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Load fluctuation characteristics have a great influence on drilling performance. So, study the influence of drilling kinematic parameters on the load fluctuation characteristics of PDC bit becomes more and more important. Gas drainage drilling experiment table was built and Gas drainage drilling experiment was carried out under different feed speed, different rotation speed and same drilling thickness with different feed speed and rotation speed. The results indicate that drilling torque decreases with the increase of rotation speed; vibration amplitude of drilling torque increases with increase of rotation speed. Vibrati on amplitude of feed resistance decreases from 90 r/min to about 240 r/min and then increases. The value of 240 r/min is minimum. Feed resistance and drilling torque increase with the increase of feeding speed. Vibration amplitude of drilling torque and feeding resistance increases with the increase of feeding speed. Feed resistance and drilling torque are almost equal with the same drilling thickness; meanwhile, vibration amplitude of drilling torque and feed resistance increases with the increase of feeding speed. It provides a good basis for setting the appropriate feeding speed and rotation speed in actual work.

All metallurgical and mechanical aspects of failure that can occur for a drill pipe are considered and discussed. And finally, this comprehensive review leads to a conclusion that categorizes the primary sources of drill pipe failure into seven major groups and makes some recommendations to avoid them [

In order to increase the drilling performer of PDC bit, lots of research have been done, however, papers about study on the influence of PDC bit drilling kinematic parameters (include feed speed, rotation speed, same drilling thickness) on drill pipe vibration under confining pressure in Gas Drainageis few. So drilling experiment table is built and carried out under confining pressure and different drilling parameters. In this paper, influence of PDC bit drilling kinematic parameters on Drill pipe vibration is researched and some conclusions are obtained.

Cutting mechanism of PDC bit is that the cutter can automatically cut the coal seams under the drilling pressure. Cutters cutting coal rock under the action of torque. The two dimensional force model was established according to the analysis of shear broken coal rock is shown in

of coal rock was researched under the coal rock properties ( θ , ψ , ϕ ) was determined. The relationship between the contact normal pressure and the rake angle of the cutter was researched. Normal pressure is on the contact surface, resultant force is F 1 , reaction force P 1 , vertical force P 2 is on the shear plane. Rake angle of cutter is α , the friction angle is θ , the included angle between the cutting surface of coal and the feed direction of the cutter is ϕ , Internal friction angle of coal and rock is ϕ , shear surfaces is S ( S A B C D ) .

P 1 = ∫ σ n d s , F 2 = ∫ τ d s (2-1)

Average shear force; τ = C + σ n tan ϕ is taken into (2-1);

F 2 = C ⋅ S + P 1 tan ϕ (2-2)

where: C is cohesion of coal rock, σ n is anti-crushing strength, τ is the residual shear force. According to the two directions of the coordinate axes, the equilibrium condition is available under the limit equilibrium state.

∑ X = 0 , F 1 sin ( π 2 − α − ψ − θ ) − F 2 = 0 (2-3)

∑ Y = 0 , P 1 − F 1 sin ( π 2 − α − ψ − θ ) = 0 (2-4)

(2-2) is taken into (2-3), (2-4)

F 1 = C ⋅ S ⋅ cos ϕ cos ( ϕ + ψ + θ + α ) (2-5)

s = b ⋅ b 0 ; b is width of cutter drill coal and rock in

F 1 = C ⋅ b ⋅ b 0 cos ϕ cos ( ϕ + ψ + θ + α ) (2-5)

b = 2 R 2 + ( R − h 3 cos α ) 2

b is drilling width, h is drilling thickness ( h = v / n ), v is feeding speed (m/ min), n is Rotating speed of bit (r/min).

When α + ψ + θ + ϕ ≥ π / 2 , F 1 is infinity or negative. The contact area

between the cutter and the coal rock increases with the increase of rake angle, so strong extrusion is occurred on coal rock. Shear failure is not occurred, and extrusion failure is occurred. Since the uniaxial compressive strength of the coal rock is much larger than that of the shear strength, the fragmentation of coal and rock need more drilling pressure and tangential force. Drilling efficiency is very low, so the affective range of the rake angle is 0 ≤ α ≤ π / 2 − ( ψ + θ + ϕ ) .

Cutting force, drilling pressure, The reaction force of rock to cutter is F 1 , friction F 3 and contact normal pressure P 2 are shown in

The coefficient of friction between the coal and the tangent plane is μ, F 3 = μ P 2 , Balance principle of force:

F c = F 1 cos ( θ + α ) + μ P 2 (2-6)

W = F 1 sin ( θ + α ) + P 2 (2-7)

F c is cutting force, W is feeding resistance, P 2 is related to the force distribution on the wear surface.

