Unseen and generally unknown by the public, underground and submerged pipelines play an important role carrying fresh water, natural gas, oil and other fluids long distances as part of the infrastructure that keeps societies and economies productive. Unseen that is, until a corroded pipeline ruptures and releases hydrocarbons or toxic chemicals into the environment or results in a fireball that makes the evening news. Fortunately, methods have been developed that, along with appropriate and timely inspections, keep corrosion at bay.
The Need for Cathodic Protection of Pipelines
Corrosion protection for buried pipelines is typically achieved by the application of an external protective coating system in conjunction with cathodic protection (CP). Coatings are the first line of defense in many corrosion control programs. Unfortunately, coatings are not perfect, and accelerated corrosion can occur at breaks (holidays) in the coating. Cathodic protection is used to prevent corrosion of the steel substrate where holidays form in the protective coating. (For an introduction to CP, see the article The Basics of Cathodic Protection.)
Cathodic protection is accomplished by causing DC current to flow from an external anode, through the electrolyte (soil or water) and onto a protected structure (pipeline). The DC current flow onto the pipeline polarizes the structure’s potential (voltage) in an electronegative direction, thereby protecting the structure against corrosion. Criteria for effective cathodic protection are defined in NACE International Standard SP0169 – Control of External Corrosion on Underground or Submerged Metallic Piping Systems, Section 6: Criteria and Other Considerations for Cathodic Protection. The protection criteria most often applied when conducting an interrupted close interval potential survey (CIPS) is as follows:
- A polarized pipe-to-soil potential of -850 mV, or more negative, relative to a saturated copper / copper sulfate reference electrode
In other words, the absence of specific data that demonstrate that corrosion of a structure is occurring, polarized pipe-to-soil potential measurements more negative than -850 mV with respect to a saturated copper/copper sulfate reference electrode indicate that corrosion protection has been successfully achieved.
Polarized pipe-to-soil potential measurements of -1200 mV, or more negative, relative to a saturated copper/copper sulfate reference electrode indicate the possible potential for pipeline coating damage, e.g., cathodic disbondment. Per NACE SP0169, the presence of excessive polarized potentials on externally coated pipelines should be avoided. To protect the coating from disbondment, polarized pipe-to-soil potential measurements should not be more negative than -1200 mV.
The Importance of Close Interval Surveys (CIS)
External corrosion direct assessment (ECDA) is a process that assesses and reduces the impact of external corrosion on pipelines in order to improve safety and ensure pipeline integrity. (Learn how to conduct an ECDA in Tips for Success in Conducting External Corrosion Direct Assessments.) A vital inspection tool in the ECDA process is the close interval survey (CIS), also known as a close interval potential survey (CIPS) or potential gradient survey, which is an inspection tool primarily used to assess the effectiveness of installed cathodic protection systems on buried and submerged pipelines and ensure the CP systems are operating per NACE SP0169 specified standards.
A close interval survey can help detect some coating defects or holidays, although it cannot detect very small holidays. Corrosion activity, such as external stress corrosion cracking and microbiologically influenced corrosion (MIC), can also be inferred from a CIS.
Pipeline cathodic protection system inspections are mandated by many governments and regulatory agencies, so close interval surveys are a tool used to verify compliance with inspection codes and standards.
In practice, a close interval survey acquires voltage measurements at specific time intervals (hundreds of milliseconds to several seconds in duration) at specific spacing intervals along the pipeline in reference to a grounded standard reference electrode (half cell). The measured voltage is the potential difference between a pipeline and the soil surrounding the pipeline.
The spacing interval is usually less than the depth of the pipeline, typically one to three meters or less, and is dependent upon field conditions, the pipeline’s physical properties and owner/operator requirements.
Types of Close Interval Surveys
There are several types of closed interval surveys. The type chosen depends on various factors, and more than one survey type is often conducted for a given pipeline.
A native potential survey is used to establish a baseline for future surveys if the 100 mV criteria is used to determine if cathodic protection has been achieved. A native potential survey measures the electrical potential of the structure before cathodic protection is applied. Once a current has been applied to a pipeline it is never again native.
A depolarized survey is used when a cathodic protection system is already installed and operational. First, the CP system is switched off for a sufficient time for the pipe-to-soil potential to stabilize to a (near) native level. Then the potential difference is measured between the ground and the pipeline.
An on/off survey measures the potential difference between the ground and the pipeline while an installed CP system is repeatedly switched on and off. Two measurements are taken – one when the CP is operating, and another when it is off. The cycle setup (how often the CP system is switched on and off) is normally based on client requirements and depends on the unique characteristics of each pipeline and what the motivation for the survey is. Cycles of 8:2, 4:1, 3:1 and 1.5:0.5 are all very common.
An on potential survey is used when it is impractical to turn off the CP system for a depolarized survey. The potential difference is measured at regularly spaced intervals while the CP continues to operate. This survey can determine if corrosion is occurring and the extent of the corrosion.
