A year ago, I saw some local work being done on city pipes. The team was using a bot with three telescoping radial wheeled legs like

\   /
 \ /
  O
  |
  |

and wheels at the end of each leg. They put it in one end of the pipe, extended the legs so the wheels touched, and then drove it around in the pipe.

It was a welding device, so they could do spot-fixes from the inside.