility of RCP increases.
- Operating temperature – as temperature decreases, the possibility of RCP increases.
- The pipe material’s resistance to impact fracture and resistance to RCP.
This RCP failure of PE pipe is rare, but the potential consequences are significant for gas distribution. Resistance is now designed into pipes, so it may be avoided under worst-case scenarios. In order to meet this requirement, GL Noble Denton has built a full-scale RCP test facility at its asset test site, Spadeadam in Cumbria, UK (see image, right). Extensive RCP testing conducted by the company has identified safe operating pressures, and as a result, RCP has never been recorded within the UK Transco PE distribution network.
In summation, PE pipeline networks have established an impressive safety record over the years, and are the preferred material of choice in the construction of gas and water distribution networks. However, they are not immune from failure for an assortment of reasons, including:
- System design
- Manufacturing practices
- Installation practices
- Accidental damage
- Incorrect material selection
- Thermal exposure
- Stressing beyond anticipated design stress
- Point loading and stress raisers
- Chemical exposure
- Soil conditions
Nevertheless, PE pipe system failure can result in explosion, fire and loss of life, resulting in costly litigation and damages. In addition, this can cause service interruption, safety concerns and loss of brand credibility. For these reasons, failure analysis must be standard practice for the utility market.
PE offers the pipe industry many benefits, including:
- Economical, high volume manufacture – extrusion, injection moulding.
- Design flexibility – easily shaped.
- Integrated design – multifunction, ready-assembled components, couplers and fittings.
- Low material cost.
- Light-weight design – ease of transport and handling.
- Flexibility – use in conjunction with trenchless technologies and resistance to seismic activity.
- Relative ease of jointing (compared to metallic pipe systems).
- Corrosion and chemical resistance.
- Biologically inert capabilities.
- Toughness, impact resistance, abrasion resistance and long-term durability – technical lifetime of >50 years.
- Low-temperature performance.
- Leak-free fusion jointing – low maintenance costs.
- Low friction bore – no scale build-up and efficient flow of transfer medium.
- Environmental benefits – recyclable.
Dr Chris O’Connor Email: Chris.O'Connor@gl-group.com