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Failure analysis of IEU drill pipe wash out
International Journal of Fatigue 27 (2005) 1360–1365 www.elsevier.com/locate/ijfatigue
Failure analysis of IEU drill pipe wash out Shuanlu Lua,*, Yaorong Fenga, Faqian Luob, Changyi Qina, Xinhu Wanga a
Tubular Goods Research Center of China National Petroleum Corporation, Xi’An 710065, China b Tarim Oilfield, Xinjiang Kuerle 841000, China
Abstract Many 127.0!9.19 mm IEU G105 drill pipe failures of wash out occurred after 2367 h of pure drilling time and 8726 m of penetration footage. This paper gives a detailed investigation on these failures and a systematic analysis is carried out on service and loading conditions of the drill pipes. Measurement and inspection were performed on configuration dimensions, chemical composition, mechanical performance, metallography, macro-fractography, micro-fractography, and corrosion products. Configuration stresses at the crack positions of the drill pipe were calculated by FEA. Crack extending velocity of the drill pipe material under corrosion medium was also measured. It is thought from test and analysis results that the drill pipe wash out or fracture accidents were premature corrosion fatigue failure accident. The failure courses were as the following: corrosion pits occurred first on the internal surface at the stress concentrating area of the drill pipe, and then fatigue cracks initiated in pit bottoms, and washed out or fractured subsequently as cracks penetrated through the wall thickness of the drill pipe. The reasons of drill pipe wash out were related to configuration, material quality, and load condition of the drill pipe string. q 2005 Elsevier Ltd. All rights reserved. Keywords: Corrosion pits; Drill pipe; Pickling test
1. Introduction Drill pipe is one kind of important tools for drilling in oil field. Drill pipe fatigue failures often occur in oil fields because drill pipes bear continuously changeable tension load, torsion load, impact load, internal pressure, and etc. during drilling. Drill pipe wash out (holes occur, and leakage takes place) is main form that results in big economic loss [1]. So it is very important and urgent to do failure analysis of drill pipe wash out in order to prevent from it. This paper reports the result of failure analysis of drill pipes.
2. Background 2.1. Accident Three pieces of drill pipes were washed out and broke at the positions of the internal upset taper subject to abrupt * Corresponding author. E-mail address: [email protected] (S. Lu).
0142-1123/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijfatigue.2005.07.012
geometric change on drill pipe during normally drilling in C1 Well. It was found by NDT that there were defects at the vanishing area of internal upset taper on 20 lengths of drill pipe among 375 lengths of drill pipe used in this well. The defects were serious on 14 lengths, and not serious on 6 lengths. 2.2. Drill pipe service history The drill pipes without internal coating were first used at Q1 Well, and then used at C1 Well. The service history of the drill pipe at Q1 and C1 Wells are as the following, respectively. 2.2.1. Drill pipe work history at Q1 well Q1 well was straight exploratory well, and top drive drilling was used. The well depth was 4500 m, and the drilling time for the well was 826.75 h. The drilling parameters were as the following: Weight on bit was 48–199 kN, rotary speed 50–122 rpm, torque 3–34 kN m, pump pressure 4.2–19 MPa, and drilling fluid density 1.08–1.38 g/cm3. 2.2.2. Drill pipe service history at C1 well C1 Well was an exploratory directional well. The well depth was 4269.82 m and drilling time was 1541 h until drill
S. Lu et al. / International Journal of Fatigue 27 (2005) 1360–1365
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Fig. 1. Planned C1 Well trajectory.
