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Using a supperficially treated 2024 aluminum alloy drill pipe to delay failure during dynamic loading
Engineering Failure Analysis 104 (2019) 261–273
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Using a supperficially treated 2024 aluminum alloy drill pipe to delay failure during dynamic loading
T
⁎
Lallia Belkacema, , Noureddine Abdelbakib, Jose Luis Oteguic, Mohamed Gaceba, Mourad Bettayeba a
Laboratory of Petroleum Equipment's Reliability and Materials, Université M'hamed Bougara de Boumerdes, Algeria University Akli Mohand Oulhadj, Bouira 10000, Algeria c Consejo Nacional de Investigaciones Científicas y Técnicas, Argentina b
A R T IC LE I N F O
ABS TRA CT
Keywords: Steel drill pipe Aluminum drill pipe Failure analysis Buckling Fatigue
For many years, a critical factor while drilling deep wells in Algeria oil fields has been the fatigue loads in drill strings used in high tortuosity sections. One solution is the use of drill pipes made of alternative lighter materials instead of conventional steel. A thorough study of superficially (ST) treated 2024 aluminum alloy joined to steel drill pipes in crooked well trajectories is presented in this article. This work discusses several limiting loads exerted on the drill pipes that affect the drilling operation such as axial, twisting and bending forces, which lead to combined cyclic loads. This study is aimed to determine whether it is possible to extend the drilling operation by joining ST 2024 aluminum alloy drill pipes into the drilling string, without losing drilling stability or reliability. It is established that the ST 2024 aluminum alloy drill pipe exerts significant influence on the allowable combined load, but affects to a larger extent the critical load at which loss of drilling string stability occurs, along with improved resistance to wear and corrosion, even at high temperature.
1. Introduction During drilling operations, drill pipes and other string components are commonly used in worn conditions, wear, fatigue, wash out and other service related defects cause the pipes to lose some original strength and this limits their ability to withstand the same stresses as when they were in their new state, since they suffer a certain degree of damage. A pipe inspection provides an assessment of the damage, and the pipe is assigned to a class corresponding to the extent of damage (outside diameter thus insures their reliability and service life in order to reduce failure costs and increase yield [1]. Field experiences show that many failures occur while rotating or while picking up off bottom immediately after drilling. They occur within 4 ft of the tool joint and are associated with severe pitting on the inside wall of the pipe. These cracks appear to have started from the inside. Failures that appear to originate from the outside of the pipe are usually associated with slip marks, or surface damage when failures occur as a result of pulling on stuck pipe. The failure frequently occurs in a location where fatigue cracks have developped but had not progressed to failure. [1,2]. On the basis of previously conducted researches, drill string fatigue is still excessively frequent. The main reason for this is combined stresses (tension, torsion and compression) and different types of vibrations caused by dynamic and cyclic loading. These loads are constantly changing depending on trajectory and dog leg severity [1,2]. Therefore, drill string components should be selected and configured to delay fatigue as much as possible and in an economical and practical manner. As well as vibration analysis ⁎
Corresponding author. E-mail addresses: [email protected], [email protected] (L. Belkacem), Jotegui@fi.mdp.edu.ar (J.L. Otegui).
https://doi.org/10.1016/j.engfailanal.2019.06.005 Received 14 October 2018; Received in revised form 14 May 2019; Accepted 4 June 2019 Available online 06 June 2019 1350-6307/ © 2019 Published by Elsevier Ltd.
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Fig. 1. Critical regions of the drill pipe and connection causing fatigue failure [6].
techniques and torque and drag modeling, hydraulic design and directional control are very important. In parallel, many researchers have attempted to address drill pipe connection failures based on the following points: [2–5]. 1) The high torque to seal the joints creates a high strain in the connection. 2) The effect of stress concentration on the pin and box threaded joint's failure of the upset drill pipes under tensile, compression and bending loads 3) The stress concentrating factor (SCF) at the root of the thread. 4) The fatigue crack nucleated at the first root of the tooth from the pin tool joint shoulder of the drill pipe, and then propagated through the wall of the tool joint. 5) The deterioration of the fatigue resistance of the tool joint is related to dog leg region where severe cyclic bending load exists due to the local deviation of the drill pipe from the vertical line. The combined effect of these factors is that drill string rotary connections invariably fail at the first engaged thread on the pin side of the joint, and at the last engaged thread in the box component, Fig. 1. Similar work was conducted by Vaisberg et al. [5] who reported that, by employing 165-ksi grade material for drill pipes, the strain concentrations at upset transitions were completely redesigned in order to prevent any fatigue failures. With standard tool joints manufactured from 120 ksi yield material, a 165 ksi drill pipe, product with 165 ksi tool-joints, offers a 38% increase in tool joint torque and tension capacities. The hydraulic loss through the drill pipe was also considered. He mentioned also that, the pipe weight has been reduced by decrease of the wall thickness using high strength steel, and the corrosion fatigue property is superior besides, the résistance to high temperature is also good. Also, Wang et al. [7] studied the manufacturing anomalies and design defects of the drilling stem and the tool joints which affect its strength. They mentioned that the defects lead to inequitable stress distribution, resulting in premature failure of the drill string. It was also observed that fatigue failure of the drilling stem caused by the extensive tensile or compressive stress acting on the drill string, resulting from drill string sticking and the improper anti-sticking methods like over pushing and over pulling of the drill string. Moreover, Pan [8] supposed that fatigue cracking initiated in the drill pipe can be propagated by the combined action of corrosion and erosion processes, and a sacrificial cathode such as aluminum or zinc coating on both the inner and outer surfaces of the drill pipe reduced the potential gap and then increased its fatigue strength despite the accumulation of H+. Likewise, H. Yan et al. [9] have reported that a fact of long-term operation of drill pipe in high H2S content condition, the problem of microstructural degradation plays an important role in susceptibility to brittle fracture for instance. Due to sulphide stress corrosion cracking and high H2S content condition smooth cracks propagating radially and axially in a 76.2 mm (S135) drill pipe that deforms to a rough and possibly fractured surface. Meanwhile Azar.J [10] stated that during a drilling process, multiple modes of failure are encountered in the drill pipes rather than those described above, such as pipe wash out and twist-off, sticking, pipe-parting, collapse and burst failure. These failures are most often encountered due to similar down hole stress problems, even with very carefully planned wells. Ductile and brittle besides fatigue failure are also taken into account in these damage mechanisms. A number of studies conducted by Lubinski [11], which identified the effect of increasing the angle by the wellbore, so does the tendency for the pipe to lie down on the lower side of the hole allows the pipe to reach a state of stability and to carry higher axial compressive stresses without buckling. However, when more energy is put into the system, buckling can occur. Although widely used the steel drill string contain various drawbacks. Therefore significant interest in using aluminum drill pipes instead of steel drill pipes is very noticeable throughout the last years especially; therefore a several processing methods were successfully developed in order to reduce many of these challenges. For instance, ref. [1, 12] applied a unique approach and workflow data into usable knowledge, supporting that light weight drill pipes, when used properly, can lead to reduced drilling loads and lower torque and drag, thus making more extended drilling depths possible. As illustrated in Fig. 2, the reduction in fatigue life experienced by steel drill pipe and aluminum drill pipe while going through gradual doglegs. Aluminum drill pipe appears to be able to pass through more severe doglegs without damage. And although aluminum drill pipes are not as strong as steel drill pipes because of their lower buckling limit, they have a significantly longer fatigue life because of their lower Young's Modulus [14]. Therefore, using lighter materials has proven to be a good alternative for weight reduction. It also allows for bigger pipe diameters, which can reduce pressure losses and therefore provide more hydraulic power for the bottom hole assembly (BHA) and bit. Furthermore, the aluminum drill pipe wears less than steel as the
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Fig. 2. The reduction in fatigue life experienced by steel drill pipe and aluminum drill pipe while going through gradual doglegs. [13].
