Send Orders for Reprints to Reprints@benthamscience.ae Research on Fracture Mechanism and Improvement Methods of Electric Submersible Pump Unit's Joint Screw

According to the problem of screw fracture of electric submersible pump unit in Shengtuo oilfield, the reasons for the screw fracture have been analyzed from three aspects: material, strength and pre-tightening force. The preliminary result shows that the imbalanced pre-tightening force of screw is the main reason of screw fracture. By means of Ansys software, the static and modal analyses for the electric submersible pump adapter have been conducted respectively under consistent and inconsistent pre-tightening forces. The numerical simulation results show that when the pre-tightening force is inconsistent, sharp increase in maximum stress of the screw accelerates the screw fracture, and the dynamic performance of screw in the vicinity of 3809.7Hz is the worst. Based on this, the special torque wrench is developed and then the pre-tightening quality of screw is improved greatly by using this wrench.


INTRODUCTION
Electric submersible pump has the advantages of large displacement, high power, and simple energy transmission way and so on, and it has become the second largest equipment for artificial lift petroleum production at home and abroad [1,2].However, with its complex structure, poor working conditions and high failure rate, the falling accidents of the electric submersible pump caused by joint screw fracture occurs occasionally, which results in the great economic loss.Shengtuo Oilfield is the first discovered and put into the development oilfield in Bohai Bay basin, and it is also the first monoblock oilfield with ultra-high watercut period in China.As Shengtuo Oilfield entered the later period of high watercut stage, the conditions of wellbore and stratum have been changed, the scaling, corrosion, erosion, and casing leakage have also increased year by year.Among them, the submersible pump falling accidents caused by screw fracture, not only affect the normal oil production seriously, but also increase the operation difficulty.In the production process, any one of the component failure will cause the oil well downtime repair.In most cases, due to work disorder of oil reservoir caused by shutdown of the pump unit, the oil well needs several days to recover its best state for oil production [3,4].Usually, the summation of the downhole operation cost and electric pump repair cost is about equal to the price of a set of new electric pump.Thus, falling accidents caused by screw fracture would cost a lot for repair.

The Fracture Status of Electric Submersible Pump Unit's Screw in Shengtuo Oilfield
The statistical results of electric pump falling accidents from 2005 year to 2010 year are shown in Table 1.It can be seen from Table 1 that the falling accidents caused by screw fracture account for as high as 45% between 2005 year and 2007 year, the total falling accidents are 330 well times from 2005 year to 2010 year, falling accidents caused by screw fracture are 117 well times, accounting for 35.5%.As shown in Fig. (1), the location of screw fracture is flanged end face, and the feature of screw fracture is the transverse shear fracture without longitudinal tension.

The Reason Analysis for Screw Fracture
From the view of the material of screw, electric pump flanges are connected by high strength M10×1.25 screws made of 35CrMoA.The test results of broken screws show that all element contents of material of screw have reached the set standard, and the material quality of screw meets the requirements of the engineering drawing.Thus, the possibility could be ruled out that the material quality of screw leads to the falling accidents of the unit.
From the view of the strength of screw, M10×1.25 screw is high strength screw, and its minimum tensile load is 8 tons.With 8 screws, the flange connection could bear loads of 64 tons, and the actual weight of the unit is only 1.8 tons, so the tensile strength of the flange connection is enough, the screw can not be broken because of insufficient strength.
From the view of the pretightening force of screw, when the screw is tightened by ordinary wrench, it is uneasy to control the torque value of every screw.The uncontrollable pretightening torgue would lead to an inconsistent pretightening force of screw, and then the inconsistent pretightening force would result in excessive stress in some or other screws.Under the condition of fatigue vibration, some screws would get loose, and then the screws would be broken because of the radial force [5,6].Thus, it is the possible main reason of screw fracture that the pretightening force of screw is out of control.

Finite Element Analysis Of Screw Fracture 2.3.1. Three Dimension Mechanics Model of Joint Screw
The joint screws of electric submersible pump unit are mainly used for connecting components.When the unit works, the model of joint screw is mainly affected by four forces: axial pretightening force Q caused by torque resulted from the initial pretightening; reactive torque T of motor shaft through connectors; pulling force F caused by weight of the parts below pump head; its own weight G.
The torque T is equivalent from pump seat to pump head, and the pump head is affected by the torque T, so that the top surface of pump seat needs to be fixed.Based on above analysis, the three dimension mechanics model of connection screw is established as shown in Fig. (2).

