Elizabeth Nicholson, Corrosion 2015, Paper 5675
ABSTRACT
Describes a fuzzy logic model intended for quantitative risk analysis to the integrity of buried pipelines. The proposed approach correlates data from combined CIPS+DCVG coating surveys to the soil resistivity, in order to define an indicator that expresses the corrosion susceptibility at a given coating defect location. Inputs used in the mathematical model include: DCVG defect severity, CIPS pipe-to-soil “OFF” potentials and local soil electrical resistivity. The output is a real number, defined in the interval [0, 3], which provides a qualitative and quantitative degree of steel exposure to corrosion activity. Easy to implement, the presented method is an additional tool to assist pipeline operators with assessing the condition of their pipelines and prioritizing corrective actions.
INTRODUCTION
Corrosion is recognized as the most important contributing factor to pipeline failure. Corrosion is likely to occur at locations where coating defects exist and cathodic protection levels are insufficient. The defect exposes the unprotected steel to the surrounding soil (electrolyte), whose aggressiveness is characterized by several parameters related to its physical and chemical properties.
NACE establishes a methodology for External Corrosion Direct Assessment – ECDA in the standard SP0502-2002 [1], which constitutes a structured and proactive process aimed towards the integrity of pipelines. This process includes prevention of threats, identification of anomalies, corrosion activity assessment and corrective actions.
The SP0502-2002 ECDA methodology requires that at least two different indirect inspection techniques must be employed for integrity surveys. Any indications of concern must be analyzed along with environmental parameters that may expose the structure to corrosive activity, such as: electrical resistivity, moisture, pH, soil composition, presence of SRB etc. Based on this data, associated with pipeline history and specific operator’s criteria, priorities are defined for direct inspections (excavations) and repairs.
Two of the most widely employed indirect inspection methods are DCVG – Direct Current Voltage Gradient and CIPS – Close Interval Potential Survey. Each of these techniques has specific criteria for characterizing indications, ranking anomalies and defining corrective measures [2]. Currently, there are instruments available that allow for the combination of both techniques (CIPS+DCVG) [3], providing a complete, ECDA compliant set of indirect inspection data. Accuracy of the data is improved because the two surveys are carried out simultaneously at the same geographic point, in the same conditions, with the same equipment, by the same surveyor and without additional cost.
It is known that the cost for excavating, analyzing and repairing sections of pipe is often the most expensive step of the overall ECDA process. To define priorities for corrective actions is a complex task and of great responsibility. It requires experience of the operator and the simultaneous analysis of multiple variables, with several classification criteria. If priorities are determined without a consistent method, the pipeline operator is subject to the risk of: 1) waste of time and resources with low priority (or even unnecessary) interventions; and 2) not knowing if all necessary corrective measures were performed in order to ensure the pipeline integrity.
This paper proposes a fuzzy logic model to evaluate the variety of ECDA data in a consistant manner. The concept of fuzzy logic was introduced by L. A. Zadeh [4], to address the vague aspect of information. It is considered one of the most well succeeded [5] approaches for handling complex engineering problems, especially when a mathematical model is subject to uncertainties. Recently, fuzzy logic has been applied successfully to innumerous problems of environmental and civil engineering, including modelling of pipeline deterioration [6] and evaluation of soil corrosivity [7].
The fuzzy model proposed correlates data from combined CIPS+DCVG surveys to the soil resistivity. Inputs are: DCVG defect severity (IR%), CIPS pipe-to-soil “OFF” values, potential dips with respect to the profile and local soil electrical resistivity. The output is a real number, named “Priority Index”, defined in the interval [0, 3], which quantifies numerically the degree of risk of corrosion activity at a given defect location. The presented method may provide a complementary indicator to assist pipeline operators with assessing the condition of their pipeline and prioritizing corrective actions.
