By: Devon Brendecke, P.E., Consulting Engineer, Quest Integrity Group
As seen in the July 2013 issue of World Pipelines Magazine.
Damage mechanisms such as internal and external corrosion, dents and cracks can now be accurately quantified due to more advanced in-line inspection (ILI) tools which yield better data on pipeline condition. The drastic improvement in data quality and accompanying quantity drives the need for advanced assessment capabilities to leverage the improved data quality and accuracy. The combination of better inspection data and superior assessment procedures is rapidly demonstrating the weaknesses in older procedures. Using these more advanced assessment technologies, which are validated and supported through extensive field research, operators can now assess defects in pipelines and determine fitness-for-service (FFS) quickly and confidently.
Fitness-for-service assessments have become increasingly accepted across the pipeline industry over the past few years. FFS standard API 579/ASME FFS-1 (API 579-2007) provides guidelines for assessing types of damage affecting pipelines across all industries. Two common damage mechanisms are internal and external corrosion and cracks forming near welds, where residual stresses from the welding process have the potential to rapidly progress crack growth. Fitness-for-service assessments are an alternative to costly repair or replacement.
ILI data, particularly ultrasonics (UT), radius and thickness readings, conveys a lot about the condition of a pipeline. Both wall loss due to corrosion and out of roundness due to external damage pose a risk to the integrity of a pipeline and must be assessed or repaired. Knowing the damage is present is the first step in remediation. Applying FFS rules to assess the damage as a second step can save enormous amounts of time and money in prioritizing critical regions for digs and repairs.
UT data can quickly overwhelm an integrity engineer carrying out an assessment with a spreadsheet. There simply isn’t a practical way to leverage the full value of the ILI data in an assessment computed by hand or even within a spreadsheet. However, commercially available software programs provide the best option for maximizing the results of the inspection.
In the example below, corrosion metal loss was discovered in a 12-inch liquid transmission pipeline. The ILI data recorded an inside radius and wall thickness measurement every 0.05 inch encompassing the region of metal loss. The corrosion extended about 10 inches along the pipe.
An API 579 Part 5 Local Metal Loss assessment was completed using fitness-for-service software to determine if the section of pipeline required repair. The assessment involved the calculation of a remaining strength factor, which is a function of the measured corrosion, material tensile properties and operating pressure.
The corrosion passed the API 579 Level 2 assessment and no repair was required. Despite the amount of data, the assessment was completed in a matter of minutes with the help of the software. Figure 1 shows a 3D contour plot of the thickness readings used in the assessment.
Figure 1. Thickness readings for local metal loss assessment.
Crack-like flaws may develop in pipelines for a variety of reasons, in a variety of locations. For example, in 1950s vintage steel plates connected by low-frequency electric resistance welds, hook cracks are common in weld metal and the heat affected zone. Other examples of crack formation and propagation are:
The high number of variables makes assessing crack-like flaws complex and highlights the need for more comprehensive fracture mechanics assessment methods rather than a one-size-fits-all approach. Figure 2 illustrates several possible crack locations and orientations.
Figure 2. Possible crack locations and orientations.
Assessing crack-like flaws with hydrostatic testing has been a common practice in the pipeline industry for decades. Traditional models for crack assessment are considered conservative because they tend to predict a smaller than actual critical crack size. However, underestimating the maximum flaw size that will survive a hydrostatic test means that larger than expected flaws can remain in the pipe. This scenario is particularly hazardous because large cracks grow more rapidly than smaller cracks. Those larger than expected cracks remaining in the pipeline can subsequently grow to a critical size under normal operating conditions, resulting in a failure during service rather than during the less risky hydro test. This model is described in more detail and illustrated later.
Crack-like flaws are described by length, typically surface breaking, and a depth in the through-wall direction. While the sensitivity of modern ultrasonic inspection tools is good with a probability of detection (POD) of about 90%, there remains some uncertainty in the measurement of the crack depth. There are companies with commercially available ILI tools which utilize shear wave ultrasonics and detect cracks less than 40 mils in depth.
Once flaws have been identified and sized, an assessment calculation is required to evaluate the stability of the crack or determine a critical size. Modern fracture mechanics use the Failure Assessment Diagram (FAD) described in API 579-2007. The FAD enhances linear elastic fracture mechanics (LEFM) assessments by incorporating ductility.
The FAD extends the crack stability assessment to structures experiencing both brittle and ductile fracture. The FAD is a plot with a limiting curve and points representing the structure of interest; an example FAD is shown in Figure 3. The x-axis of the plot is the load ratio (Lr) which is the ratio between the reference stress and the material yield strength. The reference stress is proportional to the far-field stress and is computed based on the loading condition, the component geometry and the crack configuration. The y-axis of the plot is the toughness ratio (Kr) which is the ratio of the stress intensity factor (KI) computed for the primary and secondary loads and the fracture toughness of the material (KIC). The through-thickness stress profiles from the FEA model are incorporated in the computation of Lr - Kr.
For a particular crack size of length 2c and depth a, an Lr - Kr point is computed and plotted on the FAD.
· A point falling under the limiting curve is considered acceptable or safe.
· A point falling on the curve is considered critical.
· A point falling outside the curve is considered unacceptable or unsafe.
· A point lying towards the right end of the diagram fails due to plastic collapse.
A point lying towards the upper left corner of
the diagram fails due to brittle fracture.
Figure 3. Example of the FAD method of assessing the stability of crack-like flaws.
Assessing one or two flaws is reasonable when done within a spreadsheet. The calculations are not overly complicated and many of the critical parameters are publicly available. However, assessing hundreds of flaws along miles of pipelines is a different story. It can take weeks of an engineer’s time to create a spreadsheet tool that would be mildly reusable and it becomes obvious very quickly that the amount of data collected with an ILI tool requires automated assessment.
Commercially available software programs are capable of assessing hundreds of flaws at once as well as computing a range of critical flaw sizes. An automation tool like this allows the computation of the "what if” conditions in a matter of minutes. A user can not only assess hundreds of flaws along an entire pipeline at once, but the operating parameters, pipe sizes and material properties may all be varied for a probabilistic approach. A critical flaw size curve, describing the entire range of critical crack sizes for a particular set of operating conditions, can be completed as quickly as a spreadsheet or more manual-based assessment of a single flaw.
Automated flaw assessments can also be helpful in predicting the remaining life of a crack or determining the initial size of an existing flaw. Pipelines are typically subject to pressure cycles during normal operation and fatigue crack growth occurs during these cycles. Therefore, this type of calculation can prove to be much more cost effective and accurate than periodic hydrostatic testing in predicting the remaining life of a pipeline.
The partnering of advanced inspection technologies with innovative assessment methods makes solutions to complex problems substantially more accessible. Costly repairs to pipelines may be averted or delayed immediately following in-line inspection and rapid completion of a fitness-for-service assessment. Internal or external corrosion can be promptly assessed for reliability and automated crack assessment tools provide a probabilistic type solution to all potential cracks along an entire pipeline.