Monitoring Offshore Cathodic Protection Systems in the Gulf of Mexico: A Review of Available Technologies and Regulatory Requirements
Authors: Britton J.N. 
Published: October 2008

The purpose of this paper is to outline some available technologies and methods for monitoring of offshore pipeline and platform cathodic protection systems. There is discussion on local regulatory compliance and the best ways to get the most from an offshore survey dollar. Some recent advances in ROV assisted surveys for pipelines and subsea completed facilities is also discussed. Finally a section on the potential impact of remote data aquisition technology in the monitoring of offshore cathodic protection is addressed.

INTRODUCTION

Industry awareness as to the benefits and need of cathodic protection systems in the offshore sector has increased in recent years. This has been due mainly to two factors. Firstly, many facilities in the Gulf of Mexico are operating beyond their original design life, and, as a consequence failures are increasing. Secondly, many new structures are destined for deep water. In these locations, repair costs can be prohibitive, there is therefore a real &sire to design cathodic protection systems which will work reliably the first time.

The only ways to verify the adequate performance of a cathodic protection system are either to visually inspect the steel to ensure that no corrosion is occuring, or to make electrochemical measurements to determine the steel to environment electrical potential.

Clearly the latter offers a more convenient alternative. The following sections will describe how such measurements can be made in various offshore situations.

CATHODIC PROTECTION SYSTEMS OFFSHORE

There are two basic methods for applying cathodic protection to offshore submerged or buried structures. The first and most widely used method is to attach galvanic or sacrificial anodes directly to the structure to be protected. These anodes are made from alloys of Aluminum, Zinc or Magnesium cast onto steel cores to facilitate attachment by welding or clamping. The second method is known as impressed current, here a dc voltage is impressed upon a remotely or semi-remotely located anode usually made from a relatively inert material such as cast iron or platinized titanium. The use of these systems has not gained significant popularity in the offshore industry largely due to increased maintenance requirements and inherant unreliability.

Whichever system is utilized the basic principles are the same, simply stated, the object of a cathodic protection system is to &liver electrical current from an anode to be collected evenly onto the surface of the cathode (steel structure), this causes a series of complex reactions on the surface of the steel which essentially arrest the natural corrosion reactions. The most easily measured result of this reaction is a shift in the surface electrical potential of the steel to the surrounding seawater or saline mud. Figure 1 shows a schematic cathodic protection system.

REFERENCE ELECTRODES

In order to measure the surface potential of the steel to seawater, it is necessary to have a reference with which to compare the steel potential. Such a device is called a reference electrode or half-cell.