Science Of Mud, Systems In HDD Ops

March 2012, Vol. 67 No. 3

“You start with determining the size of bore you’re going to be building in the ground,” explains Heinen. “This is not the diameter of the product, but the diameter of the actual hole.”

Hole diameter in inches squared divided by 25 gives you the gallons per foot. Next figure the fluid-to-soil ratio, which is based on the soil type and is where the expertise in your ground conditions comes into play.

“The type of ground will have a bearing on how much fluid you need to pump downhole. If you’re in sand, gravel, cobble or rock, typically your fluid ratio to gallons per foot is about 1-to-1,” says Heinen. “If you’re drilling in clay, the ratio can range anywhere from 2-to-1 up to 6-to-1 depending on the soil, and reactive shale is anywhere from 10-to-1 or 20-to-1.”

The last parts of the calculation are the rod length and pump output. So the formula is Gallons per Foot x Fluid Ratio x Rod Length ÷ Pump Output.

“This is the generally accepted formula in the HDD industry. Not following it is like not having Generally Accepted Accounting Practices in accounting,” says Heinen. “It will help the contractor estimate how much fluid will be needed and how much time it may take on the job.”

Sizing equipment, proper maintenance
As stated before, mud systems are married to particular HDD machines, but systems need to be sized according to the hole diameter rather than the capacity of the drill.

“The mud cleaning system is a function of the hole diameter,” says Heinen. “The rule of thumb is the pump should provide 1 liter per minute per millimeter in diameter (6 gallons per minute per inch in diameter). Your annular velocity should be between 3 to 6 feet per minute to maintain a clean hole.”

As an example, 132 gallons (500 L) per minute would be needed for a 20-inch (508 mm) bore. As for fluid supply, there should be enough clean fluid in the mixing hopper to pump a minimum of 10 minutes of fluid at the maximum pump capacity. An example here would be 4,000 gallons (15,152 L) of clean mud to supply a 400 gpm (1514 L) pump. The mixing capacity, which is how much mud the system can create at a given time, should be three times the capacity of the pump. If we are still working with a 20-inch (508 mm) hole and a 400 gpm (1514 L) pump, the mixing capacity should be 12,000 gallons (45,452 L).

Mud systems and reclaimers typically have similar structure no matter the manufacturer. The engine needs fluids checked daily. “Just like any machine, the engine is a daily maintenance check,” says Heinen.

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