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Cathodic Shielding Effects of In-Trench Pipeline Supports

Published: 20150316 by NACE International
Author(s): Daniel Fingas

When installing pipelines in rocky areas, additional mechanical safeguards may be necessary to protect the coating. These safeguards may take the form of intermittent supports which elevate the pipeline off the trench bottom. As such supports are non-metallic and are installed directly against the pipeline coating, the pipeline at the location of the supports may be shielded from cathodic protection current.

The cathodic shielding effects of two such supports, a plastic pipeline support and a standard spray-in polyurethane foam support, are investigated and compared. Coating defects were simulated with coupons installed under both supports on the surface of a fusion bonded epoxy coated NPS24 pipe section, with an additional coupon installed to act as a control. The pipe, coupons and supports were immersed in a tap water electrolyte until stable corrosion potentials were reached. Cathodic protection, supplied by a magnesium anode, was applied until the system reached a steady-state. At suitable intervals, the cathodic protection current for each coupon was measured independently and the cathodic protection current density was calculated. In addition, coupon instant-off potentials were measured in order to assess the polarization characteristics of each coupon. The current density and potential results are compared to one another to assess the relative cathodic shielding effects.

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