tension in the string is lower, thereby reducing the axial and torsional drag, and has great flexibility when the drill stem is subjected to unusual stretching, bending and twisting loads as demonstrated [15]. Pan, [16] on the other hand, stated that aluminum drill pipes did not penetrate the market as expected, despite their distinct advantages over steel drill pipes; First, because the relatively low yield strength compared to high-strength steel limits its applicability in complex wells. The aluminum alloy used for drill pipes has yield strength of 58,000 psi (400 MPa). The tool-joints for aluminum drill pipe are manufactured from steel. Secondly he mentioned that aluminum drill pipes have experienced serious wear and corrosion problems in the middle of the body, mainly due to the pH of the mud, chlorides and oxygen, high temperatures and storage. Similarly he assumed that fatigue cracking initiated in the drill pipe can be propagated by the combined action of corrosion and erosion processes.as well other scientists assumed that a sacrificial cathode such as an aluminum or zinc coating on both the inner and outer surfaces of the drill pipe reduced the potential gap and then increased its fatigue strength despite the accumulation of H+. The aim of the present work is to conduct a comprehensive review of most failure modes that occurred in the string during the drilling process of the Algerian oil field, explaining the main reasons for these failure modes in order to manage risks and achieve high well performance throughout the drilling process. It aims also to verify that the use of a new set of surface-treated drill pipe materials (aluminum alloy 2024) can solve this problem. 2. Material and methods Concrete examples of the degradation of steel drill pipes in Algerian oil fields are presented. It is followed by a summary of the 203.2 mm (8.5 in.) drill section. A dynamic analysis of the drilling operation is then performed to identify the uniform distribution of stresses across the borehole of the drill ring using well design software based on the stiff String" model. The evaluation of the actual data sets will be conducted to explore the validity of the aluminum alloy integration hypothesis in order to determine an optimal solution to the problems encountered. 2.1. Case studies of steel drill pipe damage in Algeria oil gas fields 2.1.1. Case 1: drill pipe shearing During drilling a 12–1/4″ hole section with weight on bit from 8 to 21 ton, RPM = 130–170, Q = 177 m3/h, SPP = 20,684.27 kpa, ROP = 14 m/h, the driller reamed each drilled joint. The lithology at 2070 m was 20% Clay, 60% Anhydrite and 20% Dolomite. It was decided to pull out the drill string for wiper trip from 2327 m to 2091 m and try to unscrew, a very tight 263
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Fig. 3. The 5 in drill pipe (127 mm) shearing.
connection. The 5 in drill pipe (127 mm) had sheared and was caught by the drill pipe slip; consequently the drill string broke within the body of the drill pipe. The string was hanged by the drill pipe, the Annular & Pipe rams BOPs was closed for safety. The failure investigation revealed that the combined applied stress at the moment was less than the yield strength of the material [15]. (Fig. 3). 2.1.2. Case 2: drill pipe failure After drilling 12–1/4″ in hole section with weight on bit 8 to 21 ton, RPM = 200 rev/min, ROP = 32 m/h, from 2466 to 2865 m (total section depth), the drilling string was pulled out to casing shoe to perform a formation integrity test (FIT). During pulling out, the driller had a hard back-reaming at six different intervals between 2740 m and 2483 m. After circulation and cleaning hole (density in equal to density out), operators pulled out the drilling string to top BHA (Bottom Hole Assembly). Three joints of DP (100 m above Heavy Weights) had been over-torqued and there was a gap between pin and box (Fig. 4). When the BHA was raised to surface, some broken thread in middle part of Jar (mechanical section) was observed. The investigation revealed that the Bottom Hole Assembly was properly designed in terms of neutral point and sufficient available weight. The WOB applied while drilling was within the design range, kept below 20 tons, similarly for the Bending Strength Ratio and the Stiffness Ratio. This BHA is commonly used in this area (offset wells) for drilling 12–1/4 in sections. - Down hole torque while drilling was higher than that before in offset wells. This seems to be the most important contributing factor in these drill pipe connection failures. In addition, Top Drive System stalling during back-reaming and the back torque may have contributed to drill pipe connections partially backed off. Down hole torque was within the range for the Jar in terms of torsional capacity and the jar inspection sheet shows that the jar is in good condition. Total hours on the jar is 154, were less than allowable (250 h. max). This will make the jar failure investigation inconclusive, but the RPM used to drill was up to the limit (200 rev/min). This might have contributed to the high torque. 2.1.3. Case 3: wash out During drilling 12–1/4″ in hole section from 2470 m to 2516 m with increased weight on bit from13 to 20 t, RPM = 130 rpm, Q = 8 m3/h, torque =2 ton.m, SPP = 12,755 KPa, the driller reamed each stand and increased mud weight from 1897 kg/m3 to increase 2204.80 kg/m3, the lithology was: 0–20% clay, 0–30% dolomite 50–100% anhydrite. The stand pipe pressure dropped suddenly, so it was decided to pull out the drill string in cased hole to 1950 m. A wash out problem was discovered in 127 mm (5in), S135 drill pipe number 177(Fig. 5).
Fig. 4. The 5 in drill pipe (127 mm) connection damage. 264
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Fig. 5. The 5 in drill pipe (127 mm) wash out, the crack propagated radially and circumferentially.
Due to the high pressure drilling mud passing through cracks and openings, fracture surfaces were washed away and no useful information was obtained from these surfaces. It was assumed that the washout was due to a fatigue crack initiated on the internal surface of the tube body, in a region where the internal coating had been damaged thus allowing corrosion of the metal substrate (The drilling mud may serve as electrolytic medium). 2.2. Well design and integrity This study was conducted using background data gathered for the 203.2 mm (8.5 in.) drill section in the X field in Algeria. As numbered below; i) ii) iii) iv)
In this section, the goal is to drill Clay Lias (Lias Argileux) at 150 m in a Silurian formation called F6-B as depicted in Fig. 6. The reservoir formations were covered with a production coating of 0.1778 m in diameter, extending from 2942 m to 4785 m. The drilling parameters related to this drilling section are reported in Tables 1 and 2. The well path data is depicted in Table 3
Fig. 6. Hole section 203.2 mm (8.5 in) lithology. 265
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Table 1 Hydraulics data summary. Hole size (mm)
Depth (m)
Bottom hole assembly type
Recommended flow rate (m3/ h)
Minimum flow
Mud weight (kg/ m3)
Average rate of penetration (m/h)
1297–1347
7
Clean (m3/h) 203. 20
4785
Packed
84–108 m3/h
40
Table 2 Torque and drag summary. Axial Loads Hole size (mm)
Depth (m)
Bottom hole assembly type
Recommended WOB (N)
Buoyed weight below jar (N)
String weight (N)
Pick-Up weight (N)
Over pull (N)
• Does not include travelling assembly Table 3 Well path data. Measured depth (m)
Inclination (°)
True vertical depth (m)
Dog leg (°/30 m)
3168 3179 3190 3200 3211 3219 3222 3245 3256 3265 3275
6.540 8.010 9.750 10.87 11.67 12.43 13.14 16.82 19.9 19.95 23.24
3168 3178 3189 3199 3201 3217 3221 3243 3253 3262 3271
4.846 4.332 4.488 3.32 2.251 2.868 6.358 4.757 9.041 0.154 10.292
The investigation reveals that the most important problems in drilling operations are associated with torque and drag load. As the drill bit progresses through the well, forces are generated in three directions: lateral, axial and torsional. For this reason, the result of these three constraints is one of the most important data to be included in the process. Thus, the effect of these multi-axial stresses due to hydrostatic pressure and mechanical loads was assessed through this study and compared to the strength of the drill pipe material and the manufacturer's specifications as well as the dynamics of the down hole. The stresses leading to the failure of the drill pipes are indicated hereunder: i) ii) iii) iv) v)
Axial stresses due to tension, bending and buckling. Approaching bending stress of the borehole curvature. Torsional stress due to torsion Transverse shear stress due to contact force axial, lateral and torsional vibration
2.3. Stiff string model Well design software based on the Dawson cable model (The stiff-string) is based on the assumption that the drill pipe is composed of short segments connected by connections that transmit tension and the drill string is made up of short segments joined by connections which transmit tension, compression and torsion. The stiff string model traverses the string by selecting each component length or 30 ft. (9,14 m) equivalent section. It computes the side force at the center point. This side force is used to compute the multiaxial changed from one element to the next element as depicted in Fig. 7. The detailed analysis of each node involved creating a local mesh of 10 to 20 elements around this node. The end nodes of the mesh are given the following end constraints (boundary conditions):
• If the model is at the bit or the top of the string, the node is a pivot and is free to rotate. • If the end node has been previously solved in the traverse, then the node is fixed with the displacement and angle from the previous solution. If • the node is at the front end of the traverse, the node is fixed in the center of the hole. Each sub-element is given the same dimensions and properties as it would be given to the full drill string. If the node length 266
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Fig. 7. The stiff string model and stress acting on drill string.
