2018 16 Paper: Maraetai Intake Rehabilitation

The project involved the rehabilitation of the five intakes at Mercury’s Maraetai I Power Station on the Waikato River in the Central North Island. Each of the five intakes consists of a 100 tonne headgate and a pair of gate tracks on which the headgate runs. Rehabilitation sounds simple enough except that the headgate and gate tracks are submerged some 50 metres below the surface of Lake Maraetai, which creates unusual conditions and complexity as well as unusual physical and cost risks. More than 450 surface decompression dives to depths of up to 50 metres were required to complete the project.
The project was delivered safely through several innovative applications:
1. A diver screen was developed to allow the divers to work safely at the mouth of the penstock without being ingested into the unit (with very likely fatal consequences) in the event the guide vane isolations failed.
2. Bespoke tooling was developed to maximise diver safety and productivity:
a. Lightweight, mobile, aluminium platforms to enable the divers to work without having to fin in place and to provide something to react against when torqueing bolts
b. Cleat delivery sleds for ferrying the 2,000 old cleats back to the surface and new cleats down to the divers
c. Bolt coatings that could be torqued without damage thereby preventing the divers having to apply a final coating.
3. Multiple decompression chambers were deployed to enable dive works to run continuously and cause minimal disruption to generation operations.
4. The Goliath crane turned out to be undersized, so lift bags were attached to the headgates to lessen the load transmitted to the crane.

• Author: Bede Geoghegan, James Hunter
• Exchange: 2018 - Albury AGL
• Company: Mercury
• Topic: Asset Condition, Asset Management, Asset Replacement, Civil, Hydraulic Gates, Hydrology, Operation & Maintenance, Refurbishment
• Key Words: Asset, Diving, Gates, Hydraulics, Maintenance, Rehabilitation, Repair, Trash Racks, Weld