Static Analysis of Screw Under Consistent Pretightening Force
The tetrahedral element Solid187 is adopted, and the automatic mesh generation method is applied to get the model mesh.The element has 10 nodes; each node has 3 degrees of freedom.The element is suitable for complex and irregular models in CAD/CAM [7][8][9].The whole model is divided into 783218 units, 1135983 nodes as shown in Fig. (

3).
Add a local coordinate system in each screw, and apply the pretightening force 18kN on the cylindrical surface of each screw [10,11].Apply a fixed constraint on the top surface of the pump seat, and then apply gravity load and tensile load as shown in Fig. (4).
The stress distributions of screws are shown in Fig. (5).It is shown that the stress is mainly concentrated in the flanged connection, which explains the position of screw fracture.The maximum stress is 489.61MPa, and the yield limit of the material of screw is 835MPa, so the margin of safety is equal or greater than 1.71.It is clear shown that the screw is not easy to fail, when the pretightening forces of 8 screws are consistent.

Static Analysis of Screw Under Inconsistent Pretightening Force
Based on it, import the same model and mesh the model, and set the boundary same conditions as Fig. ( 4), but the pretightening forces of 8 screws are respectively set for 10kN, 14kN, 16kN, 18kN, 20kN, 22kN, 24kN and 26kN.The stress distributions of screws are shown in Fig. (6).It is shown that the maximum stress occurs at the flanged connection, is about 692.79MPa.Compared to the maximum stress under consistent pre-tightening force, the maximum stress under inconsistent pretightening force increases by 41.5%, and nearly reaches the yield limit of the material.Although screws can work under the condition, the speed of screw fracture will be accelerated working in the nearly range of the yield limit and the fatigue load.

Modal Analysis of Screw with Pretightening Force
The pretightening force 18kN is applied on each screw, a fixed constraint is applied on the top surface of pump seat, and the order of frequency analysis is 6.Then the model is solved, and the initial six order natural frequencies and the descriptions of vibration type are shown in Table 2, and the vibration mode diagrams are shown in Fig. (7).
As is shown in Fig. (7) and Table 2, it can be seen that the screw vibration is weak in the first and second modes, the screw is in strong vibration in the third and sixth modes, and the screw vibration is strongest in the fourth and fifth modes.The screw has the worst dynamic performance in the vicinity of 3809.7 Hz.The vibration of the connection position makes the pre-tightening force of screw decline or  loose, the upper and lower connecting pieces are subjected to the shear abrasion, and the fracture failure is prone to happening under the action of repeated dynamic torque.

RESULTS AND DISCUSSION
Based on it, a special torque wrench is developed to solve the problem that the pre-tightening force is inconsistent when electric submersible pump unit works.The special torque wrench is shown in Fig. (8).Torque value can be set accurately with graduated scale on the special torque wrench.When the torque reaches the set value, the wrench sends out clear alarm sound, and a slight shock could be felt on the handle.Therefore, the pre-tightening force of the screw could be determined accurately by the special torque wrench, and the pre-tightening quality of screw is improved greatly.The field experiment results show that the application of special torque wrench improves the pretightening quality of every screw greatly, and eliminates screw fracture caused by inconsistent or excessive pretightening force.Due to the application of special torque wrench, the accident caused by screw fracture declines sharply, screw fracture rate fell to zero beginning in 2011.

CONCLUSION (1)
When the electric submersible pump is assembled, inconsistent pretightening force of screw is the main cause of screw fracture. (2) The initial six modal analyses show that the screw has the worst dynamic performance in the vicinity of 3809.7Hz, and then the screw is prone to failing. (3) The special torque wrench is developed and can determine the pretightening force of screw accurately.Its application improves the pretightening quality greatly, and eliminates the screw fracture phenomenon caused by the inconsistent or excessive pretightening force.The bottom of pump head vibrates drastically along the Z axis.The bottom of pump seat, pump head, and screws vibrate along the axial with tensile trend.

3 2112. 3
The bottom of pump seat, pump head, and screws vibrate along the radial direction with expanding