exceeds the maximum column-buckling load for the section, the node is further broken into fractional lengths to keep each section below the buckling threshold. This is why the analysis can take considerably longer when large compressive loads are applied. This paper consists of an approach in which will be discussed, the effect of the side forces and all derived calculations (Torque and Drag) as well as the bending stresses together with the pipe position in the hole, the hydrostatic pressure and the well conditions on the drilling string. 3. Discussion 3.1. Load distribution in the steel drill string The Results of this analysis provide data on the mechanical integrity and elongation of steel drill pipes. All failures due to axial and bending, as well as torsions stresses, were recorded (Table 4). The total stretch in the string is computed as the sum of stretch components due to axial, buckling and ballooning which is caused by differential pressure inside and outside the string as follows:
∆L stretch = ∆LHL + ∆LBuck + ∆LBalloon + ∆L (thermal)
(1)
where: ΔLHL is the change in length due to the Hooke's Law mechanism, ΔLBuck is the Change in length due to buckling Load and ΔLBallon is the change in length due to ballooning mechanism; units (m) [5]. 1) ΔLHL Stretch due to axial load. This term is based on Hooke's Law. The first term reflects the constant load in the string, while the second term reflects the linear change in the load. [5]
∆LHL =
Fapa × Lc Ac × E
+
∆F × L 2Ac × E
(2)
where Fapa is defined as Axial force as determined by the pressure area method units MN; Lc is length of work string component element (in m); Ac is the cross sectional area of component units (in m2).E = Young's Modulus of component, unit (MPa).Fapa = Change in pressure area axial force over component length is calculated using the pressure area method. Table 4 String Analysis Summary of steel drill pipe during on-off rotation and tripping out of the hole. Load condition
Tripping out Rotating on bottom
Stress failure
Fatigue Fatigue
Buckling limits
No buckling No buckling
267
Stretch(m) Mechanical
Ballooning
Thermal
4.99 3.94
0.36 0.36
1.60 1.60
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Fapa =
∑ [LD Wair cos θ + Fdrag + ∆Farea] − Fbtm − Wbit
(3)
where: the Fbtm is the bottom pressure force which is a compressive force due to fluid pressure applied over the cross-sectional area of the bottom component, units (MN). Fdrag = Drag Force, units (MN), LD = Length of string, units (m), Wair = Weight per foot of string in air, unit (N/ft),Wbit = Weight on bit, unite (N) and ΔFarea = Change in force due to a change in area at junction between two components with different cross -sectional areas, such as the junction between drill pipe and heavy weights, or between heavy weights and drill collars. If the area of the bottom component is larger, the force is tensile. If the top component is larger, the force is compressive. 2) Stretch due to buckling If buckling occurs, the additional stretch in the buckled section of the work string is calculated using the following equation. [5]
∆LBuk =
rcl2 Fapa Lc 4EI
rcl2 Fapa Lc
+
(4)
8EI
where: ΔLBuck = Change in length due to buckling, Fapa = Axial force as determined by the pressure area method, unit (Kpa), ΔF = Change in pressure area axial force over component, unit (KPa) and E = Young's Modulus of component, unit(KPa).I = Moment of Inertia unit kg. m2 and r = Clearance between the wellbore wall and the work string component (m) 3) Stretch due to ballooning Stretch due to ballooning is caused by differential pressure inside and outside the work string, and is defined by the following equation [25]
−νLc
∆LBalloon = E⎡ ⎣
( ) dbo dbi
2
2
d d ⎡ ⎤ x ⎢ ⎜⎛ρmi − ⎛ bo ⎞ ρma ⎟⎞ Lc + 2 ⎜⎛Ps − ρma ⎛ bo ⎞ ⎟⎞ ⎥ d d bi bi ⎝ ⎠ ⎝ ⎠ ⎝ ⎠ ⎤ ⎠ − 1 ⎣⎝ ⎦ ⎦ ⎜
⎟
⎜
⎟
(5)
where ΔLballoon is the stretch due to ballooning, dbi and dbo are the inside and outside diameter of body. E = Young's Modulus of Elasticity. Lc is the length of component, ps = Surface pressure on string side and ν = Poisson's Ratio for component. ρma is the mud density in annulus at depth of string component and ρmi = Mud density inside string component unit (kg/m3) 4) Stretch Due to Temperature Effects
∆ltemp = αL c ∆T
(6)
where ΔT = Temperature change over component length (in °K), α = Coefficient of linear thermal expansion and ΔT = Temperature change over component length [25]. The Field case appears to be a problematic well that required various trajectory corrections throughout the drilling process to reach the target (side track). This is clearly observed in the fluctuation in inclination and azimuth measurements as shown in Table 3 which gives data for a complex well trajectory, drilled vertically to 3275 m and build 10°/30 m reaching 23.24° inclination at TD [3275mMD]. The latter reached about 4°/30 m 4 times at depths ranging from 3168.31 to 3190.12 m, to finally reach a maximum of 10°/30 m between 3190,12 and 3274.99 m. This leads to an increased risk of bending stress severity in the above-mentioned areas. This finding was confirmed by the fatigue ratio value of 1.87 above 1, which makes the use of steel drill pipes impractical in the area of fatigue failure, as shown in Table 5.
Rf =
(|σbend| + |σbuckle|) σft
(7)
where Rf = Fatigue ratio and σbend is bending stress corrected by the bending stress magnification factor. σbuckle is the buckling stress Table 5 Fatigue ratio for steel drill pipe. Measured depth (m)
Contact force (Kgf/length)
Torsional stress (KPa)
Shear stress (KPa)
Axial stress (KPa)
Bending stress (KPa)
Bending stress magnification factor
Von mises ratio
Fatigue ratio
3151.82 3160.94 3179.18 3224.78 3233.91 3243.03 3252.15 3261.27 3270.39
1183.00 1252.70 1204.20 1234.30 1107.50 1880.20 1015.50 131,380 1056.0
94,518 92,819 89,345 82,914.9 81,142.0 79,551.2 76,850.7 75,392.1 73,505.1
8357.9 8849.7 8507.2 8720.3 7824.3 13,283. 7174.4 9281.3 7460.1
140,245.9 139,160,9 136,997.8 131,657.4 130,602.2 129,554.0 128,531.4 127,485.9 126,510.0
105,247.2 105,870.1 107,863.6 118,899.0 121,788.7 194,997.3 117,750.1 131,913.1 140,634.4
2.932 2.921 2.898 2.841 2.829 2.818 2.807 2.795 2.785
0.445 0.442 0.438 0.439 0.439 0.529 0.427 0.442 0.45
1.027 1.031 1.047 1.144 1.17 1.87 1.127 1.261 1.342
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Table 6 Details in each specific drilling string. Section
Steel/Steel drill pipe
Aluminum/Steel drill pipe
Grade Depth (m) Yield strength (Mpa) Tensile strength (Mpa) Young module(Mpa) Fatigue Endurance limit (Mpa)
G105 4400 724 931 206,896.55 137.89
G105 850 724 931 206,896.55 137.89
AL2024 250 359 466.5 88,763 160
S135 3060 931 1138 206,896.55 137.89
and σfl is fatigue endurence limite. 3.2. Distribution of loads in aluminum drill pipe Full-scale tests were undertaken by replacing the steel drill pipe joints with lighter aluminum drill pipes in the string of drill pipes while maintaining the same hole section and tool joints made of steel. Table 6 lists details of all specifications for each drill string, followed by a summary of the stress analysis during the selected operation. The effective tension curve in Fig. 8 shows that the drilling operation curves do not cross the tension limit curve at 4400 m. There is therefore no strain on the aluminum drill pipe during this tortuous area. The pipe had attained its yield strength to a depth of 4400 m. It can therefore be inferred that aluminum drill pipes have good fatigue resistance, despite the simultaneous presence of high torque and high axial load, as well as severe doglegs. This is supported by a fatigue rate of less than 1 which is 0.931, making the use of steel pipe impractical for fatigue ratio calculation as shown in Table 7. Another area of interest in this regard is the dynamics caused by inadequate loads, and in particular the analysis of critical
Fig. 8. Effective tension curve for mixed steel -AL2024. 269
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Table 7 Fatigue ratio mixed drill string. Measured depth (m)
Contact force (Kgf/ length)
Torsional stress (KPa)
Shear stress (KPa)
Axial stress (KPa)
Bending stress (KPa)
Von mises ratio
Fatigue ratio
3167.90 3176.83 3185.76 3194.69 3203.62 3212.55 3221.47 3230.40 3239.33 3248.26 3257.19
965.8 919.6 661.5 499.6 745.4 1279.3 982.4 1196.0 1443.2 444.6 1522.9
55,802.1 54,444.2 53,151.3 52,221.2 51,518.9 50,470.8 48,672.1 47,290.9 45,609.3 43,580.1 42,955.0
6679.2 6359.4 4574.7 3454.9 5155.4 8847.2 6793.9 8271.6 9981.0 3074.9 10,531.9
108,183.9 107,126.2 106,072.0 105,018.4 103,969.6 102,925.4 101,889.3 100,853.0 99,824.1 98,813.7 97,790.4
46,247.6 49,074.9 42,184.4 32,608.3 37,487.2 59,712.7 47,456.0 46,957.6 78,591.7 14,887.6 88,954.7
0.584 0.586 0.563 0.535 0.543 0.595 0.558 0.552 0.628 0.458 0.646
0.506 0.534 0.457 0.352 0.403 0.639 0.505 0.498 0.83 0.157 0.931
velocities, which is used to determine critical rotational speeds of drill pipes and areas of high stress concentration in the entire drill string. The practice is to locate a corresponding critical rotational speed above which the system is unstable. The Forced-FrequencyResponse (FFR) analysis model presented herein enables to determine the forced frequency response (critical speeds) of a BHA due to imposed load and/or displacement excitations anywhere in the drill string. Results obtained can be used to avoid critical rotary speeds that accelerate pipe fatigue resulting in catastrophic drill string failure as provided in Figs. 9 and 10.This result reveals that the axial stress along the longitudinal axis of the drilling string is significantly greater than 5515.81Kpa for both steel and mixed drilling string compared to other loads. Bit bounce is therefore likely to occur, whereas, the bending stress in steel drilling string, revealed with two stresses peaks about 648Kpa and 2026 Kpa with different rotational speed 70 and 92 (rpm) respectively, compared to mixed drilling string,which reached one occasional peak of about
Fig. 9. Critical speed analysis for mixed Aluminum 2024 drill sting. 270
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Fig. 10. Critical speed analysis for steel drill pipe.
