Fracture failure of internal thread joint on back redressing stabilizer

Stabilizers have been one of the most critical components in the drilling string, due to their great function to prevent the unintentional sidetracking and vibrations in whole drilling assembly. In recent years, with the increasing of the well depth, the rising of the harsh service conditions have pose big challenges to the stabilizers. So it is very necessary to conduct systematic research on the failure mechanism of the stabilizer and carry out ideal failure prevention measures. In this article, the fracture failure analysis of an internal thread joint of the back redressing stabilizer was carried out, and the failure causes were investigated by nondestructive testing, mechanical property tests, chemical composition analysis, metallographic analysis and scanning electron microscope analysis. The results show that the mechanical tests and chemical composition analysis results all meet the requirements of the SY/T 5051-2016 standard, the fracture mode of the back redressing stabilizer is fatigue fracture, and the causes of the failure is mainly belong to the pit shape defects on the surface of internal thread and the quenching crack in the internal thread.


Introduction
As an important component in the drill string, stabilizer plays a significant role in the stabilization of the bottom hole assembly, and the improvement of the drill bit's service condition [1][2][3][4] .In the practical drilling operation, certain amount of stabilizers are placed in the bottom hole assembly as required, which can give an effective limitation of the lateral oscillation of the drill bit, and thus improve the quality of the wellbore configuration, also, the stress condition of the whole drill string could also be adjusted by the stabilizers based on their fulcrum function.Moreover, in some condition, blades also could be assembled on the outer surface of the stabilizer, which is called the back redressing stabilizer, and could conducted the back redressing operation of the wellbore when there is a blockage during tripping.During the process of well drilling, the stabilizer would endure harsh and complicate loads, including the tension load from the bottom hole assembly (BHA), torsion load applied by the top drive, impact and vibration load induced by rock-breaking of the bit, when combined with the corrosion caused by the corrosive medium in the formation or in the drilling fluid, would bring great risk to the service safety of the stabilizer [5][6][7][8] .For the failure accidents of the stabilizer happened frequently, but little failure analysis was conducted on it.When considering the important role of the stabilizer, it is very necessary to conduct systematic analysis on some failure accidents.
In this work, nondestructive testing, mechanical property tests including tensile, chapry V-notch impact and Brinell hardness tests, chemical composition analysis metallographic examination analysis, macro and micro fracture feature analysis and etc were conducted to the failure sample, aiming to find out the failure causes and put forward correspondingly failure prevention measures.

Background of failure
A back redressing stabilizer fractured at the internal thread joint when conducted back redressing operation to the depth of 3661m in a well.The photo of the fractured back redressing stabilizer internal thread joint is shown in Fig 1 .The failure process is as follows: back redressing conducted in a well after drilling to the depth of 4800m, when back redressing to the depth of 3661m, the pumping pressure suddenly declined from 2796psi to 1223 psi, the torque also reduced by 4kft-lbs.When fishing the string fish, and found the back redressing stabilizer fractured at its' internal thread position.The material of the back redressing stabilizer is 4145H, the type of the internal thread is 6 5/8 REG, the total service time of it was 453.75 hours.Furthermore, the micro-morphology of the fracture surface and cracks were analyzed using the scanning electron microscope (SEM) and energy dispersive spectrometer (EDS).

Metallographic analysis
The microstructures of the failed back redressing stabilizer internal thread joint characterized by optical microscopy are presented in Fig. 4. Samples were taken from the fractured location and the un-failed location respectively.The microstructures in both the two locations are all tempered sorbite, which is the normal microstructure of the iron in quenched and tempered heat treatment state.

Characterization of cracks
Crack sample which illustrated in Fig. 3 was taken for metallographic analysis, the results are shown in Fig. 5.There are pits along the thread surface, the crack is initiated from the pit which located on the root of the thread, and propagated towards the inner surface of the back redressing stabilizer internal thread joint.The crack exists a zigzag appearance and branched at the tip of the crack.Metallographic microstructure on both sides of the crack are all tempered sorbate, and no coarse grain or decarburization could be seen.

