Engineering Failure Analysis 79 (2017) 1–7

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Engineering Failure Analysis

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A comprehensive analysis on the longitudinal fracture in thetool joints of drill pipes

Wang Xinhu ⁎, Li Fangpo, Liu Yonggang, Feng Yaorong, Zhu LijuanTubular Goods Research Institute, China National Petroleum Corporation (CNPC), State Key Laboratory for Performance and Structural Safety of Petroleum Tubular Goods andEquipment Materials, No. 89 Jinyeer Road, Xi'an 710077, Shanxi, PR China

a r t i c l e i n f o

⁎ Corresponding author.E-mail address: wangxinhu002@cnpc.com.cn (X. W

http://dx.doi.org/10.1016/j.engfailanal.2017.03.0191350-6307/© 2017 Published by Elsevier Ltd.

a b s t r a c t

Article history:Received 23 February 2016Received in revised form 28 November 2016Accepted 24 March 2017Available online 25 March 2017

The Longitudinal fractures or splits in tool joint box of drill pipe often occur, because the num-bers of deep, directional, extended and horizontal oil wells are increasing. The mechanism offrictional heat check cracking of drill pipe tool joints was discussed in the standard API RP7G. However, authors have identified that heat check cracking was just one of the cracking ini-tiation mechanism, not crack propagation mechanism. This paper has reviewed 21 cases of thiskind of failure analyzed by authors from year 2000 to 2015. Fracture surfaces and mechanicalproperties have been examined in this paper. It was found that there were various other causesof crack initiation, in addition to frictional heat check cracking, such as tong tooth bite marksand friction damage in internal threads. Such cracks propagated mostly via stress corrosioncracking (SCC) mechanism, although two cases was brittle cracking due to poor materialtoughness. The stress corrosion mechanism was related to hydrogen sulfide (H2S) or ionic sul-fur (S2−), which came from the degradation of applied thread greases or drilling fluid ingredi-ents. Although heat check cracks and tooth bite marks as crack initiation are common-place,the failure can be prevented through prevention of crack propagation. Failure data showedthat although improved transverse material absorbed energy avoided longitudinal brittle crack-ing of the tool joint, stress corrosion cracking still occurred. The statistical analysis resultsshowed that material hardness was related to the longitudinal cracking of drill pipe tool joints.It was demonstrated that if material hardness was restricted to less than HB310, the crack didnot propagate. It was suggested that the material hardness of tool joints box should be revisedto HB285–HB310, in addition that the transverse material Charpy absorbed energy of the tooljoint should be specified.

© 2017 Published by Elsevier Ltd.

Keyword:Drill pipeTool jointCrackFracturePrevention

1. Introduction

Drill pipe is one kind of important tools for drilling in oil and gas field. The drill pipe's failure often occurs in the oil and gasfield because drill pipe bears continuously changeable tension load, bend load, torsion load, impact load, internal pressure and cat-astrophic downhole corrosion. Although the wash out caused by fatigue is most popular type of drill pipe failure [1–3], the lon-gitudinal fracture or split of the tool joint box is also one of main failure type of drill pipe [3,4]. The standard API RP 7G [5]explains that friction heating and drilling liquid quenching frequently resulted in the cracking through the tool joint box of

ang).

Table 1The chemical composition of 35CrMnMo steel (wt%).

Elements C Si Cr Mn Mo P S

Content 0.32–0.38 0.15–0.35 0.90–1.20 0.85–1.00 0.28–0.35 ≤0.015 ≤0.008

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drill pipe, and controlling hole angle and lateral force is able to minimize or eliminate the longitudinal fracture of the joint boxtool. However, the lateral force on the drill pipe is inevitable during drilling operation. Sometimes, although the lateral forcewas below the load capacity of the tool joints, the split of tool joints box still occurred. There are growing interests in preventingthis kind of failure, and one effective way is to enhance the transversal Charpy absorbed energy of the tool joint box. Xinhu Wangproposed the transversal Charpy absorbed energy requirement [6], which had been accepted by Chinese standard SY/T 5561 [7].However, the longitudinal fracture of tool joints still occurred occasionally. Therefore, this paper has analyzed 21 longitudinalcracking failures of the tool joint box in detail to find the failure mechanism, and prevention measures.

2. Analysis methods and materials

The 21 failure cases in this paper come from the cases analyzed by us since year 2000. Because our research institute (TubularGoods Research Institute of China National Petroleum Corporation) is the Oil Tubular Goods Failure Analysis Center of Chinese Pe-troleum Society, many failure analyses were finished every year. The failure analysis reports were submitted to the clients, andkept in the data base. There are 21 longitudinal cracking failure cases of the tool joint box in this data base. All informationsuch as the fracture analysis results, the metallographic analysis results, the material Charpy absorbed energy testing results, hard-ness testing results, and failure happen time in this paper were from the failure reports in this data base.

