In the oil business, what is a "mud man" and how much do they earn?

A good friend of mine has a husband who just retired from being, as he called it, a mud man. He worked for an offshoot of Halliburton and was transported by helicopter to offshore oil rigs. What did he do on the rig? He had a nice house and boat house right on a huge lake for years. He had to learn several foreign languages due to rigs far from the USA. She has no idea what he did on the rigs or I would have asked.