Fig. 2. C1 Well trajectory.
pipe washed out. The planning of drilling was shown in Fig. 1. The actual maximum overall angle change rate (dogleg) was 5.68/30 m, and the well trajectory was shown in Fig. 2. The drilling parameters were as the following: The bit weight was 276 kN, pump pressure 2.6–27.6 MPa, and rotary speed 6–189 rpm. 2.3. Failure analysis specimen
The failed drill pipe was opened along longitudinal direction, and there were severe corrosion pits on inner surface. The pits were 0.30–1.06 mm in depth. The corrosion pits were obvious, and many cracks were found on the inner surface by the washed out holes after pickling for 1 h with 50% hydrochloric solution (Figs. 4 and 5). The internal upset shape was not regular, and the profile presented as wave shapes. In order to know the resistance to corrosion fatigue cracks of the drill pipe, the comparison pickling tests
One washed out drill pipe was taken as the failure analysis specimen and the following test was performed.
3. Visual examination and dimension measurement 3.1. Visual examination There were three wash out positions at the drill pipe (Fig. 3). The wash out positions were 495, 497, and 503 mm far from pin seal shoulder, respectively. The washed out holes were 21, 33, and 35 mm in circumference direction.
Fig. 3. Washed out morphology on the drillpipe.
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that the total length of internal upset taper (Miu) is in accordance with API SPEC 5D [2], but the internal upset taper is irregular, and presents wave shapes with different radius.
4. Material tests 4.1. Chemical analysis The chemical composition analysis result of the drill pipe material was in accordance with API SPEC 5D. Fig. 4. The morphology on inner surface of failed drill pipe after pickling test.
4.2. Mechanical tests The test result of mechanical performance was in accordance with API SPEC 5D. 4.3. Metallography
Fig. 5. The contrast between failed drill pipe and the compared drill pipe for streamline metal after pickling test (the failed drill pipe is beneath).
between failed drill pipe and the compared drill pipe were done for 1 h with 50% hydrochloric acid solution. The test result indicated that resistance to corrosion of the failed drill pipe was worse than that of the compared drill pipe, and the resistance to corrosion by the inner surface of the failed drill pipe was worse than that by the outer surface (Fig. 5). It is shown from wash out morphology analysis and pickling tests that the wash out positions are all limited to the internal upset taper, and there are severe corrosion pits in this area, and the cracks bud from corrosion pits stemming from internal surface. 3.2. Main dimension measurement The main dimension measurement result of the failed drill pipe was shown in Table 1. It is indicated from Table 1
The metallography result is shown in Figs. 6–8. It was indicated from metallography result that there were severe corrosion pits and cracks at the vanishing area of internal upset taper, and cracks were 1.25–7.33 mm in depth (Figs. 6–8). The microstructure was tempered S, and inclusions were A1.0, B0.5, and D1.0. It is known from metallography that cracks initiated from corrosion pits, and featured corrosion fatigue cracks. Some crack tops extended along rolling direction.
5. Micro-fractography and corrosion product analysis 5.1. Micro-fractography The cracks opened were hemicycle flat area with black color originating from corrosion pits on inner surface. There were corrosion pits and deposits on the origination area and extending area of cracks. There were fatigue striations on the flat area front (Fig. 9). It is clear from the micro-fractography result that cracks originated from corrosion pits on inner surface, and there were fatigue striations on the fracture, and corrosion deposits contain Cl, S, K, Ca, and etc., and the crack nature is corrosion fatigue crack.
Table 1 The main dimension measurement result (mm) Item
Pipe body
Upset area
D
d
Dou
dou
Miu
R
Internal taper shape
Result
127.1
108.6
129.7
88.0
113
543
API SPEC 5D [2]
C1:3 127:0K 0:6
–
131:8C3:2 K0:8
90.5
R76:2
–
Wave shape, and radiuses were 181, 209, 53 and 128 mm, respectively –
S. Lu et al. / International Journal of Fatigue 27 (2005) 1360–1365
Fig. 6. The corrosion pit morphology on inner surface by wash out hole.
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Fig. 8. The crack top morphology (the main crack was vertical to the rolling direction, and branch cracks were parallel to rolling direction).
5.2. Structure identification of corrosion deposits The components of corrosion deposits were Fe, Cl, Cr, Mn, S, and Si per Energy Dispersive Analyzer. The structure identification results of corrosion deposits are shown in Table 2 per XRD. It is known from Table 2 that oxides occupy high proportion among the corrosion deposits. The oxides are produced frequently per electrochemistry corrosion because drill pipes are dunked in drilling fluid with oxygen during drilling.
