When tubing is freely suspended, it can be buckled by an upward force applied at the bottom of tubing. A section of tubing exposed to compression force will have a chance of being buckled. However, a part which is under tension will not face a buckle issue.
The neutral point is the boundary below which buckling can possibly be occurred and above which buckling will not happen.
Figure 1 – Wellbore Diagram with Tubing Buckling Due to Compression Force
Difference in temperature causes steel to contract or expand. If tubing is free to move, length of the tubing will be either longer or shorter due to thermal expansion. On the other hand, if the tubing is not free to move, there will be a change in axial force due to the temperature effect.
Figure 1 illustrates an increase in length due to heat and Figure 2 demonstrates a decrease in length because of cooling.
Figure 1 – Tubing Lengthen by Temperature Increase
Figure 2 – Tubing Shorten by Temperature Decrease
Ballooning is a change in average pressure which causes a radial contraction or swelling. If the tubing is free to move, the length of tubing will be either longer or shorter. If tubing cannot be moved, stress will be created in the tubing body.
Figure 1 illustrates ballooning of tubing. This will happen when internal pressure is higher than external pressure. Tubing will shorten if the tubing is free to move. If the tubing cannot be moved, ballooning will create a tension force on the packer.
Figure 1 – Ballooning
This article demonstrates a piston effect and how it will change the length of tubing with full detailed calculations.
For the analysis, Lubinski’s sign conventions are used.
Compression force = ⇑(+)
Tensile force = ⇓ (-)
Shorten in Length = (-)
Elongate in length = (+)
Note: The tubing is free to move.
Please see this in the reference section to get a full paper.
Total force change
Total length change
In many situations, a completion string may consist of several size of strings and components so total axial force must be carefully considered by applying a free body diagram concept. When the tubing string is free to move, forces will affect the tubing above where the force is applied.
Tapered String
Tapered string is a string which consists of more than one size of tubing. Hence, forces acting at tapers must be accounted for. Force depends on configurations of pipe and pressure and it will either decrease or increase surface load.
Figure 1 illustrates diagram of a tapered completion string. Wherever in the string there is a difference in outside diameter or inside diameter, there will be downwards or upwards force acting against the tapered edge. It is imperative to analyze each ledge and combine all the results in order to see the total effect. From Figure 1, F1 and F2 cause compression, but F3 creates tension force.
Figure 1 – Tapered String Configuration