Tool joint material is mainly 35CrMnMo steel in China. Table 1 shows the steel chemical composition.

3. The crack initiation

Among the 21 failures, there were three kind of cracking initiation, which caused the split of tool joint box. The first one is thefriction heat cracks on outside surface of tool joint box. The second one is mechanical damage on outside surface of tool joint box.In addition, the third one is the inside thread teeth damaged.

It was realized that the friction heat cracking was main cause which results in longitudinal cracking through tool joint box inthe standard API RP 7G [5]. A typical appearance of the longitudinal heat checking through tool joint is shown in Fig. 1. The tooljoint was worn gleamed, and there were many friction heat cracks on the outside surface. Drill pipe tool joints bore high lateralforce and large friction load due to the serious friction in deep well and directional well. In addition, the friction energy could betransformed into heat energy. Once the heated tool joints suffer fast cooling process in drilling liquid, brittle martensite phasewould form on the surface of the tool joints. Cracks in the martensite structure could be initiated during thermal cycles in service.

The mechanical damage on the outer surface is another cracking initiation that results in longitudinal fracture through the tooljoint box of drill pipe. Among the 21 longitudinal fracture cases analyzed by the authors, five failures were induced by the frictionheat cracking, and six failures were induced by the mechanical damage on outside surface. The tong tooth bite marks was onetype of the mechanical damage. Fig. 2(a) shows many bite marks and a long longitudinal crack. Fig. 2(b) shows another tooljoint box, which had many tool bite marks, and fractured at one of bite marks. Individual drill pipe must be connected intodrill string by tong in order to drilling well. Tong is the tool of tightening the screws of tool joint (making up tool joint), theclamping part of the tong have many teeth. When holding and making up or breakout the tool joint by a set tong, its teethwill bite the outer surface of tool joint, then the bite marks formed on outer surface of the tool joint box. Due to the effect ofstress concentration, cracks initiated from the location of bite marks.

Fig. 1. The friction heat crack appearance of the box tool joint.

Fig. 2. The fracture appearance of the tool joints and the tong tooth bite marks on its outer surface. (a) Tooth bite marks and longitudinal crack on tool joint box.(b) Tooth bite marks and fracture at one of bite marks.

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Among the 21 failure cases analyzed by the authors from 2000 to 2015, 10 failure cases were induced by the cracks initiatedfrom inside thread teeth damaged. As the example displayed in Fig. 3, the chevron marks on the fracture point back to the threadteeth (the arrows point to the crack origin in Fig. 3(b)) and the crack extends toward outer surface of tool joint. Fig. 3(c) is thefracture of the tooth pointed by arrows in Fig. 3(b), the tooth was crack initiation location, and its crest was damaged. Fig.

Fig. 3. The crack at the inside surface of tool joint and its cracking initiation. (a) The crack appearance. (b) The chevron marks point back to crack initiation. (c) Thecracking initiation area. (d) The cracked white martensite in the surface of thread crest. (e) The crack in the martensite structure propagating into thread basestructure.

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3(d) and (e) are fractographic pictures of the crack origin and its neighboring location in Fig. 3(b) and (c), cracked white martens-ite on the surface was fracture initiation. A large crack propagated through the wall thickness as displayed in Fig. 3(e), the crack isparallel with the fracture in Fig. 3(b) and (c). Because friction between pin and box while making up and breaking out of the tooljoint, heating and cooling happened at the surface of threads, and a layer of martensite formed in the surface material of threads.Plenty of cracks formed in brittle martensite as displayed in Fig. 3(d). Some cracks go into the material structure of wall thicknessand propagating to outer surface of tool joint because maximum local stresses is near the root of thread teeth as displayed in Fig.3(e).

4. Failure mechanism

The friction between the tool joints of drill pipe and the wall of well hole is inevitable, especially in the directional wells, ex-tended wells and horizontal wells. The scratches and manufacturing defects, and tong tooth bit marks at the outer surface of thetool joints are the potential initiation of cracks. The thread teeth are a potential initiation area of cracks due to friction-inducedmartensite. Although it is hard to avoid these crack initiation such as heat check cracking, the split failure of tool joints can beeliminated if the further propagation of the crack can be stopped.

