Coating resistance or conductance values are critical input parameters for the accurate modelling and design of AC mitigation and cathodic protection (CP) systems for pipelines. However, there are relatively few sources for real-world coating resistance values in the literature. To attempt to fill this gap, data from coating quality tests performed on thousands of trenchless pipeline installations was used to characterize the coating resistance of new, primarily fusion bonded epoxy (FBE)-coated pipelines. The tests assess damage incurred as part of the trenchless installations and cover the full range from pristine coating to significant scratches with bare metal. This allows the dataset to provide insight into both a baseline for open-trench coating quality and a range of practical coating qualities for trenchless installations.
It was found that specific coating resistance values varied from less than 100 Ω·m2 to >1 MΩ·m2 but that there was no specific threshold that could be associated with pristine FBE coating. However, the data clearly validated the ‘300 mV shift’ design guideline for CP systems, which also demonstrates the link between CP current requirement and coating quality. Considering this, the implication of requiring a certain current density irrespective of coating quality is illustrated.
Coating conductance tests for pipeline trenchless crossings provide information about coating damage sustained during installation. The test essentially measures the pipeline resistancea to the earth and a relationship between the coating conductance and the percentage of bare pipeline was developed.1 The test methodology was later improved with the addition of a visual assessment and a current requirement test.2 The applicability of the methodology to deep HDDs was also confirmed experimentally3,4. While the primary purpose of these coating conductance tests, which have now been conducted on thousands of trenchless installations varying from track bores to kilometre-long HDDs, is to assess and mitigate for in situ coating quality when traditional coating inspections are impossible, the data is also applicable to more general design practices. Although trenchless installations are expected to sustain more damage than traditional open trench installations, this paper explores the possibility of establishing a practical target for specific coating resistance based on the aggregate coating quality data, with the best trenchless installations representing the excellent coating quality representative of open trench installations.
