Analysis of causes of casing elevator fracture.pdf

Analysis of causes of casing elevator fracture Luo Faqian a, Lu Shuanlua,b,*, Li Helinc, Qin Changyic, Zhou Jiea, Tang Jiping a, Chi Juna a Tarim Oil Field, Korla, Xinjiang 841000, China b China University of Petroleum, Changping, Beijing 102249, China c Tubular Goods Research Center of CNPC, Xian, Shaanxi 710065, China Received 8 April 2005; accepted 12 March 2006 Available online 7 September 2006 Abstract A fracture accident occurred with a 244.5 mm350 ton casing elevator and a traveling hook during casing running down. This paper gives an investigation of this accident, and analyzes the causes based on fracture surface examination and material tests. Some simulation tests are performed in order to validate the fracture mechanism. It is concluded that the casing elevator fracture originated from quenching cracks caused by surface carburization. Calculations of the fracture load acting on the casing elevator showed that the casing elevator broke fi rst, resulting in hook fracture. ? 2006 Elsevier Ltd. All rights reserved. Keywords: Casing elevator; Fracture; Quenching crack; Surface carburization 1. Background The 244.5 mm casing was running down as the well depth reached 3702 m. The weight of the casing string was 215 ton with the 331 lengths of casing. The upper casing elevator and hook broke as the lift load of the casing elevator reached 170 ton after the lower casing elevator was taken apart from the well head for running down a new length of casing. The casing elevator broke at the left hanging ear position, and the hook broke up into many pieces 1. The right link fell down on the front of the slope road, and the left link was (left) hanging on the neck of the hook by a length of 12.5 mm diameter steel wire rope. This accident caused a delay of the well completion and was submitted for failure analysis at TGRC. 2. Macrofractography and fractography 2.1. Macrofractography The casing elevator broke at the left hanging ear position, and the fracture morphology is shown in Figs. 1 and 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: (L. Shuanlu). Engineering Failure Analysis 14 (2007) 606613 sents 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 where the crack originated (Fig. 1). Area B was severely corroded and produced some original cracks which had not extended. Area C was the extension from Area A and was the fi nal fracture presenting a rougher fracture surface than that of Area A. The angle between A and C was about 30?. At the lower edge of the fracture the plastic 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 A extended fi rst, 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) 606613607 2.2. Fractography A big diff erence appeared in micro-morphology between Areas A and C (Fig. 4) in the scanning electron microscope. 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) 606613 age + secondary cracks, and the micro-morphology was mainly intergranular at the right lower corner of Area A. 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). The micro-morphology in Area C was cleavage + dimples (Fig. 6). 3. Metallography It 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 microstructure of the cracks at the fracture was the same as in other areas (Fig. 7). The crack features were indicated to be quenching 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 carburized layer was very clear (Fig. 8) after treatment in a vacuum furnace at 900 ?C. The hardness distribution in the carburized 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 these quenching 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) 606613609 4. Material tests 4.1. Composition The material of the casing elevator was 20SiMn2MoVA. 4.2. Mechanical properties The 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) 606613 The 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 tests In order to validate the nature of the fracture, some supplementary simulation tests were carried out in the laboratory on specimens from the broken casing elevator. 5.1. Temper test The temper test for the CVN specimen was done for 5 h at 230 ?C according to the temper specifi cation of the casing elevator, and it was found that the CVN specimen was grass yellow which was similar to that on Area A of the casing elevator fracture. 5.2. Fracture section oxygen atom analysis Fracture Auger spectral energy meter and PHI-600 scanning spectral energy meter analysis were carried out on Areas A and C of the fracture and the CVN specimen tempered at 230 ?C. Oxygen atom concentration at Area A of the fracture was 60%, and oxygen atom concentration was 53% on the CVN specimen tempered at 230 ?C. The oxygen atom concentration at Area C of the fracture was only 26%. 5.3. Oxide structure analysis Per analyzing by PHI-540 X-ray electron energy chart instrument, the oxide on Area A of the fracture and the 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 similar to that of the CVN specimen fracture tempered at 230 ?C. The result above could confi rm that Area A of the fracture was generated after quenching and before tempering. 6. Discussion 6.1. Fracture accident process analysis A 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 links that 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 fi rst If the hook ears broke fi rst during the lifting process, then the wire ropes that were linked with the neck of the hook would have also broken because the strength of the 12.5 mm wire ropes were weaker than that of the casing elevator ears. The casing elevator ears were not tensioned by the links in this case. So it would not have broken. Thus, this assumption is not consistent with the facts, and is not correct. 6.1.2. Supposing right hook ear broke fi rst There would have been only one link to load the casing elevator if the right hook ear broke fi rst. The frac- ture would take place from the weak position in this case. If the strength of the left hook ear was weaker than Table 1 Mechanical properties ItemYield strength (MPa)Tension strength (MPa)Elongation (%)Charpy absorbed energy (J) Mechanical property1144143315.531 L. Faqian et al. / Engineering Failure Analysis 14 (2007) 606613611 that of the left casing elevator ear, then the left hook ear would have broken fi rst, and it was vice versa. It was impossible that both of them broke at the same time in this assumption. In fact, both the casing elevator and hook actually broke in the accident. So this assumption is not correct. 6.1.3. Supposing left hook ear broke fi rst If the left hook ear broke fi rst, then the left casing elevator ear would not have broken because it was not loaded by the hook in this situation. So this assumption is impossible. 6.1.4. Supposing left casing elevator ear broke fi rst The right single link load would act on the casing elevator if the left casing elevator ear had broken fi rst, and then the hook was forced to break up. The original cracks on the left casing elevator ear would extend quickly to fracture when the elevator suff ered over load. The left link was still suspended on the neck 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 suspended from the right single link. The wire rope hanging the right link was tensioned to failure as the traveling block rose continuously after the hook broke, and then the right link fell down to the front of the slope way. This process is well matched with the actual conditions. In addition, there was a plastic deformation area hit by the left link at the lower edge of the casing elevator fracture. It indicated that the hook was not broken when the left casing elevator ear broke. 6.2. Casing elevator break load analysis The casing elevator broke as the load was up to 170 ton during lifting, and the left and right casing elevator ear would bear 85 ton load, respectively, in this situation. 107.2 ton load would be laid on the left and right casing elevator ear, respectively, according to the total load of 214.4 ton as 331st piece of casing was run. The actual fracture load acting on the left and right casing elevator ears should be more than 107.2 ton at least instead of 85 ton, resulting in fracture, according to the above. Extra tension, or impact load worked on the casing elevator during fracture. The load acting on the casing elevator will be increased because of lifting acceleration 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) 606613 6.3. Eff ect of original cracks on load capability of the casing elevator The weaker position of the casing elevator is at its ear circumference edge connected with the link during string lifting, and the fracture position is just at the critical section. The left casing elevator ear would have not broken even though 170 ton was acting it if there were no original cracks on it according to the casing elevator strength 3. The original crack area is about 1/3 of the critical area, but the fracture load is only 1/2 of the rated load. It indicated that the cracks in the critical section not only reduce the area of loading, but also cause severe stress concentration that makes its carrying capacity decrease. 6.4. Analysis on causes of quenching cracks There were severe quenching cracks on the casing elevator before the fracture accident. The quenching cracks were due to carburizing in the surface layer of the casing elevator. The cause of carburizing is that the carbon content of carbon potential was too high in the furnace during heating for quenching. The surface layer material of the casing elevator is determined as high carbon alloy steel with Si, Mn, Mo, and V after carburizing, and the quenching heat temperature in carburizing layer should be much lower than that of 20SiMn2MoVA steel. The casing elevator is desi