Discussion
The tests results above show that chemical composition, mechanical properties of the back redressing stabilizer internal thread joint all meet the requirements of the SY/T 5051-2016 standard.The failure of the stabilizer is fatigue fracture.Considering the pits on the surface of the thread and the cracks initiated from the pits on the root of the thread, it can be concluded that the fracture failure should be in relationship with the pits and cracks.
Generally speaking, the root of the thread which can be seen as a structure change could induce stress concentration mechanical component, that is the reason for most of the drilling tools all fractured at the internal or external thread joint.As for the internal thread of the back redressing stabilizer, there are many pits on the surface of the thread surface, and EDS analysis result indicates that there is no corrosive medium in the pits, so it can be concluded that pits on the thread surface were mechanical damage which induced by irregular operation.
The microscopic stress concentration on the bottom of the pits would definitely increase the stress concentration extent of the thread root, thus more easily for the initiation of the cracks.
Cracks initiated from the pits bottom on the root of the thread, and exits an inter-granular crack feature, because EDS analysis results shown that no corrosive element was found on the crack, so the cracks should be in no relationship with stress corrosion crack (SCC), and the type of the crack could be quenching cracks concluded by its morphology [9][10][11] .Because back redressing stabilizer made of stabilizer which has blades welded on its outer surface, during the drilling process, the blades are often worn by the friction of the stone in the downhole, so the operation of blades welding often conducted before the drilling of each wall.The thermal stress induced by the un-uniform cooling after welding would results in the initiation of quenching at the pits bottom on the root of the thread, and propagated under the combined load during the drilling process, until the final fracture happened.

Conclusions and suggestions
The following conclusions and suggestions could be drawn according to the investigation the failure causes : (1) The failure of the stabilizer is fatigue facture, which induced by the quenching cracks on root of the thread.The pit defects on the root of the thread would increase stress concentration, during the drilling process, such positions would have higher stress distribution, so cracks would preferentially initiated at the thread roots which have pit defects.
(2) The alternating load during drilling would force the cracks to propagate towards the outer wall, also, un-uniform cooling after blades welding would also give an acceleration of the propagation of the crack.The effective wall thickness decreased with the propagation of the crack, when the remaining wall thickness unable to bear the load, the final fracture occurred.
(3) A comprehensive inspection of the thread should be conducted before drilling to discover the defects on the thread, and a regulation of the blades welding should also be established.

Fig. 1 .
Fig. 1.Macro morphology of the fractured back redressing stabilizer internal thread joint.(a) Fractured sample; (b) Schematic diagram of the fracture position

Fig 2
Fig 2 shows the fracture macro-morphology of the back redressing stabilizer internal thread joint.The fracture surface is flat, and the fracture feature on the surface was destroyed by the flushing of the drilling fluid from the time cracking initiated till the final fracture, the morphology of the fracture surface have the feature of fatigue fracture.The fracture surface can be divided into three sections based on the height difference, and the fracture could also be concluded as multiple initiation from this.

Fig. 5 .Fig. 6 .
Fig. 5. Metallographic microstructure of crack1 illustrated in Fig. 3. Scanning electron microscope and energy dispersive spectrometer were used to find out the chemical element composition of the substance in the cracks.were used to analyze the chemical element distribution at different position along the crack.As shown in Fig.6a -Fig.6b,elements such as C, Fe, O, Al, Si, K, are detected.When opened the crack by mechanic methods, the micro-feature of the crack surface exists an inter-granular crack feature, as shown in Fig.6c, chemical elements composition on the crack surface are C, O, Cl, Si, K, Fe.

Table 1
. All the elements contained in the material are in accordance with the SY/T 5051-2016 standard requirement.

Table 1 .
Chemical composition analysis results of the failed back redressing stabilizer internal thread joint (wt.%) Tensile, chapry V-notch impact and Brinell hardness tests results of the failed back redressing stabilizer internal thread joint are shown in Table2, 3 and 4, respectively.The results reveal that the tensile and Brinell hardness and chapry V-notch impact properties test results are all in accordance with the SY/T 5051-2016 standard requests.

Table 2 .
Tensile test results of the failed back redressing stabilizer internal thread joint

Table 3 .
Brinell hardness test results of the failed back redressing stabilizer internal thread

Table 4 .
Charpy V-notch impact test results of the fractured top drive side cover joint