P 2 = b ⋅ σ s ⋅ [ 0.6 cos α + 1 3 ( Δ L f − 0.6 cos α ) ] [

Δ L f ―PDC cutter wear length. R is the radius of cutter, σ s is uniaxial compression yield force of coal, usually the value is tenfold than coal Platt factor, σ s = 10 f , f is coal rock hardness.

(2-6) and (2-7) are brought into the (2-8):

W = 2 C b 0 R 2 + ( R − ν 3 n cos α ) 2 ⋅ cos ϕ ⋅ sin ( θ + α ) cos ( ψ + α + θ + ϕ ) + b ⋅ σ s [ 0.6 cos α + 1 3 ( Δ L f − 0.6 cos α ) ] (2-9)

F c = 2 C b 0 R 2 + ( R − ν 3 n cos α ) 2 ⋅ cos ϕ ⋅ cos ( θ + α ) cos ( ψ + α + θ + ϕ ) + μ b ⋅ σ s [ 0.6 cos α + 1 3 ( Δ L f − 0.6 cos α ) ] (2-10)

It’s obvious drilling torque and feeding resistance are not only correlated with rake angle, cutting condition, but also correlates with feed speed and rotating speed. This paper studies load fluctuation characteristics of PDC bit during gas extraction borehole in soft coal seam.

According to the theory of rock cutting and breaking, the establishment of the gas drainage drilling test bench (

The dynamic system of the test bench adopts ZDY1900S full hydraulic drilling machine. The rig is mainly composed of a host machine, pumping station and control station. Pump station is driven by the motor rotation, the hydraulic oil will be transferred to the feed passage and the rotary actuator, the control console on the pole realizes the drill rotation and feed. The drilling machine not only provides power for the drilling tool, but also provides the confining pressure load for the confining pressure loading device on the top of the coal wall support. At the top of the coal wall support, HSG90/63 engineering hydraulic cylinder is provided, the rated working pressure of the hydraulic cylinder is 16 MPa, and the stroke is 640 mm, so as to meet the requirement of applying the confining pressure to the coal wall. The hydraulic circuit of the hydraulic circuit is additionally provided with two-way hydraulic lock, so as to ensure the stablity of the confining pressure in the operation of the hydraulic cylinder.

YSV dynamic signal acquisition and analysis software were used in testing system

for drilling test data acquisition system; FST800-201G222A-16B pressure sensors are arranged in the hydraulic motor and hydraulic cylinder oil import to drill drilling torque to tests the feed resistance signal. The main performance parameters of the sensor: range: 0 - 31.5 Mpa; sensitivity: 158.73 output voltage: 0 - 5 V; standard engineering unit: mv/Mpa; measurement accuracy: 0.5% FS; the flow of the hydraulic system using the model for the turbine flow sensor LWGY-15A, the nominal pressure 6.4 MPa, maximum pressure loss is 0.0035 MPa, the turbine flow sensor and the hydraulic motor return oil pressure sensor on the pipeline in series.

According to the above power system and the test system composed of drilling test bench, provide the maximum torque of 1900 Nm and maximum speed of 300 r/min, to the process of 600 mm. According to the working performance of the drilling rig, the displacement of the main oil pump in the pump station is changed to change the rotating speed of the drilling machine. The speed range of the drill bit is 85 - 300 r/min. Adjust the displacement of the auxiliary oil pump in the pump station to change the feed rate of the drilling mechanism. The feed rate range of drilling mechanism is 0 - 3 m/min.

To research the influence of rotation speeds on load fluctuation, drilling coal rock experiment is carried out under different rotation speeds. Working condition parameters in the process of drilling coal rock are as follows: protodyakonov coefficient of coal is f:f = 1.03, confining pressure is 7 MPa, feed speed is 0.7 m/min, rake angle of PDC bit is 15˚, rotation speed is 90 r/min, 140 r/min, 190 r/min, 240 r/min and 290 r/min. Feeding resistance and drilling torque is got under different rotation speed is shown in

Rotation speed (r/min) | Drilling torque (N・m) | Feeding resistance (KN) | ||||
---|---|---|---|---|---|---|

Maximum value (N・m) | average value (N・m) | standard deviation | Maximum value (KN) | average value (KN) | standard deviation | |

90 | 552.8 | 525.6 | 12.4 | 6.95 | 5.77 | 0.56 |

140 | 469.4 | 420.6 | 14.7 | 6.14 | 5.35 | 0.45 |

190 | 373.7 | 344.3 | 15.3 | 4.82 | 4.12 | 0.41 |

240 290 | 337.3 221.8 | 309.5 178.4 | 15.7 16.8 | 4.22 2.86 | 3.66 2.86 | 0.23 0.53 |

r/min is Minimum.

