This chapter presents a selection of representative previous work on the mechanics of drill strings. The limitations, results and applicability of each formulation are emphasised and the most important of these are reproduced, as briefly as possible, to give some insight into the development of the research literature. Chapter 2 is sub-divided in to five items: section 2.1 deals with static analysis, hence time dependent forces are not included. Section 2.2 treats the case of dynamic analysis where rotary speed, inertia and dissipative forces are considered. Several modes of vibration are reviewed in section 2.3 whereas the instability of drill strings leading to buckling is presented in section 2.4. Finally some practical aspects and experiments related to drilling are reported in section 2.5.
2.1 - Static Analysis
Early work on static analysis of drill strings was dominated by solution of the governing equations using iterative or polynomial series methods. However, complex assemblies containing several stabilisers and different drill collars needed to be analysed by numerical methods. Sub-section 2.1.1 below examines the stability of drill strings in vertical holes, the critical weight on bit and the post-buckling shape. This leads on in 2.1.2 to the problem of drill strings in straight inclined holes with the solution for one stabiliser presented in 2.1.3. The method applied in 2.1.2 is then adapted and extended to constant curvature holes in 2.1.4. Finally some numerical solutions for general cases are reviewed in 2.1.5.
2.1.1 - Stability of Drill Strings in Vertical Holes
By the beginning of this century, "Vertical holes" were obtained through time consuming operations of varying the weight on bit and redrilling, with the maximum acceptable deviation, relative to the vertical, being five degrees. However, it was argued that the rate of change of angle was more important than the absolute angle itself because a small rate of change w ould not interfere in drilling, completion or production while a larger rate of change could be "dangerous" for the drill string. At that time only slick assemblies
(i.e., drill strings without stabilisers) were employed - thus buckling due to self weight was a major concern. If the weight on bit is zero the drill string remains vertical, on increasing the weight on bit drilling progresses vertically and any small lateral excitation vanishes because the equilibrium is stable. As more load is applied, the system becomes unstable and any lateral perturbation tends to buckle the drill string leading on to buckling at second and higher order modes if the load is continuously increased (Figure 2.1.1.1). The nature of the problem suggests that, at least for the first modes, buckling occurs in one plane.
Buckling may affect drilling in many ways: generation of caves, fatigue damage of pipes and rapid bit wear. The buckled drill string touches the bore hole wall, rotates and rubs the surrounding formation. Contact forces increase with apparent radius and vice-versa, hence this self-feeding process may lead to large caves in soft formations. The apparent radius, or radial clearance, is defined as the difference between the hole and drill string radii. Note that the bending moment is also proportional to the apparent radius and high stresses could develop inside caves. Furthermore alternating compression and tension stresses, resulting from rotation of the buckled pipe around its own axis, may induce fatigue damage and failure. Also, the bit tilts after buckling and for isotropic formations it tends to drill in the direction of the resultant force on bit, i.e., the bit tends to deviate from the vertical. Lubrication and force distribution on the bit teeth become asymmetric when the drill string buckles and consequently accelerated bit wear may take place. Despite these inconveniences, Lubinski (1950) advocated that it may be better to apply high weights on bit to improve penetration rate. He argued tha a perfect vertical hole can only be drilled if critical weights are not exceeded.
Lubinski’s paper (1950) on the stability of vertical drill strings is recognised as laying the mathematical foundations for the mechanics of drill strings. Previous research was restricted to a description of the problem and drilling had to rely on empirical methods and experience derived from trial and error. Lubinski investigated drill string buckling up to second m ode - calculating forces on bit and contact points, bending stresses and mode shapes. The governing equation, given by (2.1.1.1), to describe the stability of vertical drill strings is derived and solved in Appendix 1. The solution involves a power series function as given by equation (2.1.1.2), w ith
boundary conditions that assume bending moments and displacements are zero at extremities. Thus :
Y(X) = (2.1.1.2)
n=0
where :
Y(X) describes the shape of the buckled drill string,
E is Young's m odulus,
I is the second moment of cross-sectional area,
p is the weight "in mud" of the drill pipe per unit length,
p = ( p - p J g A ,
p and Pm are respectively steel and mud densities,
A is cross-sectional area,
g is the acceleration due to gravity,
F2 is the component of force on bit transverse to the hole centreline