By John — Desalination News
Western Australia’s $2.8 billion Alkimos Seawater Desalination Plant has passed one of its most technically demanding construction milestones, with crews completing two of the longest undersea tunnels ever built in Australia to carry seawater to and from the future plant north of Perth.
Two tunnel boring machines, named Mary and Karli, spent roughly 15 months excavating the plant’s intake and outfall tunnels beneath the seabed off Alkimos, a coastal suburb in Perth’s northern growth corridor. The intake tunnel runs about 2.5 kilometres, extending 1.6 kilometres offshore, while the outfall tunnel stretches around 4 kilometres, reaching roughly 3 kilometres out to sea. Both tunnels sit approximately 20 metres below the seabed. Combined, the two bores account for close to 6.5 kilometres of new undersea tunnelling, among the longest such infrastructure built in the country.
The machines advanced at up to 50 metres a day and installed around 32,000 precast concrete segments, manufactured locally, to line the tunnel walls as they progressed. Separately, Australia’s largest registered jack-up barge, named Beverley, was used to install the undersea intake and outfall structures that connect the tunnels to the open ocean — an operation the project has described as a marine engineering milestone in its own right, given the scale of equipment required to work safely on the seabed.
The tunnelling work is being delivered by the Alkimos Seawater Alliance, the delivery partnership overseeing construction, with Western Australia’s state-owned Water Corporation as the project owner. Once complete, the plant is designed to supply up to 50 billion litres of drinking water a year — enough to serve more than 2.5 million Western Australians — making it one of the largest desalination investments in the state’s history alongside the existing Perth and Southern Seawater plants. The project remains on schedule for completion in 2028.
Water Corporation has framed the plant as a hedge against Perth’s increasingly unreliable rainfall and groundwater supplies, which have declined markedly over recent decades as the region’s climate has grown drier. Desalination already supplies a substantial share of Perth’s drinking water, and Alkimos is intended to add further climate-independent capacity as the population of the northern corridor continues to grow. The tunnels themselves have also been sized to accommodate a future expansion of the plant’s capacity beyond its initial design output, reducing the cost of any later upgrade.
The project is also being cited as a significant driver of local employment and manufacturing activity, with the Alkimos Seawater Alliance crediting the works with supporting more than 1,500 local jobs, including the precast concrete manufacturing that supplied the tunnel-lining segments.
With the tunnelling now finished, attention shifts to the above-ground treatment plant itself and the pipeline network needed to connect Alkimos to Perth’s Integrated Water Supply Scheme. Completing the undersea works was widely regarded as the highest-risk phase of the build, given the technical difficulty of long-distance marine tunnelling at depth, so finishing both machines’ runs without major incident removes one of the project’s largest schedule risks heading into 2027 and 2028.
For a state that has invested heavily in desalination since opening its first large-scale seawater plant near Perth in 2006, Alkimos represents a further bet that engineered, climate-independent water supply — rather than rainfall-dependent dams — will underpin Western Australia’s water security as its population keeps growing.
(Source: Marine Business News) (Source: Utility Magazine)