The mechanism of the friction heat cracking in the tool joint of drill pipe was discussed in the standard API RP 7G. Authorsrealized that heat checking was just one of cracking initiation mechanism, but not the mechanism of cracks propagation. Chevronmarks were always observed on the fracture surface as displayed in Fig. 3 among the 21 failure cases, and these fracture mech-anism was brittle fracture. It was well known that higher fracture toughness could prevent the brittle cracking. In order to im-prove tool joints' reliability, the longitudinal material Charpy absorb energy requirement of tool joint had be put forward inAPI SPEC 5DP [8]. However, tool joint's longitudinal mechanical property is different from its transversal mechanical propertydue to steel rolling. Longitudinal Charpy sample's fracture surface is perpendicular to longitudinal axis of tool joint while Trans-versal Charpy sample's fracture surface is parallel with longitudinal axis of tool joint, higher longitudinal toughness has little effecton preventing the longitudinal cracking of tool joint box. Because longitudinal fracture also is mainly parallel with longitudinalaxis of tool joint, higher transversal Charpy absorbed energy is more helpful for preventing longitudinal cracking of the tooljoint box. Effective way is to enhance the transversal Charpy absorb energy of the tool joint box. Xinhu Wang put forward thetransversal toughness criterion [6], which had been accepted by Chinese petroleum standard of SY/T 5561 [7]. The SY/T 5561 isthe only standard that introduced the transversal Charpy absorb energy requirement in the world at present. The Charpy absorbenergy of the 21 failure tool joint box was shown in Fig. 4, and the serial number of the tool joints was in sequence with the fail-ure occurrence date. The size of Charpy impact absorbed energy testing samples were 10 mm × 10 mm × 55 mm. It was indicat-ed that the transversal toughness of the tool joints has been improved significantly since the year 2008.

As shown in Fig. 4, the longitudinal Charpy absorbed energy of the No.1 tool joint box was only 48 J. The fracture of the No.1was caused by material brittleness. Unfortunately, the transversal Charpy energy was not tested for first five tool joints. Althoughlongitudinal toughness criterion of the tool joint was introduced into the API SPEC 5DP, the transversal Charpy absorbed energy oftool joints box was still quite low. For example, the transversal Charpy absorbed energy of the No.11 tool joint was only 40 J whileits longitudinal Charpy absorbed energy was 90 J. Therefore, the transversal Charpy absorbed energy requirement was specified inSY/T 5561 in that the requirement of minimum average absorbed energy for a set of three transverse specimens was 60 J at

Fig. 4. The Charpy absorbed energy of 21 cracked tool joint box.

Fig. 5. The longitudinal fracture appearance of the tool joint.

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−20 °C, and minimum absorbed energy for one of three transverse specimens was 50 J at −20 °C. The transversal Charpyabsorbed energy of the box was greatly improved since then. This is an effective measurement preventing from the brittlenesscracking of the tool joint box. Since then, the fracture numbers of tool joint box decreased obviously.

However, the longitudinal cracking through tool joint box still occurred occasionally, even when the transversal materialCharpy absorbed energy of tool joints exceeded 70 J at −20 °C (Fig. 4). Therefore, it was impossible to eliminate the fractureof tool joints only by improving the material toughness.

Among the 21 failure cases, the fracture features of most tool joints were observed by scanning electron microscopy (SEM)with energy dispersive X-ray spectrometer (EDX). It indicated that most fractures were intergranular, the typical feature wasshowed in Fig. 5. This was in accordance with metallographic analysis results, typical intergranular cracks were showed in Fig.6. The transversal Charpy absorbed energy of the tool joints was high enough to prevent it from brittle cracking except forNo.1 and No.11 tool joints. The transversal Charpy absorbed energy of some tool joints even exceeded 70 J. Therefore, it was re-alized that the stress corrosion crack was the main factor, which induced cracking in the tool joint box with high transversalCharpy absorbed energy. Some tool joints of drill pipe suffered catastrophic corrosion, and it is difficult to distinguish the crackingfeatures.

The corrosion products on the fracture surface of tool joints were analyzed by EDS, and the result was showed in Fig. 7. Theelemental sulfur was detected on cracking initiation area of the each fracture sample, such as crack in bite mark, crack in threadroots, heat cracks. Elemental sulfur in the corrosion products usually comes from the hydrogen sulfide in oil or gas, or degradationof drilling fluid additives and thread compound additives [9]. Therefore, the longitudinal cracking in tool joints was probably in-duced by the sulfide stress corrosion crack [9–11].

Fig. 6. Optical micrograph of stress corrosion cracks in the tool joint.

Fig. 7. EDX analysis result of corrosion products on the fracture of tool joint.

