The Kwinana Gas-fired Power Station Project is a 320 MW combined-cycle
Gas fired power plant located at the Kwinana Industrial Estate 30 kilometres south of Perth, Western Australia.
A key element of the new power station, the outfall comprised 320 m of 1.8m diameter concrete pipe, extending from a seal pit weir behind the sand dunes through the beach and into Cockburn Sound. The last 100 m of the pipeline included 21 diffuser ports in a constant water depth of 9 m.
Brady Marine & Civil were engaged as a single ECI contractor a year prior to the commencement of construction. Our contributions to the design and the preparation and submission of project plans to the authorities ensured that both elements reflected the intended method of construction, leading to a project which finished on time, below budget and to the complete satisfaction of all relevant stakeholders.
Construction was facilitated by a substantial temporary bridge structure along the full length of the pipeline giving safe, all-weather access for cranes and other construction equipment, delivery vehicles and personnel.
An innovative method of “leapfrogging” a short length of cofferdam along the length of the pipeline was successfully used, providing less risk, cost benefits, better control and environmental benefits. The cofferdam completely contained any sedimentation associated with the pipeline excavation, an important requirement given the close proximity of the intake structure for the adjacent Perth desalination plant.
The use of shoring minimised seabed disturbance, with an area of disturbance to the existing seabed no more than one fifth of what it would have been had the pipeline been laid in an open trench requiring rock armour protection on the resulting side batters. Completing the permanent works in short lengths also meant that only a small portion of works was exposed to storm damage at any given moment, thus addressing another substantial risk to the project.
Key elements of the project scope included:
- Design management of all works including permanent and temporary works.
- Supply & installation of 320 m of 1.8m diameter concrete pipe, comprising 131 nos 2.44m pipe lengths.
- Supply & installation of 21 duckbill diffuser ports in a constant water depth of 9 m.
- Placing of 4,000 m2 of flexible concrete mattress scour protection.
Special Features
Substantial reuse of materials and components used in the construction of the works was achieved. The sheet-pile waling system, for example was purchased second hand from the Perth Metro rail project and subsequently sold to the Alkimos project north of Perth on project completion.
Constructing the works progressively in short cofferdams required an innovative solution at the junction between cofferdams. Issues to consider such as retaining backfill behind the pipe starter length from the previous cofferdam and avoiding damage during subsequent excavation required careful thought. The solution adopted fulfilled all of these criteria and successfully completed the joints 14 times along the length of the pipeline.
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