3350 KPa at rotational speed of 84 rpm. 3.3. Drill string dynamics The 127 mm, 29Kg/m G105 steel drill pipe experienced to fatigue failure during rotating on/off bottom operations, which may be the result of unintentional deviations and loading conditions. For instance, rotation in a dangerous dogleg region where the severity exceeds 10°/9.14 m [158,16] induces large alternating stresses in the drill pipe, which becomes a potential site for fatigue cracks nucleation. Thus risky results were obtained by increasing the severity of the moment bending curvature on the aforementioned zones [17], from3151.82 m to 3270.39 m. For this reason, the drill pipe is under tension and has been pulled towards the inside of the bend with a high lateral contact force that is greater than 8896 N (2000 lb) with a rotational speed of 150 rpm. Accordingly, the abrasion problem in the drill pipe connections is very significant. [18–20]. In fact, the calculation of bending stress is the key criterion of the cumulative fatigue damage model. The cumulative fatigue damage model as defined in API RP7G has been incorporated into an advanced torque and strength model, which tracks any variation in stress at any given point in the drill pipe during drilling, such as the transition zone, connections and drill pipe body [8,19,21]. For instance the G105 alloy drill pipes provides an alteration in bending forces throughout the drilling process, due to side forces resulting from dog leg area. Thus the first set of analyses highlighted the impact of dog leg interval in the steel drill pipe, since bending pipe around dog leg rises tension load on outside section of the pipe and declines on the inside of the curve,. Subsequently, the rotation causes oscillating bending stresses which can quickly lead to fatigue failure. As a result, the high stress concentration mentioned in Table 5 above has resulted in an increase in the fatigue ratio from 1.26 to 1.78, which is high. (The minimum fatigue ratio is equal to 1 as a safety limit). Consequently, fatigue failure of the drill pipes is accelerated since the efficiency of axial force transfer is reduced [4,20–22]. On the other hand, although the 2024AL alloy drill pipe has low mechanical properties in terms of density, Young's modulus, yield and tensile strength, compared with 127 mm, 29Kg/m G105 alloy drill pipes, have high flexibility when the drill pipe is subjected to abnormal stress and bending, during tortuous conditions. The latter is confirmed by a fatigue rate of 0.931 less than 1, which makes the use of steel pipes impractical in the event of wear and tear, which makes it impracticable for such an operation. Since the lateral 271
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force in the area of the dog's legs has decreased compared to the lateral force of a steel drill string. The mixed drill pipes therefore provides balanced tensile and bending strength, while taking into account the compressive load during operation of the drill pipes and the axial load on the bottom of the well. [12,14,20] It can then be hypothesized that the 2024AL alloy drill pipe is a powerful torque and drag reduction tool for very tortuous wells and solves many of the problems encountered during drilling, as it extends its fatigue life and reduces wear for both casing and drill pipe. This result is confirmed by the output obtained in the Torque Drag Effective Tension Graph Fig. 8 which has included the next graphical curves on Tension vs. Distance along String. Accordingly; it is obviously seen that all operations curves do not cross the Tension Limit curve, the drill string is located into a safe window; therefore there is no danger of exertion the aluminum drill pipe in tortuous interval. [19,20] As a final point, the uniform rotation is unstable if the drill- string is longer than the critical length or if it rotates below the critical rotational speed. Accordingly, it is clearly seen that when the drill string rotates at a speed of 150 rpm, the axial stress along the longitudinal axis of the string is significantly substantial compared to other stresses for both mixed and not mixed drilling string. Bit bounce is therefore likely to occur followed by Bending Stress which is revealed with two peaks for different rotational speeds 70 and 92 (rpm) respectively in the steel drilling string compared with one peaked in the aluminum steel drilling string. 4. Conclusions In conclusion, using a 2024 surface-treated aluminum alloy drill pipe mixed with G105 steel alloy drill pipes has a number of technical benefits for reducing drill pipe malfunction and assuring successful drilling operations due to the many constraints that affect the integrity of steel drill pipes as summarized below: 1. Well path tortuosity: Aluminum drill pipe has a lower friction coefficient compared to steel drill pipe in open and cased holes, which results in a decrease of both torque and side forces. 2. Doglegs severity: Aluminum drill pipe has significantly good fatigue resistance even in severe doglegs. This is due to a lower fatigue ratio (0.931) than that of steel drill pipe. 3. Side force: The side force in a dog leg region is reduced in surface treated 2024 aluminum alloy compared to steel drill string. For that reason, the mixed drill string assembly provides a balance strength regarding both tensile and bending forces as well as considering buckling compression load during drill string running and well bottom axial loading. 4. Vibration due to bit bounce on the cutting surface may occur and damage the bit. This may cause severe bending moments to develop in the BHA leading to fatigue failures. It was verified that the 2024 aluminum alloy drill pipe exerts significant influence on the stress distribution but to a larger extent affects the amount of critical loads at which the loss of stability of the drilling string does not occur. Good resistance to wear and corrosion, even at high temperature, have also been verified. Eliminating all these restraints is impossible. But they can be controlled and reduced. Knowledge from the drilling engineers is crucial to successfully address these problems and experience is vigorous to improve best field practices. Nomenclature W.O.B rpm Lpm R.O.P BHA API H2S PEO FFR
Weight on bit Revolution per minute Liter per minute Rate of penetration Bottom hole assembly American petroleum institute Hydrogène sulfur Plasma electrolytic oxidation Forced-frequency-response
Acknowledgment This research was conducted at Laboratory of Petroleum Equipment's Reliability and Materials at University M'hamed Bougara, Boumerdes, Faculty of Hydrocarbons and Chemistry Independence Street, 35000; Algeria. [email protected], lfepm@univ_boumerdes.dz References [1] American Petroleum Institute, Recommended Practice 7G, Recommended Practice for Drill Stem Design and Operating Limits, (1998), pp. 96–98. [2] M.T. Albdiry, M.F. Almensory, Failure analysis of drill string in petroleum industry: a review, Eng. Fail. Anal. 65 (2016) 74–85, https://doi.org/10.1016/j. engfailanal.2016.03.014. [3] S. Luo, S. Wu, Effect of stress distribution on the tool joint failure of internal and external upset drill pipes, Mater. Des. (2013), https://doi.org/10.1016/j.matdes. 2013.05.073. [4] M. Abdel Salam, A. Mahmood, Handbook of Materials Failure Analysis with Case Studies from the Oil and Gas Industry, Applied Thermal Engineering Journal, Elsevier, 2016 (ISBN: 9780081001172).
