NORTHERN TERRITORY Ecosystem engineering The local project team overcame complications in supply, design and risk to create a new ecosystem consisting of four engineered reef structures situated across Beagle Gulf which were installed in 2019. The reef complexes are large concrete structures that are five metres tall and four metres square at the base, weighing about 24 tonnes. They are expected to last up to 100 years and will attract a variety of species to suit a range of fishing techniques and abilities. Innovation was demonstrated in the concrete mix that was developed for pouring into fully complete module moulds (3.7). A specific mix using nylon fibre reinforcement and additives allowed for a consistent, selflevelling flow through the mould from a single injection point which achieved well over 40 MPA. This mix had not been used in Darwin before this project. Fisheries’ Evan Needham described the specific design of the modules as providing a ‘high rise apartment complex for fish’. The upwelling of the current lifts nutrients through the module and attracts foraging fish while allowing habitat protection from predation. Location. Location. Location. A key aim of the project was to increase the diversity and abundance of fish whilst minimising the attraction of fish from the closest natural reef. To minimise the potential for attraction, the artificial reefs needed to be placed outside the home ranges of fish on natural reefs. A home range is the distance a fish travels from their reef in search of food. There were several target species to consider, however, there was no known home range for them. Cardno found the home ranges and habits of demersal reef fish in other countries associated with artificial and natural reefs. A collection of secondary research
Testing showed the upwelling of the current lifting nutrients through the module.
Reef Pyramid with and without antenna.
reports were utilised to identify a suitable separation distance between a natural reef and an artificial reef. For the areas under consideration, Cardno identified the optimal separation was a minimum of 500 m away from the natural reefs. Installation issues and smart solutions The installation process was another risk that needed to be mitigated. Subcon would typically use an anchor moored barge and crawler crane to install large modules. This process involves a dumb barge being towed to site by a tugboat at a speed of 5 knots. This approach wouldn’t work for several reasons: • the number of reef modules on the project meant that at least six full resupplies would be required, therefore increasing handling and the risk of incidents • infield vessel to vessel transfers would be required for the crew which are statistically the most dangerous offshore operation
• the installation vessel would be subject to swinging around the anchor • removing anchors could damage modules The project team had to design a safe and timeeffective installation procedure. After market investigation, a Dynamically Positioned vessel became the ideal option and Guardian Offshore, who had been interested in buying this kind of vessel, purchased one from Montego Bay. To ensure the installation could take place safely, Subcon provided Shorecast with support by: • developing the novel A-Frame installation technique • performing the engineering to upgrade the capacity of the existing A-Frame • providing experienced crew and supervision for the installation To further ensure safe deployment, Shorecast developed a cradle arrangement to hold the modules still on the
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