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Analysis of causes of casing elevator fractureLuo Faqiana, Lu Shuanlua,b,*, Li Helinc, Qin Changyic, Zhou Jiea,Tang Jipinga, Chi JunaaTarim Oil Field, Korla, Xinjiang 841000, ChinabChina University of Petroleum, Changping, Beijing 102249, ChinacTubular Goods Research Center of CNPC, Xian, Shaanxi 710065, ChinaReceived 8 April 2005; accepted 12 March 2006Available online 7 September 2006AbstractA fracture accident occurred with a 244.5 mm350 ton casing elevator and a traveling hook during casing runningdown. This paper gives an investigation of this accident, and analyzes the causes based on fracture surface examinationand material tests. Some simulation tests are performed in order to validate the fracture mechanism. It is concluded thatthe casing elevator fracture originated from quenching cracks caused by surface carburization. Calculations of the fractureload acting on the casing elevator showed that the casing elevator broke first, resulting in hook fracture.? 2006 Elsevier Ltd. All rights reserved.Keywords: Casing elevator; Fracture; Quenching crack; Surface carburization1. BackgroundThe 244.5 mm casing was running down as the well depth reached 3702 m. The weight of the casing stringwas 215 ton with the 331 lengths of casing. The upper casing elevator and hook broke as the lift load of thecasing elevator reached 170 ton after the lower casing elevator was taken apart from the well head for runningdown a new length of casing. The casing elevator broke at the left hanging ear position, and the hook broke upinto many pieces 1. The right link fell down on the front of the slope road, and the left link was (left) hangingon the neck of the hook by a length of 12.5 mm diameter steel wire rope. This accident caused a delay of thewell completion and was submitted for failure analysis at TGRC.2. Macrofractography and fractography2.1. MacrofractographyThe casing elevator broke at the left hanging ear position, and the fracture morphology is shown in Figs. 1and 2. Area A shows the original cracks with a depth of 65 mm, taking about 1/3 of the total fracture. It pre-1350-6307/$ - see front matter ? 2006 Elsevier Ltd. All rights reserved.doi:10.1016/j.engfailanal.2006.03.012*Corresponding author.E-mail address: lusl (L. Shuanlu)./locate/engfailanalEngineering Failure Analysis 14 (2007) 606613sents a yellow-gray appearance and shows corrosion at the edge and then a tempered color after cleaning.There was an obvious radiating band that contracted to the right lower corner of the fracture from wherethe crack originated (Fig. 1). Area B was severely corroded and produced some original cracks which hadnot extended. Area C was the extension from Area A and was the final fracture presenting a rougher fracturesurface than that of Area A. The angle between A and C was about 30?. At the lower edge of the fracture theplastic deformation area was 40 mm in length and 10 mm in width. Many cracks were found on the left hang-ing ear surface of the casing elevator (Fig. 3) by NDT. The cracks were mostly concentrated on the bevel posi-tion at the right lower corner of the fracture.The fracture features showed that Areas A and B were original cracks, and the original crack in Area Aextended first, causing the fracture.Fig. 1. Fracture morphology and position of the casing elevator.Fig. 2. Fracture appearance.L. Faqian et al. / Engineering Failure Analysis 14 (2007) 6066136072.2. FractographyA big difference appeared in micro-morphology between Areas A and C (Fig. 4) in the scanning electronmicroscope. The micro-morphology close to the crack origin of Area A was intergranular interface + cleav-Fig. 4. The micro-morphology of the boundary between Areas A and C.Fig. 5. The micro-morphology of the extension of Area A.Fig. 3. Cracks on surface at right lower corner of the fracture.608L. Faqian et al. / Engineering Failure Analysis 14 (2007) 606613age + secondary cracks, and the micro-morphology was mainly intergranular at the right lower corner of AreaA. The intergranular micro-morphology decreased with distance from the original crack. The micro-morphol-ogy far away from the crack origin in Area A was intergranular + cleavage + secondary cracks (Fig. 5). Themicro-morphology in Area C was cleavage + dimples (Fig. 6).3. MetallographyIt was found that the original cracks extended along an intergranular interface, and there were intergran-ular cracks near to the fracture origin. The cracks presented a gray colour, and the surrounding microstructureof the cracks at the fracture was the same as in other areas (Fig. 7). The crack features were indicated to bequenching cracks according to metallography, macrofractography and fractography.There was a carburized layer 0.44 mm in depth on the casing elevator hanging ear surface. The carburizedlayer was very clear (Fig. 8) after treatment in a vacuum furnace at 900 ?C. The hardness distribution in thecarburized layer is shown in Fig. 9.The cracks on the surface at the right lower corner of the fracture also extended along intergranular inter-faces, and the crack depth corresponded with that of the carburized layer (Fig. 10).It is obvious that many cracks were generated on the elevator surface due to carburizing and quenching,especially at the edge of the hanging ear. The original cracks on the fracture were actually part of thesequenching cracks.Fig. 6. The micro-morphology of Area C.Fig. 7. The crack and micro-structure near the edge of Area A 400.L. Faqian et al. / Engineering Failure Analysis 14 (2007) 6066136094. Material tests4.1. CompositionThe material of the casing elevator was 20SiMn2MoVA.4.2. Mechanical propertiesThe mechanical properties are shown in Table 1.Fig. 8. The micro-structure of the carburized layer 125.Fig. 9. Hardness distribution in carburized layer.Fig. 10. The micro-morphology of cracks on surface and micro-structure at right lower corner of the fracture 125.610L. Faqian et al. / Engineering Failure Analysis 14 (2007) 606613The surface hardness of the carburized layer was 60.060.5 HR45N, equal to 54.555.0 HRC. Section hard-ness was 44.546.5 HRC. The former is 10 HRC higher than the latter.5. Simulation testsIn order to validate the nature of the fracture, some supplementary simulation tests were carried out in thelaboratory on specimens from the broken casing elevator.5.1. Temper testThe