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[5] Decision Space Well Engineering Software, EDT_ 5000.1. Software System. Landmark Products, (2014). [6] O. Vaisberg, O. Vincke, G. Perrin, J.P. Sarda, J.B. Fay, Fatigue of drill string: state of the art, Oil Gas Sci. Technol. 57 (2006) 7–37, https://doi.org/10.2516/ ogst:2002002. [7] R.H. Wang, Y. Bin Zang, R. Zhang, Y.H. Bu, H.Z. Li, Drillstring failure analysis and its prevention in Northeast Sichuan, China, Eng. Fail. Anal. 18 (2011) 1233–1241, https://doi.org/10.1016/j.engfailanal.2011.03.005. [8] L. Pan, Stability Analysis of the Rotary Drill-String, The University of Tennessee, Knoxville, 2014 PhD thesis. [9] H. Yan, Z. Xuehu, B. Zhenquan, Y. Chengxian, Science direct 20th European conference on fracture (ECF20) failure analysis on fracture of a S135 drill pipe, Procedia, Mater. Sci. 3 (2014) 447–453, https://doi.org/10.1016/j.mspro.2014.06.075. [10] J.J. Azar, Drilling Problems and Solutions, Petroleum Engineering Handbook, SPE International Publication, 2015, pp. 433–454. [11] A. Lubinski, A Study on the Buckling of Rotary Strings, API Drilling Production Practice, (1950), pp. 178–214. [12] B. Legarth, J. Lehne, H. Okibe, V. Koinkar, Z. Mammod, Changing the game from steel to aluminum drill pipe: results from full string aluminum alloy drill pipe deployment to enhance extended reach drilling performance, Technol. Conf. Exhib. SPE/IADC-173088, 2015. [13] M. Jenkins, A.C. Rodriguez, Field Tests Show Aluminum Drill Pipe Can Extend Operating Envelope for Extended-Reach Drilling, (2010). [14] S. Menand, J.K. Lehner, A. Oil, N. Evans, A. Palmer, A. Metcalfe, P. Virginia, Successful Use of Mixed Aluminum-Steel Drill Pipe String in Complex Horizontal Wells : Case Study, (2014), pp. 1–13. [15] I. Diyashev, V.Y. Lishchuk, G. Aker, O. Bustos, L. Prakash, Successful horizontal drilling in Western Siberia: use of appropriate, cost-effective technology solutions to increase well productivity, IADC/SPE Drilling Conference, 2004, https://doi.org/10.2118/87122-ms. [16] A. Lubinski, Maximum permissible dog-legs in rotary boreholes, J. Pet. Technol. 13 (02) (1961) 175–194, https://doi.org/10.2118/1543-g- pa. [17] P. Paslay, E. Cernocky, P bending stress magnification in constant curvature doglegs with impact on drill string and casing, IADC/SPE Drilling Conference, 1991, https://doi.org/10.2118/22547-MS. [18] S. Moradi, K. Ranjbar, Experimental and computational failure analysis of drillstrings, Eng. Fail. Anal. 16 (2009) 923–933. [19] L. Belkacem, N. Abdelbaki, M. Gaceb, E. Bouali, Drill String Fatigue Failure and Lockup Risks Assessment in Tortuous Trajectory Well in Algeria, vol. 61, (2017), pp. 1099–1104, https://doi.org/10.3303/CET1761181. [20] L. Belkacem, N. Abdelbaki, M. Gaceb, E. Bouali, Integrating Superficially Treated 2024 Aluminum Alloy in Steel Drill String to Deal with Fatigue Problem in Crooked Trajectory for Vertical Deep Well, (2017), https://doi.org/10.1007/978-3-319-45677-5_30. [21] S. Menand, SPE 151279 a New Buckling Severity Index to Quantify Failure and Lock-up Risks in Highly Deviated Wells, (2012), pp. 1–15. [22] S.M. Zamani, S.A. Hassanzadeh-tabrizi, H. Sharifi, SC, EFA, (2015), https://doi.org/10.1016/j.engfailanal.2015.10.012.
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Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Using a supperficially treated 2024 aluminum alloy drill pipe to delay failure during dynamic loading
T
⁎
Lallia Belkacema, , Noureddine Abdelbakib, Jose Luis Oteguic, Mohamed Gaceba, Mourad Bettayeba a
Laboratory of Petroleum Equipment's Reliability and Materials, Université M'hamed Bougara de Boumerdes, Algeria University Akli Mohand Oulhadj, Bouira 10000, Algeria c Consejo Nacional de Investigaciones Científicas y Técnicas, Argentina b
A R T IC LE I N F O
ABS TRA CT
Keywords: Steel drill pipe Aluminum drill pipe Failure analysis Buckling Fatigue
For many years, a critical factor while drilling deep wells in Algeria oil fields has been the fatigue loads in drill strings used in high tortuosity sections. One solution is the use of drill pipes made of alternative lighter materials instead of conventional steel. A thorough study of superficially (ST) treated 2024 aluminum alloy joined to steel drill pipes in crooked well trajectories is presented in this article. This work discusses several limiting loads exerted on the drill pipes that affect the drilling operation such as axial, twisting and bending forces, which lead to combined cyclic loads. This study is aimed to determine whether it is possible to extend the drilling operation by joining ST 2024 aluminum alloy drill pipes into the drilling string, without losing drilling stability or reliability. It is established that the ST 2024 aluminum alloy drill pipe exerts significant influence on the allowable combined load, but affects to a larger extent the critical load at which loss of drilling string stability occurs, along with improved resistance to wear and corrosion, even at high temperature.
1. Introduction During drilling operations, drill pipes and other string components are commonly used in worn conditions, wear, fatigue, wash out and other service related defects cause the pipes to lose some original strength and this limits their ability to withstand the same stresses as when they were in their new state, since they suffer a certain degree of damage. A pipe inspection provides an assessment of the damage, and the pipe is assigned to a class corresponding to the extent of damage (outside diameter thus insures their reliability and service life in order to reduce failure costs and increase yield [1]. Field experiences show that many failures occur while rotating or while picking up off bottom immediately after drilling. They occur within 4 ft of the tool joint and are associated with severe pitting on the inside wall of the pipe. These cracks appear to have started from the inside. Failures that appear to originate from the outside of the pipe are usually associated with slip marks, or surface damage when failures occur as a result of pulling on stuck pipe. The failure frequently occurs in a location where fatigue cracks have developped but had not progressed to failure. [1,2]. On the basis of previously conducted researches, drill string fatigue is still excessively frequent. The main reason for this is combined stresses (tension, torsion and compression) and different types of vibrations caused by dynamic and cyclic loading. These loads are constantly changing depending on trajectory and dog leg severity [1,2]. Therefore, drill string components should be selected and configured to delay fatigue as much as possible and in an economical and practical manner. As well as vibration analysis ⁎
Corresponding author. E-mail addresses: [email protected], [email protected] (L. Belkacem), Jotegui@fi.mdp.edu.ar (J.L. Otegui).
https://doi.org/10.1016/j.engfailanal.2019.06.005 Received 14 October 2018; Received in revised form 14 May 2019; Accepted 4 June 2019 Available online 06 June 2019 1350-6307/ © 2019 Published by Elsevier Ltd.
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Fig. 1. Critical regions of the drill pipe and connection causing fatigue failure [6].
techniques and torque and drag modeling, hydraulic design and directional control are very important. In parallel, many researchers have attempted to address drill pipe connection failures based on the following points: [2–5]. 1) The high torque to seal the joints creates a high strain in the connection. 2) The effect of stress concentration on the pin and box threaded joint's failure of the upset drill pipes under tensile, compression and bending loads 3) The stress concentrating factor (SCF) at the root of the thread. 4) The fatigue crack nucleated at the first root of the tooth from the pin tool joint shoulder of the drill pipe, and then propagated through the wall of the tool joint. 5) The deterioration of the fatigue resistance of the tool joint is related to dog leg region where severe cyclic bending load exists due to the local deviation of the drill pipe from the vertical line. The combined effect of these factors is that drill string rotary connections invariably fail at the first engaged thread on the pin side of the joint, and at the last engaged thread in the box component, Fig. 1. Similar work was conducted by Vaisberg et al. [5] who reported that, by employing 165-ksi grade material for drill pipes, the strain concentrations at upset transitions were completely redesigned in order to prevent any fatigue failures. With standard tool joints manufactured from 120 ksi yield material, a 165 ksi drill pipe, product with 165 ksi tool-joints, offers a 38% increase in tool joint torque and tension capacities. The hydraulic loss through the drill pipe was also considered. He mentioned also that, the pipe weight has been reduced by decrease of the wall thickness using high strength steel, and the corrosion fatigue property is superior besides, the résistance to high temperature is also good. Also, Wang et al. [7] studied the manufacturing anomalies and design defects of the drilling stem and the tool joints which affect its strength. They mentioned that the defects lead to inequitable stress distribution, resulting in premature failure of the drill string. It was also observed that fatigue failure of the drilling stem caused by the extensive tensile or compressive stress acting on the drill string, resulting from drill string sticking and the improper anti-sticking methods like over pushing and over pulling of the drill string. Moreover, Pan [8] supposed that fatigue cracking initiated in the drill pipe can be propagated by the combined action of corrosion and erosion processes, and a sacrificial cathode such as aluminum or zinc coating on both the inner and outer surfaces of the drill pipe reduced the potential gap and then increased its fatigue strength despite the accumulation of H+. Likewise, H. Yan et al. [9] have reported that a fact of long-term operation of drill pipe in high H2S content condition, the problem of microstructural degradation plays an important role in susceptibility to brittle fracture for instance. Due to sulphide stress corrosion cracking and high H2S content condition smooth cracks propagating radially and axially in a 76.2 mm (S135) drill pipe that deforms to a rough and possibly fractured surface. Meanwhile Azar.J [10] stated that during a drilling process, multiple modes of failure are encountered in the drill pipes rather than those described above, such as pipe wash out and twist-off, sticking, pipe-parting, collapse and burst failure. These failures are most often encountered due to similar down hole stress problems, even with very carefully planned wells. Ductile and brittle besides fatigue failure are also taken into account in these damage mechanisms. A number of studies conducted by Lubinski [11], which identified the effect of increasing the angle by the wellbore, so does the tendency for the pipe to lie down on the lower side of the hole allows the pipe to reach a state of stability and to carry higher axial compressive stresses without buckling. However, when more energy is put into the system, buckling can occur. Although widely used the steel drill string contain various drawbacks. Therefore significant interest in using aluminum drill pipes instead of steel drill pipes is very noticeable throughout the last years especially; therefore a several processing methods were successfully developed in order to reduce many of these challenges. For instance, ref. [1, 12] applied a unique approach and workflow data into usable knowledge, supporting that light weight drill pipes, when used properly, can lead to reduced drilling loads and lower torque and drag, thus making more extended drilling depths possible. As illustrated in Fig. 2, the reduction in fatigue life experienced by steel drill pipe and aluminum drill pipe while going through gradual doglegs. Aluminum drill pipe appears to be able to pass through more severe doglegs without damage. And although aluminum drill pipes are not as strong as steel drill pipes because of their lower buckling limit, they have a significantly longer fatigue life because of their lower Young's Modulus [14]. Therefore, using lighter materials has proven to be a good alternative for weight reduction. It also allows for bigger pipe diameters, which can reduce pressure losses and therefore provide more hydraulic power for the bottom hole assembly (BHA) and bit. Furthermore, the aluminum drill pipe wears less than steel as the
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Fig. 2. The reduction in fatigue life experienced by steel drill pipe and aluminum drill pipe while going through gradual doglegs. [13].
