This is a standard combo stress problem; 6 DTC 4 has a cheat sheet that is faster to use than the ellipse of plasticity, and this page walks you through the calculation as well:
1) (σz + pi)/σyield = (40M + 20M)/80M = 0.75
2) Chart: --> 0.75 ---> -0.385 = (pi - pcrr)/pcr (note negative sign for collapse)
3) pcrr = pi - (-0.385)pcr) = pi + 0.385(pcr) = 20M + 0.385(pcr)
4) Redbook: 6 in. N-80 collapse rating of 7.180M psi.
5) pcrr = pi - (-0.385)pcr) = pi + 0.385(7,180) = 20M + 2.764M = 22.8 M (C).
How do you find collapse rating in the Redbook? I'm looking at section 200, and afraid my Redbook may be older and missing some key information (like section 240). Using section 200, you need 3 inputs, OD, Grade and Weight to find Collapse Resistance. Collapse Resistance for 6", N-80, 23lb/ft casing is 7,180 psi which matches what's provided above. However, I cannot find collapse rating (or resistance) for pipe specified in other similar problems, like 2017 #14, or for problems that don't provide casing weight, so I cannot solve the problem. Am I doing something wrong, or is my Redbook lacking casing information that I need? Thanks in advance.
ReplyDeleteYou are doing fine; your Redbook (and TS2) will always show the collapse if you know the weight.
ReplyDeleteOn some problems, I leave it off because any of the selections will give the right answer. However, on the 2017 Problem 14 the very first printing was missing "A 5-1/2” 26 lb/ft N80 casing" text. It was fixed pretty fast, so you must have been one of the very early users. Just follow the info on the blog.
Thank you, but I'm still having trouble. I cannot find 5-1/2", 26 lb/ft, N-80 casing in the Redbook (or TS2). In the Redbook, I'm looking at the Dimensions and Strengths of Casing (Table No. 203). Am I looking in the correct place?
ReplyDeleteI see. That's a non-API pipe so I guess some older Redbooks don't have it (or TS2). It's on page 18 of my Redbook. But don't sweat it for the exam since it will be given for sure if it's not API. Good question.
DeleteThank you so much.
ReplyDeleteEllipse of plasticity is not covered in in the SPE reference. How do you suggest we solve without the shortcut?
ReplyDeleteKeep in mind: just because it's not in the provided reference doesn't mean it won't be tested. In fact, it could be more likely to be tested, but using word problems or conceptual questions. This is why one should take practice exams over and over until it just clicks and you know you will do well compared with your fellow test-taker.
DeleteBut it's also very easy for any specific problem to just give you the necessary inputs on the problem. On this one, they could merely give the ellipse in the problem; that's how I would do it were I writing the exam, and I would make it so you had to "understand" how it all works with the needed API equations. Remember, exams target a failure rate, and if everyone can find it in the reference, it's too easy IMO. When using the Guidebook, I try to make it as simple as possible how it all "fits together". This is why I would just run with whatever study methods you are using and once familiar with the provided reference just ignore it. Testers can drive you crazy with problems not in the reference that you really should know how to do, and 90% of the folk will be searching in vain. It's better to just ignore references unless you know exactly where and what it is, and accept that many problems will not use the reference at all so don't even go there unless you know everything about the problem.
thank you for your response.
ReplyDeleteWould that table on 6 DTC 4 be provided as a part of the question?
ReplyDeleteProbably the ellipse of plasticity. But I like to use the chart to help better understand the ellipse. Honestly, i find the ellipse confusing if I don't work through it in chart format. YMMV.
DeleteGreat, thank you
DeleteIf the rating is reduced by 0.385, isn't the reduced collapse rating = (1-0.385)(7,180 psi)+20,000 psi internal = 24,416 psi?
ReplyDelete