Advances in Dynamic Bottomhole Assembly Modeling and Dynamic Response Determination

Abstract

Abstract Understanding bottom-hole assembly (BHA) dynamic phenomena remains a critical drilling issue due to the substantial cost of associated failures and potential savings from increased drilling efficiency, Many industrial studies have provided useful insight into drillstring dynamics. The current work extends previous efforts by providing a novel BHA model which accounts for several critical response factors and can be augmented to address a variety of other effects, The finite-element technique is used with a frequency-dependent mass matrix to account for the fluid added-mass effect, The BHA excitation is modeled as a vector with components which are monochromatic functions of time, Results from pertinent numerical studies are presented. Introduction Annual drilling postmortems show that drillpipe and BHA failures are among the most prevalent drilling problems worldwide and cost several million dollars annually. Like other drilling problems, such as tubular failures or well control incidents, BHA failures have substantial financial consequences due both to their severe impact on the operation and their frequency of occurrence. In addition to causing drillstring failures, BHA vibrations affect the rate of penetration (ROP) and the reliability of measurement-while-drilling (MWD) systems. With these stakes, it is critical that better understanding of this complex mechanical system be pursued. Drillstring mechanics models have been developed in a number of forms to address distinct design issues. Torque/drag models, under development since 1983, use static force approximations to predict overall drillstring friction loads during rotation and reciprocation. However, these models provide limited insight into the dynamic phenomena that occur during provide limited insight into the dynamic phenomena that occur during drilling. Static directional prediction models date to the earliest days of drilling and focus on determining BHA forces under static conditions. Analytical BHA force calculations ultimately gave way to numerical methods, predominantly finite-element techniques, to allow for analysis under predominantly finite-element techniques, to allow for analysis under general conditions. Dynamic directional prediction models, initially developed in the late 1970s, attempt to model BHA motion by using time-domain simulation to establish temporal averages of BHA and bit forces. As a compromise between the computationally intensive time-domain models and the simplistic static models, quasi-dynamic models evolve In these models rotational friction loads are included in the static formulation to improve the quality of the calculated force distribution and to allow walk predictions in the azimuthal plane. Experimental drillstring dynamic studies were conducted as early as 1960 and are continuing. Measured phenomena include drillstring damping, dynamic downhole bit forces, and dynamic surface drillstring forces. Special tests have addressed the impact of shock subs, whirl, and the boundary conditions of the drillstring. To examine the excitation of the BHA, insight into bit mechanics is require.. Studies on quasi-random bit excitations show that dynamic drillstring forces may be more realistically modeled by adding a random component to the nominal bit rotation speed. A later study involved the instrumentation of tri-cone bits and the measurement of dynamic bit forces and moments. Applications for this technology include detecting bit wear, cone locking, and formation properties during drilling. These tests show that bits produce excitations with many spectral peaks and that specific "signatures" are associated with particular bit types. In 1989, another notable study found that polycrystalline diamond cutter (PDC) bits can exhibit irregular whirl behavior. P. 581