Helical buckling of Tubing

Helical buckling of Tubing

Consider a string of tubing, freely suspended in the absence of any fluid inside casing, as shown in Fig. 1 (a) . Now consider an upward force F applied at the lower end of this tubing. This force compresses the string; and if the compression is large enough (which is always the case in actual problems), the lower portion of the string will buckle into a helix. as shown in Fig. 1 (b) . The lower end of the tubing is subjected to a compression F. This compression decreases with the distance from the bottom and becomes nil (neither compression nor tension) at the neutral point. Above the neutral point, the string is in tension and remains straight.
The distance n from the bottom of the tubing to the neutral point is:
As proven in the Appendix, the pitch p (i.e., the distance between spirals, just above the lower end of the tubing) is:


Eq. 2 also gives pitch at any point below the neutral point, if F is understood as the compression at that point. The pitch is the smallest at the lower end, where it may be of the order of 20 ft, and increases as the neutral point is approached, where it becomes infinite. In most situations occurring in oil well tubing, the neutral point is located several thousand feet above the packer, and the number of .spirals may be of the order of 1QO or 200. Consider now the same tubing, but sealed in a packer which permits free motion of the tubing, as shown in Fig. 2. Consider that a pressure Pi is applied inside the tubing at the packer level. This pressure P, subjects the bottom of the tubing to a compressive force, and one would expect this compressive force to buckle the tubing inside the casing. As proven in the Appendix, however, the tubing will buckle more severely than could be expected from this actual compressive force alone. It will buckle as if it were subjected to the following compressive force Ff.



As part of this compressive force does not exist, the entire force Ff will be called fictitious. If the packer configuration is as. shown in Fig. 3, the pressure P, subjects the tubing to a tension one might think, therefore, that the string should remain straight. Actually, however, the string will buckle although under tension. This might be considered strange, but similar phenomena have been proven correct for pumping· wells:" and other instances. It is proven in the Appendix that the tubing buckles as if subjected to the compressive fictitious force Ff, given by Eq. 3. Consider now a pressure Po applied outside the tubing at the packer level. In Fig. 3, the pressure Po subjects the tubing to a compression and one might think, therefore, that the string should buckle. Actually, however, the string will remain straight. It is proven in the Appendix that, in the presence of both inside pressure P, and outside pressure Po, the tubing behaves (as far as buckling or straightness is concerned) as if it were subjected to the following fictitious force Ff.


The string will buckle if Ff is positive, i.e., a fictitious compression. It will remain straight if Ff is either negative (i.e., a fictitious tension) or if Ff is zero. Eq. 4 holds true for both the packer configurations of Figs. 2 and 3, and any other possible configuration. It is also proven in the Appendix that in the presence of liquids the weight per unit length, w, must be considered as:


The fictitious force Ff given by Eq. 4 and the weight per unit length, w, given by Eq. 5 must be used in Eq. 1 and 2 to obtain the location of the neutral point and the pitch. With regard to Eq. 1, it should be well understood that, in the presence of liquids, the neutral point is not the point at which there is neither' tension nor compression!··'·' ~ Rather, it is the point below which the string is buckled and above which the string is straight. Depending on conditions, this point may be either under tension or compression, but its location in the string is always given by Eq. 1.

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Prepared By Yasir Albeatiy 3/11/2016 1:22 PM

Reference:
Lubinski, A. & Althouse, W. (1962). Helical Buckling of Tubing Sealed in Packers. Journal Of Petroleum Technology, 14(06), 655-670. http://dx.doi.org/10.2118/178-pa





Helical buckling of Tubing Helical buckling of Tubing Reviewed by Yasir Albeatiy on 3:23:00 AM Rating: 5

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