Downhole or topside, onshore or offshore, oil rigs have some of the most difficult operational requirements in the oil and gas industry.
Drilling deeper, farther, as well as both hotter and colder in increasingly remote locations has made oil rig reliability and serviceability even more vital. Today, their critical joints must be maintained and production downtime minimised, despite ever more extreme operational shock, vibration and temperature demands.
From measurements while drilling (MWD) chassis, top drives, and set screws to high pressure compressor valves, vertical pumps, and beyond, the tapped holes and fasteners holding vital equipment together must operate with virtual 100% reliability. Otherwise the cost of production downtime and unscheduled service can be astronomically high, particularly when million-dollar equipment is thousands of feet below ground or sea level in the wellbore.
Even though engineers traditionally use adhesives, locknuts, lock wires, or lock washers to prevent loosening, these may be less effective in oil rig applications or have higher total cost when maintenance and production issues are considered. Applying adhesives, for instance, can be inconsistent, ineffective in high or low temperatures, and essentially single use, requiring cleaning off and reapplication.
Since the last few threads of typical locknuts deform, they cannot safely be reused. With interference threads, they are both difficult to run up the bolt during installation and down the bolt during servicing and maintenance. In high vibration equipment, such as top drives or shakers, typical locknuts may so deform that they are sometimes cut from the bolt with a blowtorch during servicing, requiring both the locknut and bolt to be replaced.
As an effective alternative to traditional fastening methods, a unique internal thread form is helping the oil industry enhance reliability and productivity by resisting shock, vibration, thread loosening, and extreme temperature even in hard-to-cut super alloy materials, while enabling extensive reusability in the field.
Fastening problems
Traditional locking fasteners do not address a basic design problem with the standard 60-degree thread form: that the gap between the crest of the male and female threads can lead to vibration-induced thread loosening. Stress concentration and fatigue risk at the first few engaged threads is also a problem. Temperature extremes can also expand or contract surfaces and materials, potentially compromising joint integrity.
Oil and gas industry engineers, however, have successfully attacked these challenges while easing reusability, assembly, maintenance and inventory management with innovative Spiralock self-locking threaded fasteners. The self-locking thread form is used in a range of oil and gas applications from MWD chassis, top drives, reamers, and set screws, to high pressure compressor valves, vertical and OHH pumps, and even mining shakers and separators for oil sands operations.
This re-engineered thread form, offered by the Spiralock brand of STANLEY Engineered Fastening, a provider of fastening and assembly technologies, adds a unique 30-degree wedge ramp at the root of the female thread which mates with standard 60-degree 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.
The crests of the standard male thread form are then drawn tightly against the wedge ramp, eliminating radial clearances and creating a continuous spiral line contact along the entire length of the thread engagement. 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.
Whether as tapped holes or locknuts, the innovative threaded fasteners with a locking feature integrated into the thread form enhance joint reliability and enable extensive reusability, while eliminating the need for secondary thread locking devices or procedures.
“The thread form is not only self-locking but also reusable without damaging the locknut or bolt,” says Ryan Bostick, a STANLEY Engineered Fastening, oil and gas technical sales specialist for the Spiralock brand. “Full contact along the entire engaged length of the thread reduces thread galling and enables fastener reusability up to 40 times or more, reducing the need for replacement parts. The locknuts are actually free spinning until they’re torqued down and locked in place, easing maintenance.”
NASA tests, in fact, found the Spiralock-thread fasteners delivered 50 uses with no loss of clamp load. The thread form was used in the main engines of the Shuttle orbiter, with its tremendous launch vibration and repeat servicing requirement.
Enhancing downhole to topside productivity
When a major oil services company designed MWD equipment, it sought to resolve the downhole issues of extreme shock, vibration, and temperature more effectively than using adhesives. MWD equipment, which measures properties such as pressure, temperature, and wellbore positioning in three dimensional space, is vital to enable extended-reach drilling (ERD), both offshore and onshore. If MWD equipment breaks down in the wellbore due to loose screws, and the equipment must be brought to the surface from deep below ground or sea, it can cost more than $500,000 in lost production costs.
Instead of traditional adhesives, the tapped Spiralock self-locking thread provided the oil services company vibration and temperature resistance without requiring adhesive application or clean up.
Today, after testing and deployment, the self-locking thread enables reuse on a variety of MWD chassis applications, including logging-while drilling (LWD) tools that measure ground formation variables such as porosity, resistivity, gamma ray, and sonic velocity; and photomultiplier tube (PMT) tools that measure refracted light from the soil to determine nearby resources such as oil, carbon, or water. To perform in rough downhole conditions, the self locking thread is tapped in some of the hardest super-alloys available including Inconel 718, P550, and MP35N.
In another oil and gas industry situation, one of the world’s largest top drive manufacturers was using a locknut and conical spring washer to prevent an overhead bolt from coming loose in a topside rig environment. The hardware was costly, as the prevailing torque locknut was not designed for reuse and the conical spring washer tended to be misplaced during periodic disassembly and servicing.
When the top drive manufacturer turned to the Spiralock self-locking thread, it combined the function of the original locknut and conical spring washer into a single, reusable locknut with a flange. This provided the additional locking tension of the conical spring washer, enabled reusability, and saved significantly in hardware costs.
Since then, the self-locking thread form has been used to securely fasten top drive masts and torque tubes, and recently has been identified as a reusable secondary retention device for overhead objects as listed in certain drop awareness organisations. This enhances safety in a rig environment, where the nuts, bolts, and joints of overhead equipment from cranes, hoists, and the rig frame itself must be securely fastened to avoid dangerous drops onto workers below.
While Spiralock tapped holes and locking fasteners are used in a range of oil industry equipment, engineers often become aware of its use in mission critical applications in other industries. It has been used in extreme fastening applications with virtually no chance of recall: from the main engines of NASA’s Space Shuttle; to the Saturn Cassini orbiter and Titan Huygens probe; to medical implants, artificial limbs, and heart pumps. Besides being used in high vibration, high temperature diesel engine applications, the Spiralock locking fastener has been validated in published test studies at leading institutions including MIT, the Goddard Space Flight Center, Lawrence Livermore National Laboratory, and British Aerospace.
“Since Spiralock’s locking component comes from its internal thread form, there’s no need for chemical adhesives or secondary locking features,” says Bostick. “It works with standard bolts and screws, which simplifies application and maintenance.”
Production changeovers to the self-locking fasteners are typically quick and seamless, often requiring just an exchange of traditional nuts, wire inserts or simply drilling out and re-tapping existing parts stock. For resistance to corrosive salt water environments.
