CN-121997620-A - Fatigue life prediction method based on dynamic stress of drill string

Abstract

The application belongs to the field of petroleum and natural gas exploitation, and particularly relates to a fatigue life prediction method based on dynamic stress of a drill string, which comprises the following steps of inputting basic parameters, wherein the basic parameters comprise a borehole track, a bottom drilling tool assembly, a drill string structure and drilling parameters; establishing a dynamic model of the drill string system according to a Lagrangian equation, obtaining node speed, displacement and acceleration through the dynamic model of the drill string system, calculating dynamic equivalent stress of the drill string according to the node speed, the displacement and the acceleration, calculating stress amplitude and stress mean values by adopting a rain flow counting method, correcting the stress amplitude and the stress mean values by adopting a goodman formula, and carrying out S-N curve calculation to obtain the fatigue life. The fatigue life of the whole-well drill string can be analyzed, the fatigue life of the whole-well drill string under the working condition is predicted through data analysis and processing, the specific use time of the drill string is long as the result, and the method has a strong guiding effect on field engineering practice.

Inventors

• QIAO LIHUA
• QI YU
• CHEN KUAN
• FAN SHENGLIN
• LIU BAOJUN
• ZOU BO
• YU LAIHONG
• XU BIN
• XIAO QIFU
• WANG NAN

Assignees

• 中国石油天然气集团有限公司
• 中国石油集团川庆钻探工程有限公司

Dates

Publication Date

20260508

Application Date

20241105

Claims (13)

