In this particular circumstance it falls into a mix of things. My colleague from Justice might contribute to my answer more fully from a legal perspective.
From a practical perspective, it depends on the nature of what issue might come about. If a company were bankrupt and no longer existed, and its pipeline had been abandoned, it would become a situation of provincial jurisdiction, perhaps as a disturbance of an environmental nature, for example, if something had caused damage. There might be a disturbance of a different nature.
The National Energy Board at this point in time has been pursuing a fund to look at how to manage the abandonment that might occur, which may provide some protection against what you are explaining could potentially happen.
Bill C-46 provides clarity that, under the National Energy Board Act, pipeline companies will be responsible for their pipelines even after they're abandoned, up to the point at which they're removed from the ground. Should a pipeline be abandoned and left in place, the company will be responsible for it in perpetuity until the pipeline is removed, and it must give provisions to the board to ensure there's adequate and appropriate funding to accommodate that should the pipeline company cease to exist in the longer term.
It makes explicit and clear that abandoned pipelines remain under federal jurisdiction, and that companies remain responsible and liable for those pipelines.