ABSTRACT

The effective distribution of cathodic protection current to bare or poorly coated cross country pipelines in high resistivity soils depends greatly on the type of impressed current groundbed installed. Continuous anode groundbeds provide better current distribution with lower groundbed resistances than traditional distributed anode groundbeds. Total installation and operating costs of continuous anode systems an: less expensive than that of distributed systems over the life of the groundbed.

INTRODUCTION

High resistivity soil (50,000 ohm-cm or greater) poses distinct problems for the application of cathodic protection (CP) current to pipelines. In addition to restricting the output of CP groundbeds, the ability to evenly distribute the CP current over the entire length of the pipeline is greatly reduced. This reduction in the distribution of current becomes most obvious when protecting bare or poorly coated pipelines.

Bare or effectively bare (poorly coated) pipelines require a substantial amount of CP current, as much as 2 milliamperes per square foot (21.52 mA/m2), to be cathodically protected. On large diameter pipelines this can equate to substantially high current requirements per mile (1.609 km) of pipe. As much as 55 amperes of current would be needed to protect one mile of bare 20 inch (50.8 cm) pipe at 2 milliamperes of current per square foot (21.52 mA/m2). In areas with high and/or varying resistivity soils, evenly distributing this amount of Current to the pipe becomes very difficult.

Generally, distributed impressed current anodes are installed parallel and in close proximity to the pipeline in order to distribute CP current to the pipe. Traditional distributed anode groundbeds consist of impressed current anodes placed 50 to 200 feet (15.22 to 60.9 m) apart on a common cable. An alternative to the installation of traditional impressed current anodes is the installation of a continuous anode cable groundbed. This paper will compare the two methods from a technical and financial standpoint

CURRENT DISTRIBUTION - CONTINUOUS VERSUS DISTRIBUTED GROUNDBEDS

High soil resistivity reduces the ability of CP current to flow from an anode to the pipeline. This problem is increased as the distance between the anode and pipe are increased. Traditional distributed groundbeds distribute current to the pipe at the anode; however, between anodes there is less current distribution and therefore less protection. Figures 1 and 2 show close interval survey graphs of a bare 14 inch (35.56 cm) pipeline in approximately 50,000 ohm-cm soil protected with a distributed anode grouodbed. The location of anodes can be seen at the upward peaks in the ON pipe-to-soil potential every 100 feet (30.45 m) in Figure 1 and every 200 feet (60.9 m) in Figure 2. The ON potential between the anodes however is much less than that at the anode. This indicates less current is reaching the pipe between the anodes. The level polarization of this pipeline, determined by the voltage difference between the OFF and Static potentials, is less than 100 mV at several locations between the anodes.

Increasing the output of the groundbed may not improve the current distribution between anodes.

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