/ Exo Group, LLC Thu Jan 03 17:58:05 2019
Blog Post
Handling Emergency Weld Repairs on a Vibratory Caisson

Critical weld repairs and an overall shortening of a foundation caisson are two problems you don't want to have on a transmission line construction project.

Pepco Engineers experienced this situation with its ongoing reliability improvement program, which involves replacing existing 69kV wood pole lines with steel poles. During a caisson installation for one of the projects in the program, the Pepco refusal criterion was met approximately four feet short of the driven depth intended, and the contractor discovered fatigue cracks developing on the full penetration welds and in the base metal of the caisson itself.


In order to avoid serious damage, the engineering company had to address these issues immediately with a clear resolution plan.


Pepco Engineers Takes Action


After identifying the problem, Pepco Engineers outlined its repair options, which included: 1) evaluating the shorter embedment depth of the installed caisson to determine if it could remain as installed with only weld repairs made to the damaged area, and 2) trimming the caisson to the intended height above the ground, and re-using and re-welding the existing top flange. The company conducted an analysis of the two options and determined both would work, but decided to go with the second approach because it removed the damaged section of the caisson. 

From there, Pepco Engineers tapped Exo to evaluate and execute the necessary welding repairs. The Exo team adhered to the appropriate Weld Procedure Specifications (WPSs) to make the repairs, following
a 9-step plan that included:

  1. Measuring four feet from the existing flange plate
  2. Cutting the shaft and removing the damaged portion of the caisson
  3. Preparing and re-welding the flange plate to be a 100% penetration weld
  4. Beveling the top of the caisson to the approved angle on the WPS
  5. Installing a new backing bar on the interior of the pole shaft
  6. Tack welding a pre-heated flange plate to the top of the caisson
  7. Performing an inspection of the weld
  8. Re-coating areas that had the galvanizing removed with Zinc Rich Paint


Lessons Learned


There were several lessons learned from the Pepco Engineers emergency weld repairs that can be applied to other similar scenarios. For example, the company determined that refusal criteria needs to be evaluated based on total driving time so the caisson does not become fatigued. Additionally, it realized that the original design weld details should be evaluated to enhance fatigue resistance.

By partnering with Exo, Pepco Engineers received inspection and repair services from experienced professionals who were familiar with the necessary codes and standards for the work performed. Exo collaborated with the Pepco team to complete the repairs thoroughly and in a timely manner, so the assets would remain structurally sound.

 

Download the full case study to learn more