December 2017, Vol. 244, No. 12
Features
Traces of Bentonite Prevent Bonding of Electrofusion Pipeline Joints
Underground pipelines are often installed using drilling techniques that minimize surface disruption. An investigation to determine the root cause of field failures in a polyethylene gas distribution line led to the discovery that a common ingredient in drilling mud could lead to ineffective bonding of electrofusion joints, which are common in the polyethylene pipeline industry.
Results from the investigation and a cleaning method that was developed to reduce the chance of contaminated joints are shared here by engineers at Exponent, the firm that conducted the investigation.
The gas distribution industry commonly uses saddle-tapping tees to join service branches to polyethylene mains using the electrofusion (EF) process, which involves heating and melting the plastic via electric current applied to heater wires integrated into special fittings.
In 2014, a utility company discovered leaks at several electrofusion saddle taps along distribution mains in a specific service area. As part of its statistical sampling, the utility company excavated over 100 additional tapping tees from the area. Of these, 38 tapping tees failed subsequent laboratory testing, prompting an investigation to identify the root cause of the problem.
All of the field failures occurred in areas where horizontal directional drilling (HDD) was used to install the mains. Exponent prepared electrofusion joints in the laboratory to mimic field conditions and inspected the field failures and other excavated joints. After the likely cause of failure was identified, Exponent tested additional fitting types and prepared procedural recommendations designed to reduce the likelihood of failure of electrofusions.
Under electrofusion standard operating procedures, the pipe is cleaned to remove contamination that might inhibit the fusion process, and scraped to remove the outer layer of oxidized polymer that might also inhibit the fusion process. After scraping and cleaning, the electrofusion saddle fitting is secured on the pipe. Two electrical leads from an electrofusion control unit are attached to the fitting and the control unit applies an electric current profile over a specified time interval. After the current profile is complete, the leads are removed, the joint is allowed to cool and the completed joint is inspected (Figure 1).
Figure 1: Successfully completed 6-inch electrofusion joint.
Before bentonite was identified as the contaminant that caused the observed failures at issue, numerous plausible contaminants were tested to determine their propensity to cause weak or brittle electrofusions. These contaminants included greases, oils, native soil, and various HDD fluid ingredients, including bentonite clay, a primary ingredient in drilling mud.
Whereas bentonite was found to cause weak joints even when present in visually undetectable amounts, the other contaminants were only found to cause weak joints when present in visually obvious amounts that would be detected by reasonably conscientious installers. After bentonite was identified as a potent contaminant, testing was conducted to identify mechanisms that could result in bentonite-contaminated pipe despite following standard cleaning procedures.
Electrofusion joints were intentionally contaminated and then tested to identify contaminants and procedural shortcomings that may have contributed to field failures. Through these tests, Exponent aimed to recreate the observed failures.
The first round of testing focused on plausible contaminants. After problematic contaminants were identified, the second round of testing investigated procedural deviations that would result in joint contamination and joint failure. Exponent followed the utility company’s current standard operating procedure for pipe preparation and electrofusion, with variations in the procedure as part of the test matrix.
To evaluate electrofusion success, joints were tested destructively using a strip bend test. The tower portion of the saddle tee was cut off and the remaining assembly was sawn into longitudinal strips to provide at least two strips about 25-mm (1-inch) wide having sections of the saddle attached (Figure 2).
Figure 2: Examples of strips cut from pipe with saddle joint for strip bend test.
Figure 3 shows the joint interface with the cold zones and fusion zones identified; separation in the fusion zone over the majority of the zone indicates failure according to the bend test. For testing, each strip was bent back on itself, and if the saddle detached from the pipe wall over more than 50% of the fusion zone length on either side of the joint’s axial center, the joint was deemed to have failed (Figure 4).
Figure 3: Location of cold zones (separation allowable) and fusion zones (failure occurred if separation greater than 50% of the length of the zone).
Figure 4: An example of a joint that failed the bend test – 100% of the fusion zone on one side of the branch hole has separated.
Joints that failed the bend test were typically deemed to have been acceptable, according to field inspection criteria, including stored electrofusion unit data and visual criteria. Thus, the field inspections would not have detected the joints that subsequently failed the bend tests.
The initial experiments showed that, compared to other plausible contaminants, only small amounts of bentonite were necessary to cause failure. Bentonite is a type of smectite clay often used for barriers and seals against water, and is a major component of drilling mud, the thick fluid used to support a borehole and lubricate the drill bit when drilling for underground pipe installation.
Bentonite consists of small, plate-like nanoparticles called platelets. These particles form a filter cake that blocks fluid and thus helps stabilize a horizontal borehole. The microstructure of bentonite and its hydraulic conductivity and diffusivity make it effective at preventing molecule flow.
After the strip specimens were bend-tested, a scanning electron microscope (SEM) was used to inspect samples and detect the presence of contamination. The failed saddles were completely removed from the strips and examined.