Fig. 9. The fatigue striations on crack front.
6. Corrosion fatigue simulation test Streamline metal on failed drill pipe was obvious due to severe inclusions precipitated. Theoretically, if there were severe inclusions in metal material the corrosion fatigue life of the drill pipe will be reduced. In order to quantitatively analyze the effect of inclusions precipitated on corrosion
fatigue life of the drill pipe the contrast test between the failed drill pipe and the compared drill pipe without inclusions precipitated were carried out to confirm the extending speed of corrosion fatigue crack. The test condition and result are as follows. 6.1. Test condition The full thickness specimens were taken from internal upset taper area of the failed drill pipe (1#) and the compared drill pipe (2#), respectively. The specimen was 120 mm in length, and 6 mm in width. The specimen thickness was different along internal upset taper. The corrosion fatigue test was carried out on GD test machine, and the stress ratio was 0.5, and stress breadth was sin wave type. The test solution was 3.5% NaCl at 14G2 8C. Table 2 Identification results of corrosion deposits
Fig. 7. The crack morphology initiated from corrosion pits at vanishing area of internal upset taper.
Molecular formulary of corrosion deposits
Content (%)
FeO(OH) FeFe2O4 Ca2SiO4
7.96 82.24 9.80
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S. Lu et al. / International Journal of Fatigue 27 (2005) 1360–1365
Fig. 10. The relation of crack expending speed (da/dn) versus stress intensity parameter (DK) at crack top. Note that the crack growth rate of the sample made from the failure pipe material is 1.7–2.3 times the crack growth rate of the comparison sample that do not show corrosion fatigue in service.
The extending speed of crack was inspected with microscope (error of G0.005 mm). 6.2. Test result The test result on relation of crack extending speed (da/dn) versus stress intensity parameter breadth (DK) is shown in Fig. 10. It is seen from Fig. 10 that the extending speed of corrosion fatigue crack of 1# is higher than that of 2#, The former is 1.7–2.3 times of the latter.
pure drilling time is 2367 h, and total penetration footage is 8769 m. The normal penetration footage of a drill pipe is 90, 000 m according to drilling experience in this oil field. The working life of failed drill pipe is only 9.6% of its normal life. So the drill pipes are abnormal failures. According to the research results of Dale [3] and Joosten [4] that the drill pipes are also premature corrosion fatigue failures. It is indicated from test result that the drill pipes were failed because of corrosion fatigue cracks. The failure process is as the following: Corrosion pits brought on the inner surface of the drill pipe/fatigue cracks originated from corrosion pits/ cracks extended/washed out (as depth of cracks penetrated through wall, or approached wall). The corrosion fatigue cracks of the drill pipe are caused by the stress of alternating change and corrosion medium. The failure of drill pipes is affected on corrosion, stress condition, and the stress concentration of drill pipe structure. It is known from test result that the corrosion fatigue cracks originated from corrosion pits, and cracks and pits are severe at the locations of internal upset taper subject to abrupt geometric change. The corrosion pits possess the feature of pinhole corrosion (Fig. 6), i.e. tiny batteries with Fe anode are formed first at the discontinuous positions of metal microstructure, such as inclusions, carbide, and outcrop of streamline metal etc., and the structural discontinuity, such as abrupt structure change and roughness at the internal upset taper area. Fe dissolves and corrosion origination is formed. Fe ions hydrolyze as the following:
7. FEA Fe C 2H2 O/ 2HC CFeðOHÞ2 FEA was carried out with MSC/NASTRAN software, adopting axes symmetry cell of linearity static state, and applying axial tension load. The FEA result is shown in Table 3. It is known from Table 3 that the stress concentration is less with more Miu and R. As internal upset taper is irregular with different R the maximum stress concentration locates at an area of r30.
