It involves inserting a new pipe inside the original pipe, usually by pulling the new liner by winch from the upstream end.
Since the new liner has obviously a smaller diameter than the original, the expectation is often that the capacity will be reduced. But the reality is that the quality of the new interior is so much better than the old surface that the carrying capacity will actually be greater. The new polyethylene liner is pre-welded to the required length and is drawn in immediately behind the pulling head.
Laterals are disconnected from the main pipe so that the pulling process cannot damage them. Once the new pipe is in place the laterals are re-connected.
It is this sort of trenchless technology that can enable pipeline rehabilitation to be carried out quickly and effectively when the circumstances are appropriate.