For Continuous Coring Slim Hole Drilling the basic geometry of the drilling system required to facilitate wireline retrieval of cores through the drillpipe provides much of the vibration minimisation.
The important vibration minimisation techniques of this system, which performs excellently in the hard rock mineral industry, are:
- Close annular clearance between the drill rods and the hole wall. This minimises lateral vibrations by restraining the displacement. The small annulus is considered to act as a hydrodynamic bearing so adding to the damping of lateral vibrations, and the mud flow may damp out torsional vibrations.
- High rotational speed which ensures that the tangential component of sliding velocity between the pipe and the borehole wall is above the critical value for slip-stick friction. The high tangential shear rates in the mud in the annulus absorb energy and thereby provide viscous damping of torsional vibrations. Unfortunately the energy absorbed by these high shear rates results in high ECDs.
- Drill rods which are manufactured to close tolerances, thus minimising out of balance forces. For a given unit weight and hole size the drill rods are much stiffer in torsion and bending than oilfield drillpipe, thus minimising displacement and hence strains due to vibrations.
- Use of diamond core heads which have a low aggressiveness and therefore minimise the coupling between longitudinal and torsional vibrations. As they are stabilised by the core in the throat of the core head whirling is reduced. Typically low bit weights are used to minimise buckling of the collars which can contribute to stick slip friction.
- Smooth surface feed off facilitated either by supporting the chuck type power swivel on two hydraulic pistons, or by suspending the power swivel in the mast by means of a hydraulic piston in tension, or by a hydraulic feed off on the drawworks.
- Use of a hydraulically driven power swivel driven by a prime mover with a constant power output governed by simple throttle setting, thus ensuring that to a certain extent the torsional vibrations coming up the drill rods are absorbed rather than reflected (soft torque system)
This combination of techniques enables relatively light drillstrings to be used to drill to greater depths in hard rock than is possible with conventional clearance oilfield drillstrings.