Separation moment of API connection

[julian89]

Hi guys!


I recently got asked by a client at what bending moment joint separation will occur (with and without internal pressure).

API 7G gives clear rules for calculation of separation tension (Eq. A.19), but as stated in A.10.7:

"A.10.7 CAUTION: The loads considered in this simpliÞed
approach are torsion and tension. These curves are approximations
that do not consider the effects of internal pressure or
bending. For this reason, the answers from these curves
should be derated. A safety factor of one was used."

Does anyone know of a way of quantifying the separation bending moment, with and without internal pressure? I would prefer not to an FEA

Julian

 

[Smithy_]

A very large bending moment would be my guess!

The formulae in 7G allow you to calculate the compressive stress at the torque shoulder due to make-up torque. So I suppose you could use the ol' Euler-Bernoulli bending calc, treat the box and pin like one continuous pipe, and work out the bending moment required to produce an equal and opposite stress at the tool joint OD - which would cause the contact force to be zero. In fact, if the connection torque rating is box-limited then the contact stress will just be 60% x material yield strength.

This will give a rough approximation. FEA will get you closer, but even then it won't be real-world due to the assumptions made about friction coefficient of the thread dope (0.08).