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November 13, 2017
Repair Your Piping: Fixing Transmission Pipeline Dents

Posted by: Casey Whalen

What are the options when repairing dents on transmission pipelines? Considering neither the ASME nor ISO have standards for dent repair using composites, the damage you’re dealing with will require a custom designed repair. This custom design should be based on validation testing of dent conditions.

The typical, non-cutout repair, utilizes a composite material that addresses the damage and helps extend the service life of the asset. However, there are unique considerations when specifically dealing with dents.

Design Repair Concerns

The primary purpose of a composite repair is to extend the pipe’s fatigue life. This is accomplished by reducing the increased localized stress in the pipe caused by a dent.  

For a composite repair to work effectively on dents, the operating pressure should be reduced by a minimum of 20% to enable the composite to be pre-stressed when the pressure is restored to normal operating conditions.  This pre-stressed condition enables load sharing between the composite and the existing pipe. A composite repair applied at the peak of pipe expansion will not take on any of the stress and therefore will not provide the needed support. Upon re-pressurization, the additional stress is transferred to the composite. 

A major component of the repair is filler material with a high modulus of elasticity. The filler assists in the reshaping of the dent and transfers stress to the composite while minimizing dent strain.  Failure to use adequate filler would allow cyclic fatigue failure of the pipe.

Product Considerations

Not all composite repair systems are equal. The stiffness of the fabric will determine how effectively the materials can absorb stress while minimizing strain.

An additional consideration is the composition of the fabric. The most common systems are either unidirectional or bi-directional.  With bidirectional fabric, such as the material used in our Atlas™ system, longevity is improved by addressing the multi-axial stresses present in a dent. Unidirectional fabric systems lack the ability to reinforce axial stresses and may prematurely fail. Dents with severe wall loss need to be addressed separately as they pose a greater immediate risk of failure.

Repair Process

While dent repairs are designed differently, the repair procedure is similar to standard composite repairs. The only significant change is the application of the filler material. Utilizing a product designed for dents, such as EP-420, can significantly improve the ease of application through its non-sag consistency.  Depending on the severity of the pressure cycles of the pipe and the absence of any cracks, engineered composite repairs should be designed to have a 20-year life span.

One of the most important aspects of repairing any dent is to act as soon as possible. Composite repairs will significantly increase the life of the pipe. However, left unrepaired, this flaw, coupled with any corrosion, can create far larger problems. The combination of increased localized stress (caused by the dent) and any wall loss (caused by corrosion or other damage) will combine to drastically lower the fatigue life of your asset.

Next in our Repair Your Pipe series, we’ll continue our look at transmission pipelines covering the repair of wrinkle bends and the specific considerations for an effective solution. Be sure to subscribe to our News and Thought Leadership Blog to catch that and more infrastructure repair insight from Milliken Infrastructure. 

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Category: Pipe Wrap

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