IADC/SPE Copyright 2004, IADC/SPE Drilling Conference

Transcription

1 IADC/SPE Use NC56 Connections on 8 Drill Collars and Cut 1 or ¾ Pin Relie s on Rotated BHA Connections NC38 and Larger Sean Ellis, SPE, Nicholas Reynolds, SPE, Kang Lee, PhD, SPE, T H Hill Associates, Inc. Copyright 2004, IADC/SPE Drilling Conerence This paper was prepared or presentation at the IADC/SPE Drilling Conerence held in Dallas, Texas, U.S.A., 2 4 March This paper was selected or presentation by an IADC/SPE Progra Coittee ollowing review o inoration contained in a proposal subitted by the author(s). Contents o the paper, as presented, have not been reviewed by the International Association o Drilling Contractors or Society o Petroleu Engineers and are subject to correction by the author(s). The aterial, as presented, does not necessarily relect any position o the International Association o Drilling Contractors or Society o Petroleu Engineers, their oicers, or ebers. Papers presented at IADC/SPE eetings are subject to publication review by Editorial Coittees o the International Association o Drilling Contractors and Society o Petroleu Engineers. Electronic reproduction, distribution, or storage o any part o this paper or coercial purposes without the written consent o the International Association o Drilling Contractors and Society o Petroleu Engineers is prohibited. Perission to reproduce in print is restricted to a proposal o not ore than 300 words; illustrations ay not be copied. The proposal ust contain conspicuous acknowledgent o where and by who the paper was presented. Write Librarian, SPE, P.O. Box , Richardson, TX , U.S.A., ax Abstract When designing botto-hole asseblies, the designer ust speciy both the type o connections and whether or not to use stress relie eatures. The relative atigue lives o NC56 and 6-5/8 REG API connections were studied to copare the two connections in identical service. In addition, three coon pin stress relie groove (SRG) widths were studied to deterine the width that yields the best atigue lie. The relative agnitude o iproveent in atigue lie was quantiied using inite eleent analysis (FEA) and the Morrow strain-lie odel. The cobination o a ore atigue resistant thread or and increased box stiness ro a steeper taper gives the NC56 connection a draatic atigue advantage over the 6-5/8 REG connection. In addition, the results show the atigue lie o the pins odeled with ¾ and 1 SRGs to be the best, with the 1 groove having a slight advantage. Pins cut with no groove can be expected to have the worst atigue lie, ollowed surprisingly closely by pins cut with the 1 ¼ groove. Introduction Because atigue historically accounts or roughly 80% o all drill string echanical ailures and the cost o a ailure can be quite substantial, it is in the interest o the oil and gas industry to utilize the ost atigue resistant connections. Operators will beneit ro decreased costs associated with ishing and tool replaceent, and drilling contractors and rental copanies will beneit ro increased tool lie. In order to deterine which designs will be ost atigue resistant, and to quantiy the beneits o a particular design, analyses ust be conducted that copare available options on a relative basis. Finite eleent analysis has been used or any years to deterine the stresses present in coplex odels under ultiple loads. Advances in FEA sotware and coputer technology have allowed probles that previously required expensive supercoputers and days o coputer run tie to be solved with a desktop workstation and only several hours o coputer tie. As a result o these advances, probles that were run in the past with sipliying assuptions (in the interest o tie and oney) can be revisited in the pursuit o ore accurate results. In addition, new topics previously deeed too coplex to analyze can now be investigated. Given the ultiple loads involved (e.g. ake-up torque, externally applied tension, bending, etc.), accurately analyzing the stress and strain distribution in a rotary shouldered connection is a challenging atter. In general, the areas that requently require the ost attention are (1) the esh, (2) the application o ake-up torque (MUT), (3) the application o a bending load, (4) the stress and strain output, and (5) the atigue lie odel. The esh, application o MUT, and application o bending load are discussed in detail in Appendix A. The stress and strain output and atigue lie odel are discussed below as they distinguish these analyses ro those previously conducted. and Output The stress distribution and axiu stress have traditionally been the eans by which the suitability and perorance characteristics o a odel have been rated. When evaluating the perorance o a rotary shouldered connection, however, it is soeties necessary to ocus on the strain instead o the stress, or both. is signiicant because the stresses in the critical areas o the connection (thread roots and shoulder transition region) oten exceed the yield strength o the aterial. Reerring to the stress versus strain curve in Figure 1, it is evident that once the yield strength o the aterial is exceeded, the stress changes only slightly until the ultiate strength is reached, but the strain continues to increase signiicantly. Thus, i the point strain is plastic, the strain and strain aplitude are iportant drivers that should be considered when evaluating connection perorance, not just stress.