tension in the string is lower, thereby reducing the axial and torsional drag, and has great flexibility when the drill stem is subjected to unusual stretching, bending and twisting loads as demonstrated [15]. Pan, [16] on the other hand, stated that aluminum drill pipes did not penetrate the market as expected, despite their distinct advantages over steel drill pipes; First, because the relatively low yield strength compared to high-strength steel limits its applicability in complex wells. The aluminum alloy used for drill pipes has yield strength of 58,000 psi (400 MPa). The tool-joints for aluminum drill pipe are manufactured from steel. Secondly he mentioned that aluminum drill pipes have experienced serious wear and corrosion problems in the middle of the body, mainly due to the pH of the mud, chlorides and oxygen, high temperatures and storage. Similarly he assumed that fatigue cracking initiated in the drill pipe can be propagated by the combined action of corrosion and erosion processes.as well other scientists assumed that a sacrificial cathode such as an aluminum or zinc coating on both the inner and outer surfaces of the drill pipe reduced the potential gap and then increased its fatigue strength despite the accumulation of H+. The aim of the present work is to conduct a comprehensive review of most failure modes that occurred in the string during the drilling process of the Algerian oil field, explaining the main reasons for these failure modes in order to manage risks and achieve high well performance throughout the drilling process. It aims also to verify that the use of a new set of surface-treated drill pipe materials (aluminum alloy 2024) can solve this problem. 2. Material and methods Concrete examples of the degradation of steel drill pipes in Algerian oil fields are presented. It is followed by a summary of the 203.2 mm (8.5 in.) drill section. A dynamic analysis of the drilling operation is then performed to identify the uniform distribution of stresses across the borehole of the drill ring using well design software based on the stiff String" model. The evaluation of the actual data sets will be conducted to explore the validity of the aluminum alloy integration hypothesis in order to determine an optimal solution to the problems encountered. 2.1. Case studies of steel drill pipe damage in Algeria oil gas fields 2.1.1. Case 1: drill pipe shearing During drilling a 12–1/4″ hole section with weight on bit from 8 to 21 ton, RPM = 130–170, Q = 177 m3/h, SPP = 20,684.27 kpa, ROP = 14 m/h, the driller reamed each drilled joint. The lithology at 2070 m was 20% Clay, 60% Anhydrite and 20% Dolomite. It was decided to pull out the drill string for wiper trip from 2327 m to 2091 m and try to unscrew, a very tight 263
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Fig. 3. The 5 in drill pipe (127 mm) shearing.
connection. The 5 in drill pipe (127 mm) had sheared and was caught by the drill pipe slip; consequently the drill string broke within the body of the drill pipe. The string was hanged by the drill pipe, the Annular & Pipe rams BOPs was closed for safety. The failure investigation revealed that the combined applied stress at the moment was less than the yield strength of the material [15]. (Fig. 3). 2.1.2. Case 2: drill pipe failure After drilling 12–1/4″ in hole section with weight on bit 8 to 21 ton, RPM = 200 rev/min, ROP = 32 m/h, from 2466 to 2865 m (total section depth), the drilling string was pulled out to casing shoe to perform a formation integrity test (FIT). During pulling out, the driller had a hard back-reaming at six different intervals between 2740 m and 2483 m. After circulation and cleaning hole (density in equal to density out), operators pulled out the drilling string to top BHA (Bottom Hole Assembly). Three joints of DP (100 m above Heavy Weights) had been over-torqued and there was a gap between pin and box (Fig. 4). When the BHA was raised to surface, some broken thread in middle part of Jar (mechanical section) was observed. The investigation revealed that the Bottom Hole Assembly was properly designed in terms of neutral point and sufficient available weight. The WOB applied while drilling was within the design range, kept below 20 tons, similarly for the Bending Strength Ratio and the Stiffness Ratio. This BHA is commonly used in this area (offset wells) for drilling 12–1/4 in sections. - Down hole torque while drilling was higher than that before in offset wells. This seems to be the most important contributing factor in these drill pipe connection failures. In addition, Top Drive System stalling during back-reaming and the back torque may have contributed to drill pipe connections partially backed off. Down hole torque was within the range for the Jar in terms of torsional capacity and the jar inspection sheet shows that the jar is in good condition. Total hours on the jar is 154, were less than allowable (250 h. max). This will make the jar failure investigation inconclusive, but the RPM used to drill was up to the limit (200 rev/min). This might have contributed to the high torque. 2.1.3. Case 3: wash out During drilling 12–1/4″ in hole section from 2470 m to 2516 m with increased weight on bit from13 to 20 t, RPM = 130 rpm, Q = 8 m3/h, torque =2 ton.m, SPP = 12,755 KPa, the driller reamed each stand and increased mud weight from 1897 kg/m3 to increase 2204.80 kg/m3, the lithology was: 0–20% clay, 0–30% dolomite 50–100% anhydrite. The stand pipe pressure dropped suddenly, so it was decided to pull out the drill string in cased hole to 1950 m. A wash out problem was discovered in 127 mm (5in), S135 drill pipe number 177(Fig. 5).
Fig. 4. The 5 in drill pipe (127 mm) connection damage. 264
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Fig. 5. The 5 in drill pipe (127 mm) wash out, the crack propagated radially and circumferentially.
Due to the high pressure drilling mud passing through cracks and openings, fracture surfaces were washed away and no useful information was obtained from these surfaces. It was assumed that the washout was due to a fatigue crack initiated on the internal surface of the tube body, in a region where the internal coating had been damaged thus allowing corrosion of the metal substrate (The drilling mud may serve as electrolytic medium). 2.2. Well design and integrity This study was conducted using background data gathered for the 203.2 mm (8.5 in.) drill section in the X field in Algeria. As numbered below; i) ii) iii) iv)
In this section, the goal is to drill Clay Lias (Lias Argileux) at 150 m in a Silurian formation called F6-B as depicted in Fig. 6. The reservoir formations were covered with a production coating of 0.1778 m in diameter, extending from 2942 m to 4785 m. The drilling parameters related to this drilling section are reported in Tables 1 and 2. The well path data is depicted in Table 3
Fig. 6. Hole section 203.2 mm (8.5 in) lithology. 265
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Table 1 Hydraulics data summary. Hole size (mm)
Depth (m)
Bottom hole assembly type
Recommended flow rate (m3/ h)
Minimum flow
Mud weight (kg/ m3)
Average rate of penetration (m/h)
1297–1347
7
Clean (m3/h) 203. 20
4785
Packed
84–108 m3/h
40
Table 2 Torque and drag summary. Axial Loads Hole size (mm)
Depth (m)
Bottom hole assembly type
Recommended WOB (N)
Buoyed weight below jar (N)
String weight (N)
Pick-Up weight (N)
Over pull (N)
• Does not include travelling assembly Table 3 Well path data. Measured depth (m)
Inclination (°)
True vertical depth (m)
Dog leg (°/30 m)
3168 3179 3190 3200 3211 3219 3222 3245 3256 3265 3275
6.540 8.010 9.750 10.87 11.67 12.43 13.14 16.82 19.9 19.95 23.24
3168 3178 3189 3199 3201 3217 3221 3243 3253 3262 3271
4.846 4.332 4.488 3.32 2.251 2.868 6.358 4.757 9.041 0.154 10.292
The investigation reveals that the most important problems in drilling operations are associated with torque and drag load. As the drill bit progresses through the well, forces are generated in three directions: lateral, axial and torsional. For this reason, the result of these three constraints is one of the most important data to be included in the process. Thus, the effect of these multi-axial stresses due to hydrostatic pressure and mechanical loads was assessed through this study and compared to the strength of the drill pipe material and the manufacturer's specifications as well as the dynamics of the down hole. The stresses leading to the failure of the drill pipes are indicated hereunder: i) ii) iii) iv) v)
Axial stresses due to tension, bending and buckling. Approaching bending stress of the borehole curvature. Torsional stress due to torsion Transverse shear stress due to contact force axial, lateral and torsional vibration
2.3. Stiff string model Well design software based on the Dawson cable model (The stiff-string) is based on the assumption that the drill pipe is composed of short segments connected by connections that transmit tension and the drill string is made up of short segments joined by connections which transmit tension, compression and torsion. The stiff string model traverses the string by selecting each component length or 30 ft. (9,14 m) equivalent section. It computes the side force at the center point. This side force is used to compute the multiaxial changed from one element to the next element as depicted in Fig. 7. The detailed analysis of each node involved creating a local mesh of 10 to 20 elements around this node. The end nodes of the mesh are given the following end constraints (boundary conditions):
• If the model is at the bit or the top of the string, the node is a pivot and is free to rotate. • If the end node has been previously solved in the traverse, then the node is fixed with the displacement and angle from the previous solution. If • the node is at the front end of the traverse, the node is fixed in the center of the hole. Each sub-element is given the same dimensions and properties as it would be given to the full drill string. If the node length 266
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Fig. 7. The stiff string model and stress acting on drill string.
