Oil pipelines have "pigs" running through them all the time. Basically a computer donut on wheels that travel throughout the inside of pipes with scanners on the outside that constantly check the pipe walls for holes, corrosion and problems with pipe thickness, etc.
What happened with those here?
Pigs may be run all the time, but smart pigs less often. But I am sure they will be run more often by many.
They didn't run the "cleaning pigs" frequently enough, and the sludge built up to the point that it was considered potentially detrimental to the downstream pipeline to run the "cleaning pig" (would dump too much sediment load at once). With the sludge buildup, they could not run the "smart pig".