In order to research the influence of feed speeds on load fluctuation, drilling coal rock experiment is carried out under different rotation speeds. Working condition parameters in the process of drilling coal rock are as follows; Rotation speed is 290 r/min, Plattcoefficientf = 1.03 experimental coal wall, confining pressure is 7 MPa, bit rake angle of PDC bits is 10˚, feeding speed is 0.3 m/min, 0.5 m/min and 0.7 m/min. Feed resistance and drilling torque is got under different rotation speed is shown in

Feeding speed (m/min) | Drilling torque (N・m) | Feeding resistance (KN) | ||||
---|---|---|---|---|---|---|

Maximum value (N・m) | average value (N・m) | standard deviation | Maximum value (KN) | average value (KN) | standard deviation | |

0.3 | 358.7 | 344.6 | 11.6 | 1.61 | 1.44 | 0.20 |

0.5 | 384.3 | 366.4 | 14.3 | 5.0 | 2.55 | 1.11 |

0.7 | 421.6 | 356.1 | 15.3 | 4.6 | 3.56 | 1.56 |

0.9 | 423.6 | 399.6 | 20.6 | 6.0 | 4.11 | 2.30 |

In order to research the influence of same drilling thickness on Load fluctuation, drilling coal rock experiment is carried out under different rotation speeds. Working condition parameters in the process of drilling coal rock are as follows; drilling thickness h = v/n = 2.63 mm/r, protodyakonov coefficient of coal is f:f = 2.2:, confining pressure is 7 MPa, rotation speeds are 290 r/min, 240 r/min, 190 r/min, 140 r/min, rake angle of PDC bit is 15˚. Feeding speed are 0.76 m/min, 0.63 m/min, 0.37 m/min, 0.24 m/min. Feed resistance and drilling torque is got under different combinations is shown in

Feeding speed (m/min) | Drilling torque (N・m) | Feeding resistance (KN) | ||||
---|---|---|---|---|---|---|

Maximum value (N・m) | average value (N・m) | standard deviation | Maximum value (KN) | average value (KN) | standard deviation | |

0.24 | 570.2 | 525.5 | 15.7 | 3.61 | 3.52 | 0.08 |

0.37 | 580.1 | 530.7 | 19.8 | 3.63 | 3.51 | 0.10 |

0.63 | 541.3 | 520.4 | 20.7 | 4.21 | 3.65 | 0.28 |

0.76 | 544.2 | 518.9 | 23.9 | 5.82 | 3.63 | 0.31 |

Load fluctuation characteristics have a great influence on drilling performance. So, study the influence of drilling kinematic parameters on load fluctuation characteristics of PDC bit becomes more and more important. Gas drainage drilling experiment table is built and carried out under different feeding speed, different rotation speeds and same drilling thickness with different feed speeds and rotation speeds. Drilling torque and feed resistance are decreased with the increase of rotation speed. Vibration amplitude of drilling torque increases with increase of rotation speed. Vibration amplitude of feed resistance decreases from 90 r/min to 240 r/min then increases. The value of 240 r/min is Minimum. Feed resistance and drilling torque increase with the increase of feed speed. Vibration amplitude of drilling torque and feeding resistance increases with the increase of feeding speed. Feed resistance and drilling torque are almost equal with the same drilling thickness; meanwhile, vibration amplitude of drilling torque and feed resistance increase with the increase of feeding speed. According to the paper, the relationship between kinematic parameter and vibration amplitude of drilling torque and feed resistance was analyzed and load fluctuation characteristics of PDC bit during gas extraction borehole in soft coal seam was analyzed. It provides a good basis for setting the appropriate feeding speed and rotation speed in actual work to avoid blocking which caused by excessive vibration and improve work efficiency. In the paper, vibration amplitude of drilling torque and feed resistance was researched, but many forms of vibration include torsional vibration, longitudinal vibration and lateral vibration during drilling drills coal rock, study these forms is next important task.

This research was financially supported by the Henan province science and technology project (Grant No. 162102210229), China Postdoctoral Science Foundation Funded Project (Grant No. 2016M592289) and Henan province education department applied research project fund (Grant No. 15A440004 and 16A460005).

Wang, S.F., Han, X.M. and Zhang, Q.Q. (2017) Study of Load Fluctuation Characteristics of PDC Bit during Gas Extraction Borehole in Soft Coal Seam. Open Access Library Journal, 4: e3693. https://doi.org/10.4236/oalib.1103693