2 2 IADC/SPE Figure 2 aplitude versus cycles to ailure or typical drill ste steel. A sall decrease in stress can net a substantial increase in atigue lie. Thereore, when aking decisions about which design to use that concern atigue, base decisions on relative atigue lie rather than relative stress or strain levels. Figure 1 versus strain curve or typical drill ste tubular steel. Ater the yield strength o the aterial has been exceeded, stress increases only a sall aount relative to the increase in strain. Fatigue Perorance and the Morrow -Lie Model Absolute values o stress and strain are oten used as easures o how uch better one design is relative to another. When only considering overload or deterining a actor o saety, this practice is acceptable in ost instances. When evaluating dierences in the atigue perorance o rotary shouldered connections, however, looking only at the agnitude o the stress or strain does not tell the whole story. An approxiation o the atigue lie should be ade to better understand the ipact that both have on the perorance o the connection. As an exaple, consider the two cases shown in Figure 2 1. The dierence in the agnitude o the stress aplitude between case 1 and case 2 is -20%, but the dierence in atigue lie between case 1 and case 2 is 110%. Relatively sall dierences in stress and strain can have a draatic ipact on atigue lie. Thereore, it is necessary to consider the atigue lie rather that just the stress or strain when coparing design options. For the analyses to be discussed herein (taking plastic strain, stress, and atigue into account), the Morrow strain-lie odel 2 is used. The Morrow strain-lie odel was chosen as it accounts or the atigue crack initiation phase, not just the crack propagation phase. This is iportant as the connection geoetry (thread design or stress relie groove width) heavily inluences atigue perorance. Using a atigue odel such as the Foran odel, which assues an initial crack size, would not capture the eects o connection geoetry. Equation 1 is the Morrow strain-lie equation used to calculate atigue lie. b ε ( ) ( )( ) c a = 2N + ε ε p 2N... (1) E Where: ε = aplitude a = Material atigue constant deterined experientally = stress (psi), when ys = E = Young s Modulus (psi) N = Nuber o cycles to ailure b = Material atigue constant deterined experientally >, set ys ε = Material atigue constant deterined experientally ε p = plastic strain, when ys c, set p ε = 0 = Material atigue constant deterined experientally A quick study o this equation shows that i the ean stress in the critical area is above the yield strength o the aterial (100 KSI in the case o the drill collars used in these studies), the variables that drive the atigue lie calculation are strain aplitude and ean plastic strain. Thereore, the odel with the highest cobination o strain aplitude and ean plastic strain will have the lowest atigue lie. I the ean stress in the critical area is less than the yield strength o the aterial, the strain aplitude and ean stress are the variables that drive the atigue lie calculation. In these cases, the odel

3 IADC/SPE with the highest cobination o strain aplitude and ean stress will have the lowest atigue lie. Fatigue lie coparison: NC56 versus 6-5/8 REG connections Regular connections have been cut on down-hole tools or any years. In 1968, the API introduced a series o nubered connections (NC), whose undaental dierences ro those that existed at the tie were the naing convention and the connections thread design. The nubers that ollow the NC designation represent the irst two nubers o the pitch diaeter o the connection. For exaple, an NC38 connection has a pitch diaeter. All NC connections were speciied with the V-0.038R thread or. Despite the act that the NC connections were touted as better atigue perorers, other connection types (ainly REG on larger collar sizes) continue to be cut on BHA coponents. Three distinct dierences between the NC56 and 6-5/8 REG connections are shown in Table 1. Table 1-NC56 and 6-5/8 REG Connection Diensions Thread Root Radius Thread Taper Pitch Diaeter NC in. 3 in./t in. 6-5/8 REG in. 2 in./t in. The diensional dierences ay appear to be relatively inconsequential. However, a drawing o the two connections brings into ocus just how dierent these connections are, and how these dierences ultiately ipact atigue perorance. Figures 3 and 4 illustrate this point. The larger root radius o the NC56 connection translates into a lower stress concentration actor, all else equal. Its 3 / taper yields ore cross sectional area in the back o the box and a saller notch when copared with the 2 / taper o the 6-5/8 REG. This is advantageous because ost o the bending the connection experiences will occur in this area. A larger cross-sectional area eans less bending or a given load, lower stresses and strains, and longer atigue lie. To quantiy the atigue advantage a NC56 connection has over a 6-5/8 REG, the coon 8 OD x 2 13/16 ID drill collar was selected. Two diensional axis-syetric odels were ade o each connection type. The API recoended MUTs (49,153 t-lbs or NC56 and 53,934 t-lbs or 6-5/8 REG) were applied to the odels along with a bending load that induced an equivalent connection curvature o 6 /100. Higher stress and strain aplitudes coupled with exposure to corrosive drilling luids dictate that atigue ailures on large drill collars will occur ore requently in or near the last engaged thread o the box as opposed to the pin. This area should be used to copare relative dierences in atigue lie. Table 2 contains a suary o the stress and strain data generated by the inite eleent analyses. Figure 4 shows in detail the resultant eect o the diensional dierences between the NC56 and 6-5/8 REG connections. Table 2-NC56 and 6-5/8 REG and Data NC56 103,755 73,471 Aplitud e 30, Aplitud e /8 123,005 80,935 42, REG Nubers in red are values critical in deterining atigue lie. Nubers with indicate the lowest value in the set (best or atigue) Figure 4 iu stress plot o the last engaged 6-5/8 REG box thread root (let) and NC56 box thread root (right). Note the distinct dierence in the agnitude o the stress concentration in the 6-5/8 REG box thread root relative to the NC56 box thread root (120 ksi (red) / 100 ksi (yellow) = 1.2). The NC56 connection clearly has uch lower stress in the last engaged box thread root. Lower stress translates into longer atigue lie, all else equal. To quantiy the relative dierence in atigue lie, the stress and strain data ro the FEA odels were used in equation 1. Since the ean stress in the critical area is less than the aterial yield stress, the strain aplitude and ean stress drive the atigue lie calculations. The resulting relative atigue lives o the boxes are shown in Figure 5. Figure 3 Diensional dierences between 6-5/8 REG and NC56 connections. Dierences in thread taper and root radius drive the dierence in box atigue lie. (The dierence in thread root radius can be seen ore clearly in igure 4.)

