If you have a Company observer, it's basically just some fun flying. With the risks associated with flying low over inhospitable forced landing areas under conditions in or near the H-V curve, of course.
If you don't have an observer, i.e., you are flying and inspecting the line, it can be relatively high workload. The workload goes down once you are very familiar with the route, hazards along the right-of-way, airspace/airports in proximity to the route, etc.
I assume you mean by helicopter and not fixed wing. Obviously the flexibility of the helicopter makes it more effective than a fixed-wing. But of course the helicopter will take somewhat longer (speed factor), although it tends to make for a more thorough inspection and offers the pipeline company quite a few options, including the possibility of landing on the right-of-way for an up-close look at something, if desired. The airplane will cost about 1/3 of what the helicopter will--the pipeline company will make the choice and will get what it pays for, if you do your part.
Unless it is a very special-case operation funded by a well-heeled client, you will be in a single-engine machine. So be ready and willing to accept the hazards associated with that. In the very unlikely event you get hired to fly a twin turbine helicopter on a pipeline patrol, I can't imagine a much better scenario.
If you get the job, enjoy it and remember, fog is the enemy, and in the cold months, icing is just as much a potential problem at 100 AGL as it is at 10,000 MSL, you can just land quicker, that's all.