Pipe-cleaners
As the gas is coming out of the ground, and contains solids and liquids, as well as gas, the pipe can get covered in all sorts of mess inside. So how would you clean it? Send someone in with a scrubbing brush?
Pigs
The inside of the pipe is cleaned by sending through a speical piece of equipment that scrapes the build-up from the inside of the pipe.
This is known as a pig - but it hasn't got a curly tail and doesn't go oink. The photos show show two types of pig. For one of them, bits of pipe have been cut away so you can see the pig inside the pipe. The other just looks like a large yellow ball.
Enter, the pig...
There has to be a special arrangement of pipes and valves to get the pig into the pipeline. It can be quite complicated!
Now see if you can control the pig...