Talking Technically
An introduction to Reverse
Circulation drilling
Reverse Circulation, or RC, drilling
is a fast and cost efficient method
of retrieving high quality samples
from exploration and mine drilling.
The system has been continuously
developed since its inception in
Australia in the early 1970s, and
is now a preferred method for
initial exploration, ore body development and in-pit grade control.
Any method is only as good as the
equipment developed around it,
and Atlas Copco has called on the
experience of drillers worldwide in
order to perfect its offering. Rigs
such as the Explorac 220RC are
revolutionizing exploratory drilling, producing samples faster from
deeper holes and in more difficult situations.
RC methodology
The RC method employs dual wall
drill rods that comprise an outer drill
rod, with an inner tube located inside
the drill rod. The inner tubes overlap,
and seal on the tube below with O
rings when the drill rods are screwed
together. These inner tubes provide a
continuous sealed pathway for the drill
cuttings to be transported from the bit
face to the surface.
The circulating medium, in most cases high-pressure air, enters the annulus between the rod and tube via the air
swivel, which is normally part of the
drill string, or sometimes mounted on
top of the rotation head. The air travels
down the annulus to the drilling tool,
which is usually an RC hammer, or can
be a blade bit or tricone roller bit.
As in conventional open hole drilling, the air powers the drilling tool and
the exhaust air carries the cuttings. In
RC drilling the cuttings are returned to
the surface through the inner tubes in
the drill string and rotation head.
Once through the rotation head, the
air and cuttings comprising the sample
change direction at the discharge blast
box and are transported through the
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Explorac 220 in Australia.
sample hose to the cyclone. The cyclone
slows the sample, separates it from the
air, and collects it.
RC history
Difficult drilling conditions in some
types of soft iron ore and mineral sands
using conventional open hole techniques
led to the development of RC drilling
in the early 1970s for sampling. A dual
tube configuration, occasionally used
in the US oil industry, was adopted as
the basis for the RC drill rod. The first
RC drill rods were made in 1972 by
Bruce Metzke and John Humphries in
Kalgoorlie, Western Australia.
Shrouded tricone roller bits were
initially employed in softer formations,
returning previously unheard of sample
accuracy and target depth achievement.
The development of the crossover sub
facilitated the use of conventional DTH
hammers, and thereafter RC could
be applied to almost all ground conditions. Speed and cost advantages over
diamond drilling led to a boom in RC
drilling, and by the late 1980s more than
2 million m/y of RC exploration drilling were being completed in Western
Australia alone.
The need for cleaner samples led to
the development of the RC hammer
in 1990. High-pressure boosters and
auxiliary compressors were introduced
for deeper holes and faster penetration.
Air pressures up to 100 bar (1 500 psi)
were available, driving necessary advances in all aspects of RC drilling and
RC systems.
In the late 1990s, gold processing
improved, viable ore grades became
lower, but mining costs were higher,
so many mines looked to improve their
ore selection processes.
One of the easiest ways to do this was
grade control drilling, and RC drilling
was the most cost efficient and accurate
method available.As a result, RC drilling is now being used for initial exploration, ore body development drilling,
and in pit grade control drilling.
exploration drilling
Talking Technically
RC benefits
RC drilling provides virtually uncontaminated cuttings to the cyclone. As the
cuttings travel directly from the drill bit
through the steel inner tubes and sample
hose, there is no cross contamination
from other areas of the hole. Using good
sample splitters and sampling procedures, RC results rival the accuracy of
diamond core assays.
Drilling penetration rates are similar to open hole drilling, and are often
faster at greater depths. The sample velocity through the inner tubes can be up
to 250 m/sec, so retrieval of the sample
and hole cleaning is rapid.
Production rates of up to 200-300m/
day are common at rates exceeding 10
m/h, many times faster than diamond
drilling, and achieving rapid results for
the customer.
Unconsolidated formations can often
be drilled and sampled without casing.
Washing and scouring of the hole is
minimized, because there is normally
no fluid or cuttings flow against the
walls after the drill bit has passed. Low
impact bits, such as RC blade or RC
roller, are ideal in these soft or loose
formations.
With good drilling techniques, samples can be kept dry, even several hundred
metres below the water table. Dry samples are preferred as they split more
accurately for assay, and are easier to
handle. RC sample content ranges from
dust to 25 mm chips, and is already
partially processed for analysis.
Wireline surveying of the hole is still
possible through the drill rods. With the
use of a stainless steel rod at the bottom
of the string, hole azimuth readings are
also possible.
