Buckling of Tubing in Pumping Wells, Its Effects and Means for Controlling It Downloaded from http://onepetro.org/trans/article-pdf/210/01/73/2176282/spe-672-g.pdf by guest on 01 October 2021 ARTHUR LUBINSKI MEMBER AIME PAN AMERICAN PETROLEUM CORP. K. A. BLENKARN TULSA, OKLA. JUNIOR MEMBER AIME T. P. 4482 ABSTRACT INTRODUCTION ancy.":; It was shown that the com­ monly used concept of a neutral It is explained why the bottom It is well known that a column point, as being a point in the string portion of freely suspended tubing must be subjected to some compres­ with neither compression nor tension, in a pumping well buckles and sion in order to buckle; and also that is inadequate, and a more relevant straightens in succession during the a structural member does not buckle definition of the neutral point was in­ pumping cycle. Field evidence of re­ if subjected to a tension. When con­ troduced. The findings were also gen­ sulting rod-on-tubing wear, excessive sideration is given to a tubular col­ eralized to cases of different pres­ polished rod load, and excessive umn, the question may be asked, sures inside and outside a pipe."" horsepower are given. The possibility "How is the buckling phenomenon Later, in a further generalization, al­ that buckling hastens pump wear is affected by inside or outside pres­ lowance was made for variations of strongly suggested. Means for either sure?" At first, it seemingly should stress and pressure with depth.' The prevention of buckling or for mini­ not be affected. However, this is not findings were applied to the new tech­ mizing its effects are explained, their so. In order to arrive at a correct de­ nique of hydrostatic high pressure relative merits compared, and field duction one should first understand testing of lengths of pipe in the results of their use given. These the basic reasons for buckling of a mill.s means are: tension anchors, tail pipe, column under loading*. Then he sucker rod guides, and corrosion in­ should add to the effect of that load­ In 1952 it was observed that pre­ hibitors. Charts and formulas for best ing the effect of pressure. Follow­ vious theoretical findings apply to use of tension anchors, tail pipe, and ing this procedure, one may discover tubing in pumping wells. The lower guides are given. Use of tension an­ quite unexpected phenomena. In part of freely suspended tubing chors, or compression anchors with some cases, when subjected to more buckles during the upstroke portion a heavy tail pipe, prevents both buck­ pressure inside than outside, a pipe of the pumping cycle. Later, it was ling and breathing, thus improving may buckle under tension. In other ascertained that this phenomenon is volumetric efficiency. Without tail cases, a pipe may remain straight, responsible for wear and malfunc­ pipe, buckling above compression an­ although subjected to a very large tioning of the equipment. Various chors precludes such improvement. compression. remedial measures were devised and successful field trials conducted. Original manuscript received in Sodety of In 1950 and 1951 it was explained Petroleum Engineers office on Sept. 15, 1956. why the bottom portion of a string Only brief mentions on the subject Revised manuscript received Jan. 24, 1957. Paper presented at Petroleum Branch Fall of pipe freely suspended in a well have been published:" Meeting in Los Angeles, Oct. 14-17, 1956. Discussion of this and all following tech­ does not buckle even when SUbjected In this paper, the phenomenon of nical papers is invited. Discussion in writing (3 copies) may be sent to the offices of the to a large compression due to buoy- tubing buckling is explained, field Journal of Petroleum Technology. Any dis­ cussion offered after Dec. 31. 1957, should be in the form of a new paper. :·'Using, for instance. Ref. 1. ZReferences given at end of paper. VOL. 210, 1957 SPE 672-0 7.1 evidence is reported, various preven­ tive measures are analyzed and their respective merits compared. The au­ thors believe that presentation of this '-b':::::::::~::: ::::::::1- , paper will be useful to the industry FIG. 2-BUCKLING EFFECT OF INTERNAL for the following reasons: PRESSURE. 