API A.8.1 Pure torsion equation (A.14) and wash out in DP.....?

[mhldrillpipe]

Guys

API pure torsion fomula Q = [0.096167 J Ym]/D, where J is polar moment of inertia, J=pi/32(D4-d4), Y minimum yield of material and D is outside diameter. d is inside diamenter of tube.

If a wash out in drill pipe occurs the size of the wash out, i.e. wash out area, can be estimated with hydraulic calculations.

So if an area of wash out is estimated in drill string tube, how to set a new limit for torque in DP when pulling out of hole (praying the tube does not twist off)?? any ideas or suggestions/experiences?

hoping for reply

thanks

 

[DrillerNic]

Interesting question! I'm sure there is an equation sowmwhere for torsional strengths of sectioned tubes, possibly in API RP7G appendix A.1, where there is an equasion for torsional strength of eccentrically worn drill pipe. However

1)Why would you be rotating while POOH? Backreaming isn't usually recommended!
2) Although the size of the wash out can be estimated, the location of it (in the tube or in the connection?) can't. As API tool joints are usually designed to be have about 80% of the torsional strength of the pipe, which is why torsional failures usually occur in the connection (strtched pins or belled boxes). Washouts usually occur in the pipe wall, so a washout would have to be quite large before the tool joints stopped being the torsional weak point in the drill string.

 

[mhldrillpipe]

trying to answer:

1) Not backreaming. Driller was doing 10-15 rpm to while rotating out. Torque were eratic, but increasing and got 30 t overpull and torque increased towards make-up torque (poor job pulling out).
2)YES! pure TORSIONAL failures will occur in connection. But NOT combined loading (torque tension) as connections are designed to make tube the weaker element when in torque and tension. Also: IF PURE TORSION: i am not sure (this is acctually part of my original queation) that the torsional strength is purely linear. That is, if a slim crak was machined in the tube going around 79 % of the OD (or ID) of the tube.....the connection would still fail first in pure torsion (because of it beeing 80 of tube).


I wanted to set a torque LIMIT for pulling out when discovering a wash out, but can figure out how the torsion formula can be redused (if it can at all?)...


hope to hear your thougths...