Dando have been involved in the manufacture of drill rigs for coal exploration since at least 1900. The techniques and equipment of this very specific type of drilling have changed a great deal from the cable percussion approach that rigs of this era relied on. (Although the cable percussion technique is still very relevant in other areas of exploratory drilling)

Mineral exploration, and in particular open cast coal exploration requires numerous holes to be drilling very rapidly to allow logging and, if required, to enable core sampling quickly and effectively. Dando has been engaged in constant development of techniques and equipment with the aim of improving speed and accuracy since the mid 1960s.

The most recent open cast coal exploration machines to be deployed (to remote jungle regions in the far-east) are open-hole drilling 4 ¾” holes to depths of 400m in very arduous conditions. Once drilled, the open holes are logged with wireline devices and the results of the gamma logging used to determine whether it will be necessary to return to the site in order to retrieve core samples. To achieve this with a single drill rig, the rig therefore needs to be equipped for high speed air-flush open-hole drilling, conventional coring – also using air flush - and wireline coring using water or mud flush.

The often difficult terrain often dictates the type of vehicle that the rig is mounted on. For example, a rig intended for central European farmland might be mounted on to a tractor type vehicle with large rubber tyres, while the rigs that are for supply to the dense forests of the far-east generally benefit from the flexibility of hydraulic crawler chassis units.

Dando have recently supplied several rigs that have been deployed to open cast coal prospects in eastern Kalimantan (Borneo). One of the most striking characteristics of the rigs is the scale. In order to increase productivity the rigs are larger than would normally be seen drilling this type of hole. The drill rigs have a 7.5m stroke length to allow full length drill rods to be used at all times, they have a hoist capacity of 12,800kgf (28,200lbf) and are equipped with very rapid hoist mechanisms to allow the speeding up of tripping times. The rotary head is designed to offer both high torque AND the availability of high speed, this particular head offering 6,800Nm (5,000 ftlb) torque and up to 400rpm. The mast has a complete hydraulic clamping/breaking system, again speeding up operations and a hydraulically deployed diverter box that can be forced into the ground around the borehole to entirely divert the flow from the borehole of cuttings, fluid and groundwater and is particularly useful when working in gas-bearing formations. (see photographs)

Self containment of the rig is important in the remote locations. On the deck of the rigs are 500hp caterpillar power unit, 900cfm 350psi and 30gpm, 600psi diamond coring pump and foam injection pump. The machine has integral fuel and oil tanks and can carry enough fuel to complete at least one shift without re-fuelling.

The whole rig rides on caterpillar excavator tracks with single plate blades which give the machine the ability to tackle the steep slopes and wet conditions typical of tropical jungle areas.

In tramming (travelling) position, the driver is position is seated close to the mast so the rig can be positioned over an existing hole if required.

The full-hole drilling is carried out with specially developed PCD (polycrystalline diamond) bits which can achieve a 400m hole in a single shift – dependant on the geology. (see photograph)

The coring is either performed using a 5m “H” size core barrel with a split inner tube using air, or HQ wireline techniques using water flush – according to depth of the sample required.

The key to large scale coal exploration seems to largely depend on the speed that the holes can be drilled, thus giving as much information as possible about the coal deposit being explored. Over time, Dando have developed techniques that will drill and sample coal measures as quickly as current technology will allow. As with all drilling operations, the speed of drilling is dependent on several factors. These are:-

Bits

To get the best from the coal exploration package, it is essential to use the right type of drilling bit and Dando have developed a full hole 4¾” bit specifically for this purpose. The bit has increased an larger diameter flushing holes to allow a larger amount of air to pass and a larger than usual number of polycrystalline diameter cutters that are positioned in a way that, from long experience, has been found to be the most effective.

Flushing Medium

High pressure air is used in larger volumes to give a much faster up-hole velocity than would usually be the case – up to and sometimes exceeding 12,000 feet per minute (61ms-1). Although there is a slight disadvantage in increased equipment wear due to erosion from the high speed of the returning drill cuttings, the penetration rate is dramatically increased in the mud-stones and sand-stones often encountered in coal measure due to the drilled material “exploding” away from the face of the bit as the borehole progresses. Additives, in the form of polymer drilling fluids and foaming agents are injected into the air-stream when required as drilling damp mud-stone formations can produce extremely sticky cuttings that can be hard to clear from the borehole.

High rpm and accurate bit weight control

These are necessary to obtain the best performance from the specialist diamond bits and the increased flushing medium. This requires that a certain amount of attention is given to the hydraulic system in the rig to allow a relatively small amount of load to be placed on the bit, when the drill string may be quite heavy, at depths up to 400m.

Tool Handling

The penetration rate of any drilling technique is dependent upon the bit design, flushing media, rig drilling controls etc. but another large part of the operation on which speed depends is the handling of tools – i.e. drill pipes, bits etc. when tripping in and out of the borehole. Dando has found that the fastest method for this application is to equip the rig with an extremely rapid hoist facility and a rotary head design that can be tilted to 90º and easily moved off the centre line. The tools are then run in and out of the borehole using high speed hydraulic controls. The drill pipe is then tripped out in 6m sections and, as it is relatively light, can be laid flat on the trestles by the crew.

Dando’s expertise has been gained over a century of mineral exploration projects from Europe to South America to the Far East. Other drill techniques have been tried with varying levels of success. For example, dual wall reverse circulation drilling using air flush has been used and the experience of Dando’s engineers has been that, although this technique produces an uncontaminated albeit a very disturbed sample from an easily identifiable depth, the weight of the equipment makes the drilling operation slower and the much reduced annular space on the outside of the drill pipe can cause problems in mud-stone formations. This technique has however been used with good success in exploring for metals such as gold and nickel in harder rock formations, and for silica sand in extremely soft formations.