1. 1. The fatigue life prediction method based on the dynamic stress of the drill string is characterized by comprising the following steps of: S1, inputting basic parameters including a borehole track, a bottom hole assembly, a drill string structure and drilling parameters; S2, establishing a drill string system dynamics model according to a Lagrangian equation; s3, obtaining node speed, displacement and acceleration through the drill string system dynamics model in the step S2, and calculating the dynamic equivalent stress of the drill string; s4, adopting a rain flow counting method to count stress amplitude and stress average values; S5, correcting the stress amplitude and the stress mean value by adopting a goodman formula; S6, carrying out S-N curve calculation to obtain the fatigue life.
2. 2. The fatigue life prediction method based on the dynamic stress of the drill string according to claim 1, wherein the specific content of the step S2 is: The method comprises the steps of dispersing a continuous drilling string of a borehole with a complex structure by adopting a double-node beam unit, wherein each node has 6 degrees of freedom, each node comprises 3 translation amounts (x, y and z), two transverse rotation angles (theta y ,θ z ) and a torsion angle theta x , i and j represent the upper node and the lower node of the beam unit, and the movement of the drilling string is represented by a displacement vector of the node of the beam unit: {U i } e ＝[x i ,y i ,z i ,θ xi ,θ yi ,θ zi ,x j ,y j ,z j ,θ xj ,θ yj ,θ zj ] (1) The Hamiltonian principle is used for expressing the relation between the kinetic energy, potential energy and external force working of a drill string system, expressed as Wherein T is the kinetic energy of the drill string system, V is the potential energy of the drill string system, W is the external force acting of the drill string system, delta represents the variation operation and refers to the tiny variation of a certain quantity, and delta T represents a certain time interval.
3. 3. The fatigue life prediction method based on dynamic stress of drill string according to claim 2, wherein the drill string system is discretized into a plurality of continuous euler-bernoulli beam units containing two nodes by using finite element method, and the discretized beam units are rewritten into lagrangian equation for controlling drill string movement Wherein U i is node displacement, and F i is node external force.
4. 4. The fatigue life prediction method based on drill string dynamic stress according to claim 3, wherein the translational kinetic energy of the beam unit, the rotational kinetic energy of the beam unit, the potential energy of the beam unit, the gravity of the beam unit and the centrifugal force of the beam unit are substituted into (3) to obtain a drill string dynamic control equation, and the drill string dynamic control equation is written into a matrix form In the formula, { U }, { F } is the generalized acceleration, velocity, displacement and external force vector, respectively, [ M ] is the mass matrix, [ C ] is the damping matrix, and [ K ] is the stiffness matrix.
5. 5. A fatigue life prediction method based on dynamic stress of drill string according to claim 3, wherein the boundary conditions of drill string system include the tensile force of drill string under and torque provided by rotary disc, axial excitation and resistance moment generated when drill bit and rock interact with each other, and the forward contact force, tangential friction and friction torque of shaft to drill string.
6. 6. The fatigue life prediction method based on drill string dynamic stress according to claim 4, wherein the total kinetic energy of the beam unit comprises translational kinetic energy of the beam unit and rotational kinetic energy of the beam unit, and the expression is: in the formula, Respectively, the translation speed on X, Y, Z shafts, m/s, ρ is the density of the drill string, kg/m 3 , A is the cross-sectional area of the beam unit, m 2 , e is the section eccentricity, m. The potential energy expression of the beam unit is: Where u x is the x-axis displacement, θ x is the torsion angle around the x-axis, and θ y 、θ z is the rotation angle around the y-axis and the z-axis, The linear stiffness matrix is respectively the translation speed on X, Y, Z shaft, the elastic modulus of the drill string is E, the shear modulus of the drill string is Pa, the first 4 items in the formula (5) are linear stiffness matrices, the 5 th and 6 th items are nonlinear stiffness matrices of coupling of axial deformation and bending deformation of the drill string, and the last two items represent nonlinear stiffness matrices of coupling of torsional deformation and bending deformation of the drill string; The components of beam element gravity in x, y, z can be expressed as: Wherein q represents the equivalent gravity of a 1m drill string, N/m, and alpha is the included angle between the axis of the beam unit and the vertical direction, so that the equivalent node force of the gravity vector is as follows: Where L denotes the drill string unit length, m, the cross section of the drill string is not a centrosymmetric model, so that there is a centrifugal force when rotating, and the two unit centrifugal forces in the x, y, z directions are: where β is the phase angle of the center of gravity, rad, and for the centrifugal force vector of the node, its equivalent force can be expressed as:
7. 7. the fatigue life prediction method based on dynamic stress of drill string according to claim 1, wherein the specific content of step S3 comprises: The displacement, speed, acceleration and external force vectors are the results of iterative solution of the drill string dynamics control equation and are stored in { U }, { F } in four matrices; The normal stress σ x and the shear stress τ acting on the drill string are expressed as: Wherein F x is the dynamic axial force born by the drill string node, the unit is N, A x is the cross section area of the drill string unit, the unit is M, M y is the bending moment of the drill string in the section of the y axis, M z is the bending moment of the drill string in the section of the z axis, d o is the outer diameter of the drill string, the unit is M, d i is the inner diameter of the drill string, the unit is M, T 0 is the torque born by the drill string, the unit is N.m, I y is the moment of inertia of the y axis, and I z is the moment of inertia of the z axis.
8. 8. The method of claim 7, wherein the first principal stress sigma 1 , the second principal stress sigma 2 , and the third principal stress sigma 3 of the drill string are calculated according to the Moire stress circle criterion, The equivalent stress expression at a certain position of the drill string is written as
9. 9. The fatigue life prediction method based on dynamic stress of drill string according to claim 1, wherein the specific content of step S4 comprises: And dispersing the continuous load-time course into a series of peak values and valley values by adopting a rain flow counting method, and carrying out cycle counting statistical treatment to obtain a random load spectrum, so as to obtain the stress average value and stress amplitude value of the whole-well drill string at each position.
10. 10. The fatigue life prediction method based on dynamic stress of drill string according to claim 1, wherein the content of step S5 is: The general expression for goodman curves is: Where S a is the stress amplitude, S e is the fatigue limit, S m is the stress average, σ b is the tensile strength, and the fatigue limits S e and the tensile strength σ b are determined by the material properties itself.
11. 11. The fatigue life prediction method based on drill string dynamic stress according to claim 10, wherein the equivalent mean value of amplitude-mean-times under goodman curve is obtained by a rainfall method, namely, the stress amplitude S i when the mean value is 0 is obtained: Where S i is the zero mean stress amplitude after conversion, S ai is the ith stress amplitude, S mi is the ith stress mean, and σ b is the tensile strength.
12. 12. The method for predicting fatigue life based on dynamic stress of drill string according to claim 1, wherein the content of step S6 comprises: obtaining an S-N curve through a fatigue experiment, wherein S represents stress amplitude and N represents cycle times; S=2513.6N -0.132 (16) N=S -7.576 *2513.6 7.576 (17) And finally substituting the corrected zero mean stress amplitude mean value into an S-N curve, solving the fatigue life by utilizing a Miner linear cumulative damage theory, and finally combining the calculated time length to directly obtain the service life of the drill string.
13. 13. The method of claim 12, wherein each stress magnitude is effective to damage the fatigue life of the material when the material is subjected to stress cycles of different magnitudes, and wherein the total fatigue damage of the material is predicted by accumulating the damage, and wherein the damage Di generated by the cycle times Ni corresponding to each stress magnitude S i is defined as: where ni is the number of cycles that the material actually experiences at the stress amplitude Si; The total damage D of the material is calculated by the following formula: When D=1, the material reaches a critical state of failure or destruction, and when D <1, the material has not reached fatigue destruction; The service life P of the drill string is calculated by the following formula: P=D×t (20) wherein P is the service life of the drill string, and t is the calculated time length.

