A Pipeline Designer's Guide to Cathodic Protection

As part of our ongoing free webinar series with The Pipeline Industries Guild, our latest presentation “Pipeline Designer’s Guide to Cathodic Protection” is now available on YouTube.

The aim of the presentation is to provide information to pipeline designers on the factors that can influence the effectiveness of the external corrosion protection system - coatings, cathodic protection and electrical interference.

The technical information is given in such a way that it is understandable to people that are not corrosion specialists.

Effect of Earth Resistivity on Evaluating the Corrosion Risks from AC Interference to Pipelines

Introduction

This paper shall be presented as part of the Virtual EuroCorr Conference 2020. The paper has been jointly written by Kenneth Lax (Corroconsult), Charalambos A. Charalambous (University of Cyprus) and David Boteler (Natural Resources Canada).

Abstract

Currents in power line conductors induce currents in nearby pipelines and in the earth beneath. The pipe-to-soil potential (PSP) variations produced by this AC interference depend on two factors:

(1) the electromagnetic induction into the pipeline, and

(2) the electrical response of the pipeline.

Evaluating Part 1 involves using information (amplitude, phase relationship and frequency) about the currents in the power line; the height of the power line and lateral distance to the pipeline; and the earth resistivity to calculate the electric field induced in the pipeline. This calculation can be done using Carson’s formula or by using the complex image method.

Determining the pipeline response in Part 2 involves using the calculated electric field value as input to a transmission line model of the pipeline. This model involves the series impedance along the pipeline and the parallel admittance to ground. The admittance to ground is a function of both the coating conductance and resistivity of the soil surrounding the pipeline.

Thus, soil resistivity features in the calculations both in Part 1, dealing with electromagnetic induction, and in Part 2, dealing with the pipeline response.

The purpose of this paper is to examine the effect of different Earth resistivity values on AC interference to pipelines and to assess which resistivity values are important. Deeper resistivity values feature in the electromagnetic induction calculations of the electric fields, while the near-surface resistivity of the soil surrounding the pipeline features in the pipeline modelling. The conclusion is that the resistivity of the soil has no material impact on the calculations of the induced electric field and consequent pipe-to-soil potential (PSP) variations produced by the steady state AC interference.

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Cluden to Brighouse Bay Pipeline Project

Corroconsult have commenced their supervisory and consultancy role on behalf of Arup for the Cluden - Brighouse Bay Pipeline Project in Scotland, UK.

The project is to construct a 50km high-pressure steel pipeline between Cluden and Brighouse Bay in Scotland, including a 7.5km re-route of the pipeline between Cluden and Lochfoot.

This is the final step to complete the full twinning of the two gas interconnectors between Ireland and the UK. Twinning of the on-shore section, which feeds the 2 subsea pipelines in place since 1993 and 2004 respectively, will secure this vital link to the UK gas market.

The pipeline will also boost the operational flexibility of the Irish gas network which is essential to providing backup to intermittent renewable electricity generation. Natural gas, as the cleanest and most flexible fossil fuel for power generation, is an ideal partner to renewables as Ireland seeks to meet its energy and climate change targets.

The pipeline will be constructed using 900mm diameter high-grade steel pipe, buried to a minimum of 1.2 m below ground. Pipeline construction activities are being supervised by Arup and the main construction works are being undertaken by Sicim Roadbridge on behalf of GNI (UK) Limited.