FeðOHÞ2 C OHK/ FeðOHÞ3 Fe(OH)3 deposits at corrosion pits and covers pitheads, so that PH will decrease because of HC, and Fe will dissolve quickly. On the other hand, ClK in mud enters the pits from pitheads in virtue of ionization, and chemical reaction appears as the following: ClK C HC/ HCl
8. The analysis on cause of drill pipe washed out Pure drilling time is 826.75 h, and penetration footage is 4500 m for these drill pipes in Q1 well. Pure drilling time is 1541 h, and penetration footage is 4269 m in C1 well. Total
So that PH reduces, and solution accelerates. i.e. catalyzing itself action takes place. The pickling test results showed that there were severe streamline metal on longitudinal section and the streamline metal at area closing with internal surface was more severe
Table 3 FEA result Item
Length of internal taper (Miu), mm
Radius at vanishing area (R), mm
Small radius at position of 58 mm far from start position of Miu (r), mm
Stress coefficient of pipe body, so
Maximum stress coefficient and position, smax
smax/so
Result
80 110
300 647
30 –
1.591 1.591
1.828 (at r30 position) 1.62 (Miu vanish point)
1.149 1.018
S. Lu et al. / International Journal of Fatigue 27 (2005) 1360–1365
than that at the region near outer surface; but there was no obvious streamline metal for compared drill pipe. Hence there must be discontinuous microstructure (such as bandings, inclusions, carbonizations, and etc.) and discontinuous configuration in micrography for failed drill pipes. There were severe corrosion pits on the internal surface of the drill pipe. The more severe pits of the area were, the more severe fatigue cracks of that area were. There were dissolved oxygen, ClK and etc. in the mud, so that corrosion concentration occurred at locations of the internal upset taper subject to abrupt geometric change. Corrosion fatigue test showed that the crack extending speed of the failed drill pipe was 1.7–2.3 times by that of the compared drill pipe. I.e. the drill pipe failure is related with the material impurity separated out that makes the drill pipe life reduction of 41.2–56.5%. In order to prevent drill pipes from corrosion, the internal surface coating is an effective method. Severe corrosion must take place on the internal surface of drill pipe without internal coating at locations subject to abrupt geometric change because drill pipes were dunked in the mud containing dissolved oxygen and ClK during drilling. As the corrosion pits are formed, the stress concentration becomes more severe, and the corrosion fatigue cracks will nucleate and extend. It is clear from the test result that the cracks are limited to the locations of internal upset taper subject to abrupt geometric change. This indicates that the section of internal upset taper is weak position, and the abrupt geometric changes of the configuration accelerate corrosion fatigue cracks budding and extending, which are verified by failed drill pipes and FEA. On the other hand, C1 Well is a slanted well with obvious tilted angle and dog-leg, so that the drill pipe must have borne high stress, and budding and extending of fatigue cracks must be accelerated.
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To summarize, the drill pipes were premature failures due to the working condition and the drill pipe quality. Considering working condition, there were severe corrosion medium, and the drill pipes had borne large load in this well. Thinking over the pipe quality, the internal upset tapers were irregular configuration with severe corrosion and stress concentration, and the material resistance to corrosion was low.
9. Conclusion and suggestion (1) The drill pipes were premature failures due to corrosion fatigue cracks. (2) The main reason of drill pipe washed out was bad quality of drill pipes with irregular internal upset taper configuration subject to abrupt geometric change and low material resistance to corrosion, and rigor condition of the well with severe corrosion medium and obvious tilted angle and dog-leg. (3) Use of drill pipes with internal coating, and improving internal upset taper configuration and material quality of the drill pipe were recommended.
References [1] Lu Shuanlu. Investigation and analysis of drill pipe wash out accident. Oil drilling and production technology, 2005. [2] API SPEC 5D, Specification for drill pipe, API, Washington, DC. [3] Dale BA. Inspection interval guidelines to reduce drill string failures. SPE17207, 1988. [4] Joosten MW. New study shows how to predict accumulated drill pipe fatigue. World Oil, October 1985.
