The integrity management of challenging pipelines is a problem for all operators. Even with the extensive technology toolbox now available, there may not be an economic internal inspection solution for some pipelines or segments. For these cases, the industry generally applies a process known as Direct Assessment (DA). To support operators, whatever their need, the ROSEN Group provides DA services that are unique because of the combinations of technology we deploy, and the fundamental understanding we have of threats and degradation rates learned from dealing with internal inspection of many thousands of pipelines around the world.
For pipelines where internal inspection is considered beyond challenging, or where the costs to implement inspection are very high, methods are still needed to evaluate condition and ensure safe operation. In addition, where there are numerous challenging-to-inspect segments, there can be uncertainty regarding which segments should be prioritized. Operators worldwide face this problem. For example, Gazprom states that there is no economic internal inspection solution for 47% of its 159,500 kilometers of pipelines. At Enbridge, while 97% of the length of its pipeline system is piggable, today some 40% of its segments cannot be internally inspected economically (Mirzoev, 2015).
These segments are primarily dead legs, branch pipelines, in-field lines and parts of transmission pipelines commissioned in times when in-line inspection was not a commonly known practice.
Figure 1: Total length of piggable and non-piggable pipelines at Gazprom (Aksyutin, Vazyagin)
Aboveground survey techniques such as Direct Current Voltage Gradient (DCVG) and Close Interval Potential Survey (CIPS) provide some information on the condition of the coating and the cathodic protection system. However, they do not provide direct evidence of the condition of the steel pipe and, as with all techniques, they have limitations. For example, they are not intrinsically sensitive to coating disbondment, which is a significant threat to an asset’s integrity.
An additional aboveground survey method is the Remote Magnetometric Method (MTM). MTM defines stress characteristics of pipe sections by registering changes in the magnetic field of the pipeline, based on the converse magnetostrictive effect. MTM is an indicative (screening) survey technique and, like any other technique, MTM has a number of limitations.
To improve the integrity assessment of challenging pipelines where ILI is not an option, ROSEN has introduced an inspection approach based on a combined DCVG, CIPS and МТМ survey. This gives an evaluation of coating and pipe integrity in one pass, and their combination goes some way to compensate their respective limitations.
WHAT ARE DCVG, CIPS AND MTM?
DCVG is based on the principle of introducing a direct current between the pipe and the earth, and then detecting a potential drop in the neighborhood of a coating fault (holiday). Once a coating fault is located, its severity or size is estimated depending on the potential difference and general conditions.
The following classification of coating defects is commonly adopted:
- Category 1 – likely to be a large coating defect that should be repaired;
- Category 2 – coating defects possibly deserving repair;
- Category 3 –small coating defects that do not need to be repaired.
To add value to the data collected during a DCVG survey, a combination of coating-defect location with pipe-to-soil potential measurements is made using a DCVG/CIPS hybrid technique. Data from the CIPS is used to support the DCVG-based coating condition assessments. In addition, hybrid DCVG/CIPS surveys can determine whether the exposed pipeline wall (where there is a coating fault) is effectively protected from external corrosion by the cathodic protection (CP) system – or not.
Remote Magnetometric Survey (MTM) is based on the change of the magnetic susceptibility of a material when subjected to mechanical stress. The method uses the “natural” magnetization of steel pipes by the magnetic field of the earth. MTM plots the attributes and characteristics of pipe sections by monitoring and analyzing changes in the magnetic field of the pipeline. These changes are related to stress, which, in turn, is related to the loading on the pipe (pressure, temperature, external loads) and defects in the metal. Magnetic measurement data is collected from the ground surface, and anomalies detected are a function of stress, mechanical loading and structural changes in the metal. In addition to providing an indication of the stress caused by defects, MTM identifies their character, location and orientation.
The assessment of magnetic anomalies identified using the MTM method is performed based on an integrity index F, which reflects the exceedance of registered values over a baseline, the density of peak values and their distribution pattern. Anomalies are typically classified into three ranks as shown in Figure 2.
Figure 2: Magnetic anomalies classification according to index F (Guidance for pipeline inspection using remote magnetometric method, 2002)
The significant advantage of MTM is that it does not require any preparation of the pipeline for inspection, such as cleaning, opening the pipe or interrupting pipeline operation. Magnetic field measurements are performed with the pipeline operating as usual.
However, MTM has the following limitations:
- Errors when the measuring equipment is too far from pipeline (> 10-15 pipe diameters [D]);
- Errors associated with residual over-magnetization of pipeline due to production effects or ILI;
- Errors generated by magnetic masses (metal objects) located close to pipeline (< 2D);
- Calibration required (1-2 excavations required per line);
- Low accuracy for detecting pipe features with stress level less than 5% of the SMYS (e.g. pitting corrosion);
- Screening technique, no feature sizing.