exceeds the maximum column-buckling load for the section, the node is further broken into fractional lengths to keep each section below the buckling threshold. This is why the analysis can take considerably longer when large compressive loads are applied. This paper consists of an approach in which will be discussed, the effect of the side forces and all derived calculations (Torque and Drag) as well as the bending stresses together with the pipe position in the hole, the hydrostatic pressure and the well conditions on the drilling string. 3. Discussion 3.1. Load distribution in the steel drill string The Results of this analysis provide data on the mechanical integrity and elongation of steel drill pipes. All failures due to axial and bending, as well as torsions stresses, were recorded (Table 4). The total stretch in the string is computed as the sum of stretch components due to axial, buckling and ballooning which is caused by differential pressure inside and outside the string as follows:
∆L stretch = ∆LHL + ∆LBuck + ∆LBalloon + ∆L (thermal)
(1)
where: ΔLHL is the change in length due to the Hooke's Law mechanism, ΔLBuck is the Change in length due to buckling Load and ΔLBallon is the change in length due to ballooning mechanism; units (m) [5]. 1) ΔLHL Stretch due to axial load. This term is based on Hooke's Law. The first term reflects the constant load in the string, while the second term reflects the linear change in the load. [5]
∆LHL =
Fapa × Lc Ac × E
+
∆F × L 2Ac × E
(2)
where Fapa is defined as Axial force as determined by the pressure area method units MN; Lc is length of work string component element (in m); Ac is the cross sectional area of component units (in m2).E = Young's Modulus of component, unit (MPa).Fapa = Change in pressure area axial force over component length is calculated using the pressure area method. Table 4 String Analysis Summary of steel drill pipe during on-off rotation and tripping out of the hole. Load condition
Tripping out Rotating on bottom
Stress failure
Fatigue Fatigue
Buckling limits
No buckling No buckling
267
Stretch(m) Mechanical
Ballooning
Thermal
4.99 3.94
0.36 0.36
1.60 1.60
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Fapa =
∑ [LD Wair cos θ + Fdrag + ∆Farea] − Fbtm − Wbit
(3)
where: the Fbtm is the bottom pressure force which is a compressive force due to fluid pressure applied over the cross-sectional area of the bottom component, units (MN). Fdrag = Drag Force, units (MN), LD = Length of string, units (m), Wair = Weight per foot of string in air, unit (N/ft),Wbit = Weight on bit, unite (N) and ΔFarea = Change in force due to a change in area at junction between two components with different cross -sectional areas, such as the junction between drill pipe and heavy weights, or between heavy weights and drill collars. If the area of the bottom component is larger, the force is tensile. If the top component is larger, the force is compressive. 2) Stretch due to buckling If buckling occurs, the additional stretch in the buckled section of the work string is calculated using the following equation. [5]
∆LBuk =
rcl2 Fapa Lc 4EI
rcl2 Fapa Lc
+
(4)
8EI
where: ΔLBuck = Change in length due to buckling, Fapa = Axial force as determined by the pressure area method, unit (Kpa), ΔF = Change in pressure area axial force over component, unit (KPa) and E = Young's Modulus of component, unit(KPa).I = Moment of Inertia unit kg. m2 and r = Clearance between the wellbore wall and the work string component (m) 3) Stretch due to ballooning Stretch due to ballooning is caused by differential pressure inside and outside the work string, and is defined by the following equation [25]
−νLc
∆LBalloon = E⎡ ⎣
( ) dbo dbi
2
2
d d ⎡ ⎤ x ⎢ ⎜⎛ρmi − ⎛ bo ⎞ ρma ⎟⎞ Lc + 2 ⎜⎛Ps − ρma ⎛ bo ⎞ ⎟⎞ ⎥ d d bi bi ⎝ ⎠ ⎝ ⎠ ⎝ ⎠ ⎤ ⎠ − 1 ⎣⎝ ⎦ ⎦ ⎜
⎟
⎜
⎟
(5)
where ΔLballoon is the stretch due to ballooning, dbi and dbo are the inside and outside diameter of body. E = Young's Modulus of Elasticity. Lc is the length of component, ps = Surface pressure on string side and ν = Poisson's Ratio for component. ρma is the mud density in annulus at depth of string component and ρmi = Mud density inside string component unit (kg/m3) 4) Stretch Due to Temperature Effects
∆ltemp = αL c ∆T
(6)
where ΔT = Temperature change over component length (in °K), α = Coefficient of linear thermal expansion and ΔT = Temperature change over component length [25]. The Field case appears to be a problematic well that required various trajectory corrections throughout the drilling process to reach the target (side track). This is clearly observed in the fluctuation in inclination and azimuth measurements as shown in Table 3 which gives data for a complex well trajectory, drilled vertically to 3275 m and build 10°/30 m reaching 23.24° inclination at TD [3275mMD]. The latter reached about 4°/30 m 4 times at depths ranging from 3168.31 to 3190.12 m, to finally reach a maximum of 10°/30 m between 3190,12 and 3274.99 m. This leads to an increased risk of bending stress severity in the above-mentioned areas. This finding was confirmed by the fatigue ratio value of 1.87 above 1, which makes the use of steel drill pipes impractical in the area of fatigue failure, as shown in Table 5.
Rf =
(|σbend| + |σbuckle|) σft
(7)
where Rf = Fatigue ratio and σbend is bending stress corrected by the bending stress magnification factor. σbuckle is the buckling stress Table 5 Fatigue ratio for steel drill pipe. Measured depth (m)
Contact force (Kgf/length)
Torsional stress (KPa)
Shear stress (KPa)
Axial stress (KPa)
Bending stress (KPa)
Bending stress magnification factor
Von mises ratio
Fatigue ratio
3151.82 3160.94 3179.18 3224.78 3233.91 3243.03 3252.15 3261.27 3270.39
1183.00 1252.70 1204.20 1234.30 1107.50 1880.20 1015.50 131,380 1056.0
94,518 92,819 89,345 82,914.9 81,142.0 79,551.2 76,850.7 75,392.1 73,505.1
8357.9 8849.7 8507.2 8720.3 7824.3 13,283. 7174.4 9281.3 7460.1
140,245.9 139,160,9 136,997.8 131,657.4 130,602.2 129,554.0 128,531.4 127,485.9 126,510.0
105,247.2 105,870.1 107,863.6 118,899.0 121,788.7 194,997.3 117,750.1 131,913.1 140,634.4
2.932 2.921 2.898 2.841 2.829 2.818 2.807 2.795 2.785
0.445 0.442 0.438 0.439 0.439 0.529 0.427 0.442 0.45
1.027 1.031 1.047 1.144 1.17 1.87 1.127 1.261 1.342
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Table 6 Details in each specific drilling string. Section
Steel/Steel drill pipe
Aluminum/Steel drill pipe
Grade Depth (m) Yield strength (Mpa) Tensile strength (Mpa) Young module(Mpa) Fatigue Endurance limit (Mpa)
G105 4400 724 931 206,896.55 137.89
G105 850 724 931 206,896.55 137.89
AL2024 250 359 466.5 88,763 160
S135 3060 931 1138 206,896.55 137.89
and σfl is fatigue endurence limite. 3.2. Distribution of loads in aluminum drill pipe Full-scale tests were undertaken by replacing the steel drill pipe joints with lighter aluminum drill pipes in the string of drill pipes while maintaining the same hole section and tool joints made of steel. Table 6 lists details of all specifications for each drill string, followed by a summary of the stress analysis during the selected operation. The effective tension curve in Fig. 8 shows that the drilling operation curves do not cross the tension limit curve at 4400 m. There is therefore no strain on the aluminum drill pipe during this tortuous area. The pipe had attained its yield strength to a depth of 4400 m. It can therefore be inferred that aluminum drill pipes have good fatigue resistance, despite the simultaneous presence of high torque and high axial load, as well as severe doglegs. This is supported by a fatigue rate of less than 1 which is 0.931, making the use of steel pipe impractical for fatigue ratio calculation as shown in Table 7. Another area of interest in this regard is the dynamics caused by inadequate loads, and in particular the analysis of critical
Fig. 8. Effective tension curve for mixed steel -AL2024. 269
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Table 7 Fatigue ratio mixed drill string. Measured depth (m)
Contact force (Kgf/ length)
Torsional stress (KPa)
Shear stress (KPa)
Axial stress (KPa)
Bending stress (KPa)
Von mises ratio
Fatigue ratio
3167.90 3176.83 3185.76 3194.69 3203.62 3212.55 3221.47 3230.40 3239.33 3248.26 3257.19
965.8 919.6 661.5 499.6 745.4 1279.3 982.4 1196.0 1443.2 444.6 1522.9
55,802.1 54,444.2 53,151.3 52,221.2 51,518.9 50,470.8 48,672.1 47,290.9 45,609.3 43,580.1 42,955.0
6679.2 6359.4 4574.7 3454.9 5155.4 8847.2 6793.9 8271.6 9981.0 3074.9 10,531.9
108,183.9 107,126.2 106,072.0 105,018.4 103,969.6 102,925.4 101,889.3 100,853.0 99,824.1 98,813.7 97,790.4
46,247.6 49,074.9 42,184.4 32,608.3 37,487.2 59,712.7 47,456.0 46,957.6 78,591.7 14,887.6 88,954.7
0.584 0.586 0.563 0.535 0.543 0.595 0.558 0.552 0.628 0.458 0.646
0.506 0.534 0.457 0.352 0.403 0.639 0.505 0.498 0.83 0.157 0.931
velocities, which is used to determine critical rotational speeds of drill pipes and areas of high stress concentration in the entire drill string. The practice is to locate a corresponding critical rotational speed above which the system is unstable. The Forced-FrequencyResponse (FFR) analysis model presented herein enables to determine the forced frequency response (critical speeds) of a BHA due to imposed load and/or displacement excitations anywhere in the drill string. Results obtained can be used to avoid critical rotary speeds that accelerate pipe fatigue resulting in catastrophic drill string failure as provided in Figs. 9 and 10.This result reveals that the axial stress along the longitudinal axis of the drilling string is significantly greater than 5515.81Kpa for both steel and mixed drilling string compared to other loads. Bit bounce is therefore likely to occur, whereas, the bending stress in steel drilling string, revealed with two stresses peaks about 648Kpa and 2026 Kpa with different rotational speed 70 and 92 (rpm) respectively, compared to mixed drilling string,which reached one occasional peak of about
Fig. 9. Critical speed analysis for mixed Aluminum 2024 drill sting. 270
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Fig. 10. Critical speed analysis for steel drill pipe.
