It’s common to think of HDD drilling spreads in terms of rig size, but the true workhorse of the spread is in fact the mud pump – a high efficiency positive displacement piston pump. Without these pumps, the drilling fluid cannot be pumped into the bore to either jet drill or motor drill, the bore would not have any stability, and the cuttings would not be removed from the bore.

In the Australian HDD sector there is a limited number of available maxi-rigs and powerful mud pumps. Having access to additional maxi-rigs  and powerful mud pumps is a key means to maintaining project productivity and mitigating the impact of unscheduled repairs. If the pumps are down, the drilling stops!

Mud pressure is lost as it moves through the surface piping, and a lot more as it moves down the drill string. Most of the pressure is expended in a jet stream at the drill bit and also as it passes through the stages of a downhole motor, if being used. At this point, the mud pump needs to provide additional pressure to push the mud back along the annulus to the surface, while maintaining an operational long-term duty cycle.

It is also important that the mud pump is sized appropriately to adequately cope with the volumes of drilling fluid required and to maintain adequate annular velocity in the borehole to ensure cuttings remain in suspension in the drilling fluid until the fluid exits the borehole.

Mud pumps capacity

The capacity of the mud pumps is commonly misunderstood and misrepresented. It is common for people to promote their mud mumps as having a 500gpm (1,892lpm) capacity and a 500psi pressure rating. While both numbers may be on the spec sheet, promoting the pumps as a 500gpm pump at 5,000psi is almost certainly incorrect.

For example, a common HDD pump such as the EWECO 446 pump, which is a good all round pump for smaller projects, is often quoted as having an output of 565gpm with pressure rating of 5000PSI. While both numbers are true, they are not true together.

The spec sheets show that the pump will do 565gpm at 1200psi at 440rpm max with six inch diameter liners. Or if the liners are changed to three inch diameter, the pump will output 5000psi, but even at a max of 440RPM the flow output is only 141gpm.

For a long duration longevity on a project it is good drilling practice to limit the operation to 60 – 70 per cent of the capacity, particularly pressure capacity. Assuming for small HDD projects where flow is more important than pressure, the minimum pump in the example above with the largest liners which should be considered is a 565gpm x 65 per cent = 367gpm pump.

Applying the same logic to the pressure rating 1200psi x 65 per cent = 780psi. While there are many contributing factors to pressure such as choke points, valves, drill pipe joint ID, pipe internal roughness, jet nozzle diameter and number, to name a few, it would not be uncommon to see 500psi of pressure on a 1000m jetting hole running three x #16 jets at a flow rate of 360gpm flow rate.

If a downhole motor forms part of the BHA where the formation is rock it would not be unreasonable to add 150 – 200psi to the pressure to operate the motor effectively on bottom i.e. 200psi + 500psi =700psi. For long-term operation the pump is effectively at maximum capacity.

In general terms, additional pumps can be coupled together to increase flow rate but not increase pressure. A longer bore or a higher flow motor would break a single pump in a short time. Double pumps don’t provide additional pressure!

To solve the problem, the pump liner diameter must be reduced, which in turn increases pressure output but decreases flow output. So to drill a longer bore (>1500m) with large downhole motors (>8”) triple or quadruple pumps would be required to provide operational longevity.

Or get bigger pumps

Maxibor has a fleet of four of the largest pumps in the HDD industry. Two Gardner Denver PZ9 pumps with 1000HP engines and Two Gardner Denver PZ8 pumps with 750 HP engines. These are 100 per cent duty rated oil well servicing pumps. Primarily due to the low speed design (130rpm stroke rate compared to the 440rpm in the previous example).

They have proven project after project to operate at high flow and high pressure all day every day for months on end. From a HDD perspective, dual PZ 8/9 pumps have delivered bores in Australia at lengths of 2,500m in the civil industry and 4,000m in the gas drainage industry.

These pumps allow very long bores to be drilled to solve particular infrastructure installation challenges or they allow forward motor reaming, which is another technique to solve particular requirements where exit site sensitivities exist or it is not possible to drill a mud return line. It is these types of pumps that allow high performance cutting edge HDD bore designs to be achieved.

Pumps of this capacity are invaluable, if not a prerequisite, on long bore (+1,000m) and large diameter hole (+800mm) projects requiring larger maxi-rigs such as the Gallagher 660e, Gallagher 600, American Auger 660 and the Vermeer D330x500 which are a key part of the Maxibor HDD fleet.

They are most often required in Australia on river and harbour crossings and long and deep water and sewer projects. Maxibor will be using its Gardner Denver mud pumps on two landmark projects requiring a total of seven bores each averaging over 2.2km in length.

Availability of the pumps has been one of the key factors in the selection of Maxibor as the HDD provider on these projects. An HDD provider like Maxibor with its sizable fleet of powerful pumps and maxi and other rigs provides added comfort to project stakeholders that these key plant items will be available to enable the construction schedule to be maintained.

For more information, please visit or contact David Turner on 0499 375 511.

This sponsored editorial is brought to you by Maxibor. For more information, visit

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