The weight of the issue

Chelsey Reschke explains how Keymay Industries engaged Charter Coating Service to design a test protocol to assess the possible shielding effects of geosynthetic set-on weights.

By Chelsey Reschke, VP Business Development, Keymay Industries, Sherwood Park, AB, Canada

Crossings through aquatic and semi-aquatic environments are universally common in pipeline construction. Areas with high water tables, rivers, marsh or wetlands, and flood plains rely on buoyancy-control products to maintain downward force on the pipe, and counteract the buoyant force of materials with relatively higher specific gravity. Like many engineered products used in pipeline construction, the options for buoyancy control have evolved.

From early cast iron shells and envelope-style saddle weights, to modern geosynthetic set-on weights, concrete coating, and screw anchors, the pressure to adapt these products has come from enhanced knowledge about the long-term effects of each product on pipeline integrity. The integrity industry has made significant strides in understanding cathodic protection (CP) and the various factors that impact its performance, yet there remains no active standard on shielding potentials of materials placed over pipe coatings. This article provides new data from field installers and independent laboratories on the cathodic shielding considerations for geosynthetic set-on weights.

chelsey
Chelsey Reschke, Keymay Industries VP Business Development

Modern geosynthetic weights have become a popular choice due to their technical performance, ease of installation, minimal environmental impact in sensitive areas, and cost per unit. Hundreds of thousands of these weights have been installed on pipelines around the world; however, it is only in recent years that investigations into the possible CP shielding effect have been explored.

Concerns stem from the hypothesis that permeable 400 GSM geosynthetic material, when placed in direct contact with the coated surface of the pipeline, will impede the cathodic protection due to lack of contact with the native electrolyte (water). Furthermore, material permeability is directly affected by the type of fill material used in the weight (the screened rock used in this study, or the most common option of sand), meaning that using tests like ASTM D4491 Water permeability of a geotextile is not a direct assessment of shielding potential.

The phenomenon that prevents the use of ASTM D4491 as a measure of shielding potential has to do with the particle size of the fill material. Fines, like sand, have a tendency to fill the voids of the woven geosynthetic material over time, meaning that even if the permeability of the woven material meets engineering requirements, the use of sand to fill the weight will affect the overall shielding potential of the system when installed over a pipeline.

For these reasons, Keymay Industries, a manufacturer of geosynthetic weights, sought the expertise of Charter Coating Service (2000) Ltd (CCS) to design a test protocol and method to assess the possible shielding effect of the Keymay EcoBag filled with 0.75 inch of screened rock (CSA 20-5 mm).

CCS has provided an independent source of technical and practical expertise for coatings, paints, and mastics since 1982. It has highly trained personnel with excellent knowledge in coating analysis and failure prevention who can conduct laboratory tests, inspections, and failure analysis/consultations for a broad range of coatings and paints, with an emphasis on materials that mitigate corrosion. CCS is an ISO/IEC 17025:05 accredited laboratory, and can offer full compliance with the common standards in the oil and gas industry, as well as other industry specifications.

figure_1
Charter Coating’s test protocol: the ‘standard’ CD tests were conducted as per a modified CSA Z245.20-2014 section 12.8.

The test designed by CCS used a 4.5 inch (114 mm) OD pipe coated in fusion bond epoxy (FBE) with 3.2 mm intentional holidays drilled through the FBE. The exposed holidays were covered by an EcoBag filled with 0.75 inch screened rock and submerged in solution of 3 per cent NaCl, with a -1.5V CP pipe potential during the 28-day test duration.

The results are summarised below:

  • There was no sign of corrosion for all specimens tested in cathodic-protection shielding and the ‘standard’ cathodic-disbondment tests, indicative of effective cathodic protection.
  • The cathodic-shielding tests showed significantly lower current than the ‘standard’ cathodic-disbondment test, indicating that the EcoBag filled with 5-20 mm screened rock will partially block CP current. This may be attributed in part to the higher resistivity (lower conductivity) of screened rock compared to salt water. But the level of current passing through was sufficient to polarise the pipe surface and prevent corrosion, because no corrosion was observed at the intentional holidays.
  • The average radius of disbondment from the EcoBag CP Shielding test was slightly lower than for the ‘standard’ cathodic-disbondment test and much higher than the soak test with intentional holiday. The results confirmed there was a CP reaction taken place at the intentional holidays during the cathodic-shielding test.

Studies conducted using a -1.5V CP potential on test pipes covered and not covered with a Keymay EcoBag filled with 5-20 mm screened rock showed that this bag does not totally shield the pipe from CP and the pipe continues to be CP protected, although the current was significantly reduced. The intentional holidays created in the FBE remained corrosion free after the 28-day test. It is concluded that the permittivity of the bag is sufficient to allow cathodic current to pass through at this voltage, and thus to afford continued cathodic protection of a pipeline covered with the EcoBag.


Cathodic shielding investigation

Question: Does the Keymay EcoBag geotextile pipeline weight (filled with 5-20 mm screened rock) shield the cathodic protection on the underlying FBE-coated steel pipe?

Hypothesis: If EcoBag is non-shielding, expect to find:

  • no sign of corrosion in intentional holiday;
  • negligible difference between standard vs sample CD test; and
  • negligible difference between standard vs sample current measurements

Test method: Modified CSA Z245.20-2014 Section 12.8

Test materials:

  • 4.5 inch (114 mm) OD pipe with 3.2-mm intentional holidays
  • Keymay EcoBag filled with 0.75 inch screened rock

For more information visit the Keymay Industries website.

This article was featured in the December edition of Pipelines International. To view the magazine on your PC, Mac, tablet, or mobile device, click here

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