Basic Tubular Buckling – Work

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Basic Tubular Buckling, release 5, issued 16 May 2008. Work version.

This worksheet exists in two versions. They are identical apart from the way they are formatted. The Work version hides intermediate calculations and allows the user to see the results just below the inputs. This is useful for quick “what-if” games, changing various inputs to see what works best. The JIT version displays all intermediate calculations, plus adds tutorial text to explain the methodology.

Description

This worksheet takes a bunch of inputs for a tubular and it’s parameters. It uses the accepted engineering formulae to calculate the Axial Force F_a and the Stabilising Force F_s which arise from the axial (tensile/compressive) force and the internal and external pressures. These two forces added together make the Effective Force, F_eff. Either of the two component forces can be negative and where the Effective Force becomes negative, it is possible for the casing to buckle. With thin walled tubes like casings and tubings, it is assumed that the tubular has no mechanical resistance to buckling.

If both ends of the tubular are fixed (such as in a cemented casing), a temperature or internal pressure increase will decrease F_a and if this decreases F_eff to below zero, then buckling can occur. The temperature increase can be entered in this worksheet and the results calculated, allowing options to be explored to avoid buckling due to changes in temperature.

User input

Select a casing

Select a connection type.

For the OD and connection selected,

additional weight per 

If no data exists, you can enter a figure here for the additional weight per connection. If no value is entered in this field, then the worksheet will instead use the API nominal weight per foot to calculate downhole stresses.

Manually enter the additional weight per connection. Wt_conn =

Enter the average joint length. L =

Enter the depth of the highest fixed point (eg top of cement or tubing anchor), TVD. TVD_fixed =

Enter the pipe weight average with connections. w_ij =

This value is given in the worksheet “Casing Strengths”. If mixed string above TVD_fixed, enter highest weight.

Enter the axial force at the surface (tensile is +ve). F_a =

Enter the internal pressure at the surface. p_i =

Enter the external pressure (annular pressure) at the surface. p_o =

Enter the fluid gradient inside the tube. ρ_i =

Enter the fluid gradient in the annulus outside the tube. ρ_o =

Enter the expected average temperature increase with the well on production. T_Δ =

(Enter -ve if well cools down instead of heats up).

Click here when any values are modified to update the result.

Results

Axial force at the top of the pipe

Top of cement or tubing anchor depth

Neutral point for buckling at the initial temperature (depth above which the tubular cannot buckle)

If the neutral point for buckling occurs within the free pipe (the pipe above the upper fixed point eg top of cement) then the NP_buckling is plotted as a blue dotted line, below which buckling can occur.

The axial force at surface was entered as   or   . The minimum axial force required at the surface to avoid     .

If the minimum axial force required < Fa, then the options include;

1) Picking up an additional surface force after the cement has set to make F_a exceed the minimum required surface force.

2) Raise the top of cement above   , or

3) Support the casing laterally, such as by using rigid centralisers up to the NP_buckling depth.

The situation shown in the 

Neutral point for buckling after the temperature change

This replots the graph above after the temperature change. The same advice as above applies, if the Effective Force goes below zero at a depth above the fixed point in the tubing.

After the temperature change, the

new 

Worksheet references

Benham PP and Warnock FV. Mechanics of Solids and Structures. Pitman 1976.

Version 1 of this worksheet released on 5 February 2008.

Version 4 released after testing against API 5C2 found an error in one of the 8.625″ casing wall thicknesses.

Version 5 released to correct an error in the conversion factor for kPa/m.

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