Morris manufactures and supplies 510 ton Portal Crane for Kariba Dam

The dam built over 60 years ago.  Over this time the plunge pool has been eroded and was found to be eroding back towards the dam foundations.  The Independent Consultants who monitor the dam recommended enlarging the plunge pool and reconfiguring it to minimise swirling and strengthening the walls to minimise erosion effects.

In addition, the six spillway gates above the plunge pool are also being refurbished as concrete, exposed to water, expands over time making the raising and lowering of the spillway gates a challenge. A new gate infrastructure is being installed.  They also recommended that a movable back up gate be installed on the upstream side of the dam as an additional safeguard should any of the gate’s jam.  As a result, a completely unique hoist arrangement was required to handle the various gates now being installed.

In May 2020 Morris was approached by Freyssinet, a French Company who were engaged in the refurbishment of the spillway and emergency backup gates. They provided us with the highly detailed Client specification and Morris made an offer to them.

Whilst the original offer was tendered against serious overseas competition, the Client was concerned that over the long term, relying on overseas Suppliers could create long response times if a breakdown occurred.

A delegation from the Zambesi River Authority, the Engineering Consultant from Stucky and the Chief Inspectors of factories for Zambia and Zimbabwe visited Morris’s 11 000m2 factory in South Africa and reviewed both our extensive manufacturing facility and design capabilities.  This meeting confirmed that the technical details, quality and design met the Engineers requirements using components fully supported long term by well-established South African Companies.  The contract was awarded in July 2022.

The crane is designed to withstand a load of 510 Tonnes, a load that is determined by the Engineers as being the force exerted by the flow of water when lowering the gates. What makes the crane unique are the three individual hoist trains which, individually provide torque to the two hoist drums.  The emergency gates and stop log items are handled via a two point lifting arrangement set at 5.2 metre centres. The hoist has three operating modes Stop Beam operating speed is 0.7 mpm driven via a 9.2 kW motor.  Emergency gates operating speed is 0.23 mpm via a 30kW motor and the lowering of the Emergency gate is at 1 mpm via a hydraulic retarder.  All hoist sections are individually connected when required via clutches and castell interlocks thus eliminating the risk of one drive back driving the other drives. These clutches are monitored by the PLC control to ensure correct engagement/disengagement.

The crane bridge is 5.5 metre span and has a total height of lift of 37 metres. The crane rails follow the curvature of the dam wall and therefore have a radius of 272.552 metres downstream and 277,702 metres upstream.  Encoders monitor the speed of each long travel drive and the PLC will balance the speed of each drive chain to ensure the crane travels Tangentially down the curved track. They also provide feedback to the Operator to indicate the crane is positioned exactly over the gate openings. Stamp down storm anchors are provided one each side of the span to anchor the crane when not in use and designed to resist a wind speed of 47 metres per second.

A total of four off emergency overspeed brakes acting on the extended hoist drum flange provide a safety backup in case of overspeed. The hoist is fitted with an encoder which will record the height as the gates are raised/lowered and display this value on the HMI screens so that the Operator knows at all times the position of the gates as they descend the spillway guides.

Motions operate via Invertors providing  infinite  variable speed control. Operation is via a radio control unit.  Cameras are provided for visual gate positioning and checking the track is clear of obstructions.  These cameras are displayed on a monitor inside the cranes air conditioned cabin.

Power is provided to the crane via a motorised cable reeling drum and a cable guide system which will guide the cable either side of the turn over anchor and which will also detect any tension in the cable so as to stop the crane in the case of a cable obstruction or cable reel drive fault.

A floor mounted slewing jib crane and pully block is mounted on the upper deck to raise and lower tools and material as required from floor level.

The complete upper machinery deck is totally enclosed inside a machinery house complete with windows to supplement the internal lighting.

The successful refurbishment of the dam’s spillway infrastructure and the installation of the advanced emergency gate system mark a significant step forward in ensuring the long-term safety and operational reliability of this critical water resource. The custom designed Morris crane, built to meet exact engineered standards, stands as a testament to local expertise and innovation.