It’s common to think of horizontal directional drilling spreads in terms of rig size, but the true workhorse of the spread is in fact the mud pumps. A highly efficienct, positive displacement piston pump. Without the 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.

If the pumps are down, the drilling stops

In the Australian horizontal directional drilling (HDD) sector there is a limited number of available maxi-rigs and powerful mud pumps. Having ready 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.

The capacity of the mud pumps is commonly misunderstood and misrepresented. It is common for people to promote their mud pumps as having a 500 gpm (1,892l pm) capacity and a 500 psi pressure rating. While both numbers may be on the spec sheet, promoting the pumps as a 500 gpm pump at 5000 psi is almost certainly incorrect.

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

The spec sheets show that the pump will do 565 gpm at 1200 psi at 440 rpm max with 6 inch diameter liners. Or if the liners are changed to 3 inch diameter the pump will output 5000 psi but even at a max of 440 rpm the flow output is only 141 gpm.

For longevity on a project, it is good drilling practice to limit the operation to 60 to 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 565 gpm x 65 per cent = 367 gpm pump. Applying the same logic to the pressure rating 1200 psi x 65 per cent = 780 psi.

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 500 psi of pressure on a 1000 m jetting hole running 3 x #16 jets at a flow rate of 360 gpm flow rate.

If a downhole motor forms part of the BHA where the formation is rock it would not be unreasonable to add 150 to 200 psi to the pressure to operate the motor effectively on bottom i.e. 200 psi + 500 psi =700 psi.  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 increase pressure output but decreases flow output. So to drill a longer bore (>1500 m) with large downhole motors (>8-inch) triple or quadruple pumps would be required to provided operational longevity.

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 (130 rpm stroke rate compared to the 440 rpm 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 an HDD perspective, dual PZ 8 / 9 pumps have delivered bores in Australia at lengths of 2500 m in the civil industry and at lengths of 4000 m 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 (+1,000 m) and large diameter hole (+800 mm) projects requiring larger maxi-rigs such as the Gallagher 660e, Gallagher 600, American Auger 660 and the Vermeer D330x500, which are key compnents 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 2.2 km 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.

+61 3 9690 8766 info@primecreative.com.au Visit on LinkedIn Editorial Editor Molly Hancock Molly.hancock@primecreative.com.au Journalist Gabrielle O'Hagan Advertising nick.lovering@primecreative.com.au 11-15 Buckhurst Street South Melbourne Victoria 3205 Australia