Trevallyn Trash Rack Replacement

Client: Hydro Tasmania
Location: Launceston, Tasmania, Australia
Date: 2022-2026

Delivering a smarter trash rack replacement without draining Lake Trevallyn, minimising outages and stakeholder impacts.

Background

The trash rack at Trevallyn Power Station, located around 18 metres below the surface of the lake, acts as the first line of defence for the station, preventing debris from entering the intake and damaging the power station infrastructure.

With parts of the current trash rack approaching 60 years old, replacement of the trash rack was required to ensure the power station continued to reliably provide clean energy.

The project, however, presented significant constraints. Draining the lake was not the preferred option due to its use for drinking water, irrigation, recreation and power generation. This meant the replacement trash rack had to be installed underwater, with the lake at normal operating levels.

The challenge was further complicated by the original design, with steel support beams cast directly into the concrete structure. As such, none of the existing components or fixings could be reused.

Solution

Entura was engaged to design a replacement trash rack and support Hydro Tasmania through delivery, working closely with divers and contractors to develop a solution that considered the needs of all stakeholders.

A key part of the project's success was the application of a staged installation approach based on what was known by the lead engineer as the “big frame” concept. This large steel structure was installed around the intake opening to create precise mounting points for the rapid installation of the new trash rack components.

The project was delivered in two stages:

• Stage 1 (station operational):
  The large frame was installed upstream of the existing trash rack, allowing the station to continue operating at reduced capacity while also avoiding lost generation revenue during the works. During this stage, many of the time-consuming activities were completed, including underwater drilling, installation of around 50 anchors, alignment of the big frame and concrete repairs.
• Stage 2 (station outage):
  With the frame in place providing precise fixing points, the old trash rack could be removed and the replacement installed during a ten-day window.

To further reduce underwater work, Entura designed the trash rack to be installed as large pre‑assembled panels. This reduced the number of underwater connections required, from around 250 bolts to approximately 30. Optimising the design in this way not only reduced the duration of the Trevallyn power station outage, thus minimising generation losses, but also greatly reduced the time divers spent underwater, leading to cost savings and reduced exposure to risk for the divers

Close collaboration with the dive team was important to the successful installation of the trash rack. An installation guide based on a 3D model of the trash rack and intake, along with workshop sessions, ensured the divers were well acquainted with the installation, equipping them with the instructions they needed to work safely in the low-visibility underwater environment. Entura’s specialist was also present for most of the removal and installation of the new trash rack, providing onsite engineering advice as the project progressed.

Outcome

The new trash rack was successfully installed with minimal disruption to power station operations, resulting in an outcome that balanced the needs of all the affected stakeholders.

The design of the new trash rack avoided the need to drain Lake Trevallyn, significantly reduced the duration of the full station outage and associated generation losses and minimised the time divers spent underwater. The design also provides long-term benefits, such as smaller maintenance-related outage windows as modular components can be easily replaced, as well as reusable anchor points ensuring the next time the trash rack needs to be replaced it will be much more straightforward.

The completed trash rack in the workshop before installation