Needing to extend the depth of a sand mine from 21m to 60m, an operator in Florida turned to a dredging technology manufacturer which developed a suction pump dredge using couplings to form the drive shaft

With the existing pump dredge becoming ineffectual due to the depth in which it was being used, the operators of a sand mine in Florida thought they would have to move to a clamshell-style dredging method to extend the mine from 21m to 60m deep. This would allow a further 40 years of mining.

For the solution, the operators turned to dredging technology designer and builder, Dredge Supply Co. The company was aware of a pump dredge that was capable of mining to depths of 36m, but was confident that, given the right drive components, a 60m pump would be possible.

Following a year of design, a 90m floating dredge was proposed that would be capable of operating uninterrupted for the next 40 years. The dredge would excavate, suction and pump to the surface 190 to 230 cubic metres of sand per hour carried through 40cm OD high density, polyethylene piping in a slurry of water flowing at 38,500 litres per minute.

Sourcing parts

The mining process begins by excavating the sand at the seabed using high pressure water cutting jets. The sand is then suctioned by an underwater pump which is driven by the longest drive shaft ever built in the dredging industry. Sourcing a drive shaft of this length, which was capable of operating in the harsh underwater conditions, was a major challenge.

Damon Gonzalas, director of engineering for Dredge Supply Co., said: “One of the key issues we had was designing a drive shaft which could transmit the 450kW from the electric motor on the dredge’s hull to the underwater pump on the sea bed. Not only would this be the longest ever drive chain built for a mining dredge, we also had to modify it for operation in the vertical position. In the end we worked closely with TB Woods to design a bespoke solution.”

TB Woods (one of the brands that Altra Industrial Motion’s Altra Couplings group consists of) supplied six 6m long disc couplings which would bolt together to form the drive shaft. The Form-Flex couplings use a hollow tubular shaft – which reduced the overall weight of the solution – with specialised taper lock mounted hubs which allow for easy field service, including axial adjustment at the pump end.

Meeting project demands

To meet the demands of this application, TB Woods customised the hubs with a flange disc pack and tapered bushing mounting to facilitate ease of installation and adjustment to the underwater pump impeller. Because of the abrasive material being pumped, axial adjustment of the impeller is necessary when it becomes worn down. The couplings are able to handle parallel offset and angular misalignment which is essential in an application that will be subjected to unavoidable external forces.

“This project required us to build what is currently the deepest dredge pump in the world. It then had to operate in an abrasive, underwater environment until 2049. Any unscheduled maintenance would be very costly in terms of both man hours and downtime, which means that a vital component, such as the drive shaft, needs to be reliable,” concluded Gonzales.

Altra Industrial Motion