Failures in the Bottom joints of Surface and Intermediate Casing Strings
Authors: Schuh F.J. 
Published: December 2007

Abstract
The drilling industry long has been plagued by failures in the bottom few joints of surface and intermediate casing strings. This paper presents an analysis of the various possible causes of failure and concludes that failures are caused by short-lived, high-energy torque impulses delivered from the drill string through the bit while drilling out the cementing plugs, cement and floating equipment. The magnitude of these torque impulses is shown to be a function of the rotational momentum of the drill string, and a method is derived to calculate the magnitude of these impulses. The available methods of strengthening the bottom joints are reviewed. It is concluded that, while present methods are ineffective, a combination of improved procedures for strengthening and minor restrictions on drillout practices will prevent failures.

Introduction
In most cases, failure in the bottom few joints of casing strings is not discovered until electric logs record that the bottom one, two or three joints have parted from the casing string and slipped down the hole. However, in some cases the parted section of casing uncovers a high-pressure or lost circulation zone, or shifts laterally restricting the passage of drilling equipment. In these instances extensive remedial work is required to realign the parted pieces and seal the exposed formations.
Several methods used to strengthen the bottom joints of casing strings include locking set screws in the couplings, putting wedges in special collars, welding straps across the couplings, welding the couplings and, most recently, using epoxy resin-based, thread-locking compounds. Since none of these techniques has eliminated the problem, this study was initiated to find the cause of failures and to evaluate the available methods of prevention.

Mechanics of Casing Failures
An analysis of the various possible causes of failure indicates that the casing is unscrewed rather than broken and that therefore failure must occur before the shoe is drilled. Since there is no general agreement as to the causes of casing failure, some possible mechanisms were considered in this analysis. Failure of the bottom joints of casing can occur by only one of three types of stress: tension, torsion or bending. The possible mechanisms of failure were evaluated by determining the forces required to cause casing failure by each of the stresses; and, where applicable, the possibility of failure by fatigue was considered.
For casing to part under tension requires a downward load of more than the tensile strength of the casing. The lightest API weight H-40 grade casing requires a bit load on the bottom of the casing of 27,000 to 34,000 lb per inch of bit diameter (Table 1). Since maximum drill collar weights seldom exceed 10,000 lb per inch of bit diameter, it is apparent that casing strings cannot be parted by the downward load of the drill string.
The second mechanism considered was failure by bending. This mechanism requires that the lowermost casing joints be free to be deflected laterally until the bending stress exceeds the strength of the pipe. To fail, the casing (at a height L above the shoe) requires a force and deflection at the shoe of
(1)
(2)
To deflect the casing shoe with the drill string obviously requires a dog-leg at the casing shoe. This dog-leg must have a rate of change equal to or greater than the change in curvature of the deflected casing, which is given by
(3) or
(4)
TABLE 1 - TENSILE LOADS REQUIRED TO PART THE MINIMUM API WEIGHT H-40 GRADE CASING IN TENSION
Required Casing Parting Load Bit Weight per Size Weight ST and C Bit Diameter (in.) (lb) Casing (lb/in.)7 17 160,000 27,3007 5/8 24 227,000 34,0008 5/8 28 252,000 32,0009 5/8 32.3 279,000 32,000 10 3/4 32.75 265,000 27,000 11 3/4 42 336,000 30,600 13 3/8 48 352,000 28,700
JPT
P. 93ˆ