Yelp - good resurrection, min!
Firstly remember 'Offshore' isn't a job - offshore is a place where people do loads of different jobs.
Find out what interests you and what is related to what ye did in the army.
Me, I work in drilling, started offshore but now on land rigs - really like it - but that's what I was sure I wanted to do, I wasn't interested in getting into anything else and that nearly scuppered my career before it started. But there's also Production which I believe could be more related to yer traditional trades type work.
Getting a survival ticket will probably help but is no guarantee. Try Nutec or Petrofac training in Aberdeen for the course.
Agencies definitely a good way to go but ye'd probably need your "Greenhands" roustabout course and getting that first trip is always going to be difficult.
But could I go back offshore? Nae chance, I've done my time on the rusty tin cans (for now)