Drill string
The components in an RC drill string
have a similar arrangement to those of
a conventional drill string, but are designed specifically for RC drilling, with
all components having a central inner
tube. As these inner tubes carry almost
all of the cuttings from the hole at high
velocity, they are subject to wear.
The rate of wear is governed by the
air volume and pressure of the rig, and
the type of formation being drilled.
exploration drilling
Explorac 220 with cyclone and cone splitter.
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Talking Technically
RC rod with inner tube.
Apart from the hammer inner tubes,
which may only last a few hundred metres in extreme conditions, the remaining
components should last for several
thousand metres of drilling.
Drilling tools
From top: RC bit face, RC bit with a shroud,
RC saver sub, digout sub, RC blade with skirt,
Adapter sub.
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There are generally only three types
of down hole tools used in RC drilling:
hammer, roller and blade.
The RC hammer is the most common
method used, drilling almost all formations with few changes required. The
commonest hammers are in the 4-5 in
range as these meet the power, standard
drill strings, and sample size requirements.
The hammers work on the same
principles as conventional hammers,
but with a hardened, replaceable inner
tube through the centre. The inner tube
extends into the top of the drill bit. A
conventional hammer exhausts the air
through the bit, whereas a RC hammer
exhausts around the outside of the bit
splines and around the head of the bit,
forcing the sample through the holes in
the face of the bit and upward through
the inner tubes.
To help create a higher pressure
zone above the bit face, and to force the
sample up the inner tubes, a sealing
ring is situated above the bit. This ring
can be described as a shroud, sleeve
or compensator ring and is mounted
on the drive sub, or bit chuck, and is
usually replaceable.
The RC hammer bit is similar to conventional hammer bits, but with two
large ports in the face and a large bore
through the centre to accept the hammer
inner tube. There are deep channels on
the outside of the drill bit head to allow
the exhaust air to flush the sample into
the ports in the face.
An RC roller setup comprises a sub
onto which a bit and skirt similar to a
hammer shroud are screwed directly
to the drill string. The bit is normally
a standard mill tooth tricone roller bit,
modified to allow a shroud to be fitted.
The RC roller is only suited to softer
formations, but can be extremely fast
and produces a very accurate sample
and very little disturbance in the hole.
It requires minimal air volume, and
down hole costs are low, so it is a very
economical method of drilling. RC
blade uses a sub and skirt setup similar
to RC roller, but with a drag blade as
the cutting tool. Used in heavy clay
formations, which can be difficult or
impossible with hammer or roller, it can
be very quick and produces an accurate
sample.
Rotating parts
RC drill rods consist of an outer tube,
the rod, and an inner tube. The rods are
externally flush and provide the strength
exploration drilling
Talking Technically
for the assembly, and also the pin and
box threads. RC drill rod threads have
been developed to retain strength while
maintaining a large hole through the
centre for the inner tube and airway.
The inner tube is installed into the
rod through the box, or female, end, and
usually sits on a shoulder in the rod and
is retained with a circlip. Each inner
tube has a male and female end, one of
which has O-ring seals. Once the rods
are screwed together the inner tube
ends overlap, and the O-rings seal the
tubes.
The annulus between the rod and
inner tube carries the high-pressure air
to the drilling tool, while the inner tube
provides a smooth bore sealed tube to
carry the cuttings to the surface.
Most drill rods are 3 m or 6 m long,
and run pin down because of the inner
tube installation.
The most commonly used rod size is
4.5 in (11.43 cm), coupled with a 5 in
(12.7 cm) hammer and 5.25 (13.3 cm)
to 5.75 (14.6 cm) in bit, but rods are also
available from 3.5 (8.9 cm) in to 5.5
in (14 cm) to suit other rig or drilling
requirements.
The inner tubes are a wearing, but
easily replaceable, item.
As with any drill string there are various subs used for adapting, reducing,
and stabilizing. These are all available
for RC drill strings.
The air swivel feeds air into the drill
rod annulus, while still retaining an
inner tube to allow sample flow. They
can be either in-line in the drill string
immediately beneath the rotation head,
or mounted on top of the head as an
integral part of the head.
The rotation head on an RC rig has a
large bore through the spindle to allow
for the replaceable sample inner tube.
RC heads are usually built to provide
high torque at moderate speed, with at
least 10 000 Nm and 100 rpm normal
for larger rigs.
