Analysis of Critical Buckling Loads For Tool-Jointed Drillstrings in Deviated Wellbores

Excessive torque and drag, buckling and shear forces on downhole strings and tubulars are often encountered in the drilling of longer reach or deviational wells. Buckling of drillstring and BHA occurs in drillstring mainly due to high compressive forces. A point may be reached where these compressive forces rise and exceed the critical buckling loads leading to buckling of the drillstring/BHA or tubulars. This study focuses on the evaluation of the effect of tool-joint on the buckling of drillstrings for highly deviated wells. Tool-joint in pipes changes the pipes geometry in the wellbore thus affecting its hydraulics, orientation and stress distribution. A notable error will arise when straight pipe (with uniform outside diameter (OD) models are used to model pipes with end couplings and connections (such as tool joints). These errors may impact critical buckling loads, buckling initiation points, and post-buckling analysis of the pipe or BHA, thus affecting the success of drilling and completion operations. Torque and drag simulation and analysis was carried out for drillstring and BHA components in 9 5/8 in casing and 8.5 in open-hole sections to determine buckling loads. Two cases were considered; case 1 investigated the modeling and definition of buckling conditions for single straight body drillstrings and case 2 evaluated the buckling conditions for tool-jointed pipes. The result shows that buckling in tool-jointed pipes follows similar trend to that of straight body pipes with sinusoidal or lateral buckling being initiated first, and gradually progresses to helical buckling on increased axial force transfer. Furthermore, from the comparison of the results from two cases considered, it was observed that the presence tool-joint in the pipes led to a critical buckling load of 5.8% for sinusoidal buckling modes. The paper suggests that higher compressive force is needed to buckle the tool-jointed ends of the drillstring than the straight ends.

Experimental Investigation and Analysis of Dynamic Buckling of Drill String in Horizontal Well

Buckling of tubulars has been the subject of many researches in the past. However, the models in previous research always followed the same assumptions: the friction and rotation effects are ignored. The assumptions are relatively far from the reality. This paper focuses on the rotational drill string in horizontal well. The differential equation of the dynamic buckling is established considering some various factors, including friction, the drill string-borehole interaction, and rotation, and the equations of the critical load of the sinusoidal buckling and the spiral buckling are derived. Furthermore, the friction curve of the drill string and the critical load of the dynamic buckling during rotation are obtained. In addition, the influence of the rotational speed on the dynamic buckling and the axis orbit of the dynamic buckling are also gained. The results show that the critical load of the sinusoidal buckling has nothing to do with the rotational speed while the amplitude and the frequency of the dynamic buckling become larger with the increase of the rotational speed, and the dynamic buckling presents a completely different state with the development of the axial load at different speed ranges.

The Sinusoidal Buckling Mechanical Analysis for the Coiled Tubing Based on Energy Method

Columns are prone to sinusoidal buckling and repeated instability on the effect of drilling pressure and fluid pressure during drilling operation. The resulted large area of random contact to borehole wall would affect the drilling efficiency. It made the backreaming and loading hard and the drilling pressure low. Using energy method, taking flexural deformation energy and external potential energy of strings, hole section (slant hole, bending hole, level hole) and annular clearance etc. into consideration, derived the counting function of force that sinusoidal buckling occurred on coiled tubing when it contact with the wellbore, establishing the mechanical analysis method of calculating for sinusoidal buckling of slim lined construction roof bolt and string borehole. It proves an effective method for the applying of coiled tubing drilling pressure and the optimization of process parameters.

First-Principles Finite Element Modeling of Coiled Tubing in Directional Wellbores

Coiled tubing is utilized in a variety of oil well operations. For applications such as drilling, completions, and remediation, coiled tubing offers the benefits of reduced costs, increased insertion speed, and reduced environmental impact. Coiled tubing possesses a limitation, however, in that it can buckle in service causing damage to the tube and disruption of operations. There have been numerous papers published during the last 50 years on helical buckling. Numerous fast running engineering codes have been developed to determine the onset of sinusoidal buckling, helical buckling, and lock-up of drill pipe in a wellbore, with particular emphasis on coiled tubing. We provide a methodology for evaluating the complete nonlinear mechanical behavior of coiled tubing insertion from a first-principles finite element modeling perspective. Using this approach the buckling, post-buckling, and lock-up behavior of the drill pipe can be studied. Additionally, post lock-up methods such as vibration loading and downhole lubrication can also be evaluated.

ELASTIC BUCKLING OF EXTERNALLY-PRESSURIZED IMPERFECT TOROIDAL SHELLS

This paper summarizes the results of numerical studies into the effects of initial geometric imperfections on the elastic buckling behaviour of steel circular and elliptic toroidal shells subjected to follower-type external pressure. The types of initial imperfection studied are (a) axisymmetric localized ones and (b) sinusoidal buckling modes. The principal localized imperfections studied are (i) circular increased-radius "flat spots" and (ii) smooth dimples. The buckling pressures pcr of circular toroidal shells were not very sensitive to initial imperfections. With elliptic toroids, whether the shell was sensitive to initial imperfections or not depended on the ratio k(≡ a/b) of major to minor radii of the section. The shells on the ascending part of the pcr versus k curve behaved like circular toroidal shells, i.e. they were not sensitive to initial imperfections. However, the behaviour of elliptic toroids on the descending part of the versus k curve was very different. The numerical results quoted in the paper are for limited ranges of the geometric parameters. It would be useful to extend these ranges, to explore the effects of plasticity and to conduct model tests on imperfect steel models to verify the conclusions of the numerical studies.