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Providing stable TR protection for well coated pipes – Bode Plot or Backhoe?

Paper Number: 25
Author/s: Richard A Brodribb
Organisation: M. Brodribb Pty Ltd, Melbourne

Abstract

The coating on modern pipelines  for the oil and gas industry combine durability with high resistivity to provide environmental and electrical protection against corrosion.   However well coated pipes are often cathodically protected by impressed current rectifiers to provide additional protection should coating defects occur during the life of the pipe.

The rectifier has to be designed to deliver sufficient current for the worst case of coating degradation at end of asset life and yet also reliably provide the very small amounts of current required to give protection when the pipe coating is new and the current requirement is low.

The requirement for the rectifier control system to operate in a stable manner over a very wide dynamic range creates design problems because the load has significant resistive, capacitive and inductive elements and is also a good antenna.  

This paper will examine the electrical characteristics of a general well coated pipe and the interaction of these characteristics with a transformer rectifier using conventional damped closed loop control.

Solutions given by classical control methods (Bode Stability Plot) and practical solutions such as increasing the load current by dummy resistors to simulate defects (Backhoe) will be discussed.

A conclusion relating to these solutions suitability and other methods for improving stability will be provided.