During a routine ROV inspection performed by a contractor, it was discovered that a head gate seal plate had fallen off. Upon further inspection, it was discovered that the attachment hardware had corroded to the point of failure on the other intake slots as well. This was most likely due to dissimilar metals being used for the seal plates and attachment hardware, causing an anodic effect on the hardware.
In 2004, Underwater Construction Corporation developed a plan to utilize a custom-built ROV as well as divers to replace the seal plates and hardware. The water depth was between 200’ and 215’ for the duration of the project. This work was performed during winter pool to minimize the working depth. Winter lake levels are typically elevation 1640 while the summer pool levels are approximately elevation 1705.
The ROV was built with the ability to drill out the old hardware and reattach the existing seal plates with new hardware. A few of the seal plates were replaced with new ones. When a new seal plate was needed, the ROV would be brought to the surface with a custom-built gantry crane attached to the front of the work barge, and the new guide would be placed in the ROV’s grip claw. The ROV would then be lowered back into place where it would hold the new seal plate in place while drilling, counter sinking, and threading the new hardware holes. The new hardware would be installed by the ROV utilizing a magazine-fed drill head. This was all accomplished without the ROV returning to the surface. The ROV utilized a turret-style head, which could use up to five tools without retuning to the surface for retool or calibration. All of these processes were controlled from the surface utilizing eight cameras with several joystick-style controllers.
Divers would assist the tool by pressure washing the work area and changing out a tool if broken. A few times the drill bit or tap broke while the ROV was in the middle of a seal plate installation evolution, and it was more prudent to dive on the tool rather than bring the ROV to the surface. Having divers on site ready to assist the ROV was a huge advantage to the success of the project.