Discharge system
The discharge system is the non-rotating part of the sample path that carries
the sample from the rotation head to the
sample cyclone. It normally consists of
a mud swivel, blowdown valve, blast
box, discharge manifold and sample
exploration drilling
Standard roller bit and drag bit.
hose. The mud swivel, blowdown
valve, manifold and blast box all mount
rigidly to the top of the rotation head.
The mud swivel seals the rotating head
shaft and inner tube from the stationary
parts of the discharge system. The seals
in the mud swivel are critical as they
need to contain the pressurized flow
of sample.
Most RC rigs now have a blowdown
valve fitted to the discharge system.
This is usually a hydraulic or air driven
valve that closes off the sample inner
tube and redirects the downhole air
flow down through the sample inner
tubes.
This function is used to clear blockages in the bit ports or the inner tubes,
and to force all air up the outside of the
drill hole, hence cleaning the hole.
It is done without having to depressurize and unscrew the drill string
to add a sub, so is a very useful tool in
difficult drilling conditions.
The blowdown is mounted on top
of the mud swivel. The sample stream
can be travelling at up to 250 m/sec
and needs to be redirected towards the
cyclone. The blast box usually turns the
sam-ple flow about 90 degrees to meet
the sample hose. This direction change
also reduces the energy of the sample
considerably, but in doing so incurs very
high wear. Most systems have easily replaceable wear components in this area.
The discharge manifold extends sideways from the blast box. It helps slow
the sample to reduce sample hose wear
and also holds the hose clear of the drill
rig as the head travels up and down.
There is often provision to inject small
amounts of water into the manifold to
mix with dry sample and reduce the
dust at the cyclone. The sample hose is a
heavy materials handling hose specially
manufactured for RC drilling. The hose
transports the sample from the discharge manifold to the sample cyclone,
and is long enough to allow for the
movement of the rotation head up and
down the mast.
Sampling
The majority of RC drilling is done to
obtain mineral samples for analysis, so
correct sampling equipment and practices are necessary when undertaking
this type of drilling.
There are two main components to
the sampling system: the cyclone; and
the splitter.The cyclone serves to reduce
the speed of the sample stream, and to
separate the sample from the air, allowing it to be collected. It is important
to have an efficient cyclone to remove
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Talking Technically
Side inlet air swivel.
Tiered riffle splitter.
Cone splitter.
as much of the sample as possible, and
also to avoid contamination of samples.
A good cyclone will typically collect
greater than 99% of the sample, with
the remaining dust and air going to a
dust collector or to atmosphere.
The cyclone should be able to hold
two complete sample intervals without
contamination. An area of the cyclone
called the dump box is equipped with
a door or knife valve for this purpose.
The sample interval is normally 1 m or
2 m of hole. As one sample has collected in the dump box, another is being
drilled and is collecting in the cyclone.
The lower dump box doors are opened
to allow the sample to fall through the
splitter. The lower doors are then closed,
and the upper doors are opened to drop
the next sample into the dump box. In
this way sampling becomes a continuous
process, with little or no interruption to
drilling. Processing of the sample is one
of the most important aspects of RC
drilling. The sample from 1 m of a 5.5 in
drill hole is about 30 lit, or up to 50 kg.
The purpose of a splitter is to divide
the sample down to a smaller size that
is an accurate representation of the
complete sample. This assay sample is
collected in a bag and sent to a laboratory to be analysed for various minerals.
Two main types of splitter are in use.
Riff le splitters use several tiers of
dividers that halve the sample at each
level, until the assay size is reached.
This usually involves 3 or 4 tiers to
give 12.5% or 6.25% of the total sample.
Riffle splitters are easy to use and clean
with dry sample, but do not perform
too well with wet samples. Tiered
riffle splitter, sometimes known as a
Jones riffle splitter. These essentially
divide the sample in two at each tier.
Half the sample goes to waste and the
other half to be split at the next tier and
so on. The number of tiers dictate the
final assay sample size, see picture above.
Cone splitters drop the entire sample
over the point of an inverted cone and
allow it to run down the cone. The assay
sample is taken by collecting a segment
of the sample as it runs off the edge
of the cone. This segment size can be
adjusted to collect the required percentage for assay. Cone splitters can give a
more accurate split, but are more sensitive to setup than riffle splitters. The
con splitter works by dropping sample
through a 120 mm hole over the point
of a cone in an “hourglass effect”. This
provides an even flow of sample over
the cone. Beneath the bottom of the
cone are 2 segment shaped chutes that
direct a percentage of the sample to the
assay bags. These chutes are adjustable to take between 3 and 12% of the
total sample. One is used as the assay
sample, the other for a dublicate sample.
