Background: For pipelines that are transporting oil and gas, data collection is conducted from time-to-time to quantify the damage incurred by the pipeline over the years. Most of the time, an in-line-inspection (ILI) tool is used to identify the corrosion areas (Level 0, Level 1 analysis) and then pit-gauge measurements are conducted after excavating the pipeline section (Level 2, Level 3 analysis) to quantify the deepest points of the damages and calculate the burst pressure on the pipe section. These calculations are vital in making integrity decisions about the remaining life or fitness-for-service of the pipeline. In this blog, we’ll briefly discuss the pressure calculation methods and introduce you to the new PSQR pressure calculations which are more efficient and also save time and resources.
Figure 1. Typical corrosion defect on a pipeline [2]
Pressure calculations: The industry standard methods for calculating the burst pressure calculations for in-service pipelines are given below:
ASME B31G (1991)
Modified ASME B31G (2009)
Effective area method (RSTRENG)
The major difference between these three methods is in the shape approximation of the corrosion anomaly (see Figure 1) and flow stress formula used. It is independently proven by various researchers [1] that the above-mentioned methods lead to undue conservatism in calculations when it comes to making fitness-for-service decisions which the PSQR method aims to alleviate.
Figure 2. River bottom profile (left) vs. plausible profiles (right) [3]
PSQR (plausible profiles) method: This method was developed by researchers at TC Energy [3] to calculate the burst pressure more accurately & precisely and to safely reduce the undue conservatism in integrity decisions. The overview of the model is explained below:
PSQR model is built upon the fundamentals of the effective area method
Defect profile: (0.85 x d x L) for depth ≤ 0.8(wt)
Flow stress: [Specified minimum yield strength (SMYS) + 10,000] psi
Instead of using a single profile to calculate the burst pressure for a corrosion anomaly, PSQR method calculates multiple corrosion paths (known as plausible profiles). The purpose of this is to consider the circumferential separation between the pits to determine the plausible interaction between them and to eliminate any underprediction of the failure pressure (See Figure 2).
The effective area pressures originating from multiple ‘plausible profiles’ are plotted in a statistical distribution and the lower 5th percentile value of the distribution is selected as the failure pressure of the anomaly.
The method will prove to be useful in reducing the number of excavations while still ensuring safety of operation. The TC Energy team has experimentally validated the method on 30 corroded pipe samples and the method is also peer-reviewed and validated by industry experts and deemed as a novel method which avoids over conservatism without compromising safety [4]. Now let's look at how we can do these calculations without picking up a calculator.
Figure 3. VXintegrity modules [6]
PSQR pressure in Creaform’s VXintegrity software: Since the ILI tool data is validated and complimented by using highly accurate and precise 3D laser scan data, it only makes sense to calculate the PSQR pressure using the 3D laser scan data of the corroded pipe. It is common knowledge in the NDT industry that the 3D laser scan data can be easily converted to fully analyzed reports containing deepest point grids and burst pressure calculations by the click of a button using Creaform’s Pipecheck software [5]. Now, some of the you are aware that Pipecheck was replaced with VXintegrity in 2021 to become the umbrella software for integrity analysis with different modules for specific applications (see Figure 3). The NDT industry heavily relies on the Pipeline module to do all that they used to do in Pipecheck and more, like PSQR calculations which was introduced to VXintegrity in November 2023.
Figure 4. Deepest point grid and burst pressure calculations in VXintegrity’s Pipeline module
A typical deepest point grid and burst pressure analysis results in VXintegrity’s Pipeline module are shown in Figure 4. The two new results highlighted in red pertain to the PSQR calculations, so let's try to understand what these are.
Axial alignment:
NO: When the river bottom path is circumferentially displaced, this will lead to the PSQR pressure value to be HIGHER than the effective area pressure value.
YES: When the river bottom path is NOT circumferentially displaced, this will result in the EQUAL values of the effective area pressure and PSQR pressure
So these two indicators will automatically give an added layer of information about the circumferential alignment and the pressure calculations of that particular corrosion anomaly, which can help in increasing the accuracy and efficiency of the integrity decisions for that pipe section.
If you think 3D laser scanner based corrosion analysis for NDT applications is something you would like to explore further, don’t hesitate to get in touch with us. We exist to solve complex 3D measurement challenges, empowering confident decision making.
Read more about the 3D scanning based solutions for NDT pipeline inspection for oil and gas industry here.
References:
[1] Zhu XK. A comparative study of burst failure models for assessing remaining strength of corroded pipelines. Journal of Pipeline Science and Engineering. 2021 Mar 1;1(1):36-50.
[2] Al-Amin M, Zhou W. Evaluating the system reliability of corroding pipelines based on inspection data. Structure and Infrastructure Engineering. 2014 Sep 2;10(9):1161-75.
[3] Zhang S, Yan J, Kariyawasam S, Huang T, Al-Amin M. A more accurate and precise method for large metal loss corrosion assessment. InInternational Pipeline Conference 2018 Sep 24 (Vol. 51869, p. V001T03A059). American Society of Mechanical Engineers.
[4] Kiefner J. Peer Review of the Plausible Profiles Corrosion Assessment Model. InInternational Pipeline Conference 2020 Sep 28 (Vol. 84447, p. V001T03A002). American Society of Mechanical Engineers.