Recovery of a Pipe Coupon Located 4,000’ from the Access Point

Surge Shaft for Mohican Access

This project involved a hydroelectric intake tunnel where the customer had hot tapped the pressure piping in order to divert a portion of the water for another purpose. During the hot tapping by the contractor, there was a malfunction, and the pipe coupon could not be extracted by the hot tap equipment. The coupon fell into the pressure pipe. Since the coupon was of significant size, it posed a hazard to the hydroelectric facility and its turbines. The decision was made that it would have to be removed from inside the pipe. Since the location of the hot tap was approximately 4,000’ from the location of the nearest access point into the tunnel at the surge shaft, Hibbard Inshore was called to provide an ROV and extraction tooling that could not only fly the distance to the coupon and locate it but also remove it through the pressure piping and shafts while the tunnel and piping remained flooded. Hibbard Inshore was up to the task, quickly devised a method, designed ROV tooling, and mobilized to site to assure the coupon would be removed.

Mohican ROV with 4 Function Manipulator

Due to the need for a Remotely Operated Vehicle (ROV) with the ability to retrieve the coupon, a larger light work ROV was used instead of a standard inspection ROV. It was decided that Hibbard Inshore’s Sub-Atlantic Mohican ROV with 4 function hydraulic arm would be the proper vehicle to retrieve the coupon.

Upon arrival at site, it was assumed that the coupon was either in the immediate area of the hot tap or slightly downstream at the lower elbow of the vertical tunnel section. While traveling in the horizontal section, the ROV’s forward looking sector scan sonar was used to image the invert of the tunnel and look for the coupon. When the coupon was located, the ROV manipulator picked up the coupon with a special hook. The hook was built specifically for the retrieval of the coupon by inserting the hook through the hole in the center of the coupon.

Mohican and Coupon Lowered to Ground

The ROV searched for the coupon as it entered the tunnel but focused on the lower elbow below the vertical section. Then, the ROV travelled up the vertical section and located the hot tap on sonar. The ROV moved to the hot tap location and quickly found the coupon in the invert of the pipe, immediately below the hot tap. A detailed visual inspection was performed of the hot tap to tunnel interface which was on the left spring line of the tunnel when facing up stream. Once the inspection was complete in the hot tap area, the manipulator engaged the coupon. Once engaged, the ROV travelled with the coupon downstream to the lower elbow of the surge shaft.

Recovered Steel Coupon

After the coupon was recovered, a tunnel inspection was performed from the surge shaft upstream (approximately 3,800’) to the lower elbow of the vertical section. The area above the vertical section was inspected during the coupon retrieval stage. Both the coupon retrieval and the tunnel inspection were performed with very low water clarity. The ROV Used two sonar heads, one recorded the cross section of the tunnel while the other scanned for features forward of the ROV during the inspection phase.