Failure analysis of IEU drill pipe wash out Shuanlu Lua,*, Yaorong Fenga, Faqian Luob, Changyi Qina, Xinhu Wanga a
Tubular Goods Research Center of China National Petroleum Corporation, Xi’An 710065, China b Tarim Oilfield, Xinjiang Kuerle 841000, China
Abstract Many 127.0!9.19 mm IEU G105 drill pipe failures of wash out occurred after 2367 h of pure drilling time and 8726 m of penetration footage. This paper gives a detailed investigation on these failures and a systematic analysis is carried out on service and loading conditions of the drill pipes. Measurement and inspection were performed on configuration dimensions, chemical composition, mechanical performance, metallography, macro-fractography, micro-fractography, and corrosion products. Configuration stresses at the crack positions of the drill pipe were calculated by FEA. Crack extending velocity of the drill pipe material under corrosion medium was also measured. It is thought from test and analysis results that the drill pipe wash out or fracture accidents were premature corrosion fatigue failure accident. The failure courses were as the following: corrosion pits occurred first on the internal surface at the stress concentrating area of the drill pipe, and then fatigue cracks initiated in pit bottoms, and washed out or fractured subsequently as cracks penetrated through the wall thickness of the drill pipe. The reasons of drill pipe wash out were related to configuration, material quality, and load condition of the drill pipe string. q 2005 Elsevier Ltd. All rights reserved. Keywords: Corrosion pits; Drill pipe; Pickling test
1. Introduction Drill pipe is one kind of important tools for drilling in oil field. Drill pipe fatigue failures often occur in oil fields because drill pipes bear continuously changeable tension load, torsion load, impact load, internal pressure, and etc. during drilling. Drill pipe wash out (holes occur, and leakage takes place) is main form that results in big economic loss [1]. So it is very important and urgent to do failure analysis of drill pipe wash out in order to prevent from it. This paper reports the result of failure analysis of drill pipes.
2. Background 2.1. Accident Three pieces of drill pipes were washed out and broke at the positions of the internal upset taper subject to abrupt * Corresponding author. E-mail address: [email protected] (S. Lu).
0142-1123/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijfatigue.2005.07.012
geometric change on drill pipe during normally drilling in C1 Well. It was found by NDT that there were defects at the vanishing area of internal upset taper on 20 lengths of drill pipe among 375 lengths of drill pipe used in this well. The defects were serious on 14 lengths, and not serious on 6 lengths. 2.2. Drill pipe service history The drill pipes without internal coating were first used at Q1 Well, and then used at C1 Well. The service history of the drill pipe at Q1 and C1 Wells are as the following, respectively. 2.2.1. Drill pipe work history at Q1 well Q1 well was straight exploratory well, and top drive drilling was used. The well depth was 4500 m, and the drilling time for the well was 826.75 h. The drilling parameters were as the following: Weight on bit was 48–199 kN, rotary speed 50–122 rpm, torque 3–34 kN m, pump pressure 4.2–19 MPa, and drilling fluid density 1.08–1.38 g/cm3. 2.2.2. Drill pipe service history at C1 well C1 Well was an exploratory directional well. The well depth was 4269.82 m and drilling time was 1541 h until drill
S. Lu et al. / International Journal of Fatigue 27 (2005) 1360–1365
1361
Fig. 1. Planned C1 Well trajectory.
Fig. 2. C1 Well trajectory.
pipe washed out. The planning of drilling was shown in Fig. 1. The actual maximum overall angle change rate (dogleg) was 5.68/30 m, and the well trajectory was shown in Fig. 2. The drilling parameters were as the following: The bit weight was 276 kN, pump pressure 2.6–27.6 MPa, and rotary speed 6–189 rpm. 2.3. Failure analysis specimen
The failed drill pipe was opened along longitudinal direction, and there were severe corrosion pits on inner surface. The pits were 0.30–1.06 mm in depth. The corrosion pits were obvious, and many cracks were found on the inner surface by the washed out holes after pickling for 1 h with 50% hydrochloric solution (Figs. 4 and 5). The internal upset shape was not regular, and the profile presented as wave shapes. In order to know the resistance to corrosion fatigue cracks of the drill pipe, the comparison pickling tests
One washed out drill pipe was taken as the failure analysis specimen and the following test was performed.
