Finite Element Analysis for Large-Diameter Pressure Pipelines

Finite Element Analysis (FEA) is a process that estimates under what operating pressures a distressed pipe will fail. The process allows operators to determine the risk associated with keeping a distressed pipe section in service under various conditions. It also provides the operator with crucial information about when the pipe is likely to fail under different operating conditions. FEA is a very detailed method of determining pipe condition and is best used to compliment the results from structural inspections.

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Why Use FEA?

As water and wastewater infrastructure around the world continues to age and reach the end of design life, many utilities are beginning proactive condition assessment programs to determine a baseline condition of their assets.

While condition assessment technologies can provide various levels of detail on the condition of a pipe section, FEA is used to further examine distressed pipe sections identified through inspection and should be used in conjunction with a condition assessment program.

Some pipeline rehabilitation decisions can be made using only condition assessment data. However, more challenging pipelines often warrant a more thorough assessment before a decision is made.

FEA can determine if a distressed pipe section can be safely operated and estimates under what operating conditions it will fail, providing operators with the necessary risk-based data to make a final decision. By completing FEA on specific distressed pipe sections in critical areas, operators are able to make responsible and scientifically defensible rehabilitation decisions on their most critical assets.

Below is a video showing a FEA of a large-diameter pressure pipe as it responds to changes in pressure.

This video shows a large-diameter PCCP pipe responding to increases in operating pressure; the blue color represents the pipe in good working condition. As pressure is increased incrementally, we begin to see areas of distress as the color shifts towards red and eventually white showing complete failure. The video shows that this particular pipe will eventually fail at the joint as pressure increases beyond the yield limit.

What is needed to Complete FEA?

FEA can be completed without dewatering the pipeline; in general, the following inputs should be gathered for the most accurate results:

  • Cover depth around the pipe section
  • Wire gauge and spacing (for prestressed concrete pipes)
  • Pipe wall thickness (for metallic pipes)
  • Operating pressures
  • Pipe material properties (e.g. concrete has a certain yield to be considered)

What does the operator get from FEA?

After completing FEA, the operator will know when the pipe section is likely to fail under various operating conditions. This is represented in terms of how long the pipe will last in relation to changing conditions, not time.

FEA also provides information on how much additional deterioration the pipe section can sustain under specific operating pressures, also accounting for pressure surges. This is referred to as the yield limit.

Yield limits are estimated conservatively to allow operators enough time to intervene before a pipe fails. Once a pipe reaches its yield limit, it is in a state of incipient failure. Completing FEA is essential for operators that want to know when their pipe is near failure, or past its yield limit. A conservative estimate is made to provide a buffer between when a pipe needs to be replaced and when it will fail.

Below is the performance curve for a 60-inch Lock Joint ECP pipe with 1-foot of earth cover.

Fea curve

The Limits of FEA

Like any condition assessment tool, there are limitations with FEA, which is why the process should be used in combination with structural inspection and monitoring to ensure pipelines are operated safely.

  • The rate of corrosion and wire breaks in pressure pipes is non-linear. Therefore, FEA cannot be used to predict the future rate of deterioration and make a time-estimate on when a pipe will fail. The remaining useful life estimate combines pipe condition with operating conditions, and estimates under what operating conditions a pipe is likely to fail.
  • Because deterioration is non-linear, it is recommended that operators create a re-inspection schedule or monitor the pipeline in partnership with FEA to determine when the pipe section reaches the yield limit, allowing the pipe to be rehabilitated before it fails

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