Drilling out Cement Plugs from Horizontal Dam Drain Holes

I'm submitting this question on behalf of a colleague involved in cleaning up a closed mine site in Montana. Her description of the problem:

"The site has a tailings dam with two steel decant pipelines that extend from the downstream face of the dam, through the dam and into the impoundment upstream of the dam. The pipes are over 1000ft long. The pipes are essentially laid flat (0% grade) where they daylight on the downstream face of the dam, but may increase grade up to 5% as you move upstream. During closure of the mine, concrete was pumped from the downstream end of the pipes creating a plug of an unknown length. One decant pipe is 12 inches in diameter and the other is 15 inches in diameter, both with 3/8” pipe wall thicknesses. We know the concrete plugs are at least 25 ft long but could be hundreds of feet long. We also don’t know the exact alignment of the pipes.

 The ultimate goal is to gain access into the pipes in order to perform a camera inspection. We are considering drilling horizontally through the downstream end of the pipes in hopes of drilling completely through the plugged portions, which would allow us to push a CCTV camera from the downstream end of the pipes. The main concern with drilling horizontally through the concrete plugs is the risk of drilling through the steel pipe wall. We will need to consider methods and technology to reduce this risk as much as possible."

I would guess that they won't have a very capable drilling rig available, and that this may be a cost sensitive operation.  It seems to me that the right drill bit should be able to drill out the cement without any danger of damaging the pipelines.  Any advice or referrals would be appreciated.  

Thanks in Advance - Neil Smith Bergstrom,  MagVAR / H&P Technologies, Denver

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