Good thread, Harald, thanks for starting it.
One thing I have always wondered about is, with the string stuck, do we use the buoyed weight or the weight in air for our calculations?
Then, of course, in deviated holes the regular recording of weights whilst drilling are: up weight, free rotating weight, down weight.
Here in Indonesia, I was sitting in an office and overheard the (expat) DS telling the DSV never to pull more than 80% of the strength of the weakest connection. They were stuck and had a combination string (5" x 3½") and the DS would not let the rig have an upweight > 80% of the 3½" pipe.
Needless to say they weren't even getting the bottom of the 5" dp into tension and stood no chance of ever freeing that pipe.
The DSV and the fishing hand both pleaded but to no avail. An Indonesian DS and I spent ages explaining to this expat, that it's like a spring (5" dp) with a weight on it (3½" dp and BHA); you need to get the string in tension before you can lift that weight.
To his credit, the expat DS did look up in his notes at home that night and came back in the next today to admit he was wrong (never an easy thing to do).
Simply put, as a general question to all our members ...... when was the last time your team had proper Stuck Pipe Training? (Prevention by Design and Operation, Reaction if stuck, Recovery).
In this case you have to be clear of the weights of the DP sections to be able to put in neutral position the connection depth you need to disconnect. In a deviated well should be complicated if you didn't take to string weight every 8 or 12 hours to have an idea.
Basically if the total weight of your string is 150 klbs, TDS is 90 klbs, total weight on hook will be 240 klbs. If the 3½" string weight was 50 klbs, then the hook load yo have to have on the weight indicator to release the first 3½"connection should be 190 klbs (240-50 klbs), when you have this reading you can start to slowly rotate to left in 5 turns, when you reach 20 turns, release and observed how many turns were absorved by string and repeat steps until you disconnect the 3½" string. This procedure is not so precise but if you don't have any more options will wok.