As contractors move up to maxi drills — drills with more than 100,000 lbs of pullback capability —  there are several items that need to be decided before the rig ever leaves the yard. The following are some of the key items:

Drill Stem


There are several items to consider when sourcing drill stem for a maxi drill, such as:
Understanding the rotational torque capability of the drill and matching the drill stem to this torque rating.
Knowing the bore profile so that the drill stem is flexible or steerable enough to make required corrections.
Determining how much flow is needed to get downhole and making sure that the drill stem joint selection will accommodate the flow.

Drill stem diameters for maxi drills vary from 3 ½ to 6 5/8 in. O.D., while joint diameters vary from 5 to 8 ½ in. in O.D. Three of the more common joints are 4 ½-in. IF (internal flush joint), 5 ½-in. full hole and 6-in. full hole.

The length of drill stems used on maxi drills are typically manufactured in 31.5-ft lengths, which are referred to as Range 2 pipe.

Since drill stem is a large investment for maxi drills, it is important to get an inspection report from the drill stem supplier if you are purchasing refurbished drill stem. A contractor needs to consider the risks on the job, soil conditions and if the drill rig will be running in high torque conditions.

Tooling


The obvious and first item to consider when choosing the pilot bore and pullback tooling is what soil conditions you will be boring in. Gather all the geological information you can for the planned bore path.
A mud motor is the preferred method for boring in harder formations and typically ranges in size from 6 ½ to 8 ½ in. in diameter. It is important to match the mud motor to your mud pump specs so it performs effectively. Typically, a higher torque rating on a motor equates into higher production rates, but you will need the required flow and pressure from the mud pump. When renting a mud motor, be sure to fully understand the total cost to get in and out of the motor, as this typically includes not only the rental cost, but also any inspection or rebuild costs.

A variety of heads are available for boring in softer formations. Hole openers and reamer selection depends on the formation in which you are boring and what size of pre-reaming passes you plan to do.
Always double check to make sure that you have the proper transition subs to match reamer, swivels, pullers, drill heads and mud motors to your drill stem.

A commercially available breakout/make up tool will need to be used for removing tools from the drill stem.

Because most maxi drill bores are typically high exposure, it is always a good idea to have spare tooling onsite.

Locating systems used with maxi drills can either be walkover or steering tools (non-walkover). The selection process should be based off of accessibility to the borepath, boring depth requirements and accuracy requirements. A steering tool is normally offered as a service, which an operator is provided when you rent the steering tool.

High-pressure fluid pumps come with many different flow and pressure combinations. Knowing your soil conditions and borehole size are necessary to calculate pumping requirements.

Recycler sizing needs to be matched to the size of your high pressure pump. A good rule of thumb is to have a minimum 2:1 ratio. This means that if the maximum flow output of your pump is 500 gpm, then you should have a minimum of 1,000 gpm processing rate on your recycler.

As with any size of horizontal directional drill rig, the most important thing you can do is plan ahead and know the formations in which you will be drilling.

Marv Klein and Ed Savage are with Vermeer Corp. All Drillmaster Reports are reviewed by the Drillmaster Advisory Board: Savage; Frank Canon, Baroid Industrial Drilling Products; Richard Levings, The Charles Machine Works Inc.; Ron Lowe, Myers-Aplex, a Pentair Pump Co.; and Trevor Young, Tulsa Rig Iron.


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