Description

Fatigue life prediction method based on dynamic stress of drill string Technical Field The application belongs to the field of petroleum and natural gas exploitation, and particularly relates to a fatigue life prediction method based on dynamic stress of a drill string. Background In ultra-deep well drilling, the down-hole drill string is subjected to complex loads such as stretching, compressing, twisting, bending, colliding, hydrostatic pressure and the like with higher strength, and the number, the position and the induction time of the loads are continuously changed. In addition, the load and stress changes of the well with the complex structure are larger and more complex, the fatigue strength is reduced, and the failure probability is greatly increased. The drill string is worn, broken, thorn leaks, inefficacy accidents such as deformation, not only greatly increased the well drilling cost, but also increased and pulled down time, greatly hindered normal well drilling operation, reduced the mechanical drilling rate. There is therefore a need for a method of predicting fatigue life of a drill string for quantifying drill string life and providing guidance for drill string use and maintenance. Statistics show that of all the reasons for drill string failure, the fatigue induced failure rate is 68%, the drill string overload induced failure rate is 17%, the drill string corrosion induced failure rate is 4%, and the rest causes induced failure rate is only 1%. Thus, drill string fatigue and overload are the primary causes of failure, while drill string vibration is the most important factor causing drill string fatigue and overload. Heretofore, numerous students have analyzed drill string vibrations, including transverse, axial, torsional, etc., but most of these studies have been qualitative analyses and have failed to provide quantitative guidance for drill string use. On this basis, a few students have also proposed related drill string fatigue studies. In 2020, china university Yang Chunxu sets up a bottom hole assembly dynamic stress calculation model considering contact collision and dynamic excitation of a leg and a well wall, and the safety coefficient is used for representing the safety performance of a drill string. In 2020, the university of south China Yin Mao uses finite element software to conduct numerical simulation and fatigue life prediction analysis on drill rod models containing cracks with different geometric parameters, and the fatigue condition is judged qualitatively through the use of the size of the cycles. In 2021, the university of Yangtze river Wu Yuan lamp adopts ABAQUS software to establish a finite element model for calculating the fatigue life of the continuous pipe in the process of drilling and grinding bridge plug operation, and calculates the fatigue life of the continuous pipe under the action of internal pressure and torque composite load, and the fatigue method is similar to the assessment method of university of Nantong river Yin Mao. In 2022, the paper "study of fatigue life of bottom hole assembly based on drill string dynamics" published by southwest petroleum university Mao Liangjie, which proposes to use the drill string dynamics calculation result to study the fatigue life of the drill string, but the fatigue calculation result is also a week, which has little meaning to the actual use in the field and cannot quantify the service life of the drill string. In 2024, shanghai university Wang Wenchang published paper "ultra deep well drilling column dynamic fatigue failure characteristics and parameter optimization", authors put forward fatigue safety coefficients based on the whole well drilling column dynamics characteristics and fatigue damage theory, authors only qualitatively judge the fatigue failure risk of a drill string although the authors have fatigue safety judgment standards, quantitative analysis cannot be carried out on the fatigue life and the service time of the drill string, and the use reference value on site is low. In addition, some scholars have issued related invention patents in recent years, petroleum Ma Xiaocheng issued patent "method for analyzing fatigue life of horizontal directional drilling rod" in 2020, and Mao Liangjie issued patent "study on fatigue life of bottom hole assembly based on drill string dynamics" in 2022, which also have similar problems to previous studies, the fatigue life of drill string is represented by the number of weeks, and effective reference cannot be provided for on-site drill string use and maintenance due to no timeliness. Through research, the conventional fatigue life prediction method is mostly theoretical analysis, the specific service time of the drill string cannot be quantitatively estimated, and effective reference is difficult to provide for the site. The invention discloses a bottom drilling tool combination fatigue life prediction method based on drill string dynamics, which relates to the te