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Case Study #17 — Fastener Failure Workshop

Off-Shore Drilling
 

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.

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