COMBINED SURVEY SOLUTION
To realize the full value of the combination of DCVG, CIPS and MTM surveys, we have incorporated them into a Direct Assessment (DA) methodology based on NACE guidelines. This provides a rigorous, structured and industry-recognized approach to evaluate the effect of external corrosion on the integrity of a pipeline system. DA is a holistic approach to corrosion integrity management of unpiggable pipelines that covers both internal and external corrosion (Figure 3). In this article, we focus on external corrosion, NACE SP 0502.
Figure 3: DA Methodology suite; Direct Assessment based on NACE guidelines allows for a holistic corrosion management approach
This process has been used by ROSEN on multiple occasions with a high success rate. It has also been used for pipelines where only an incomplete ILI data set was available. The data available from these incomplete ILI data sets were then used to benchmark DA models and provide confidence in the results before extrapolating the DA model to the areas of limited or no data. This is an area of ongoing research for us, where we are benchmarking and modifying DA models using ILI results for multiple examples of similar pipelines to improve estimates of condition and degradation rate.
The DA process, following NACE guidance and incorporating the combined DCVG, CIPS and MTM surveys described above, is summarized as:
STAGE 1 – Pre-Assessment (desk study)
- Data collection and review
- DA feasibility assessment
STAGE 2 – Indirect Examination (DCVG-CIPS-MTM surveys, in this case)
- Performing DCVG, CIPS and MTM surveys
- Alignment and comparison of the DCVG, CIPS and MTM data
- Evaluate the Indirect Inspection and Pre-Assessment data in order to identify and prioritize locations for Direct Examination (STAGE 3)
STAGE 3 – Direct Examination
- Excavations and data collection at areas where corrosion activity is most likely
- Direct pipeline inspection and measurements of coating damage and corrosion defects
- Evaluations of remaining strength (severity) and remaining life calculations
- Analysis and evaluation of discovered condition(s)
- Process evaluation
STAGE 4 – Post-Assessment
- Provide an estimate of the remaining life of each section of the pipeline
- Identify appropriate reinspection methodologies and set a suitable reinspection/reassessment interval
- Critically review whether the Indirect Inspection methodology has produced valid results in terms of successfully identifying and prioritizing defect locations. Determine whether reclassification and/or reprioritization of indications is necessary based on all direct and indirect inspection data. Provide a statement on the overall effectiveness of the DA process, as determined by the correlation between detected corrosion and the DA predicted locations.
- Analyze the results from all previous stages to identify any trends or patterns which would assist the targeting of integrity management resources.
- Develop detailed recommendations for the integrity management of the pipeline in order to achieve the required future operational life, based on all direct and indirect inspection data collected as part of the DA process.
Figure 4 illustrates a standard DCVG, CIPS and MTM survey process, where five surveyors are involved.
The results of DCVG, CIPS and MTM data alignment are presented in Figure 5.
Direct examination (excavation) sites are selected based on the correlations of interacting defects to give a relative pipeline “severity rating” (Figure 6). A baseline severity rating is used to filter low-level indications; locations are ranked for excavations that are above this baseline.
Detailed consideration of DCVG, CIPS and MTM survey data allows us to identify “hotspots,” where appropriate preventative maintenance and direct inspection activities are recommended (as shown in Figure 7).
Having performed combined DCVG, CIPS and MTM surveys as a part of Direct Assessment studies for a number of operators worldwide, we are now confident that the sum is greater than the parts. The approach has proven its efficiency as a comprehensive integrity management instrument for non-piggable pipelines. The benefits that can be achieved are:
- The limitations of each method are compensated;
- A qualified statement on the current pipeline condition and integrity can be delivered;
- Active degradation mechanisms and probable causes of corrosion are identified;
- Appropriate corrosion mitigation and control strategies are recommended;
- Remaining safe working life is calculated;
- Locations where excavations will provide the maximum value are selected;
- Segments can be prioritized for modification and internal inspection;
- Effective integrity management plans are defined.
Aksyutin, O.E. Increasing the operational reliability of Gazprom gas transmission system // Gazovaya Promyshlennost’.- 2010.- 03/643/2010.- p. 23-25.;Guidance for pipeline inspection using remote magnetometric method. (2002). p. 26, Table 2 RD 102 - 008 - 2002.;Mirzoev, J. (2015). A Comprehensive Approach to Integrity on Non-Piggable Pipelines Based On Combined DCVG/CIPS/MTM Survey. In: Pipeline Technology Conference 2015. [online] EITEP Institute. Available at: https://www.pipeline-conference.com/abstracts/comprehensive-approach-integrity-non-piggable-pipelines-based-combined-dcvgcipsmtm-survey [Accessed 21 Oct. 2019]. NACE SP 0502.;Vazyagin S.V., Zakharov A.V., Popov A.N. Engineering activities for inspection and integrity assessment of linear part of transmission lines // Gazovaya Promyshlennost'.- 2011.- 09/664/2011.- p. 69-72.