APPROACH TO CASING DESIGN

Casing design is basically a stress analysis procedure which is fully described in the ‘Casing Design Manual’.

As there is little point in designing for loads that are not encountered in the field, or in having a casing that is disproportionally strong in relating to the underlying formations, there are clearly four major elements to casing design:

• Definition of the loading conditions likely to be encountered throughout the life of the well.

• Specification of the mechanical strength of the pipe.

• Estimation of the formation strength using rock and soil mechanics.

• Estimation of the extent to which the pipe will deteriorate through time and quantification of the impact that this will have on its strength.

4.7.1. Wellbore Forces

Various wellbore forces affect casing design. Besides the three basic conditions (burst, collapse and axial loads or tension), these include:

• Buckling.

• Wellbore confining stress.

• Thermal and dynamic stress.

• Changing internal pressure caused by production or stimulation.

• Changing external pressure caused by plastic formation creep.

• Subsidence effects and the effect of bending in crooked hole.

• Various types of wear caused by mechanical friction.

• H₂S or squeeze/acid operations.

• Improper handling and make-up.

This list is by no means comprehensive because new research is still in progress.

The steps in the design process are:

1) Consider the loading for burst first, since burst will dictate the design for most of the string.

2) Next, the collapse load should be evaluated and the string sections upgraded if necessary.

3) Once the weights, grades and section lengths have been determined to satisfy the burst and collapse loading, the tension load can then be evaluated.

4) The pipe can be upgraded as necessary as the loads are found and the coupling type determined.

5) The final step is a check on biaxial reductions in burst strength and collapse resistance caused by compression and tension loads, respectively. If these reductions show the strength of any part of the section to be less than the potential load, the section should again be upgraded.

4.7.2. Design Factor (DF)

The design process can only be completed if knowledge of all anticipated forces is available. This however, is idealistic and never actually occurs. Some determinations are usually necessary and some degree of risk has to be accepted.

The risk is usually due to the assumed values and therefore the accuracy of the design factors used.

Design factors are necessary to cater for:

• Uncertainties in the determination of actual loads that the casing needs to withstand and the existence of any stress concentrations, due to dynamic loads or particular well conditions.

• Reliability of listed properties of the various steels used and the uncertainty in the determination of the spread between ultimate strength and yield strength.

• Probability of the casing needing to bear the maximum load provided in the calculations.

• Uncertainties regarding collapse pressure formulas.

• Possible damage to casing during transport and storage.

• Damage to the steel from slips, wrenches or inner defects due to cracks, pitting, etc.

• Rotational wear by the drill string while drilling.

The DF will vary with the capability of the steel to resist damage from the handling and running equipment.

The value selected as the DF is a compromise between margin and cost.

The use of excessively high design factors guarantees against failure, but provide excessive strength and, hence, cost.

The use of low design factors requires accurate knowledge about the loads to be imposed on the casing.

Casing is generally designed to withstand stress which, in practice, it seldom encounters due to the assumptions used in calculations, whereas, production tubing has to bear pressures and tensions which are known with considerable accuracy.

Also casing is installed and cemented in place whereas tubing is often pulled and re-used.

As a consequence a of this and due to the fact that tubing has to combat corrosion effects from formation fluid, a higher DF is used for tubing than casing.

4.7.3. Design Factors

The following DF’s must be used in casing design calculations:

Casing Grade Design Factor

Note The tensile DF must be considerably higher than the previous factors to avoid exceeding the elastic limit and, therefore invalidating the criteria on which burst and collapse resistance are calculated.

4.7.4. Application of Design Factors

The minimum performance properties of tubing and casing from the ‘API’ bulletin are only used to determine the chosen casing is within the DF.

Burst For the chosen casing (diameter, grade, weight and thread) take the lowest value from API casing tables columns 13-19.

This value divided by DF gives the internal pressure resistance of casing to be used for design calculation

Collapse Use only column 11 of API casing tables and divide by the DF to obtain the collapse resistance for design calculation.

Tension Use the lowest value from columns 20-27 of the API casing tables and divide by the DF to obtain the joint strength for design calculation.

Note: It should be recognised that the Design Factor used in the context of casing string design is essentially different from the ‘Safety Factor’ used in many other engineering applications.

The term ‘Safety Factor’ as used in tubing design, implies that the actual physical properties and loading conditions are exactly known and that a specific margin is being allowed for safety. The loading conditions are not always precisely known in casing design, and therefore in the context of casing design the term ‘Safety Factor’ should be avoided.

4.8. DESIGN CRITERIA