Industry:
Off-Shore Drilling
Gulf of Mexico Operations
Problem:
Fasteners holding segmented portions of drilling tools were breaking.
When they broke, the segments fell to the bottom of the
hole and drilling
had to be stopped.
Injuries:
None.
Damages:
This seemingly simple failure had huge consequences, because when
the
fasteners broke, they were over 4000' (1219m) below the surface
of the ocean. Drilling couldn't continue until the broken fasteners
and any other metal parts were removed from the hole, as this debris would
ruin the diamond tips on the cutters. The first step was to remove the
entire drilling assembly so that "fishing" for the broken
pieces could take place. The drill head is at the bottom of a series of
screwed together shafts. Each drill shaft section is 30' (9m) long,
meaning that over 133 drill shaft sections had to be manually disassembled
just to access the hole. Then magnets and grasping devices were lowered
on cables to remove the debris. The drill head was then repaired. These
failures happened on three different occasions. "Fishing"
operations took as little as a day, or as long as seven days. Each day
was at the cost of at least US$100,000.
Observations:
The products failing were 1" – 8x2" socket cap
screws. All product
fractures were at the fillet radius and helical spring lock washers were
used under the heads to counteract the extreme vibration.
Investigation:
The tools were assembled using an as received dry torque value with
slippery
anti-seize compound used on the threads. These were fatigue fractures
that initiated on the fillet radius, exactly where the lock washers touched.
Calibrated torque wrenches had been used.
Conclusion:
After investigation, we know that potent lubricants were used on
the threads,
and thus with the torque applied, the screws were clearly overloaded.
However, the most damaging factor was the use of a lock washer under the
head. The pointed edge of the lock washer contacted the fillet radius
and scratched it, and a subsequent stress riser was created. The fact
that the screw was over tightened exacerbated the opportunity for the
fillet damage. That's because when a lock washer is over tightened,
one leg can splay out due to torsional friction, and the other leg can
then dig into the fillet. The pointed edge becomes like a cutting tool
on a lathe. The failure likely transpired in this manner: the fillet radius
of the screw became scratched by the lock washer, a groove then developed
from the scratch. Cracking then began in the groove, as forces in the
application acted upon it. Then, through subsequent cyclical loads during
drilling, the cracks continued to propagate. The forces in the application
finally exceeded the strength of the steel remaining, and there was sudden
and complete failure.
Recommendations:
Remove the lock washers and provide normal fillet radius clearance.
To
handle the vibration, use a medium strength anaerobic thread locker, and
adjust the torque to coordinate with the lubricity factor. Then torque
the assembly smooth, quick, and allow curing.
As appeared in Fastener Technology
International magazine. |
