Research On Fatigue Life Prediction Of Push-the-Bit Rotary Steerable Drilling Tool

China’s external dependence on scarce strategic resources generally exceeds 50%,posing a serious threat to national economic security.It is necessary to extract a large number of deep and ultra deep wells under complex geological conditions to explore offshore oil and gas resources.Rotary steering system combined with LWD technology can accurately judge formation,accurately control wellbore trajectory,improve directional well timeliness,control downhole risk,and complete downhole operations safely and reliably.However,the core technology of rotary steerable system has been monopolized by three international oilfield service companies for a long time,and there are obvious technical barriers.Compared with the existing rotary steerable drilling system abroad,the deviation capacity of the domestic system is generally low,limiting the application range,and the application time is short,the safety and reliability of the rotary steerable system is difficult to guarantee,and the fatigue failure problem of the rotary steerable system appears in the application,causing the downhole equipment to fall and even the entire hole to be abandoned,resulting in millions or even billions of economic losses.Therefore,the influence of extreme operating conditions on dynamic response and fatigue life of static push-the-bit rotary steerable tool is studied in this paper.(1)The whirl model,stick-slip model,and fatigue life prediction model of the static push-the-bit rotary steerable drilling tool are constructed.The effect of drilling fluid is considered in the model,which is calculated using viscous damping.The dynamic characteristics and whirl motion of RSBHA under sliding friction are analyzed by whirl model.The nonlinear friction and self-excited vibration are considered in the stick-slip model to study the torsional vibration response characteristics.The torsional vibration level of the bit is defined to evaluate the torsional vibration strength of the bit during rotation.The measurement and calculation method of fatigue cumulative damage are considered in fatigue life calculation.The research results provide a theoretical basis for nonlinear vibration and fatigue life prediction of the subsequent static push-the-bit rotary steerable tool.(2)The whirl finite element model of the static push-the-bit rotary steerable tool is established,and the fatigue life is predicted.The initial defects,movement gap contact,coulomb friction model,and coupling to drilling fluid are taken into account.The effects of rotational speed,frictional coefficient,and steering force on the whirl characteristics are studied,and the effects of whirl phenomen on the fatigue life are discussed by numerical model.With an increase of rotational speed and friction coefficient,the whirl phenomenon gradually changes from forward whirl to random whirl and then to reverse whirl phenomenon,and the fatigue life decreases obviously.Compared with the forward whirl phenomenon,the effect of the reverse whirl on the fatigue life is particularly obvious,and the fatigue life reduction rate can reach 80% ～ 84.5%.The fatigue life of rotary steerable tool at different positions is significantly related to drilling parameters.The fatigue life of the flexible sub is relatively low and can be regarded as the weak position.As the rotational speed and friction coefficient increasing,the fatigue life in the middle of the drill string and near the stabilizer decreases.When the speed is 200 rpm,the friction coefficient is 0.2,and the steering force is 6.5 k N,the reverse whirl phenomenon can be avoided,and the fatigue life is about 106.05 cycles.Researches on the whirl phenomenen and fatigue lives of BHA with rotary steerable tool are carried out to provide references for improving the performance rotary steerable tool.(3)The stice-slip model of rotary steerable tool is established,and the self-excitation and randomness of torque are developed by Fortran language.A new method combined deterministic excitation and random excitation is proposed to predict the fatigue life of bottom-hole drilling tools.The initial defects,Coulomb friction model,frictional contact between drill bit and rock,and random cutting process of rock are considered in the model.The effects of rotational speed,WOB,steering force,and random excitation on stick-slip characteristics are studied.The mean dynamic response of stochastic model differs little from that of deterministic model.The flexible sub is the weak position.The 95%confidence interval for torsional vibration strength is inversely proportional to rotational speed and proportional to WOB and steering force.The maximum confidence intervals of the torsional strength coefficients of the three drilling parameters are 12.4%,12.0%,and 10.4%,respectively,indicating that the sensitivity of guiding force is lower than that of rotational speed and WOB.The decrease of rotational speed and the increase of WOB will aggravate the stick-slip phenomenon and reduce the fatigue life.The steering force further increases the nonlinear phenomenon at the bottom and increases the fatigue damage of the rotary steerable tool.A safer stochastic model is recommended to predict the fatigue life of RSBHA at relatively high WOB and low rotational speed.(4)The deterministic and random excitation shaking table tests of the bottom hole of the rotary steerable tool are carried out,and the fatigue life changes of the tool under different drilling parameters are analyzed.The scaled test model of the rotary steerable tool is established through dimensional analysis,and the vibration characteristics of the rotary steerable tool are studied.The results of sweep frequency test are in good agreement with the numerical results of the original size.Different drilling parameters affect vibration response and fatigue life.A certain degree of WOB and steering force can reduce the discrete response of the tool,but relatively large values will increase the mean stress and tool nonlinearity.The relatively large random strength makes the stress response dispersed in a large range.The peak value moves towards large amplitude,which increases the transverse vibration damage and decreases the fatigue life of the structure.In practical engineering,it is recommended to use the total damage value of 0.6 to predict the safe fatigue life.