1. Perhaps few people are aware ling effect of internal pressure. For today that tubing buckles in pumping this reason, this column load will be wells. The use of adequate preven­ referred to as "fictitious." As shown tive means is rare. in the Appendix, its magnitude is 2. Frequently preventive means equal to pressure times piston area. are used improperly. The correct If the pressure is large enough, the amount of tubing pickup for tension pipe will buckle. It is interesting to anchor installation is not known and note that it buckles, although sub­ spacing for sucker rod guides is gen­ jected to actual tension due to pres­ erally guessed. Information for bet­ sure acting on the annular area ter use of anchors and rod guides is marked AB in Fig. 1. given in this paper. Consider now a pumping well as 3. The choice of preventive means shown diagrammatically in Figs. 3U may often be made without full Downloaded from http://onepetro.org/trans/article-pdf/210/01/73/2176282/spe-672-g.pdf by guest on 01 October 2021 and 3D. During pump upstroke (Fig. knowledge of their respective merits. FIG. 4---BUCKLING EFFECT OF PRESSURE 3U), the standing valve is open and IN TUBING. the traveling valve is closed. This PROCEDURE means that the plunger acts in the if subjected to a fictitious column Through analytical means, the same way as the pistons of Fig. 1. load, a drilling string buckles under phenomenon of tubing buckling in Therefore, if the pressure is large the influence of an actual upward pumping wells has been investigated enough, the tubing will buckle as if column loading, commonly called and quantitative relationships have subjected to an upward fictitious col­ "weight on bit." Both strings are been developed to permit design of umn load or buckling force, t. shown buckled below the neutral point and corrective means. in Fig. 4 and equal to: essentially straight above it. * But the t = a f':..P . (1) Through field observations both neutral point should not be con­ the detrimental effects of buckling in which a is plunger cross-section strued as the point where there is and the improvements due to use of area, and f':..p is the pressure differ­ neither longitudinal compression nor corrective means have been estab­ ential across the plunger. tension. This confusing subject has lished. Results are included in each During pump downstroke (Fig. been largely covered in the liter­ item as discussed. 3D), the traveling valve opens and ature.""" For the purpose pursued in the standing valve closes. Thus the this paper, the neutral point may be DISCUSSION AND RESULTS tubing no longer acts as if terminated considered as that point in the string by a piston, and straightens. below which it buckles. In a drilling BUCKLING OF FREELY SUSPENDED It is evident that there is a simi­ string, the location of the neutral TUBING IN A PUMPING WELL larity between a drilling string and point is obtained from the fact that Consider a pipe lying on the the tubing string of Fig. 3U. Just the weight in fluid of the portion of the string below the neutral point is ground terminated by pistons, as as the tubing of Fig. 3 U buckles as shown in Fig. 1. Pressure is applied equal to the weight on bit.' inside the pipe. Pistons are connected In other words, by a rod in order to prevent their W being expelled by pressure. 11 = -. (2) q It is proven in the Appendix that in which 11 is the distance to the neu­ pressure inside the pipe terminated tral point, w is the weight on bit, by pistons exerts a buckling effect and q is the weight per foot in fluid on the pipe. Furthermore, this buck­ of drill collars. Eq. 2 holds true only ling is the same as if the pipe, in­ if a sufficient length of drill collars stead of being subjected to internal is carried, i.e., if the neutral point is pressure, were subjected to a column in the drill collars. load t, as shown in Fig. 2. Although If w is replaced by the fictitious this column load actually does not force, and if q is taken as tubing exist, it is introduced as a device to t, permit easy calculation of the buck- weight per foot in fluid, Eq. 2 may be applied to a tubing string. How­ ever, for tubing, the equation holds true only if the working fluid level IS sufficiently high, i.e., if the neutral ';'Actually a string of drill collars buckle, helically up to the neutral point in a vertical hole and does not in a slightly inclined hole. FIG. 3U (U:FT)-HELlCAL BGCKLl"i; uF The criterion which led to this conclusion i~ TeBING DURING PU~IP UPSTROKl:. given in Ref. 8. Application of this criterion to buckling of tubing leads, however, to th. FIG. l-PIPE WITH PISTON CLOSED, ENDS FIG. 3D (RIGHT)-TuBING STRAIGHTENS conclusion that tubing buckles helically even SUBJECT TO INTERNAL PRESSURE. DURING PUMP DOWNSTROKE. in inclined wells. .. I PETROLEI'M TR'\,'''ACTIO'''~. AIME point is below the working level. The is subjected only to a small pressure tributed the trouble to crooked hole tubing weight per foot in fluid may differential. In low working fluid conditions. The then common belief also be written as follows: level wells, where the pressure dif­ that hole deviation from vertical could q = q.