Mechanical Performance And Fatigue Failure Of Bottom Hole Assembly

Drill string mechanics is very important to the trajectory control and fatigue life prediction of drill string. Therefore, based on the previous works and the engineering experience of drilling technology, the mechanical performance and fatigue failure of Bottom Hole Assembly have been studied systematically. The content includes:(1) According to the work condition of Bottom Hole Assembly (BHA) inthe well, the geometrical and contacting nonlinear static mechanical model of BHA is presented in this paper. In the model, considering the contact condition between BHA and hole wall and the vertical bending caused by the BHA weight and weight on bit(WOB),the iterative method of "restrict" and "release" is used to deal with the problem of contact between BHA and wall of the well. The method have good precision and convergent rate. Moreover , it can figure out the position of upper tangent point, which present a nice way to analyze the force and deformation of BHA.(2) The preprocessor of the contact nonlinear static mechanical model is compiled by ABAQUS, which is called Inp program file. The Inp program file has the following functions: ①The BHA model can be translated into the CAE interface of ABAQUS; ②Static mechanical model can be established by Inp file according to different BHA; ③For a certain BHA, the parameters affecting the inclined capacity of BHA can be changed easily; ④the iterative method of "restrict" and "release" is used to figure out the position of upper tangent point.(3) The inclined capacity of BHA is analyzed, which is affected by various parameters. It is shown that the lateral force of drill bit and inclined capacity of BHA have something to do with the parameters(the distance between drill bit and the first stabilizer, outside diameter of the first stabilizer, WOB etc). Moreover, taking the BHA with two stabilizers for example, based on analyzing the displacement, bending moment and stress on the nodes of BHA, the force and deformation of BHA are obtained. At the same time, counterforce ,displacement and axial stress of a section on the nodes of BHA are listed in the form of sheet, which provides a basis on studying the fatigue life of BHA.(4) Based on mechanical analysis of BHA, fracture mechanics is used todiscuss the stress intensity factor .￡,and the critical stress intensity factorKir of the opening crack propagation mode ( I type crack). At the crack tip , thispaper takes the plastic deformation into account and corrects plastic zone to broaden the applied scope of the linear—elastic mechanics. The Forman fatigue life prediction model and Miner linear accumulative fatigue model are presented for the fatigue life prediction of BHA.(5)A kind of stress-relief groove of box thread joint has been developped. Finite Element models of several thread joints have been established by ABAQUS. The Morrow Strain-life Model is used to evaluate the fatigue life of 3 kinds of connections with different stress-relieaf grooves of box. The results show that the fatigue life of box with stress-relief groove developed in the paper is the longest.