Q1: In the Guidebook, page 1 RIG 5 says: "RTTM equation assumes DP to bit, so wdp is subtracted from wbha." What are the mechanics behind this? Is there anything online to help understand this?
This formula is found many places and formats; I think this version is from Lapeyrouse's Formulas and Calculations for Drilling, Production, and Workovers. It's a good field book. And while this sort of problem may be too "real world" for a typical PE Exam question, I include it because something like it could easily show up.
The equation used assumes the DP goes all the way to the bit, then subtracts the DP over the BHA length later. There are other ways to skin this cat but I prefer this method because I find it faster. And speed is everything on the exam.
Q2: On 1 RIG 5, why is tripping RTTM multiplied by 1, coring by 2, & drilling by 3?
For drilling or "connection" TM, use the TM for actual drilling, plus the TM for picking up, connecting, and starting to drill again. This is 3 lengths, so multiply by 3 over the drilling depth.
For coring, you don't need to pick up again; just add another stick. So you multiply by 2.
This would be a great word problem for the PE Exam, to test one's understanding of how the TM equation actually works. So it's an excellent question. It's very easy for a test writer to nail those who survive by "plug-and-chug" and lack understanding.
