In November 2011 a Root Cause Analysis (RCA) was conducted in order to determine the source of multiple leaks in a 6-inch pipeline that supplies Nitrogen to a glass plant in the municipality of Los Garza NL in Mexico. Leaks were detected after the rehabilitation of damaged sections of the pipeline as a consequence of Hurricane Alex which affected the city of Monterrey in July 2010. Following the re-commissioning of the pipeline a loss of pressure occurred due to multiple leaks in the last section of the pipeline close to the destination facility.
After a complete assessment that involved soil conditions damaged pipeline examination and inspection of corrosion control systems it was possible to identify all the factors involved in the corrosion processes that led to the loss of wall thickness in the nitrogen line.
The physical and chemical analysis revealed the presence of corrosive species in the soil and in the water deposited by a processing plant over the right of way. A profile of acid pH and a low resistivity ground were measured helping to create a more aggressive environment for steel in this area.
The evaluation of corrosion control systems was conducted under the scheme of the External Corrosion Direct Assessment methodology (ECDA). Both cathodic protection systems and the pipeline coating were evaluated by indirect measurements and direct inspections. The cathodic protection coverage in the surveyed area was poor while the evaluation of the coating revealed severe damage due to deficient application a significant degradation in the time of operation and lack of maintenance.
Additionally direct inspection showed shielding effects due to coating disbonding from the substrate allowing penetration of the electrolyte favoring the formation of corrosion cells by differential oxygen concentration while obstructing the flow of cathodic protection current given the dielectric properties of the coating itself.
The conclusions presented a set of solutions ranging from technical actions such as improvement of cathodic protection systems development of work programs for maintenance inspection and early detection; to implementation of new practices in the construction engineering as in the maintenance of the pipeline including activities that are non-mandatory given the nature of the product however they´ll contribute to increase the reliability of the pipeline.
Keywords: Root Cause Analysis Direct Assessment shielding