Figure 5 shows the saddle from the failed joint of Figure 4, in which the bentonite contamination is visible as gray patches. Area 1, at the top of the image, shows the energy-dispersive spectroscopy (EDS) spectrum of the EF saddle, which contains only carbon and oxygen. Areas 2 and 3 show EDS spectra that also identify aluminum and silicon, which is consistent with the EDS spectrum of bentonite-based drilling mix.
Figure 5: The SEM image and EDS spectra for B4 saddle after bentonite-based drilling fluid contamination.
This finding is consistent with the chemical composition of Wyoming-type bentonite, which is primarily SiO2 and Al2O2, and may also contain Fe2O3, MgO, CaO, Na2O and K2O, depending on the source of the bentonite.
Because bentonite applied to pipe surfaces through the application of commercial drilling muds resulted in failed joints during lab tests even when the bentonite was present in invisible amounts, Exponent conducted a suite of procedural tests to identify how bentonite might be transferred to the joint area before the electrofusion procedure, as well as testing variations in scraping and cleaning the pipe to identify improvements.
For procedural tests, the contaminant was applied to the pipe before scraping and cleaning, reflecting the contaminant transferred to the pipe during initial installation. Cleaning methods varied, from wiping only within the scraped area to wiping including the surrounding area.
From these tests, it was confirmed that very small amounts of bentonite were sufficient to cause joint failures, even when it was not visible to the eye. Most important, the standard cleaning procedure of using isopropyl alcohol beyond the scraped area effectively spread small amounts of bentonite from the unscraped area to the fusion area and thus prevented successful fusion.
Many of the joints failed after they were prepared with drilling mud and then “cleaned” over more than the scraped area (including wiping material into the cleaned area from the unscraped edges where dried drilling mud was still present). Of the 19 procedural tests that included bentonite, 42% of the samples failed. However, when limiting the procedural tests to those that include both bentonite contamination and wiping beyond the scraped area (10 tests total), the failure rate was 80%.
Based on the combination of contamination tests and procedural tests, it was found that bentonite residue was likely to cause joint failures, and that bentonite residue could end up in the electrofusion fitting area if the pipe was cleaned by wiping over too broad an area. Thus, we conclude that the field joint failures were due to bentonite contamination, exacerbated by the pipe-cleaning procedure.
Even diligently following the existing cleaning procedure risked contaminating the pipe if bentonite residue from the unscraped area was wiped onto the scraped area during the cleaning step. Accordingly, Exponent was assigned with developing and confirming the effectiveness of a revised cleaning procedure designed to reduce the risk of bentonite contamination in the preparation of field joints.
The new procedure for both saddle and coupling fittings involves washing the pipe with water and isopropyl alcohol (IPA) before scraping, and then cleaning only the scraped area with IPA, using a new cloth or disposable wipe, ensuring that the wipe is used only on the scraped portion of the pipe. The cleaning procedure after scraping was designed to ensure that no contamination from the unscraped area was dragged onto the scraped EF area.
Fifteen joints were prepared using IPA and clean cotton cloths, and five joints were prepared using disposable dry wipes wetted with IPA. There were no failures due to bentonite contamination when the new procedure was used to clean the 20 bentonite-contaminated pipes.
Based on the findings, failed joints that appear to have been properly prepared and installed using HDD most likely failed due to contamination from a bentonite-based drilling mud. EF joints prepared using a typical procedure in which the pipe was scraped and then cleaned with IPA after cleaning often failed if bentonite contamination was present. EF joints prepared using the revised cleaning procedure, in which the pipe is cleaned before scraping and then cleaned with a new wipe over only the scraped area, were successful.
A procedure that involves first cleaning, then scraping the pipe, and then cleaning only the scraped area is more effective at removing bentonite contamination than cleaning a larger area of the pipe. Because the procedure uses water in the initial washing step, care should be taken to ensure that the pipe is adequately dried to prevent void formation from water vapor (steam), which could compromise the joint.
The characteristics that make bentonite effective for preventing fluid migration through borehole surfaces also make bentonite effective at preventing electrofusion by blocking the wetting or inter-diffusion of melted plastics between pipe and fitting.
Therefore, particular care should be taken when using EF procedures on pipes that may have contacted bentonite-based drilling muds because even small amounts of contamination, (not visible to the naked eye) can prevent successful electrofusion. P&GJ
Acknowledgments: The authors are grateful to Pacific Gas and Electric Company for providing technical, material and financial support for this study. We particularly thank Abisai Gonzalez, Michael Kerans and Raymond Thierry for their technical and material support, and for their many helpful discussions.
Authors: Kaitlin Spak is a licensed professional engineer who specializes in the mechanical engineering of systems and structures. She completed her Ph.D. from Virginia Tech while working at NASA’s Jet Propulsion Laboratory. Spak’s work since joining Exponent includes analysis and testing of industrial machinery, heavy equipment, engines, drilling operations, pipelines, and consumer products and appliances.
Richard W. Klopp specializes in mechanical engineering and the mechanics of materials. He has particular expertise in laboratory-based testing, mechanical design, failure analysis and prevention, and manufacturing. Klopp has extensive experience with plastic pipelines. He holds a Ph.D. from Brown University and a bachelor’s degree in mechanical engineering from Lehigh University.
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