Comparing drilling rigs can be time consuming and somewhat confusing. With so many manufacturers offering all sizes and types of equipment, how do you know which is right for your business? The drilling rig market is full of options – drilling units for geothermal drilling, micropiling, deep foundation work, geotechnical, water well and the list goes on. Generally though, a rig advertised for water well drilling is capable of much more than just drilling water wells. So why is it just called a water well rig?

Most drilling rigs are built and advertised to be application specific, based on a variety of criteria from max drill depth to the diameter of the borehole. However, a lot depends on the industry application and ground conditions, both of which generally dictate drilling methods and, in turn, required power specifications. These variables mean that based on your industry, geographic location, drilling method and, ultimately, your budget, the field of suitable drilling units begins to narrow. Let’s look at what specifications and options are required for various drilling methods and industries.

Water Well

Like a lot of rigs, Hammer Drilling Rigs’ K60 can work a variety of drilling jobs, depending on the subsurface conditions and operator and client needs. Source: Hammer Drilling Rigs

Many of the principals first mentioned apply equally to other industries. Soil conditions will generally determine the most efficient drilling method. When mud drilling, pushdown—also called pulldown or weight on the bit (WOB)—is key. The harder the formation, the more pushdown required. As more drill pipe is added to the drill string, more weight is applied to the bit and therefore less pushdown is required from the rig. The mud pump must supply adequate flow and pressure which is again determined by the bore depth, diameter and soil. Centrifugal pumps are cost effective and easy to maintain, but provide less pressure and volume than piston pumps. The rpm required from the rotation head is again determined by the material being drilled and the drilling method, with mud rotary generally requiring higher rpm than DTH hammer or auger drilling.

Much less pushdown is required for DTH hammer drilling since the air compressor is providing much of the force. A general rule of thumb is 500 pounds of pushdown per inch of bit diameter. However, the air compressor specifications will greatly affect penetration rates. Pullback is vital in determining depth capacity as the rig must have sufficient power to be able to retract the drill string. The required pullback depends primarily on the weight of the drill string, which is determined by the drill rod weight and bore depth; however, extra power is often necessary when trying to recover a jammed or backfilled drill string due to the borehole collapsing and to allow for a reasonable margin of error. A rig that can drive casing and drill simultaneously is advantageous, and there are several ways to accomplish this from dual rotary heads, which require lots of torque, to bottom drive casing systems.

Geothermal

Similar to water well drilling in that subsurface conditions and depth define the best drilling method and the required power specs, geothermal differs because available space on the drill site is often limited or restricted. In such cases, track mounted rigs are often preferred over larger truck mounted units. Residential retrofits, urban sites, basements, underground parking garages or commercial parking lots don’t offer convenience for the driller. So while the rig requires similar power specs to a water well rig, it needs to be compact and have a small footprint to limit intrusion and avoid costly clean up on retrofit sites. The drill method will also determine what “clean-up” equipment may be required. A mud cleaning unit will keep the site clean, reduce the amount of mud used per borehole and protect the environment from harmful or hazardous waste material. Dust suppression systems are advantageous when DTH hammer drilling.

Geotechnical

Versatility and mobility are key considerations. Drilling methods and requirements vary greatly from site to site, ranging from high speed coring to large diameter hollow stem augering requiring high torque to direct push. A rig with the ability to utilize various heads and options such as SPT hammers is generally required. Mast boom extensions for multiple winches, drill head slide functions for alternating between drill heads and clamping systems all need to be considered to effectively and efficiently complete a project. 

Shallow Foundation / Micropiling

Torque is usually the most important specification under consideration as larger diameter boreholes are often required. Depending on the depth, the weight of the rig and/or the ability to anchor the unit can be vital. However, the size of the equipment is frequently a limiting factor for micropile drilling. Compact rigs are required to drill in dense urban areas, confined spaces and on low headroom sites. Here again, the ability to drive casing and drill simultaneously is advantageous. Managing size and power (torque in this case) can be difficult for the buyer. Limited access drilling units are often limited in their power capabilities so torque should be noted before purchasing.

Generally, the specifications to start comparing are torque and pullback. These two will determine your max drill depth and bore diameter, and give you a good starting point for gauging the capabilities of a rig. Note that while torque specifications often refer to drill power, drill speed can be more important depending on your application and drill method.

Most equipment manufacturers have spec sheets available online that can be used for comparison. A word of caution: Be sure to examine the specifications carefully as some spec sheets use different units of measurement (such as inch-pounds vs. foot-pounds of torque). In the final analysis, the balance between available power specs, equipment size and final price can narrow your search to the point where the best options are clear.