3. Visual examination and dimension measurement 3.1. Visual examination There were three wash out positions at the drill pipe (Fig. 3). The wash out positions were 495, 497, and 503 mm far from pin seal shoulder, respectively. The washed out holes were 21, 33, and 35 mm in circumference direction.
Fig. 3. Washed out morphology on the drillpipe.
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that the total length of internal upset taper (Miu) is in accordance with API SPEC 5D [2], but the internal upset taper is irregular, and presents wave shapes with different radius.
4. Material tests 4.1. Chemical analysis The chemical composition analysis result of the drill pipe material was in accordance with API SPEC 5D. Fig. 4. The morphology on inner surface of failed drill pipe after pickling test.
4.2. Mechanical tests The test result of mechanical performance was in accordance with API SPEC 5D. 4.3. Metallography
Fig. 5. The contrast between failed drill pipe and the compared drill pipe for streamline metal after pickling test (the failed drill pipe is beneath).
between failed drill pipe and the compared drill pipe were done for 1 h with 50% hydrochloric acid solution. The test result indicated that resistance to corrosion of the failed drill pipe was worse than that of the compared drill pipe, and the resistance to corrosion by the inner surface of the failed drill pipe was worse than that by the outer surface (Fig. 5). It is shown from wash out morphology analysis and pickling tests that the wash out positions are all limited to the internal upset taper, and there are severe corrosion pits in this area, and the cracks bud from corrosion pits stemming from internal surface. 3.2. Main dimension measurement The main dimension measurement result of the failed drill pipe was shown in Table 1. It is indicated from Table 1
The metallography result is shown in Figs. 6–8. It was indicated from metallography result that there were severe corrosion pits and cracks at the vanishing area of internal upset taper, and cracks were 1.25–7.33 mm in depth (Figs. 6–8). The microstructure was tempered S, and inclusions were A1.0, B0.5, and D1.0. It is known from metallography that cracks initiated from corrosion pits, and featured corrosion fatigue cracks. Some crack tops extended along rolling direction.
5. Micro-fractography and corrosion product analysis 5.1. Micro-fractography The cracks opened were hemicycle flat area with black color originating from corrosion pits on inner surface. There were corrosion pits and deposits on the origination area and extending area of cracks. There were fatigue striations on the flat area front (Fig. 9). It is clear from the micro-fractography result that cracks originated from corrosion pits on inner surface, and there were fatigue striations on the fracture, and corrosion deposits contain Cl, S, K, Ca, and etc., and the crack nature is corrosion fatigue crack.
Table 1 The main dimension measurement result (mm) Item
Pipe body
Upset area
D
d
Dou
dou
Miu
R
Internal taper shape
Result
127.1
108.6
129.7
88.0
113
543
API SPEC 5D [2]
C1:3 127:0K 0:6
–
131:8C3:2 K0:8
90.5
R76:2
–
Wave shape, and radiuses were 181, 209, 53 and 128 mm, respectively –
S. Lu et al. / International Journal of Fatigue 27 (2005) 1360–1365
Fig. 6. The corrosion pit morphology on inner surface by wash out hole.
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Fig. 8. The crack top morphology (the main crack was vertical to the rolling direction, and branch cracks were parallel to rolling direction).
5.2. Structure identification of corrosion deposits The components of corrosion deposits were Fe, Cl, Cr, Mn, S, and Si per Energy Dispersive Analyzer. The structure identification results of corrosion deposits are shown in Table 2 per XRD. It is known from Table 2 that oxides occupy high proportion among the corrosion deposits. The oxides are produced frequently per electrochemistry corrosion because drill pipes are dunked in drilling fluid with oxygen during drilling.
