A Strategic Approach for Completing Engineering Critical Assessments of Oil & Gas Transmission Pipelines

Regulatory Overview

In January 2012, the Pipeline Safety, Regulatory Certainty, and Job Creation Act of 2011 was signed into law directing PHMSA to take steps to further assure the safety of pipeline infrastructure. PHMSA issued the related Notice of Proposed Rulemaking (NPRM) for Safety of Gas Transmission and Gathering Pipelines on April 8, 2016.

Included in the NPRM were significant mandates regarding:

  • Verification of Pipeline Material (§192.607); and
  • Maximum Allowable Operating Pressure (MAOP) Verification or “Determination” (§192.624)

 

The NPRM proposes requirements for operators to verify the MAOP of a gas transmission pipeline when:

  • 1. The pipeline has experienced an in-service incident (as defined by §191.3) due to select causes1 in a High Consequence Area (HCA), “piggable” Moderate Consequence Area (MCA), or Class 3 or 4 location since its last successful pressure test
  • 2. The pipeline lacks Traceable, Verifiable, and Complete pressure test records for HCAs or Class 3 or 4 locations
  • 3. The pipeline MAOP was established by the grandfather clause (§192.619 (a)(3)) for HCAs, “piggable” MCAs, or Class 3 or 4 locations.

 

To verify the MAOP of a pipeline, the NPRM provides the following options:

  • Method 1: Pressure Test
  • Method 2: Pressure Reduction
  • Method 3: Engineering Critical Assessment (ECA)
  • Method 4: Pipe Replacement
  • Method 5: Pressure Reduction for segments with small potential impact radius (PIR) & diameter
  • Method 6: Use Alternative Technology

 

The ECA Approach

Per the NPRM, Method 3 (ECA) is defined as an analysis, based on fracture mechanics principles, material properties, operating history, operational environment, in-service degradation, possible failure mechanisms, initial and final defect sizes, and usage of future operating and maintenance procedures to determine maximum tolerable sizes for imperfections. Although this analysis may seem daunting, analytical tools and systematic approaches can greatly simplify and enable an efficient, robust, and defensible analysis for completing MAOP verification via the ECA process.

Fundamental Approach and Workflow

Structural Integrity has developed a simplified approach to completing ECA that consists of the following:

  • Pre-assessment: Completing a detailed review of the material properties (incorporating mill/design/construction records as well as data from a Material Verification program as expected to be required under §192.607); construction practices and operational history of the segment
  • Field Data Collection: Utilizing past pressure test, ILI, field inspection records to identify the worst case set of flaws that may currently exist in the pipeline
  • Analysis: Implementing a flaw growth model and evaluating the predicted failure pressure with safety factor over time for the specific pipeline characteristics, and estimating safe remaining life of the pipeline assets for a given MAOP.

The proposed ECA process can help improve safety margins and provide further insight into the remaining life of the pipeline by evaluating the range of flaws that may exist, developing an understanding of the key material properties, modeling the rate of degradation and estimating time to failure and safety factors as a function of time using fracture mechanics principles.

 

To illustrate this in an example, take the case where an operator is missing pressure test records and decides to conduct an ECA to verify MAOP. In this example, pre-assessment record review indicates that the line has a history of Stress Corrosion Cracking (SCC), no prior pressure test and the operator wanted to confirm a safe operating pressure following an EMAT ILI (inspection) run using the ECA process. Assume the pipeline has an outer diameter of 24”, a nominal wall thickness of 0.344” and Specified Minimum Yield Strength (SMYS) of 60,000 psi.

 

Based on selecting a 90th-percentile value from mill reports, a full size CVN value of 31 ft-lbs was used with the modified ln-secant method for evaluating predicted failure pressure. The crack-like threats to be evaluated are SCC and crack-like manufacturing defects.

The field data collection phase consists of EMAT ILI combined with a caliper tool to detect dents and possibly Magnetic Flux Leakage for metal loss characterization. Figure 1 provides a set of flaw loci illustrating the set of flaws (length vs depth) that would fail at MAOP (red curve) and at a pressure test at 1.25 x MAOP for the example pipeline. The red data points are hypothetical crack-like flaws detected by the EMAT ILI. Depending on the operator excavation criteria, some or all of these may be repaired. Defects below or to the left of the black curve (ILI Probability of Detection (POD) threshold) will not be detected reliably and must be assumed to exist for analysis purposes.

In the analysis step, growth models can then be applied to the remaining flaws depending on the degradation type. For example, for Electric Resistance Welded seam defects, a fatigue analysis based on Paris Law fatigue growth can utilize past Supervisory Control and Data Acquisition (SCADA) data to estimate crack growth as a function of time. For SCC defects, SI has developed a bathtub growth curve model based on published research (see Figure 2) that incorporates pressure cycle data in addition to Cathodic Protection levels and other environmental data.

The possibility of interacting defects must be considered as part of MAOP verification using ECA. Using the detection sensitivities of various tools employed, the “worst case” interacting defects can be evaluated to determine predicted failure pressure for different scenarios and then used to verify MAOP. Alternately, a probabilistic methodology can be employed, taking into account the frequency of occurrence for various defect types and distribution of defect parameters. The probabilistic approach tends to be advantageous due to the extremely low likelihood of the conservative ‘worst case’ threat interaction scenario (e.g. worst case dent, corrosion, seam anomaly and SCC at the same location).

Although these fracture mechanics methodologies are not commonly used in the pipeline industry, SI has been using fracture mechanics models routinely in the power generation industry for decades. These advanced methods, when properly applied, can enhance the reliability and safety of gas transmission pipeline infrastructure.

Footnote

[1] Select causes include manufacturing defects, fabrication or construction defects, or crack-like defect (such as SCC, seam defects, hard spots, or girth weld cracking)

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