Performance analysis and mitigating the effects of stray currents on underground metal pipelines in South Africa.

Abstract

The transport, distribution and utilization of electrical energy can often negatively influence other services in the vicinity due to leakage of stray currents or the influence of varying magnetic fields causing induction in nearby pipelines. When pipeline operators are informed of electrical abnormalities in the vicinity of their facilities, it raises considerable concern and triggers priority remedial reaction. It is well known that in the vicinity of DC stray current the risk of electrolytic corrosion on buried pipelines is very high. In the case of a good external pipe coating with a small defect in the coating, electrolytic corrosion current density at the coating defect will be high and penetration of the pipe wall can result in a short period of time. Invariably the license agreement to operate a pipeline is granted on condition that all necessary steps are taken to prevent spillages and that all actions taken to ensure the integrity of the pipeline are accurately recorded and be available at all times for inspection to the applicable investigating authorities. The primary protection of pipelines against electrolytic corrosion is the external coating of the pipe. As no coating is deemed to be perfect and all coatings are subject to deterioration from aging and subject to damage during installation, a secondary or back-up means of corrosion protection is required. The most commonly used means of secondary corrosion protection for pipelines is cathodic protection. Cathodic protection works on the principle of polarizing all areas on the pipe surface to the same potential so that no corrosion cells can exist on the pipe surface. In addition the pipe surface is maintained at a potential more negative than its immediate surrounding so that it becomes the cathode with respect to the anode of the cathodic protection circuit. A cathodic protection system may consist of a combination of galvanic and impressed current technologies depending on varies factors such as, availability of power sources, soil resistivity, land availability for installation of anode beds and length of pipe to be protected. In South Africa metal pipelines are frequently subjected to the influence of stray currents as a result of the DC rail traction network than spans across the country. Stray current is known to cause the most severe form of corrosion to underground metallic structures that could result in a pipeline leak in a very short period of time with devastating damage to the environment. Stray currents may also interfere with existing cathodic protection systems rendering them inadequate of providing the necessary protection to the pipeline. These currents also interfere in such a manner that maintenance and operation of the existing cathodic protection becomes difficult and unpredictable. Knowledge of the presence of stray current is also critical when designing new pipelines and cathodic protection systems so that the necessary mitigating factors can be implemented in new designs. In South Africa cathodic protection has been proven to be an effective means of protecting both new and old pipelines. This study analyses the challenges associated with the mitigation of stray current and the existing methodology used by a major South African pipeline operator to monitor the presence of stray current. Methodologies for detection and mitigation are briefly discussed in this dissertation. Based on the information studied proposals are suggested to enhance the current methodology and create greater awareness about the damaging effects of stray currents.