I'd be careful about the liability here. If you make a mistake, then who pays for it, and what is being paid for? The contractor won't have professional liability insurance, so they're not going to pay for a design mistake. That'll be on you. For a refinery, you could be paying for the damaged platform, the injured (or worse) employee, or - the most expensive of them all - lost revenue for lost time at the refinery. If they have to shut down for a day to remove the failed platform and do an investigation and that investigation (and the follow on court proceedings) finger you as the cause, they can usually recoup that lost time from you. That's no small sum. As a result, a lot of places like that require engineers to have pretty good insurance. I worked in a steel mill briefly and I think we required outside engineering consultants to carry a minimum of $3M in coverage. So check on that.

If you can get over that and/or meet the requirements, then go for it. It could be your first step to writing your own ticket. I did it about this time last year and, though I'm exhausted and work like crazy, I have zero regrets.

Probably best that former employer hires you on an hourly basis (high hourly rate, but no benefits or guaranteed hours). Then you just do the work, and it is "their" product and liability they sell to their client.