4 4 IADC/SPE or cases with a stress relie groove) and a bending load that induced an equivalent connection curvature o 6 /100. Table 3 contains a suary o the stress and strain data generated by the inite eleent analyses. Table 3 NC56 and Data No 3/ /4 Aplitude Aplitude 139, ,014 23, , ,157 14, , ,859 13, , ,159 18, Nubers in red are values critical in deterining atigue lie. Nubers with indicate the lowest value in the set (best or atigue) Figure 5 - Relative atigue lie. Although not absolute, the predicted relative atigue lie yields a general understanding o which connection has the best atigue perorance. The NC56 connection has superior atigue perorance copared with the 6-5/8 REG connection under the sae load conditions. Based on this analysis, the NC56 connection has a distinct atigue perorance advantage when copared with the 6-5/8 REG connection under the sae loading conditions. Fatigue lie coparison: Pin stress relie grooves (no groove, 3/4, 1, 1 1/4 ) relie eatures have been eployed on BHA connections o various types and sizes or any years. These eatures include a stress relie groove on the pin (SRG) and a boreback box (BBB), and are typically cut on NC38 and larger connections. The purposes o stress relie eatures are (1) to reove unengaged threads that act as stress concentrators near the pin shoulder and in the back o the box, and (2) to provide lexibility in the connection outside o the threaded area so bending occurs across the grooved areas and not in the threaded areas. Eploying stress relie eatures can have a draatic positive ipact on the atigue lie o BHA connections. Analyses have been conducted in the past to study the eect dierent pin stress relie groove widths have on the atigue perorance o rotary shouldered connections. Previous analyses on this topic that the authors are ailiar with have been conducted with axially applied tensile approxiations o MUT, assue linear-elastic aterial behavior, and do not include actual bending loads 3. In addition, the stress state alone was used as the eans to easure the relative dierences between groove widths. For the current analyses, actual bending loads were applied to better siulate down-hole conditions (build or drop, dogleg, or buckling) and elastic-plastic aterial behavior was recognized. The NC56 connection with a box outer diaeter o 8 and a pin inner diaeter o 2-13/16 was selected or this study. Four dierent NC56 connection odels were generated. Geoetrically, only the width o the pin stress relie groove varied between odels. The connection odels included no groove, 3/4 groove, 1 groove, and 1 1/4 inch groove. The loads applied to each odel included the API recoended MUT (49,153 t-lbs or no groove, 45,547 t-lbs The 1 groove and 3/4 groove display nearly the sae agnitudes o ean strain and strain aplitude. The odel with no groove has relatively high strain ollowed closely by the odel with the 1 1/4 groove. The strain contours resulting ro MUT and bending are shown in Figure 6. Figure 6 No groove (top let), 3/4 groove (top right), 1 groove (botto let), 1 1/4 groove. Note the axial strain is the highest (gray = plastic) in the last engaged thread roots, unengaged thread roots, and in the transition near the shoulder. Instead o using stress alone as the basis o coparison, the stresses and strains generated ro the FEA were used in equation 1 to calculate the atigue lie o each connection. The calculated atigue lives o the odels with stress relie grooves relative to the odel with no SRG are soewhat surprising. Since the ean stress in the critical regions o the odels is above the aterial yield strength, the strain aplitude and ean plastic strain drive the calculated atigue lives. Figure 7 displays the agnitude o the relative atigue lives o each o the grooved connections relative to the connection with no stress relie groove.

