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March 07, 2018
Understanding a Composite Repair’s Design Life

Posted by: Natalie Swift

When you perform a composite repair on a pipe, you want to know that the repair will last as long as it’s intended to, or as long as its design life indicates. We’re going to look at questions you should be asking your composite repair designers about the repair’s serviceable life when you review your repair options.

Is this design for a temporary or permanent repair?

There are a lot of claims for composite repairs to be either temporary or permanent, but what do those terms mean? Here’s our interpretation:

  • Temporary repair – this repair will be installed for a specified—usually short—amount of time with specialized scheduled inspections or a planned service removal
  • Permanent repair – this repair will be installed without requirements for specialized scheduled inspections other than regularly scheduled inspections for the entire pipe

The life span of a repair can sometimes depend on your treatment of the repair post-installation; for example, a temporary installation could last much longer than intended if it’s regularly inspected as instructed and determined to be functioning as intended.

Is this design based on the pipe’s end of life condition?

When engineers are designing repairs, they should be designing for end of life conditions. If your pipe has external corrosion, then the pipe repair, with adequate surface preparation and installation, will stop the corrosion from the point of repair onward. If there’s internal wall loss, however, then the pipe will continue to lose its internal wall even after the repair is completed. In this case, the engineer will need to make a design with enough material to accommodate the pipe’s future wall loss, not its current amount of loss.

Are you considering cyclic fatigue in this design?

When you consider cyclic fatigue, you cannot only consider the capabilities of the composite but also of the pipe. For example, if your pressurized pipe has a dent in it, the fatigue life of that section may be dramatically reduced. The pressure-induced stresses will fluctuate with each cycle, with more aggressive fluctuation in liquid transportation lines. Depending on how severe the dented portion of pipe is, the cyclic stress could eventually fatigue the dent, causing a failure earlier than anticipated. A well-designed repair system should take the increased cyclic stresses into consideration and even minimize these stresses to achieve a desired design life.

In some forms of composite testing, the pipe will be put through extreme cycling to see when the defect fails. When designed and installed in accordance to the standards, the underlying pipe defect will fail well before the composite experiences fatigue failure. However, current standards only address cyclic fatigue concerns of the composite and not the life the pipe. If your system sees severe cycling, having a repair designed to only meet PCC-2 requirements may not be sufficient. Ask your composite provider to ensure that these concerns are addressed.

If my composite repair is supposed to last 20 years, do I have to remove it in 20 years?

If your design life is at 20 years, this means that the composite designer has done a mathematical design for the repair assuming numerical values for a 20-year design life. This value differs from intended service life, which is how long the repair is expected to be in operation. Some composites tend to weaken over time when under constant high stress (creep fatigue), while others do not exhibit this behavior. Typically, the stresses seen in the composite are very little, allowing composites to survive indefinitely. In many cases, the composite can be examined at the end of its design life and show no signs of fatigue damage, so the service life can be extended.

How do you determine a design life?

In general, having a higher design life implies a more conservative repair. However, in recent editions of the ASME PCC-2, the mathematical influences for design life have been removed. Make sure you understand the justification provided by the composite designer for using a specified design life and ask what that value really means.

When considering the practicality of a specified design life, all potential influences should be examined including: cyclic and thermal fatigue, creep fatigue, environmental conditions, potential for unreported 3rd party damage, inspection plans and the requested service life. Do not assume that a company providing a long “design life” means that it comes with a guarantee or warranty. Every installation is different and should be reviewed carefully by a qualified individual.

The ability of your pipe repair to last for its entire design life is serious, so take your conversations with your composite designer seriously. For further questions regarding design life or any other concerns you may have about a composite repair, reach out to our Pipe Wrap design team at [email protected]

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Category: Feature Articles, Pipe Wrap

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