API NC50 Thread_ Connection performace

[VenkatRajendran]

Hello,
I have a question regarding the connection performance of the 4-1/2 IF (NC50) thread.

We are designing a Test Mandrel with the 4-1/2 IF thread connection. The purpose of this mandrel is to test the BOP stack.

Tool Joint OD = 7.5 in
ID = 3.5 in
Material Yield = 75 Ksi
Working pressure = 15 Ksi

We wanted to be sure that the connection will withstand both the Working pressure (15ksi) and Test pressure (1.5 times the working pressure). We tried to look at the API 5C3, 7G and 5DP, but we could not find the Tensile, Burst and Collapse pressure ratings for this IF connection.

How to determine the connection performance (Tensile, Burst and Collapse) of this NC50 thread connection?
Could you pls help me to figure out this?

Thanks for your help!

Regards,
Venkat

 

[redpicker]

The standard Rotary Shouldered Connections are not rated for pressure. While the standard 4-1/4 IF often will hold the pressures you have indicated when new, the design of the connection does not take pressure differential across the shoulder into account and the wear limits do not insure a minimum limit of seal performance. There are manufacturers that do make Rotary Shouldered Connections that incorporate a metal seal in their design, but these are not compatible with the 4-1/2 IF.

You can find tensile ratings in RP7G. These ratings are based on a minimum yield strength of 120,000 PSI. In the appendix, you will find equations you can use to determine ratings at other material strengths. The standard Make-Up torque for a 6.625 OD x 3.25 ID would break this connection.

I hope you somehow make sure your test piece will not ever used to connect to a drill string. With those dimensions, it is extremely pin weak (rapid fatigue failures could be expected if rotated) and its tensile capacity is much less than half what a standard NC50 would have. I wouldn't want to have such a tool on a rig.