Page 181 - Failure Analysis Case Studies II

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Fig. 8 General surface condition of the bolt threads, showing evidence of pitting corrosion (arrowed).

The operating system of the raise borer must, therefore, be assessed in order to eliminate the high
cyclic stressing and/or the corrosion.
It can be seen from Fig. 7, which indicates the orientation of the various fatigue crack origins
relative to the original assembly position of the equipment, that there is no clear crack initiation
pattern, and, therefore, no definitive pattern of cyclic stressing. However, the more-or-less random
nature of the crack initiation is consistent with fracture by a corrosion fatigue mechanism. If the
cover was “dishing” upwards during operation, this would have the effect of transmitting a high
cyclic tensile stress on the inner region of the bolts, i.e. where cracking has originated on bolts 3, 4,
6, 8, 15, 16, 26 and 32. Similarly, for the downward “dishing” of the cover, the cyclic tensile stress
would be greater where cracking has originated on bolts 12, 13, 18, 21 and 22. Clearly, the stress
system in this case is complex. Measurements carried out on the cover indicated that the item was
“dished-in” (downwards) by only 0.01 mm. The contact face of the body was also found to be
perfectly flat so there was no apparent major permanent deformation of the cover or body.
The “centre-bolt” torque was found to be well below the normal figure during dismantling. This
could have had the effect of allowing more vertical movement of the drive head cover. With the
equipment working under such severe operating conditions it is essential that all cap screws and
bolts are torqued correctly in order to minimise movement.
Based on the 552 mm2 cross-sectional area of the drive head bolts (26.5 mm from thread root to
thread root) and the approximate ultimate tensile strength of 1230 MPa, each bolt could theoretically
withstand a tensile load of 679 kN before failure, and, therefore, the set of 32 bolts could withstand
a load of 21,728 kN before failure. Considering a total thrust pressure of 5033 kN (the total thrust
pressure includes the mass of the drill string) and the 32 bolts correctly assembled, the system is
therefore operating at a factor of around 4.3. This will, however, be reduced due to the combined
stress concentration effect of the thread root, and, more significantly, by the effect of corrosion
pitting.
During assembly, the drive head bolts are liberally coated with a proprietary anti-seize compound,