3350 KPa at rotational speed of 84 rpm. 3.3. Drill string dynamics The 127 mm, 29Kg/m G105 steel drill pipe experienced to fatigue failure during rotating on/off bottom operations, which may be the result of unintentional deviations and loading conditions. For instance, rotation in a dangerous dogleg region where the severity exceeds 10°/9.14 m [158,16] induces large alternating stresses in the drill pipe, which becomes a potential site for fatigue cracks nucleation. Thus risky results were obtained by increasing the severity of the moment bending curvature on the aforementioned zones [17], from3151.82 m to 3270.39 m. For this reason, the drill pipe is under tension and has been pulled towards the inside of the bend with a high lateral contact force that is greater than 8896 N (2000 lb) with a rotational speed of 150 rpm. Accordingly, the abrasion problem in the drill pipe connections is very significant. [18–20]. In fact, the calculation of bending stress is the key criterion of the cumulative fatigue damage model. The cumulative fatigue damage model as defined in API RP7G has been incorporated into an advanced torque and strength model, which tracks any variation in stress at any given point in the drill pipe during drilling, such as the transition zone, connections and drill pipe body [8,19,21]. For instance the G105 alloy drill pipes provides an alteration in bending forces throughout the drilling process, due to side forces resulting from dog leg area. Thus the first set of analyses highlighted the impact of dog leg interval in the steel drill pipe, since bending pipe around dog leg rises tension load on outside section of the pipe and declines on the inside of the curve,. Subsequently, the rotation causes oscillating bending stresses which can quickly lead to fatigue failure. As a result, the high stress concentration mentioned in Table 5 above has resulted in an increase in the fatigue ratio from 1.26 to 1.78, which is high. (The minimum fatigue ratio is equal to 1 as a safety limit). Consequently, fatigue failure of the drill pipes is accelerated since the efficiency of axial force transfer is reduced [4,20–22]. On the other hand, although the 2024AL alloy drill pipe has low mechanical properties in terms of density, Young's modulus, yield and tensile strength, compared with 127 mm, 29Kg/m G105 alloy drill pipes, have high flexibility when the drill pipe is subjected to abnormal stress and bending, during tortuous conditions. The latter is confirmed by a fatigue rate of 0.931 less than 1, which makes the use of steel pipes impractical in the event of wear and tear, which makes it impracticable for such an operation. Since the lateral 271
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force in the area of the dog's legs has decreased compared to the lateral force of a steel drill string. The mixed drill pipes therefore provides balanced tensile and bending strength, while taking into account the compressive load during operation of the drill pipes and the axial load on the bottom of the well. [12,14,20] It can then be hypothesized that the 2024AL alloy drill pipe is a powerful torque and drag reduction tool for very tortuous wells and solves many of the problems encountered during drilling, as it extends its fatigue life and reduces wear for both casing and drill pipe. This result is confirmed by the output obtained in the Torque Drag Effective Tension Graph Fig. 8 which has included the next graphical curves on Tension vs. Distance along String. Accordingly; it is obviously seen that all operations curves do not cross the Tension Limit curve, the drill string is located into a safe window; therefore there is no danger of exertion the aluminum drill pipe in tortuous interval. [19,20] As a final point, the uniform rotation is unstable if the drill- string is longer than the critical length or if it rotates below the critical rotational speed. Accordingly, it is clearly seen that when the drill string rotates at a speed of 150 rpm, the axial stress along the longitudinal axis of the string is significantly substantial compared to other stresses for both mixed and not mixed drilling string. Bit bounce is therefore likely to occur followed by Bending Stress which is revealed with two peaks for different rotational speeds 70 and 92 (rpm) respectively in the steel drilling string compared with one peaked in the aluminum steel drilling string. 4. Conclusions In conclusion, using a 2024 surface-treated aluminum alloy drill pipe mixed with G105 steel alloy drill pipes has a number of technical benefits for reducing drill pipe malfunction and assuring successful drilling operations due to the many constraints that affect the integrity of steel drill pipes as summarized below: 1. Well path tortuosity: Aluminum drill pipe has a lower friction coefficient compared to steel drill pipe in open and cased holes, which results in a decrease of both torque and side forces. 2. Doglegs severity: Aluminum drill pipe has significantly good fatigue resistance even in severe doglegs. This is due to a lower fatigue ratio (0.931) than that of steel drill pipe. 3. Side force: The side force in a dog leg region is reduced in surface treated 2024 aluminum alloy compared to steel drill string. For that reason, the mixed drill string assembly provides a balance strength regarding both tensile and bending forces as well as considering buckling compression load during drill string running and well bottom axial loading. 4. Vibration due to bit bounce on the cutting surface may occur and damage the bit. This may cause severe bending moments to develop in the BHA leading to fatigue failures. It was verified that the 2024 aluminum alloy drill pipe exerts significant influence on the stress distribution but to a larger extent affects the amount of critical loads at which the loss of stability of the drilling string does not occur. Good resistance to wear and corrosion, even at high temperature, have also been verified. Eliminating all these restraints is impossible. But they can be controlled and reduced. Knowledge from the drilling engineers is crucial to successfully address these problems and experience is vigorous to improve best field practices. Nomenclature W.O.B rpm Lpm R.O.P BHA API H2S PEO FFR
Weight on bit Revolution per minute Liter per minute Rate of penetration Bottom hole assembly American petroleum institute Hydrogène sulfur Plasma electrolytic oxidation Forced-frequency-response
Acknowledgment This research was conducted at Laboratory of Petroleum Equipment's Reliability and Materials at University M'hamed Bougara, Boumerdes, Faculty of Hydrocarbons and Chemistry Independence Street, 35000; Algeria. [email protected], lfepm@univ_boumerdes.dz References [1] American Petroleum Institute, Recommended Practice 7G, Recommended Practice for Drill Stem Design and Operating Limits, (1998), pp. 96–98. [2] M.T. Albdiry, M.F. Almensory, Failure analysis of drill string in petroleum industry: a review, Eng. Fail. Anal. 65 (2016) 74–85, https://doi.org/10.1016/j. engfailanal.2016.03.014. [3] S. Luo, S. Wu, Effect of stress distribution on the tool joint failure of internal and external upset drill pipes, Mater. Des. (2013), https://doi.org/10.1016/j.matdes. 2013.05.073. [4] M. Abdel Salam, A. Mahmood, Handbook of Materials Failure Analysis with Case Studies from the Oil and Gas Industry, Applied Thermal Engineering Journal, Elsevier, 2016 (ISBN: 9780081001172).
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