Application of a Probabilistic Model when Performing SCCDA

The Mears SCC Susceptibility Model consists of a series of algorithms that are used to assess the likelihood of stress-corrosion cracking. The model has been mainly used when performing SCCDA on pipelines. The results of the algorithms are used to prioritize segments of pipeline for excavation assessment and engineering assessment. The Model is designed to be applied to both forms of external SCC (near-neutral-pH SCC and high-pH SCC), and it can be adapted to identify mixed modes of SCC which has been lately found on some pipelines systems.

APPROACH

The approach followed during the implementation of this model is based on the guidance for SCC Threat Assessment provided in ASME B31.8S and NACE Standard RP0204 Stress Corrosion Cracking (SCC) Direct Assessment Methodology and it considers the following steps:

  1. Determination of the existence of an SCC threat.
  2. Review of Background Data.
  3. Implementation of the Probability of SCC Susceptibility Model

REVIEW OF BACKGROUND DATA

The process starts with the integration of the SCCDA pre-assessment data and the alignment of the results of the Indirect Inspection surveys and/or inline inspection data. The final results are used to evaluate the performance of the cathodic protection system, determine the possible severity of coating damage, and identify areas where corrosion or SCC activity may have occurred or may be occurring.

In order to provide details for the classification matrix and prioritized dig selection, the following documents must be reviewed:

  1. Alignment drawings, as-built records and GIS information.
  2. Manufacturer and manufacturing method.
  3. Annual Cathodic Protection Survey Data and/or recent Coating Fault Survey data.
  4. Pipeline information such as grade, manufacturer, operating parameters, coating type, etc.,
  5. Data related to topography, drainage and depth of cover,
  6. ILI data, and
  7. Dig reports from previous SCC findings.

The data collected is recorded in the Mears’s or client’s Data Elements Checklist Form for SCCDA.  Once all the data is compiled, it is then populated into the Stress Corrosion Cracking Probability of Failure Algorithm Model.

The model calculates a Probability of Stress Corrosion Cracking Susceptibility Value (PSCC), which is used to identify areas where SCC is likely to occur. Therefore, a high PSCC value corresponds to high probability of SCC susceptibility.

Some of the factors that the model uses to determine the highest priority areas for excavation are shown as follows:

  • Age of the pipeline
  • Operating Stress Level
  • Operating Temperature  
  • Distance from Compressor Station
  • Coating Type
  • Historical number of Leaks or ruptures
  • Existing or recently collected CIS, DCVG and soil resistivity data
  • Soil Type
  • Drainage
  • Existence of strain factors like dents, bends and/or hard spots.

INTEGRATION OF DATA AND ANALYSIS

Once all the factors are considered and the model is executed, the data and the model output are integrated and graphically aligned for ease of interpretation and analysis.  Integration and graphical representation of the data in RAP sheets is shown below.

The direct examinations conducted as part of an SCCDA project are intended to be completed in sections of the pipe most susceptible to SCC, to determine if SCC is present and, if so, classify the type and severity of the SCC.  The correlation of field conditions to the existence and severity of SCC is then used as feedback to the dig site selection process to refine the predictive capability of the model and project those results over the balance of the pipeline.  

It is important to note that, the model results are used as an indication of potential susceptibility to SCC, rather than an indication of SCC.  After the direct examinations are completed and sufficient data is collected from the dig sites, statistical correlation of the dig results to the existence and severity of SCC, if any, can be made and applied to the balance of the pipeline.  After completion of the initial excavations, the overall predictive capability of the model will be evaluated based upon the frequency and severity of any SCC findings. The initial direct examination list may then be expanded based upon the results from the initial excavations

The SCC Susceptibility Model has proven to be an excellent tool when used in conjunction during the implementation of the SCCDA Methodology. The Model has demonstrated to provide an excellent combination of accuracy, reliability and discrimination which contributes to the overall integrity management process.

For more information please contact Aida Garrity at aida.garrity@mears.net

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