PipeWrap

June 27, 2016
Using a Composite Repair to Combat FCG

Posted by: Casey Whalen 3:00 minute read

By removing the sharp crack tip manually, or, better, by grinding the pipe wall down beyond any crack influence, stress can redistribute itself without causing localized failure. If the root cause of the crack is removed completely and the remaining wall is still sufficient, a composite repair can be utilized using standard wall loss procedures. If, however, removal of the crack is not possible, composite repairs become much more risky. No composite repair will completely stop crack growth; the stress cycling will still occur underneath the repair.

In order to effectively limit growth, composite repairs must reduce the stress amplitude and allowable strain. These conditions are best achieved using a repair with a high modulus installed at a significantly lower pressure than the lowest planned operating pressure. This will allow the composite to absorb a percentage of the stress at any pressure above installation. In essence, when pressure is increased, the composite repair applies a compressive force on the pipe that reduces stress and allowable strain.

This reduction in allowable strain will also reduce the yielding associated with crack growth. The higher the composite modulus, the less strain occurs with an identical stress. Therefore, it is highly recommended that a high-modulus carbon fiber composite system is installed at reduced pressures.

A reduction in allowable strain may also be achieved by installing a repair that compresses the pipe, such as ratcheting metal bands on the pipe before wrapping. This procedure is not typically used for composite repairs, but can significantly reduce strain. Without constant analysis, however, these repairs should be considered temporary.

For a more detailed explanation on installation effects on pressure, please see the white paper, “Effects of Installation Pressure on Composite Repairs.”

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Category: Feature Articles, Pipe Wrap

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