Fracture Failure Analysis of a 5-7/8” Drill Pipe Pin Joint in an Ultra-deep Gas Well

In recent years, the frequency of drill pipe failure accidents has increased during ultra-deep well drilling process. The fracture failure of pin joint of a 5-7/8” drill pipe happened in an ultra-deep gas well after thirty-five hours’ service. Failure causes were investigated by physical and chemical properties test of the material, macro and micro feature analysis of the fracture surface. The results showed that the physical and chemical properties of the drill pipe were all satified the corresponding standard requirements, the fracture of mode of the dill pipe is fatigue fracture, which was attributed to the harsh load condition during the reaming operation. Furthermore, the increased inner diameter of the pin joint also gives an acceleration of the fatigue fracture process.


Introduction
Nowadays, with the depletion of conventional petroleum resources, ultra-deep energy reservoir has become a new growth point of the exploration and development of the fossil energy [1].The increasing of the well depth would lead to a continuous length increase of the drill string.The drill strings consist numbers of drill pipes which are connected through threads in the drill pipe joint, and the weakest points of the whole string usually located in such positions [2,3].Nowadays, harsh service conditions in the ultra-deep gas well such as high temperature, high pressure, high content of corrosive medium, and complex load conditions during drilling, all have induced great failure risks to the drilling pipe joints in the drilling string [4][5][6].According to incomplete statistics, drilling accidents caused by the failure of the drilling tools have reached to more than 500 times in the oil and gas field in China [7][8][9][10].
In this paper, the failure accident of a 5-7/8" drill pipe pin joint was investigated by nondestructive inspection, material properties tests, macro and micro analysis of the crack surface.In order to find out the causes of the fracture failure accident and propose failure prevention measures.

Background of Failure
Fracture accident of a 5-7/8" drill pipe suddenly occurred when drilling to the depth of 7480.57m.The failure process is as follows: the downhole working face was found unstable when drilling to the depth of 7480.57m, and when lifting the drill string to the depth of 7473.74m for inspection, the hang weight of the drill string first increased from 2827kN to 3253kN, and then decreased to 343kN.When pulled the drill string out of the well, and found a drill pipe which located 150m from the wellhead was fractured.
The fracture located at the second last thread of the pin end, and there was no evident plastic deformation in the fracture position as shown in figure 1.

Experiments
In order to make out the fracture failure causes of the drill pipe pin joint, several tests were performed.Magnetic particle inspection was performed on the outer surface of the fractured pin joint according to ASTM E709-2015 standard.Material properties tests including chemical composition, tensile and impact properties, Brinell hardness and metallographic analysis were carried out according to the SY/T 5561-2014 standard.The micro-feature of the crack surface and relative chemical elements content of the products on the fracture surface were analyzed by scanning electron microscopy and Energy Dispersive Spectrometer (VEGA-II).

Magnetic Particle Testing
Magnetic particle inspection results of the fractured pin joint reveal that there are no cracks on the thread and outer surface of the drill pipe pin joint.

Geometric Dimension Measurement
The outer diameter and inner diameter of the fractured pin joint are measured in three different section, the results are listed in table1.The inner diameter of the pin joint exceeds the requirements of the SY/T 5561-2014 standard.

Fracture position
Fracture position-Thread root

Fracture Morphology Analysis
Figure 3 illustrates the fracture macro-morphology of the fractured surface.The fracture surface has the macro-morphology feature of fatigue, and the surface can be divided into three sections, that is the fatigue crack initiation zone, fatigue crack propagation zone and final fracture zone.Fatigue crack initiation zone locates in the thread root of the pin joint, and there are a few steps in that area, which is a symbol of fatigue crack initiation.When removed the rust substance in the surface, some shell lines can be seen in the fatigue propagation zone.The slope zone exists a shear lip morphology.Figure 4 shows the micro-morphology feature of the fracture surface.The fatigue fracture initiated from the thread root of the pin joint, and there can be seen radial ridges initiated from here.Intensive fatigue striations can be observed in the fatigue propagation zone, and shear lip exists a micromorphology of dimple in the final fracture zone.The elements of C, O, Cr, Fe, Si are found on the surface of the fracture using SEM-EDS point scanning as shown in figure 5.The fracture mode of the drill pipe pin joint could be concluded as fatigue fracture based on the macro and micro morphology feature of the fracture surface.

Analysis and Discussion
The former analysis results of the pin joint material reveal that the material properities all meet the corresponding requirements of the product standard, so there is no relationship between the fracture failure accident and the drill pipe's material itself.
Morphology analysis of the fracture surface showed that the mode of the fracture is fatigue and the fracture initiated from the last two thread root of the pin joint, which is the last engagement region of the external and internal thread, and it's also the maxium stress position of the drill pipe when drilling, as shown in figure 6.During the drilling process, the upper drill pipe in the drill string suffer from tension and alternating torsion bending load.Under the action of the alternating load, the position which has the maxium stress distribution in the drill pipe are more likely to have fatigue crack initiation and fianlly lead to fatigue fracture.Further more, the boring log illustrate that due to the harsh geological conditions in the downhole, the bit bouncing and drill pipe sticking happened frequently than ever before, this would definitely lead to the deterioration of the drilling string's load conditions and thus accelerate the failure process.The geometric dimension measurement results showed that the inner diameter of the pin joint has exceed the product standard requirements by about 12.7mm.The enlargement of the inner diameter of the pin joint would reduce the effective thickness of the external thread, thus weaken the bearing capacity of the pin joint.

Conclusions
The failure of the 5-7/8"drill pipe pin joint is fatigue fracture failure, the alternating torsion bending load during drilling process and the enlargement of the inner diameter of the pin joint are the main causes for the failure.The frequent bit bouncing and drill pipe sticking during drilling is the minor cause for the failure.

Figure 1 .
Figure 1.Macro-morphology of the fractured drill pipe.

4. 5 . 2 .
Metallographic ExaminationFigure2shows the metallographic structure of the fractured pin joint.The microstructure are all tempered sorbite on both the unfailure position and the fracture surface, which is a proper metallographic structure of the quenched and tempered steel.The grade of the grain size is 9.0, and the grade of the nonmetallic inclusion is A0.5, D0.5,B0.5.(a) Non-fracture failure position (b) Fracture surface Figure Metallurgical structure of the fractured drill pipe pin joint.

Figure 3 .
Figure 3. Macro-morphology feature of the fracture surface.
(a) Fatigue crack initiation zone (b) Fatigue crack propagation zone (c) Final fracture zone

Table 1 .
Geometric dimension measurement results.The results of the chemical composition are showed in table 2. All the elements contained in the fractured pin joint are all satisfy the requirements of the SY/T 5561-2014 standard.

Table 2 .
Elemental composition of the fractured pin joint [wt%].
4.4.Mechanical PropertiesThe mechanical properties tests results, inculding tensile test, impact toughness test and Brinell hardness test are listed in tables 3-5.The mechanical properites tests results are all in accordence with the requirements of the SY/T 5561-2014 standard.

Table 5 .
Brinell hardness tests results of the fractured pin joint.