temper test for the CVN specimen was done for 5 h at 230 ?C according to the temper specification ofthe casing elevator, and it was found that the CVN specimen was grass yellow which was similar to that onArea A of the casing elevator fracture.5.2. Fracture section oxygen atom analysisFracture Auger spectral energy meter and PHI-600 scanning spectral energy meter analysis were carried outon Areas A and C of the fracture and the CVN specimen tempered at 230 ?C. Oxygen atom concentration atArea A of the fracture was 60%, and oxygen atom concentration was 53% on the CVN specimen tempered at230 ?C. The oxygen atom concentration at Area C of the fracture was only 26%.5.3. Oxide structure analysisPer analyzing by PHI-540 X-ray electron energy chart instrument, the oxide on Area A of the fracture andthe CVN specimen fracture was Fe2O3, and the oxide on Area C was Fe2O4.The simulation test results indicated that the oxide and its structure on Area A of the fracture were similarto that of the CVN specimen fracture tempered at 230 ?C. The result above could confirm that Area A of thefracture was generated after quenching and before tempering.6. Discussion6.1. Fracture accident process analysisA further analysis of the accident sequence gives us more understanding of this accident.During the lifting process, the hook carried the load of the casing elevator and the string through two linksthat were connected with the left and right hanging ears, and the hook with the 12.5 mm diameter wire ropes(Fig. 11). The assumptions as well as the possibilities about fracture processes are as follows:6.1.1. Supposing two hook ears broke firstIf the hook ears broke first during the lifting process, then the wire ropes that were linked with the neck ofthe hook would have also broken because the strength of the 12.5 mm wire ropes were weaker than that of thecasing elevator ears. The casing elevator ears were not tensioned by the links in this case. So it would not havebroken. Thus, this assumption is not consistent with the facts, and is not correct.6.1.2. Supposing right hook ear broke firstThere would have been only one link to load the casing elevator if the right hook ear broke first. The frac-ture would take place from the weak position in this case. If the strength of the left hook ear was weaker thanTable 1Mechanical propertiesItemYield strength (MPa)Tension strength (MPa)Elongation (%)Charpy absorbed energy (J)Mechanical property1144143315.531L. Faqian et al. / Engineering Failure Analysis 14 (2007) 606613611that of the left casing elevator ear, then the left hook ear would have broken first, and it was vice versa. It wasimpossible that both of them broke at the same time in this assumption. In fact, both the casing elevator andhook actually broke in the accident. So this assumption is not correct.6.1.3. Supposing left hook ear broke firstIf the left hook ear broke first, then the left casing elevator ear would not have broken because it was notloaded by the hook in this situation. So this assumption is impossible.6.1.4. Supposing left casing elevator ear broke firstThe right single link load would act on the casing elevator if the left casing elevator ear had brokenfirst, and then the hook was forced to break up. The original cracks on the left casing elevator ear wouldextend quickly to fracture when the elevator suffered over load. The left link was still suspended on theneck of the hook through the 12.5 mm wire rope because it was not over loaded after the left casing ele-vator ear broke.The hook broke due to its strength, which was weaker than that of the right casing elevator ear suspendedfrom the right single link. The wire rope hanging the right link was tensioned to failure as the traveling blockrose continuously after the hook broke, and then the right link fell down to the front of the slope way. Thisprocess is well matched with the actual conditions. In addition, there was a plastic deformation area hit by theleft link at the lower edge of the casing elevator fracture. It indicated that the hook was not broken when theleft casing elevator ear broke.6.2. Casing elevator break load analysisThe casing elevator broke as the load was up to 170 ton during lifting, and the left and right casing elevatorear would bear 85 ton load, respectively, in this situation. 107.2 ton load would be laid on the left and rightcasing elevator ear, respectively, according to the total load of 214.4 ton as 331st piece of casing was run. Theactual fracture load acting on the left and right casing elevator ears should be more than 107.2 ton at leastinstead of 85 ton, resulting in fracture, according to the above. Extra tension, or impact load worked onthe casing elevator during fracture. The load acting on the casing elevator will be increased because of liftingacceleration as the casing string was lifted by the upper casing elevator 2.Fig. 11. Installation of casing elevator and links and hook.612L. Faqian et al. / Engineering Failure Analysis 14 (2007) 6066136.3. Effect of original cracks on load capability of the casing elevatorThe weaker position of the casing elevator is at its ear circumference edge connected with the link duringstring lifting, and the fracture position is just at the critical section. The left casing elevator ear would have notbroken even though 170 ton was acting it if there were no original cracks on it according to the casing elevatorstrength 3. The original crack area is about 1/3 of the critical area, but the fracture load is only 1/2 of therated load. It indicated that the cracks in the critical section not only reduce the area of loading, but also causesevere stress concentration that makes its carrying capacity decrease.6.4. Analysis on causes of quenching cracksThere were severe quenching cracks on the casing elevator before the fracture accident. The quenchingcracks were due to carburizing in the surface layer of the casing elevator. The cause of carburizing is thatthe carbon content of carbon potential was too high in the furnace during heating for quenching. The surfacelayer material of the casing elevator is determined as high carbon alloy steel with Si, Mn, Mo, and V aftercarburizing, and the quenching heat temperature in carburizing layer should be much lower than that of20SiMn2MoVA steel. The casing elevator is desi
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