Page 101 - Practical Well Planning and Drilling Manual

Section 1 revised 11/00/bc 1/17/01 2:56 PM Page 77

1.4.13
Casing Design [ ]

could work. It can be seen that it would be worth the extra effort to cal-
culate it out if the casing string could be kept to the lower grade as a
result, 53.5 ppf N80 usually being cheaper than 47 ppf P110.
Tension due to shock loading. Various authors have proposed for-
mulae to calculate shock-loading forces on casings while running. By
experience, if the above safety factors and a reasonable overpull
allowance are used, they are adequate to cover shock loadings if the
drill crew follow normal procedures. In any case, calculating actual
shock loads probably cannot be done accurately due to the many
assumptions that have to be made or mitigating effects that have to be
ignored. The latter will include the damping effect of mud, frictional
forces against the wellbore (especially if deviated), and actual running
speed at the time that the casing is arrested.
Axial loads due to bending forces in a deviated wellbore. In a
deviated well, you could now account for extra tension due to bending.
Assume that our 15,000 ft well is kicked off at 1000 ft vertical depth,
angle is built at 2.5˚/100 ft to 25˚ inclination. The well is now drilled
tangent to 15,000 ft TVD. (See Fig. 1-16)

Fig 1-16 Effect of Bending Pipe on Casing Axial Stress

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