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Applying Finite Element Analysis the Right Way
Author: Diego Luna, Integrity Engineer, Integrity Services
Traditional inline inspection assessments are widely used across the industry, and although these assessments have been refined over the years, they continue to yield conservative predictions that produce less than optimal results. One of the assessments that is often applied to inline inspections is Finite Element Analysis (FEA), which is optimized to provide operators with an accurate estimation of pressure behaviors.
Unfortunately, the assessment is not always accurate. FEA is considered one of the most complex assessment methodologies available and is included as "Level 3" in standards such as API 579-1.
So, why are these assessments not as accurate as they should be? It's quite simple. Several engineers interpret the data based on assumptions. Naturally, this causes skepticism among the operators who are investing in this service and end up receiving results that could be open to interpretation and inaccuracy.
When an inline inspection (ILI) device or tool is the source of this input, there are multiple variables to consider: sensor types, mechanical configuration, resolution (axial and circumferential sampling), temperature, pressure, speed, rotation, and several more noise inducers. These complex components require a detailed application process that traditional inspections may not incorporate into their analysis processes.
Moreover, what about multiple inspections? The more combination inspections we perform, the more complex the data we acquire, making it vital for us to be able to accurately analyze data from multiple sources. Given this, there are several challenges that must be overcome to provide accurate results, i.e. sensor deflection, sagging, magnetic flux disturbances, or single inspections. Ignoring these challenges can cause assumptions such as oversimplification of the pipe geometry simply because pipelines are manufactured differently. For example, not all pipe joints are straight, have perfectly rounded surfaces or contain consistent wall thicknesses. ,
Fortunately, NDT Global has investigated the challenges that traditional FEA assessments do not consider along with the multiple variables that are often ignored during combined inspections.
For example, the data acquired from our robots is handled by the same pipeline integrity engineer from beginning to end. Our integrity engineers are trained in the analyses process and understand the most important aspects of the technology. They also work alongside our analysis team, who have the best understanding of our robot tolerances.
Below are a few inline inspections that can be combined into a single run:
•Ultrasonic Geometry (UG) – detection of deformations and geometric anomalies
•Ultrasonic Wall Measurement (UMp) – detection of wall thickness disturbances (Ex. Metal loss)
•Ultrasonic Crack (UC) – detection of crack and crack-like defects
With combination inspection runs, it only makes sense to analyze all the data together to differentiate the individual strengths and weaknesses and deliver a combined model for optimal value.
Here is an overview of the purpose that FEA serves for each inspection solution
Ultrasonic Geometry Detection (UG): to create a complete 3D model of the pipe joint.
Ultrasonic Metal Loss Detection (UMp): to precisely measure wall thickness across the pipe joint.
Ultrasonic Crack Detection (UC): to detect crack defects at the long seam or in the pipe body across the pipe joint.
If an operator invests in an inspection with two, or even three of all of these solutions, a complete and accurate model is born. This provides insights based on facts, ridding interpretations and oversimplifications that the common approach ignores.
Figure No. 1 above is an FEA visualization: a model of a pipe joint that incorporates wall thickness readings, real pipeline geometry and crack-like features. The image visualizes the results of a combined technology inline inspection: a complete joint (spool) incorporating the most realistic pipe geometry, accurate wall thickness measurements and crack defects that include depth profiling. The results can then be understood in terms of failure pressures, combined stresses, remaining life, and future and fatigue life.
Included in this visualization is a model predicting pressure scenarios and includes inputs such as cycle changes and hydrotest pressures. In conclusion, to leverage an FEA in the most accurate way possible, assessments should be conducted be highly skilled engineers who can both understand the data and can properly handle the data from beginning to end.
For best results, Finite Element Analysis should incorporate a combination of datasets from multiple runs and provide insights from complex data so operators can receive results that are precise and truly applicable to their integrity management program.