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Blind Back-off Procedure
22 June 2020
Esteemed Forum Members,

I was wondering if any of you would have a (proven) detailed procedure, that you could share, to perform a blind back-off using a TDS rig. Assume the BHA is stuck.

Particularly interested to see procedures to perform a blind back-off using a combination string (for example 3-1/2" and 5" drill pipe) in a deviated well with the objective to back-off the 3-1/2" drill pipe.

Many thanks in advance.

Harald Benning.
7 answer(s)
admin
Managing Director (rp-squared.com)
Relentless Pursuit Of Perfection Ltd.
Total Posts: 479
Join Date: 10/01/05
Dear all

We have the attached contribution from another member.

Please note that these procedures are no substitute for:
  1. Avoid getting stuck in the first place! TRAIN, TRAIN, TRAIN
  2. Correct and immediate response to a stuck pipe incident - Clock Time Matters!
  3. A solid engineering understanding of the physical principles involved - TRAIN, TRAIN, TRAIN.
  4. Involving your Fishing Company - but be sure they have (the correct) experience.
I hope that helps

Dave
Documents uploaded by user:
Mechanical Backoff Procedure.pdf
Harald
Senior Wells Advisor
Redstone Drilling
Total Posts: 43
Join Date: 13/09/07
All,
Many thanks for the responses to date.

The second query was related to how to transmit torque to the desired blind back-off point. Does anyone have any detailed procedures for this?
Many thanks,
Harald.
SCollard
Consultant Well Engineer and Trainer - HPHT, Deepwater and MPD Well Delivery and Well Control,
Welltrain Limited
Total Posts: 25
Join Date: 09/12/09
Ah..  the Air Weight or Buoyant weight discussion again!  

The correct answer is ...a bit of both!

I have uploaded a couple of slides from my Stuck Pipe course to illustrate the following.

OK ...some basics...   

  1. String weights calculated using buoyancy factors only give the right buoyed tension value at surface...no-where else in the string.  
  2. Buoyancy is caused by force imbalances from hydraulic pressure acting on flat surfaces of the pipe.  They start on the bottom of the string and then increase or decrease as you go up the string.   What you see at surface is the cumulative value of all of them.   
    Remember its quite easy in field units..  pressure (psi) x area (square inches ) = Pounds of more or less tension!
  3. Before the pipe got stuck it was all subject to buoyancy..   that didn't disappear when the pipe got stuck
This scenario is illustrated in the slides. 

The most critical number you need is the free weights up down and rotating just before the pipe gets stuck.  (The Driller DOES record that every stand doesn't she/he?!) 
If you have moved the actual hookload away from that value then you will have changed the tension in the pipe above the stuck point. 

At some point of the back off operation you will want to reduce the ACTUAL tension at the back off point to around zero.   So, first you need to know what it was prior to being stuck.   If there are no pipe size or weight changes between the stuck point and surface  you can see from the slides that the free tension at the stuck point would have been the free string tension at surface (free hook weight tension less weight of the blocks)  less the weight in air of the string above the stuck point.  (NB:  use TVDs for string lengths!)   If there is a change in pipe OD or ID then you also need to allow for the buoyancy acting at the point that either the ID and/or the OD changes.  It usually means the stuck point tension will be GREATER than you expected by the buoyancy force acting at the crossover.

Say this tension is 50,000lbs.  This means you will need to set the hookload to the free hookload value less 50,000lbs to get the tension at the stuck point to be around zero.  Now, in a deviated hole you need to consider drag:
  • if you have to pick up to achieve the required hookload then use the free weight up value
  • If you have to slack off the string then use the free weight down value 
I hope this makes sense.

Cheers  Steve


Documents uploaded by user:
Weight for Back Off.pdf
hank
fishing tool hand / consultant / advisor / coordinator / tech. rep.
SPREAD Associates
Total Posts: 42
Join Date: 13/03/12

Harald,


We always used weight in air as bouyancy has nothing but the TJ / tube size difference (this is minimal with tubing strings) to apply force to until the back off is completed,  you need some over pull anyways.


Weights are critical for mechanical/ manual back offs, but there is some leeway when using a properly built and sized string shot.


Keep a strain on'er...lol..


Henry


Harald
Senior Wells Advisor
Redstone Drilling
Total Posts: 43
Join Date: 13/09/07
Many thanks to Dave and Ricardo to respond to this ...

Dave makes a very good observation if one needs to use the buoyed weight or the weight in air for the calculations. Just to add a little anecdotal evidence, I was told to use buoyed weight by one fishing engineer and weight in air by another fishing engineer ... both fishing engineers were from the same service company ... So, if there are any experts out there, please share with us what is the standard....

Regarding working torque in the string, I was informed that if, for example you apply 10 K ft-lbs of torque at surface, this torque is not transmitted to the stuck point, but adsorbed by the string. To transmit the torque down the string, one has to lock in the torque and reciprocate the string (within the limits) and one would see the surface torque value reduce as the torque is transmitted down the string ....

Are there any fishing / back-off experts on the forum willing to share their knowledge on this?
Many thanks in advance.

admin
Managing Director (rp-squared.com)
Relentless Pursuit Of Perfection Ltd.
Total Posts: 479
Join Date: 10/01/05
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).
rvasquez
Drilling Team Leader
SPREAD Associates
Total Posts: 6
Join Date: 06/11/14
Harald:
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.
Posted by

Harald Benning

Senior Wells Advisor

Redstone Drilling

Total Posts: 43
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