Patent Application
20100258352
The present disclosure relates generally to drill strings used in underground drilling such as for gas or oil extraction or for geothermal exploration and activity.
Underground drilling, such as for gas, oil, or geothermal research and the like, generally involves drilling a bore through a formation deep in the earth. Such bores are formed by connecting a drill bit to long sections of pipe, referred to as a “drill pipe,” so as to form an assembly commonly referred to as a “drill string.” The drill string extends from the surface to the bottom of the bore.
In rotary drilling, the drill bit is rotated by rotating the drill string 106 at the surface—that is, at the swivel assembly 104. In addition, piston-operated pumps (not shown) on the surface pump high-pressure fluid, referred to as “drilling mud,” through an internal passage in the drill string and out through the drill bit. The drilling mud lubricates the drill bit, and flushes cuttings from the path of the drill bit. The drilling mud then flows to the surface through an annular passage formed between the drill string and the surface of the bore.
The drilling environment, and especially hard rock drilling, can induce substantial vibration and shock into the drill string. Vibration also can be introduced by factors such as rotation of the drill bit, the motors used to rotate the drill string, pumping drilling mud, imbalance in the drill string, and so forth. Such vibration can result in premature failure of the various components of the drill string. Substantial vibration also can reduce the rate of penetration of the drill bit into the drilling surface and, in extreme cases, can cause a loss of contact between the drill bit and the drilling surface.
The dynamical behavior of drill strings used in the oil or gas industry is very complex and needs to be effectively controlled to avoid undesirable destructive potential. The complexity stems from the fact that typical drill strings have diameter-to-length ratios in the order of 10−5, which is less than that of the average human hair. Furthermore, such very slender drill strings are subjected to complex vibrational phenomena that include: torsional relaxation oscillations induced by non-linear “slip-stick” frictional torques between the drill-bit at the rock surface, axial vibrations that induce “bit-bounce” which cause the drill-bit to intermittently lose contact with the rock surface, and whirling motion of the drill string and the motion of the bit in the Bottom-Hole Assembly “bit in BHA.”
A summary of the different modes of vibrations encountered by the drill strings and the associated physical mechanisms contributing to such modes is given by Spanos et al (2003) and Table 1 lists these modes and the corresponding mechanisms as reported by Besaisow and Payne (1988).
Extensive efforts have been expended during the past decades to understand the underlying physical phenomena governing such complex vibration behavior of the drill strings in order to develop appropriate means for mitigating the resulting destructive effects. These efforts include mathematical modeling, simulation, and/or experimental investigation. Examples of these efforts include the work of Aarrested et al. (1986), Jansen (1991), Chen and Geradin (1995), Yigit and Christoforou (1996-2000), Christoforou and Yigit (1997-2003), Leine et al (2002), Al-Hiddabi et al. (2003), Spanos et al, (2003), Khulief and Al-Nasr (2005), and MihajloviC et al. (2006).
In 1986, Aarrested et al. presented the first theoretical and experimental investigation of the vibration of full-scale drilling rig. In 1991, Jansen modeled the bottom-hole assembly (BHA) to study the nonlinearities due to the interaction between the drill string and the outer shell. Chen and Geradin (1995) presented a finite element model of transverse vibration of drill strings under axial loading. A linear finite element model has been developed by Khulief and Al-Naser, (2005), to predict the buckling loads and critical rotational speeds of drill strings. In 1996 and 1998, Yigit and Christoforou developed finite element models to study the coupled torsional and bending vibration as well as the axial and transverse vibrations of passive drill strings. In 2000 and 2003, their models were extended to simulate the dynamics of drill strings with active control capabilities. Similar attempts have been reported by Al-Hiddabi et al (2003) to control the nonlinearly coupled torsional and bending vibration of drill strings. The effect of interaction with the bore hole has been analyzed theoretically by Christoforou and Yigit (1997) and both theoretically and experimentally by Melakhessou et al (2003).
The effect of stick-slip and whirl vibrations on the stability and bifurcation of drill strings were studied by Leine et al (2002) using a two degrees-of-freedom model. In 2006, Mihajlovic et al presented an extensive study of the limit cycles of torsional vibrations of drillstrings subjected to constant input torque. Also, the equilibrium points are determined and related stability properties are discussed. In 2007, Khulief et al. extended their finite element model to study the dynamics of drill string system in the presence of stick-slip excitations.
In all the above-mentioned studies, emphasis has been placed on conventional drill strings of uniform cross sections. No attempt has been made to considering radically different designs such as periodic drill strings in spite of the potential of this class of drill strings in minimizing the vibration transmission.
As described therein, a drill string includes a plurality of pipe sections, each having an insert providing a material and/or geometrical discontinuity in the pipe section, the plurality of pipe sections forming periodic cells collectively operative to selectively block propagation of vibrations in the drill string.
Also as described herein, a drilling assembly includes a derrick, a swivel assembly coupled to the derrick, and a drill string coupled to the swivel assembly. The drill string includes a plurality of pipe sections, each having an insert providing a material and/or geometrical discontinuity in the pipe section, the plurality of pipe sections forming periodic cells collectively operative to selectively block propagation of vibrations in the drill string.
Also as described herein, a method for suppressing vibrations in a drill string includes providing a plurality of periodic inserts along a portion of the drill string.
The accompanying drawings, which are incorporated into and constitute a part of this specification, illustrate one or more examples of embodiments and, together with the description of example embodiments, serve to explain the principles and implementations of the embodiments.
