Shock subs, also known as vibration dampeners, are used to absorb vibrations and bit shock loads in drill collar strings. They usually feature long integral elastomeric elements, which serve to transmit torque and weight to the bit simultaneously. When drilling is being carried out at shallow depths, intermittent hard and soft streaks, along with broken formations, can transmit vibrations to the surface, where they are easily detected. With greater depths, though, these vibrations might not be detected because the drill string cushions them. However, they will still cause damage to the bit, as well as bottom hole assembly components and the drill string.
Some advantages of using a shock sub include:
- Offering faster drilling rates, since optimum bit weight and rotary speed may be used on the bit constantly.
- Increasing the bit length by reducing shock loads.
- Cutting damage to drill collars, drill pipe, and downhole tools by reducing bouncing.
- Reduced connection damage, because the elastomeric element absorbs both torsional and axial loads, so that connections are not at risk if the bit stalls.
- Reduced damage to surface equipment, including swivels, blocks, and wirelines.
With conventional drilling, the shock sub will be run between the bit and the drill collars, or else above the second stabilization point in the bottom hole assembly. An advantage of this is that it allows for any weight, pump pressure, and rotary speed to be used- it does not require and specialized operating techniques. There are also usually no weight limitations, although at particularly high temperatures, or when operating in oil based mud, prolonged exposure can take its toll on the elastomeric elements. The only type of servicing that can be performed on a rig is replacing the packing element, and anything else will have to be done in a specialist facility.
Shock subs may not usually used in drilling operations in several location because most drilling assemblies have been motor-type assemblies, whose bearing sections act as shock absorbers. Naturally, this means that additional shock absorption equipment is not needed. What’s more, since shock absorbers feature elastomeric parts integral to the string, there have been doubts about the mechanical integrity of the bottom hole assembly. Directional drillers may be uncomfortable with their use, since they are at some points less rigid than short drill collars which they replace.
As new rotary steerable systems are introduced, and oil-based muds are no longer used, then new shock absorbers are also coming into play. One of these is the Harmonic Isolation tool, which reduces vibrational loads generated by drill bit dynamics, while also helping to improve bit and BHA component life and lengthen the ROP when drilling in difficult conditions. It may be used in either rotary, rotary steerable, or mud motors, and limits radial and torsional vibration problems in hard formations. It furthermore reduces dynamic interaction between BHA and drill bit via a flexible geared connection.
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