5 IADC/SPE API RP7G, Recoended Practice or Drill Ste Design and Operating Liits, Sixteenth Edition, Aerican Petroleu Institute, Deceber 1, APPENDIX A Figure 7 Relative atigue lie. Although not absolute, the predicted relative atigue lives yield a general understanding o which groove widths peror best. The 3/4 groove and 1 groove are clearly superior to no groove and 1 1/4 groove. As a check, a siilar analysis was conducted using a 6-5/8 REG connection with the sae outside and inside diaeters. API recoended MUT and a 6 /100 equivalent dogleg were applied to each odel. The trend and agnitude o the relative atigue lives calculated or the SRG conigurations closely atched the analyses copleted on the NC56 connections. Based on the results o these analyses, cutting stress relie grooves at 3/4 +9/32, -1/32 will yield the best atigue perorance. Mesh An exaple o the type and size o esh used is shown in Figure A1. Each odel that is evaluated is broken up into discrete pieces or eleents. The esh is the lattice that is the su total o all o the eleents. In order to accurately describe the geoetry and shape o a odel, the eleents ust be sall enough to closely approxiate the contours present in the speciic area(s) o interest. In the case o a rotary shouldered connection, the threads and the stress relie eatures are the prie areas o interest. Thereore, it is iportant to have a dense esh in these regions. Regions o the connection away ro these critical areas ay have a coarser esh as the stresses and strains here are generally not o uch interest. Suary Choose the NC56 connection over the 6-5/8 REG connection on 8 coponents or better atigue perorance. Larger area and a saller notch in the back o the NC56 box, coupled with a larger thread root radius drive the iproved atigue perorance. Cut 3/4 +9/32, -1/32 stress relie grooves on BHA connection sizes NC38 and larger. The 3/4 and 1 grooves yield siilar atigue results, but are ar superior to connections cut with no groove or a 1 1/4 groove. When aking relative coparisons between connections, it is iportant to consider strain since the aterial near the thread roots (where atigue cracks or) is plastic due to MUT and externally applied loads. Use a atigue lie odel to ake coparisons, as just coparing the agnitude o stress and strain does not accurately represent the dierence in atigue perorance between connections. Reerences 1. Rollins, H.M., Drill Pipe Fatigue Failure, Oil and Gas Journal, Dowling, Noran E., Mechanical Behavior o Materials: Engineering Methods or Deoration, Fracture, and Fatigue. Prentice Hall, 1993, pp Figure A1-6-5/8 REG axis syetric connection odel. Saller eleents are necessary in the regions o interest, while coarser esh is allowable in non-critical areas. Application o MUT Applying torque to a coputer generated odel o a rotary shouldered connection is uch ore diicult than loading an actual connection into an iron roughneck and pressing the GO button. Successully odeling a three-diensional connection (getting a convergent solution) can be a daunting task because o the aount o sliding contact between the thread lanks, the helical nature o the thread path, and the boundary conditions. As a result, rotary shouldered connections are ost easily odeled in two diensions. Two diensional axis-syetric odels with asyetric

6 6 IADC/SPE (bending) loads ust be created. MUT is approxiated by either assuing a certain aount o thread and shoulder overlap, or by applying a theral gradient (to expand the aterial thereby creating thread lank and shoulder intererence) to the pin and/or box. The bulk stresses induced ro applying MUT to an actual connection are known or calculable in three key areas (Figure A2). Using these known or calculable values, correctly approxiating the MUT on the two diensional connection odel can be achieved by adjusting the overlap or theral layer teperature, and coparing the resultant stresses to those deined to be present in an actual connection o the sae type and diensions. Figure A2 6-5/8 REG 8 x 2-13/16 rotary shouldered connection critical stress areas: Seal area, 3/4 ro shoulder in pin, 3/8 ro shoulder in box. Bending Load Axial loads and pressure are generally siple to apply. Applying a bending load to a two diensional axis-syetric asyetric odel generally requires the use o a special eleent type. When odeling bending in a rotary shouldered connection, it is iportant to bend the connection ultiple ties in order to generate the best approxiation o the stress state at the thread roots. During connection ake up, the last ew engaged pin threads, any reaining unengaged pin threads, and soe areas o a stress relie groove (i present) are usually plastically deored. Since the stresses are plastic, bending the connection once will not give accurate stress and strain values. The connection ust be bent ultiple ties to allow or the eects o stress relieving and stress redistribution. In an actual connection ade up and run down hole, this will occur once the connection is rotated ore than one revolution while bent or buckled.