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17/01/2012
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Oil rig operations have some of the harshest operational requirements of any industry sector. The remote, hard-to-reach reserves hidden under the seabed demand deep drilling at high speed. And as reserves dwindle, the result is that ever-greater shock, vibration and temperature is put through critical joints.
 The cost of failure or unscheduled service when a million pound component is hundreds of metres below the ground and sea level is astronomically high. Like any mechanical system, fastened joints can be a source of weakness and a potential failure point. It is essential these stay secure with minimal maintenance requirement, particularly in the well bore.
Many design engineers gravitate toward lock wires, prevailing torque fasteners, or adhesives to prevent loosening but these can be less effective in oil rig applications and have a higher cost than traditional fasteners. However, US fastener manufacturer Spiralock says that its internal thread is being increasingly used by the oil and gas sector with great results; improving reliability by combating shock, vibration, thread loosening, and in extreme temperature.
Spiralock claims its self-locking thread provides excellent vibration and temperature resistance without the cost or complexity of a lock wire or special bolt. So when a rotating control device (RCD) prototype for a major oil services company was being designed the decision was made to use the Spiralock thread form to assess its impact on reliability and production uptime.
The previous generation of RCDs had used a lock wire with a special cross-drilled bolt to keep certain fastened joints from coming apart. But wiring the screw head on the RCD required added time, labour, cost, and component space for service maintenance. During drilling the new RCDs meet rig health, safety, and environment challenges by keeping a pressure tight barrier between personnel on the rig floor and rig drilling fluid and gas returns.
Jeff Jungmann, engineering manager at Spiralock, says: "Locking wires require a special bolt and takes time to wire together; chemical adhesive application can be inconsistent and doesn't work well in high temperature; and deformed threads can gall during initial assembly or lose effectiveness in later disassembly and reassembly.
"Spiralock tapped holes and locking fasteners perform well in a high shock, heat, and vibration environment, particularly in the hard-to-cut super alloys like Inconel, P550, and MP35N, which are increasingly needed in deeper, more demanding well bores. Since the locking component comes from its internal thread form, there's no need for chemical adhesives or secondary locking features."
Most locking fasteners do not address a basic design problem with the standard 60° thread. That gap between the crest of the male and female threads can lead to vibration induced thread loosening. Spiralock say its re-engineered thread adds a unique 30° wedge ramp at the root of the female thread which mates with standard 60° male thread fasteners.
The wedge ramp of the self-locking threaded fasteners allows the bolt to spin freely relative to female threads until tension is created in the male fastener. This continuous line contact spreads the clamp force more evenly over all engaged threads, improving resistance to vibrational loosening, axial torsional loading, joint fatigue, and temperature extremes.
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Author
Justin Cunningham
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