Powerful pumps heart of HDD maxi-rig spreads

In the Australian HDD sector, limited maxi-rigs and powerful mud pumps are available. Having ready access to these pumps is key to maintaining 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. More pressure is lost as it moves down the drill string. Most of the force is expended in a jet stream at the drill bit. As it passes through the stages, a downhole motor is used. At this point, the mud pump needs to provide additional pressure. They push the mud back along the annulus to the surface while maintaining an operational long-term duty cycle.

It is also vital that the mud pump is sized appropriately to cope with the volumes of drilling fluid required adequately. They also need to maintain adequate annular velocity in the borehole. It ensures 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 (1,892lpm) 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 prime example of a pump

A typical HDD pump such as the EWECO 446 is often quoted as having an output of 565gpm with a pressure rating of 5000 psi. While both numbers are accurate, they are not true when combined. The spec sheets show that the pump will do 565 gpm at 1200 psi at 440 rpm max with six-inch diameter liners.

If the liners are changed to a three-inch diameter, the pump will output 5000 psi. However, even at a maximum of 440 rpm, the flow output is only 141 gpm.

For long-duration longevity on a project, it is good drilling practice to limit the operation to 60 – 70 per cent of the capacity. This is particularly the case for pressure capacity.

Pumps need to account for different things

Small HDD projects tend to consider flow as more important than pressure. In those cases, the minimum pump in the example above with the largest liners should be considered as a 565 gpm x 65 per cent = 367gpm pump. Applying the same logic to the pressure rating 1200 psi x 65 per cent = 780 psi.

There are many contributing factors to pressure, such as choke points, valves, drill pipe joint ID, internal pipe roughness, jet nozzle diameter and number. It would not be uncommon to see 500psi of pressure on a 1000 m jetting hole running 3 x 16 jets at a flow rate of 360 gpm flow rate.

A downhole motor can form part of the bottom-hole assembly (BHA) where the formation is rock. It would not be unreasonable to add 150-200 psi to the pressure to operate the motor effectively on the 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 quickly. Double pumps don’t provide additional pressure.

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

Alternatively, get bigger pumps

Maxibor has four of the largest pumps in the HDD industry. They have two Gardner Denver PZ9 pumps with 1000 HP engines and two Gardner Denver PZ8 pumps with 750 HP engines. These are 100 per cent duty-rated oil well-servicing pumps. This is primarily due to the low-speed design (130 rpm stroke rate compared to the 440 rpm). They have proven 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 2,500 m in the civil industry and 4,000 m in the gas drainage industry.

These pumps allow long bores to be drilled to solve infrastructure installation challenges, or they allow forward motor reaming. This is another technique to solve requirements where exit site sensitivities exist, and it is not possible to drill a mud return line. These pumps allow high-performance, innovative HDD bore designs to be achieved.

Large capacity pumps important

Pumps of this capacity are invaluable, if not a prerequisite, on long bore and large diameter hole projects. They require larger maxi-rigs such as the Gallagher 660e, Gallagher 600, American Auger 660, and the Vermeer D330x500. They are often required in Australia on river and harbour crossings and long and deep water and sewer projects.

Maxibor’s Gardner Denver mud pumps are being utilised on various landmark projects, including one requiring multiple bores averaging over 2.2 km in length. The pumps’ availability has been a critical factor in selecting Maxibor as the HDD provider on these projects.

An HDD provider like Maxibor, with its sizeable fleet of powerful pumps and maxi and other rigs, provides added comfort to project stakeholders that these essential plant items will be available to maintain a tight construction schedule.

In the current environment of limited resources across the infrastructure sector, Maxibor is also willing to consider approaches for the dry hire of its pumps into projects.

For more information contact David Turner on 0499 375 511 or visit https://www.maxibor.com.au/