What Data is Gathered during a Close Interval Survey?
The survey itself generates large volumes of collected data, typically around 2 MB per mile (1.6 km) on a daily basis. Each data point is paired with the exact GPS location where the voltage measurement was taken (accurate to within sub-meter GPS accuracy). It is important that the data be accurate, correlated with GPS position information and that the management and reporting be efficient and timely.
Facilitating this process are data loggers, which are computerized voltmeters that record the pipe-to-soil potentials (voltages) during a close interval survey. In addition, field crews can enter special comments about various items of interest as they progress through the survey, such as terrain or pipeline features, right-of-way conditions, obstacles encountered or locations that were skipped and must be surveyed at a later time.
Measurements that are typically collected include:
- Weather conditions
- Pipe diameter
- Crew leader information
- Interruption cycle and delay settings
- Date and time stamps of the readings
- Test station readings
- Pipeline stationing
- On and off readings
- Sub-meter GPS latitude and longitude
- Position dilution of precision (PDOP) and GPS accuracy data
- Remarks for additional readings and above ground appurtenances
- AC voltage at probable sites for AC interference
- Near ground, far ground, and MIR readings at test stations
- Readings at casings
- Depth of cover readings
As an example of the comprehensiveness of the collected data, consider the MIR (metallic IR or measured IR), which measures the pipe IR drop. When the survey wire is still connected at the previous test station, the “on” and “off” voltage difference and polarity is measured between the far test station and the near test station. This value should be zero or near zero to signify that all current has been interrupted. MIR tables can provide information to determine:
- The magnitude and direction of the current flow in the pipeline
- Information on which rectifiers predominate in any given pipeline section
- Information on whether more rectifiers need to be interrupted
- How “clean” the CIS is and hence the validity of the survey data
- Assistance in the identification of test station leads
- Identification of test station leads that are not in low resistance with the pipeline
- Foreign current flow using the pipeline as a conductor
After the data is gathered, computerized reports and graphs can be generated and analyzed by technicians, corrosion experts and the pipeline owner/operator. These final reports (Figure 1) contain information such as:
- An engineering review and recommendation report
- Data alignment sheets with graphs and aerial imagery
- Exception reports of pipeline segments not meeting criterion
- MIR tables showing indications foreign influence
- Depth of coverage (DOC) tables with exceptions of pipeline segments not meeting minimum depth criterion, including pipeline exposures
- Operator qualification tables with crew information
- Equipment calibration records for equipment used on site
- Flash drives containing all of the raw data, shape files, alignment files, KML/KMZs, spreadsheets and the written reporting and data alignment sheets
Figure 1. Report from a close interval survey.
Raw data for depth of cover (DOC) tables are gathered electronically with the locator and are included in the report. They are significant because they represent the distance between the surface and the center of the electromagnetic field (EMF) on the pipe, which usually correlates to the center of pipe. The report subtracts the pipe’s radius to provide the distance from the surface to the top of the pipe.
Advantages of Timely and Effective CIS Reporting
The ability to immediately produce reports and graphs in the field or shortly after the survey’s completion contributes to pipeline integrity and lower maintenance costs in several ways.
The recently collected data can be immediately compared with historical data to identify trends and corrosion threats, or to spot anomalies that can be followed up by the current field crew, thus avoiding the expense of scheduling and outfitting a crew on a later date. The data can also be immediately compared to and correlated with data from direct examinations (e.g., excavations) that are occurring simultaneously.
Some data points that appear to indicate a coating defect can actually be caused by other factors, such as low resistivity soils, stray currents or interference from foreign electrical systems (e.g., overhead wires, CP systems from nearby pipelines, urban infrastructure and transportation systems). (Discover the dangers of stray currents in Stray Current Corrosion and Preventive Measures.) If needed, the site can be expeditiously resurveyed by the field crews that are already on site. Additionally, crews can perform remedial work, emergency field repairs or retake observations if questionable data is encountered.
Another advantage of immediate feedback is that alternative ECDA tools (e.g., voltage gradient, Pearson, and current attenuation surveys) can be performed at essentially the same time. When multiple tools are indicated then it is important that inspections be performed as close together in time as practical to minimize variations due to seasonal changes or the installation of additional or foreign infrastructure.
Managing the data’s integrity is important because it will be correlated with precise GPS location data so that historical inspection records can be compared and used to pinpoint excavation locations requiring a direct examination.
Finally, accurate records must be kept for legal and compliance purposes; these requirements are facilitated by a well-designed close interval survey reporting application.
Beyond the production of the initial reports and graphs, the collected CIS data is used for follow up purposes. ANSI/NACE SP0502-2010 recommends the following post-assessment steps after the completion of a close interval survey:
- Root cause analysis
- Prioritization of areas requiring remediation
- Determining mitigation strategies to prevent future external corrosion
- Remaining life calculations for the assets
- Continuous process improvement
- Assessing the effectiveness of the ECDA process
- Corrosion growth rate determination