In the drawings:
Example embodiments are described herein in the context of a system and method for drill string vibration control. Those of ordinary skill in the art will realize that the following description is illustrative only and is not intended to be in any way limiting. Other embodiments will readily suggest themselves to such skilled persons having the benefit of this disclosure. Reference will now be made in detail to implementations of the example embodiments as illustrated in the accompanying drawings. The same reference indicators will be used to the extent possible throughout the drawings and the following description to refer to the same or like items.
In the interest of clarity, not all of the routine features of the implementations described herein are shown and described. It will, of course, be appreciated that in the development of any such actual implementation, numerous implementation-specific decisions must be made in order to achieve the developer's specific goals, such as compliance with application- and business-related constraints, and that these specific goals will vary from one implementation to another and from one developer to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking of engineering for those of ordinary skill in the art having the benefit of this disclosure.
The vibration of drillstrings is, in general, detrimental to the drilling process and may induce premature wear and damage of the drilling equipment which eventually results in fatigue failures. As disclosed herein, a new design of drill strings is proposed for mitigating such undesirable vibrations in an attempt to avoid wear and premature failures. In the new design, the drill string is provided with optimally-designed and -placed periodic inserts which can be either passive or active. The inserts make the drill string act as a mechanical filter for vibration transmission. As a result, vibration can propagate along the periodic drill string only within specific frequency bands called the “pass bands” and the vibration will be substantially or completely blocked within other frequency bands called the “stop bands.” The spectral width of these bands can be tuned actively according to the nature of the external excitation which can be either passive or active. The inserts introduce impedance mismatch zones along the vibration transmission path to impede the propagation of vibration along the string. The design and the location of the inserts is optimized to confine the dominant modes of vibration of the drill string within the stop bands generated by the periodic arrangement of the inserts in order to substantially or completely block the propagation of the vibration.
The theory governing the operation of this new class of drill string as disclosed herein is developed to describe the complex nature of the vibration encountered during drilling operations. The developed model accounts for the bending, torsional, and axial vibrations of the drill string while operating under the influence of “slip-stick” frictional torques between the drill-bit at the rock surface, “bit-bounce” which make the drill-bit intermittently lose contact with the rock surface, and the motion of the bit in the Bottom-Hole Assembly “bit in BHA.” The disclosure herein provides invaluable analytical and experimental tools for the design of a class of drill strings that exhibits unique vibration mitigation characteristics, low wear, long service life, and improved drilling quality.
A. General Period Structures
Periodic structures, whether passive or active, are structures that consist of identical substructures, or cells, connected in an identical manner. The periodicity can be introduced either by geometrical or material discontinuities as shown in
The finite element equations of a typical periodic structure can be rewritten as:
where S and [Tk] denote the state vector ={uL FL}T and the transfer matrix of the kth cell. Note that the transfer matrix relates the state vector at the left end of k+1th cell to that at the left end of the kth cell. Also, note that uL and FL define the deflection and force vectors.
Equation (1) can also be written as:
S
k+1
=λS
k (2)
indicating that the eigenvalue A of the matrix [T] is the ratio between the state vectors at two consecutive cells. Therefore, one can draw the following conclusions:
i. If |λ|=1, then Sk+1=Sk and the state vector propagates along the structure as is. This condition defines a pass band condition;
ii. If |λ|<1, then Sk+1<Sk and the state vector is attenuated as it propagates along the structure. This condition defines a stop band condition.
A further explanation of the physical meaning of the eigenvalue λ can be extracted by rewriting it as:
λ=eμ=eα+iβ
where μ is defined as the propagation constant which has a real part (α)=the logarithmic decay and imaginary part (β)=the phase difference between the adjacent cells. Plain struts (rods) can act as wave guides that not only transmit completely the vibration from one end to the other but also can amplify the vibration at the structural resonances as seen in
Such unique characteristics can be further emphasized by considering the vibration contour plots shown in
From the above, it can be seen that vibration transmission can be effectively blocked by introducing periodicity along the structure. The systems and methods disclosed herein apply these concepts to drill strings, minimizing their vibrations and extending their service life by reducing premature wear and failure due to excessive and undesirable vibrations.
B. Periodic Drill Strings
The concept of the periodic drill string can best be understood by considering the schematic drawings shown in
As disclosed herein, a new class of drill strings that enables fast drilling with minimal down time due to premature failures because of excessive vibration is provided. The concept of periodic drill strings is a viable solution to mitigating the catastrophic problems that arise from excessive vibrations. With such a new class of drill strings, the interaction between the drill collars and the bore hole can be minimized, the whirling effect can be reduced, and the effect of the drill bounce can be decreased. It should be noted that the manufacturing of the passive periodic drill string in particular requires minimal modification of the designs of conventional drill strings as the periodic inserts can be manufactured as an integral part of each drill pipe section. For even better performance, however, these inserts can be provided with active capabilities to attenuate the vibration over wide frequency bands. The concept can also be effective for horizontal drilling. Furthermore, the concept can be equally effective for direct application in all the drill strings for offshore platforms where lateral vibrations due to fluid loading can also be effectively minimized by the periodicity arrangement of the drill strings. The outline merits and the potential application spectrum of the new class of periodic drill strings are envisioned to be beneficial to improving the quality and the state-of-the-art of drilling operations both in land and in sea.
While embodiments and applications have been shown and described, it would be apparent to those skilled in the art having the benefit of this disclosure that many more modifications than mentioned above are possible without departing from the inventive concepts disclosed herein. The invention, therefore, is not to be restricted except in the spirit of the appended claims.
