Analysis Of The Fatigue Resistance Of Rotary Shouldered Connections (2002)

Analysis Of The Fatigue Resistance Of Rotary Shouldered Connections Tony Collins; Tony Collins Schlumberger Oilfield Services Search for other works by this author on: This Site Google Scholar Francesco Vaghi Francesco Vaghi Schlumberger Oilfield Services Search for other works by this author on: This Site Google Scholar Paper presented at the IADC/SPE Drilling Conference, Dallas, Texas, February 2002. Paper Number: SPE-74564-MS https://doi.org/10.2118/74564-MS Published: February 26 2002 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Get Permissions Search Site Citation Collins, Tony, and Francesco Vaghi. "Analysis Of The Fatigue Resistance Of Rotary Shouldered Connections." Paper presented at the IADC/SPE Drilling Conference, Dallas, Texas, February 2002. doi: https://doi.org/10.2118/74564-MS Download citation file: Ris (Zotero) Reference Manager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex Search Dropdown Menu nav search search input Search input auto suggest search filter All ContentAll ProceedingsSociety of Petroleum Engineers (SPE)SPE/IADC Drilling Conference and Exhibition Search Advanced Search AbstractIt is widely accepted that understanding of the stresses, and thus fatigue resistance, of rotary- shouldered connections requires numerical stress analysis, and that the contact between box and pin threads makes this a complex problem. Previous analyseshave usually dealt with one particular connection, and have varied in their approach to the spiral nature of the thread.This paper covers a range of connections from NC38 to 7–5/8 H90, and uses multiple cases at different rotations to allow an axisymmetric model to simulate a spiral thread. The effects of alternating stress and make-up are separated, and then combined using a Gerber plot to predict relative fatigue resistance of various connection geometries. These results are consistent across connection styles, which makes it unlikely that they are due to modeling anomalies.Conclusions are drawn as to the beneficial effects of stress relief grooves, and on the significance of the geometry of the standard groove. The results can be used with simple beam-theory analysis to make useful strength predictions, and allow the evaluation of the effects of modifications to standard connections to meet the needs of drilling and measurement tools.IntroductionRotary shouldered connections are key elements of a drilling string, and are among the most common causes of failure, especially in the bottom hole assembly. Fatigue is the most common cause of failure, and has become more important as extended-reach and horizontal wells become more common, and may become even more significant with the advent of rotary-steerable systems.The earliest attempts to provide design rules for fatigue were based on simple beam theory and full scale fatigue testing1. This approach, while quite effective, is very limited in its ability to explain the effects of the various elements of the connection. It is widely accepted2,3 that understanding of the stresses, and thus fatigue resistance, of rotary shouldered connections requires numerical stress analysis, (FEA) and that the frictional contact between box and pin threads makes this a complex problem. Previous analyses have usually dealt with one particular connection, and have varied in their approach to the spiral nature of the thread. Most authors have reasoned that the shallow angle of the spiral makes it unimportant, but there have been attempts at three-dimensional models, which to our knowledge have been limited to the effects of make-up stress.There has been considerable difficulty in the interpretation of elastic stress analysis to a situation where the modeled stresses due to makeup are considerably above the yield stress. A previous paper4 described a systematic way to overcome this problem, and the approach is extended here.In this paper we will describe stress analysis of several different connections, with a variety of thread forms. This allows us to investigate some effects of the spiral nature of the thread, and the effects of the existence and the geometry of the stress relief groove.MethodsConnections modeled.The connections modeled are listed in table 1, and all of them have been used in Measurement-While-Drilling (MWD) tools. In general, these are larger in pitch diameter than the standard connections for their collar sizes, and have bending-strength ratios (BSR) of around 2.0, rather than the 2.5 typical of standard connections. Such connections are required to accommodate instrumentation inside the collar, and we have used FEA for many years to assure ourselves that the connections are of adequate strength. One of the connections analysed, 6–5/8 FH in an 8.0 inch collar is no longer used because it was found to be weak in fatigue. The 7–5/8 H90 connection modeled has a high BSR. This connection is used because it is functional in both 9.0 and 9.5 inch collar sizes.All the models we have used are axisymmetric, based on the nominal thread dimensions from API Specification 7 5 or RP7G6. In all cases the box has a boreback, and the pin is modeled both with and without a stress relief groove.Connections modeled.The connections modeled are listed in table 1, and all of them have been used in Measurement-While-Drilling (MWD) tools. In general, these are larger in pitch diameter than the standard connections for their collar sizes, and have bending-strength ratios (BSR) of around 2.0, rather than the 2.5 typical of standard connections. Such connections are required to accommodate instrumentation inside the collar, and we have used FEA for many years to assure ourselves that the connections are of adequate strength. One of the connections analysed, 6–5/8 FH in an 8.0 inch collar is no longer used because it was found to be weak in fatigue. The 7–5/8 H90 connection modeled has a high BSR. This connection is used because it is functional in both 9.0 and 9.5 inch collar sizes.All the models we have used are axisymmetric, based on the nominal thread dimensions from API Specification 7 5 or RP7G6. In all cases the box has a boreback, and the pin is modeled both with and without a stress relief groove. Keywords: srg 1 8, fatigue resistance, maximum stress, stress distribution, shoulder radius, groove, upstream oil & gas, srg 0, counterbore, iadc spe 74564 Subjects: Drilling Equipment This content is only available via PDF. 2002. IADC/SPE Drilling Conference You can access this article if you purchase or spend a download.