Fig. 9. The fatigue striations on crack front.
6. Corrosion fatigue simulation test Streamline metal on failed drill pipe was obvious due to severe inclusions precipitated. Theoretically, if there were severe inclusions in metal material the corrosion fatigue life of the drill pipe will be reduced. In order to quantitatively analyze the effect of inclusions precipitated on corrosion
fatigue life of the drill pipe the contrast test between the failed drill pipe and the compared drill pipe without inclusions precipitated were carried out to confirm the extending speed of corrosion fatigue crack. The test condition and result are as follows. 6.1. Test condition The full thickness specimens were taken from internal upset taper area of the failed drill pipe (1#) and the compared drill pipe (2#), respectively. The specimen was 120 mm in length, and 6 mm in width. The specimen thickness was different along internal upset taper. The corrosion fatigue test was carried out on GD test machine, and the stress ratio was 0.5, and stress breadth was sin wave type. The test solution was 3.5% NaCl at 14G2 8C. Table 2 Identification results of corrosion deposits
Fig. 7. The crack morphology initiated from corrosion pits at vanishing area of internal upset taper.
Molecular formulary of corrosion deposits
Content (%)
FeO(OH) FeFe2O4 Ca2SiO4
7.96 82.24 9.80
1364
S. Lu et al. / International Journal of Fatigue 27 (2005) 1360–1365
Fig. 10. The relation of crack expending speed (da/dn) versus stress intensity parameter (DK) at crack top. Note that the crack growth rate of the sample made from the failure pipe material is 1.7–2.3 times the crack growth rate of the comparison sample that do not show corrosion fatigue in service.
The extending speed of crack was inspected with microscope (error of G0.005 mm). 6.2. Test result The test result on relation of crack extending speed (da/dn) versus stress intensity parameter breadth (DK) is shown in Fig. 10. It is seen from Fig. 10 that the extending speed of corrosion fatigue crack of 1# is higher than that of 2#, The former is 1.7–2.3 times of the latter.
pure drilling time is 2367 h, and total penetration footage is 8769 m. The normal penetration footage of a drill pipe is 90, 000 m according to drilling experience in this oil field. The working life of failed drill pipe is only 9.6% of its normal life. So the drill pipes are abnormal failures. According to the research results of Dale [3] and Joosten [4] that the drill pipes are also premature corrosion fatigue failures. It is indicated from test result that the drill pipes were failed because of corrosion fatigue cracks. The failure process is as the following: Corrosion pits brought on the inner surface of the drill pipe/fatigue cracks originated from corrosion pits/ cracks extended/washed out (as depth of cracks penetrated through wall, or approached wall). The corrosion fatigue cracks of the drill pipe are caused by the stress of alternating change and corrosion medium. The failure of drill pipes is affected on corrosion, stress condition, and the stress concentration of drill pipe structure. It is known from test result that the corrosion fatigue cracks originated from corrosion pits, and cracks and pits are severe at the locations of internal upset taper subject to abrupt geometric change. The corrosion pits possess the feature of pinhole corrosion (Fig. 6), i.e. tiny batteries with Fe anode are formed first at the discontinuous positions of metal microstructure, such as inclusions, carbide, and outcrop of streamline metal etc., and the structural discontinuity, such as abrupt structure change and roughness at the internal upset taper area. Fe dissolves and corrosion origination is formed. Fe ions hydrolyze as the following:
7. FEA Fe C 2H2 O/ 2HC CFeðOHÞ2 FEA was carried out with MSC/NASTRAN software, adopting axes symmetry cell of linearity static state, and applying axial tension load. The FEA result is shown in Table 3. It is known from Table 3 that the stress concentration is less with more Miu and R. As internal upset taper is irregular with different R the maximum stress concentration locates at an area of r30.
