API RP 7G calcs - Mathcad

[texag]

I have been trying to create a Mathcad worksheet using the formulas provided in this recommended practice, namely those formulas in Appendix A.8.

As with most API specifications, the formulas do not lend themselves to use in Mathcad if you wish to use a unit system. If you attempt this, and I have, you will find yourself adding units where needed to get a result in units that you expect. I will never understand formulas that are/were built without clarification of units. With the API 7G formulas you just have to trust that they come out right - I can't stand that.

Two good examples are the values "E" and "B", there is no explanation of what they are and they do not appear on the A-2 figure. Obviously they are correction factors to compensate for the connection taper and are used in box and pin tensile/compression area.

Anyway, just wondering if anyone else has experienced similar frustrations using Mathcad for API formulas and has come up with a simple solution to making units work. I am actually re-writing the equations in terms that make more sense (to me anyway) and don't require "playing" with the units within the formula.

Even better would be if there was an e-Book for API formulas but I don't think that is offered yet

PS - I did search for this and have not found where this topic has already been covered. If anyone is aware that is has I would appreciate the thread ID.

Thanks!

 

[redpicker]

I feel you pain. It can be difficult to follow.

To sepcifically answer the issues you have,

The symbol "B" is used to calculate the critical area of the pin connection, defined as the area under the thread root at a distance of 0.750" from the the shoulder. So, B is the value subtracted from the pitch diameter at gauge to get the root diameter at 0.750". It is calculated from half the thread height minus the root truncation plus the difference in pitch diameter at gauge and pitch diameter at 0.750". This one often causes problems becasue it is subtraced from the pitch diameter and is isn't uncommon for someone to make some algebraic errors when trying to add the equation for this value to the equation to determine the area. B has units of inches if the thread elements are in inches and the taper is expressed in inches per foot.

The symbol "E" is similarly used to calculate the critical area of the box, which is defined as area 0.325" from the box face. E is the value added to the counterbore diameter to calculate the ID of the box at 0.325" from the shoulder. Since the counterbore is on the same taper at the threads, the formula is
tpr x 0.325 x 1/12,
where
tpr is the thread taper in inches per foot
0.325 is the distance (in inches) from the face, and
1/12 converts inches per foot to inches per inch
So, E also has uints of inches.

I agree these equations are not clearly explained and are much more complicated than they would be if the connections were designed differently. Unfortunately, the connections were designed by machinists long before the equations were derived by the engineers. Had the connectons been designed by engineers with complete knowledge of the theoretical equations, the pitch diameter probably have been defined at the 0.750" and the taper would have been in degrees. But, as they say, if a frog had wings...

rp

 

[texag]

redpicker,

I knew that is what those factors were for but am still dumbfounded as to why API doesn't simply explain that. I completely re-wrote those equations in terms of the taper angle and am getting the same values (with units!) as you would get just using a calculator. I am satisfied with my results. The only thing I wish I knew how to do now is have Mathcad create the cobined torque vs tension diagram, that is just a training issue though.

Actually, that brings up another question: How do you plot the "combined load pipe yield" curve? API is unclear on how that is created, go figure. I did some searching on the internet and several drill pipe manufacturers provide these charts with the combined pipe yield curve, so someone knows.

ps - I believe you meant to type 0.375, not 0.325

Thanks for the response

 

[redpicker]

Yes, I did mean 0.375". Sorry about that.

To get the combined load pipe yield curve, this is explained in section A.8. Equation A.15 allows you to calcualte the torsional yield strength for various tensile loads. I'm not sure, but I think the charts you find online were developed by rearranging A.15 to solve for P (instead of Qr) to build the data table, so they end up calculating the tensile yield for various values of torque. At least, that's how I'd do it.

As to why API does not simply explain these issues is mostly a function that it really isn't all that simple. What appears in the Appendix is an explanation of how the information in the standard was calculated. While, perhaps not as clear as it could be, it was clear enough for you to figure it out, so it served its purpose. It certianly could be made clearer, but remember, every bit of the work that goes into these standards is done by unpaid volunteers. While most have a company sponsoring them (to enable them to attend the meetings and conferences), the work the group memebers do is not billed to anyone and generally is not considered a job function by their employer (work performance is generally not evaluated on their volunteer efforts). If you think you could help, I am sure they would welcome anything and everything you could contribute.

rp

 

[texag]

You make good points, and sorry if I offended, it sounds like you have served committee time. Complaining and not being part of the solution, not very proactive of me.

Again, thanks for the help.

texag