by Pedrag Micic, Product Development Executive, Qenos and Mike Stahmer, Pipe Industry Consultant
Experience indicates that the largest threat to the pipeline structural integrity comes from external interference with the pipeline. Apart from the external interference that may result in immediate pipe rupture, the general mode of failure reported for polyethylene (PE) pipelines in service is a ‘brittle’ crack through the pipe wall by the slow crack growth (SCG) mechanism.
A slow growing crack could initiate in presence of localised stress as result of, for example, damage during trenchless installation, leading to failures before the expected service life of the pipeline. Due to the use of fairly conservative safety factors in pipe design, PE pipes have a large safety margin for pressure bearing capability and never fail in over pressured ‘ductile’ mode in operations.
Pressure pipes in service almost always fail in ‘brittle’ type failure mode by SCG failure mechanism. PE pipes can get damaged during transport, installation or later in service by point loading due to rock or root impingement.
Bending stresses due to ground movement could also cause localised stress on the pipe leading to ‘brittle’ failure. Consequently, high resistance to SCG is the most relevant property of PE to ensuring long service life of pipelines.
Pipeline integrity is dependent on the design, operation and management of the pipeline. A safe pipeline starts with good design and an important part of it is the selection of the polyethylene pipe resin with high SCG to be able to withstand the demands of installation and operation.
One of the major technical risks surrounding the use of PE piping has been mitigated by restricting the “allowable flaw size to less than 10 per cent of wall thickness”. The 10 per cent wall thickness damage rule has significant implications for installation of PE pipelines via trenchless techniques.
Trenchless installation possesses advantages, such as in cost efficiency and less disturbance to urban and environmental areas where installation is taking place. However, some techniques have the potential to introduce installation flaws in the pipe, such as scratches that could develop into cracks and result in brittle failures once the pipe is in operation.
What are the concerns?
The typical concern of pipe engineers with trenchless installation is that the process of pipe cracking exposes the new pipe to potential damage arising from contact with the pipe being replaced. Contact with broken pipe fragments can potentially lead to scratches greater than the conventional allowance of 10 per cent of the pipe wall.
The ASTT has published guidelines for trenchless construction (ASTT, 2009) in which it is stated in relation to pipe bursting:
Exterior pipe damage assessment is difficult to carry out and detect once installation is completed. Inspection for exterior damage should be carried out prior to installation to ensure the integrity of the pipe. Hydrostatic testing should also be performed prior to installing the pipe to ensure any defects are addressed.
One of the common practice testing techniques is to pull out 2-7 m of pipe and examine it after installation from the receiving chamber. This front section of the pipe tends to receive the most impact and damage from the installation process and is used as a guide for determining the general condition of the rest of the pipe.
These guidelines, while useful and adopted in practice, only partially address the potential for damage. The assumption that ‘the front section of the pipe tends to receive the most impact and damage’ is not necessarily always the case.
In addition, hydrostatic testing is unlikely to detect even severe damage to PE100 pipe as the likely failure mode is slow crack growth, which is a long-term process. Due to concerns regarding the effects of surface damage, sometimes sacrificial pipe jackets are installed to protect the new pipe. This is an expensive and more complicated approach.
To account for such circumstances, pipeline designers tend to use higher safety factors leading to thick pipe walls and which makes pipes more expensive.
How is material tolerance to SCG caused by notches measured?
For PE100, AS/NZS 4131 requires a minimum of 500 hours at a stress of 920 kPa in the notched pipe test on SDR11 pipe. In all cases, notches are to 20 per cent of pipe wall and tests conducted at 80°C.
How can the use of Alkadyne® HCR193B address these concerns?
In order to assess performance with deeper notches, Qenos has tested Alkadyne® HCR193B in the notched pipe test at deeper notches than the standard 20 per cent. Notch depth simulates the depth of damage via notches or scratches that may occur during installation of pipe.
All tests were conducted in a certified testing facility on industrially extruded pipes. Pipes made from Alkadyne® HCR193B surpassed standard requirements for slow crack resistance for PE100 even when 30 per cent of the pipe wall had been notched.
Testing also shows the boundary condition for allowable notch depth where 40 per cent notched pipe failed in ‘ductile’ mode therefore not meeting pressure containment requirement. The testing program supports the case for using slow crack resistant PE100 for pipes used in trenchless installations where there is a risk of deeper notches.
This article was featured in the June edition of Trenchless Australasia. To view the magazine on your PC, Mac, tablet, or mobile device, click here.
For more information visit the Qenos website.
If you have a technical paper you would like featured in Trenchless Australasia contact Assistant Editor Nick Lovering at nlovering@gs-press.com.au