The waste materials falls through a
chute and can be either collected in
a large bag or wheelbarrow or left as
waste, see picture above.
Rotating cone splitters are used for
wet sampling, as they reduce or eliminate
the bias that is created as a wet stream
favours one portion of the splitter.
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Drilling guidelines
RC Drilling has many similarities to
conventional DTH drilling, but there
are also many procedures and techniques that are required to achieve the
best results.
In RC drilling, the hole is of little
importance, while the sample is paramount. Holes are normally set up in a
similar way to a conventional hole, with
a short length of collar pipe or conductor casing set at the surface. A stuffing box, Tee piece or deflector box is
mounted on the collar pipe to direct any
lost sample or outside circulation away
from the drill rig. Rotation speeds and
feed weights are similar to conventional
drilling.
As much sample as possible should
be retrieved from the hole. It is preferable to have at least 95% inside circulation, so that most of the sample is
coming through the inner tubes and only
exploration drilling
Talking Technically
5% or less is being lost to outside. It is
not uncommon to retain 100% inside.
Achieving high inside circulation
sample return is achieved by having
the correct clearance between the bit
shroud and the hole wall, thus creating
a seal and forcing all sample up the
inside. It is also common to allow the
hole to collar off above the hammer to
help with sealing. This collar will often
breach when the water table is reached
in the hole, or it can easily be blown out
using the blowback sub.
If there is water in the hole, allowing
the hole to collar off helps in keeping
the samples dry. Dry samples split far
more accurately, and are much easier to
store, transport and process.
Auxiliary compressors and highpressure boosters up to 100 bar are now
commonly used with larger RC rigs.
These are necessary to keep the hole
dry, producing a dry sample and also
achieving far greater hole depths.
It is almost inevitable that material
will fall in behind the hammer or bit,
causing the drill string to become bogged. This is normal for RC drilling,
and it is common to run a short, stabilized dig-out sub above the hammer
for protection when digging through
fallback.
Blowdown valves are fitted to almost
all large rigs now to help clean holes,
and are usually used at each rod change
to ensure the hole remains clean, or to
remove excess water from the hole.
Blowdown subs are available for insertion into the drill string, but these have
to be removed to continue drilling.
Safety with reverse
Circulation Drilling
There is a wide range of safety regulations and requirements that vary from
site to site, but some general rules will
apply almost everywhere.
While there are some potential hazards associated with the RC system,
the normal safety requirements for the
Explorac still need to be observed.
RC drilling requires one or more
‘Samplers’ or ‘Offsiders’ who process
the sample from the cyclone and they
are also often required to manually
handle some of the downhole equipment.
exploration drilling
Explorac 220 under testdrilling in Australia.
They work close to the cyclone and
the rig mast, so they can be exposed to
hazards not usually encountered during
standard production drilling.
While engineering solutions have
been made for most hazards, the proximity of people to the machine in this
type of drilling demands vigilance and
management.
The customer should have a safety
management system in place and all
hazards should be assessed – most can
be managed with procedures.
• Manual handling: Strains and inju-
ries can be received with handling of
heavy samples, hammers, bits etc.
• Falling objects: Due to the vibrations
involved in percussion drilling there
is potential for objects to shake loose
from the rig mast. Correct maintenance
and regular inspections are required.
• High pressure air: Can be extremely
dangerous. All HP air hoses, inclu ding the sample hose should have
sock type restraints on each end.
Sample hose clamps should be cor rectly fitted and couplings tightened.
• Pinch points: There are many areas
around the rig with potential for pinch
or crush injuries. Mast boom and ro tation head movement, rod loader
movement, cyclone tilt and rotate, and
the cyclone doors can all present some
hazard. Procedures, and communi cation between the driller and sampler
are essential here.
• Personal protective equipment (PPE):
Appropriate personal protective equip ment (PPE) should be in use at all
times; ear plugs, dust masks, safety
boots, hard hat and gloves. These items
are considered as statutory require ments in most mining environments.
Summary
RC drilling is a well-respected method
within the exploration industry. It is
fast and efficient, providing accurate
samples for evaluation by the geologists using tried and tested techniques.
The method is undergoing continuous
technical development that will result
in RC drilling being applied to deeper
holes and more difficult geological conditions. RC drilling is frequently used
in conjunction with core drilling for a
better result in certain circumstances.
The RC drilling method uses high
torques, high pressures, big lifting
capacities and rapid collection of samples, so safety is a major factor in the
ongoing design process.
Jan Jönsson
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