| The drillstring rotates in the borehole which is full of drilling fluid during the drilling condition, bearings the loads of tension, stress, bending and moment. Meanwhile, owing to the drilling pressure on the drill bit, the dynamic characteristics of torque and the interaction collision of drillstring and borehole wall, the force of drillstring is complicated. While the inner and outer fluid of the drillstring moves as spiral flow, the drillstring and the drilling fluid interact with each other, forming a complicated fluid solid coupling system. The drilling fluid flows because of the movement of drillstring during the process of raising and putting down drillstring, it causes the fluctuation of inner pressure and the phenomenon of lost circulation and blowout. The drillstring and the drilling fluid will still interact with each other during the process of rising and putting down drillstring. Because the length of drilling fluid passage reaches thousands of meters while the diametric dimension is only hundreds of millimeters, the amount of fluid unit to be meshed is enormous in the mesh discretion of the inner and outer fluid, it is difficult to realize with a computer and it has certain difficulty to perform theoretical research. Therefore this essay develops the research of dynamic characteristics of drillstring based on fluid solid coupling.In view of the high computational accuracy of wavelet finite element method and the advantage of increasing the resolution ratio in the precondition of not changing the mesh, on the basis of researching the property of Symlets wavelet and the related numerical computation about scale function as well as the second-generation wavelet scale function fitting, wavelet scale function was adopted to replace the interpolation function of the traditional finite element method, one dimensional C0 wavelet elements and one dimensional C1 wavelet elements were constructed, then Sym-SGW spatial wavelet element was constructed based on the former wavelets.Aiming at the defect of emerging of the redundant inflection points during the interpolation of cubic spline, the interpolation method of tension spline function was adopted to interpolate and then combines the Simpson integral formula to compute the coordinate of any location on borehole line, and then the calculation accuracy was verfied. On the foundation of previous research, the mass matrix of the drillstring dynamics was revised, the drillstring dynamic model during the process of drilling, adopting the established wavelet beam element, was established, the cellular element method was employed to solve the model.Through the force analysis of drilling fluid to the drillstring, the fluid solid coupled dynamic finite element model of the drillstring was established by using Lagrange equation, the variation of the inherent frequency and lateral vibration of drillstring in the presence of drilling fluid were analyzed. Owing to the influence of the drilling fluid, the inherent frequency of each order lateral vibration of drillstring reduced. Meanwhile, under the effect of the inner and outer fluid of the drillstring, the lateral motion trajectory of drillstring differs with different location and then the revolution emerges. Owing that the collision between drillstring and borehole wall is important reason of drill premature failure, the collision mechanism under the condition of fluid solid coupling and the variation situation of stress field and strain field of drillstring during the process of collision were researched with ADINA.The drillstring and drilling fluid were regarded as a system to conduct a comprehensive analysis, the dynamic equation of underground system in the process of raising and putting down drillstring was established, the longitudinal coupling and Poisson coupling and friction coupling between drillstring and drilling fluid is consided and the influence of different factors to the differential pressure and fluctuation pressure was analyzed. |
PDF Full Text Request