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5. Measures to failure prevention

As discussed in Sections 3 and 4, it was very difficult to eliminate the cracks origin such as heat checking, tong tooth bitemarks and thread friction damage. The effective method of avoiding the longitudinal fracture through tool joints is to preventthe cracks from propagation. The crack propagation mechanism of the tool joints could be categorized into two types as analysesabove. One type was brittle fracture, the other was sulfide stress corrosion cracking. Enough material toughness could prevent thetool joints from brittle fracture, and improve the sulfide stress corrosion cracking resistance to some extent. However, it was im-possible to eliminate the stress corrosion cracking in the tool joints only by improving the toughness.

To induce the stress corrosion crack in drill pipe tool joints requires three factors including stress corrosion sensibility of ma-terial, corrosive medium such as H2S and tensile stress [9]. The complicate and rigorous service conditions promote the develop-ment of high-strength steel drill pipe. However, the tool joint material which yield strength is 120ksi is one kind of sulfide stresscorrosion cracking sensitive material [9–11]. Moreover, the stress corrosion of high-strength steels in the H2S environment was anold problem in the oil field and results in big economic loss [9]. Thus, it was significant to decrease the stress corrosion crackingsusceptibility of tool joints by optimizing material quality.

Many investigations have been conducted on improving the stress corrosion crack resistance of tool joint. The stress corrosioncrack resistance of the tool joints depends on the chemical composition, microstructure and mechanical properties of the mate-rials. There are three measures to improve the stress corrosion resistance of tool joints [9]: (1) reduce the impurity elements con-tents, especially the sulfur element content. The metallurgy technology has been significantly improved in most of the steel millsin China, the steel purity has reached the advanced world level. The chemical composition and impurity contents are conform tothe standard requirements. (2) Maintain the homogeneous distribution of the tempered sorbite, which is the optimum micro-structure. The grain sizes of the steel have exceeded a magnitude 8 in most steel mills in China. However, the control of compo-sition segregation and banded structure is an old problem, which is a main gap between the iron and steel industry in China andabroad. (3) Improve the material toughness of the tool joints. The drill pipe failure assessment demonstrates that the brittle splitcan be avoided when the transverse Charpy absorbed energy of the material is over 60 J. The transversal Charpy absorbed energyfor the tool joint box has already come up to 60 J at the current levels.

In fact, it is a tough work to evaluate the sulfide stress corrosion cracking resistance of material from the above three aspectsin the oil field. Therefore, the hardness was put forward to be a comprehensive index to measure the sulfide stress corrosioncracking resistance of material. The lower is the hardness, the better is the sulfide stress corrosion cracking resistance of thesteel material [9]. Generally, the steel material which hardness is lower than HRC22 does not suffer sulfide stress corrosion crack-ing [9–11]. The hardness of the tool joints is HB285–341(about HRC29.8–33.3) in API SPEC 5DP and SY/T5561, that is much higherthan HRC22. Therefore, the tool joint is prone to suffer stress corrosion crack. It is impossible to decrease the hardness of the tooljoints to HRC22. However, higher stress corrosion crack resistance can be obtained by comprehensive materials quality optimiza-tion even when the hardness of the tool joint is above 22HRC.

The hardness of sixteen cracked box tool joints was shown in Fig. 8. All of them are above HB 310, and five of them are aboveHB 340. Few tool joints with the hardness lower than HB 310 suffered stress corrosion cracking. Therefore, HB310 is probably crit-ical hardness for the tool joints of drill pipe with good stress corrosion cracking resistance. Authors suggest that the hardness oftool joints is below HB310 should be the demand of good stress corrosion cracking resistance, and the hardness of tool jointsshould be HB285–HB310.

6. Conclusions

API RP 7G realizes that the cause of longitudinal fracture in the tool joints box of drill pipe is friction heat checking. This paperanalyzed 21 longitudinal fractures in tool joints box in detail. The results show that the friction heat check cracking, tong toothbite marks, and friction damage thread teeth are the potential cracking initiation of tool joints box. Heat check cracking is justone of the crack initiation mechanisms, not the mechanism of the crack propagation. The propagation mechanism of crack is

Fig. 8. Hardness of the longitudinal cracked tool joints.

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mainly stress corrosion cracking, and sulfide is one of the mainly corrosive mediums. This paper suggests that the material hard-ness below HB310 is good measurement for preventing the split through the tool joints box of drill pipe. It is suggested that thematerial hardness of drill pipe tool joints should be revised to HB285–HB310 in the API SPEC 5DP and SY/T5561, in addition thatthe transverse material Charpy absorbed energy of the tool joint box should be required.

Acknowledgements

National Science and Technology Projects of China sponsor this work under Grant No. 2011ZX05021-002.

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