FeðOHÞ2 C OHK/ FeðOHÞ3 Fe(OH)3 deposits at corrosion pits and covers pitheads, so that PH will decrease because of HC, and Fe will dissolve quickly. On the other hand, ClK in mud enters the pits from pitheads in virtue of ionization, and chemical reaction appears as the following: ClK C HC/ HCl
8. The analysis on cause of drill pipe washed out Pure drilling time is 826.75 h, and penetration footage is 4500 m for these drill pipes in Q1 well. Pure drilling time is 1541 h, and penetration footage is 4269 m in C1 well. Total
So that PH reduces, and solution accelerates. i.e. catalyzing itself action takes place. The pickling test results showed that there were severe streamline metal on longitudinal section and the streamline metal at area closing with internal surface was more severe
Table 3 FEA result Item
Length of internal taper (Miu), mm
Radius at vanishing area (R), mm
Small radius at position of 58 mm far from start position of Miu (r), mm
Stress coefficient of pipe body, so
Maximum stress coefficient and position, smax
smax/so
Result
80 110
300 647
30 –
1.591 1.591
1.828 (at r30 position) 1.62 (Miu vanish point)
1.149 1.018
S. Lu et al. / International Journal of Fatigue 27 (2005) 1360–1365
than that at the region near outer surface; but there was no obvious streamline metal for compared drill pipe. Hence there must be discontinuous microstructure (such as bandings, inclusions, carbonizations, and etc.) and discontinuous configuration in micrography for failed drill pipes. There were severe corrosion pits on the internal surface of the drill pipe. The more severe pits of the area were, the more severe fatigue cracks of that area were. There were dissolved oxygen, ClK and etc. in the mud, so that corrosion concentration occurred at locations of the internal upset taper subject to abrupt geometric change. Corrosion fatigue test showed that the crack extending speed of the failed drill pipe was 1.7–2.3 times by that of the compared drill pipe. I.e. the drill pipe failure is related with the material impurity separated out that makes the drill pipe life reduction of 41.2–56.5%. In order to prevent drill pipes from corrosion, the internal surface coating is an effective method. Severe corrosion must take place on the internal surface of drill pipe without internal coating at locations subject to abrupt geometric change because drill pipes were dunked in the mud containing dissolved oxygen and ClK during drilling. As the corrosion pits are formed, the stress concentration becomes more severe, and the corrosion fatigue cracks will nucleate and extend. It is clear from the test result that the cracks are limited to the locations of internal upset taper subject to abrupt geometric change. This indicates that the section of internal upset taper is weak position, and the abrupt geometric changes of the configuration accelerate corrosion fatigue cracks budding and extending, which are verified by failed drill pipes and FEA. On the other hand, C1 Well is a slanted well with obvious tilted angle and dog-leg, so that the drill pipe must have borne high stress, and budding and extending of fatigue cracks must be accelerated.
1365
To summarize, the drill pipes were premature failures due to the working condition and the drill pipe quality. Considering working condition, there were severe corrosion medium, and the drill pipes had borne large load in this well. Thinking over the pipe quality, the internal upset tapers were irregular configuration with severe corrosion and stress concentration, and the material resistance to corrosion was low.
9. Conclusion and suggestion (1) The drill pipes were premature failures due to corrosion fatigue cracks. (2) The main reason of drill pipe washed out was bad quality of drill pipes with irregular internal upset taper configuration subject to abrupt geometric change and low material resistance to corrosion, and rigor condition of the well with severe corrosion medium and obvious tilted angle and dog-leg. (3) Use of drill pipes with internal coating, and improving internal upset taper configuration and material quality of the drill pipe were recommended.
References [1] Lu Shuanlu. Investigation and analysis of drill pipe wash out accident. Oil drilling and production technology, 2005. [2] API SPEC 5D, Specification for drill pipe, API, Washington, DC. [3] Dale BA. Inspection interval guidelines to reduce drill string failures. SPE17207, 1988. [4] Joosten MW. New study shows how to predict accumulated drill pipe fatigue. World Oil, October 1985.