Scope of work
The work scope entailed the design and construction of a multi-component artificial reef comprised of a single main structure surrounded by a cluster of modules. The deployment site specified for the work was 2NM off Tweed Heads in NSW, situated in a 300m long area with a water depth of 25m. The artificial reef was required to facilitate complex fish habitats to aid recreational fishing whilst being resistant to deterioration and loading actions.
For the project, PMG managed the design, fabrication, transportation and installation of an over 10m tall steel structure (‘PMG Fish Grotto’) and 32 x 5m high concrete reef modules (‘PMG Arrowheads’). For the design component ARUP Consulting Engineers were engaged, and under PMG direction undertook a structural design and stability study.
Design and Fabrication:
Pacific Marine Group designed the modules with various aspects being considered, such as the inclusion of a structure that deflects horizontal currents into vertical lift that is often favoured by bait fish. Further to this the design is omnidirectional, thus stable no matter the direction of waves and currents. The module has smaller void spaces in the ‘blades’ for smaller fish and organisms to shelter in and the interior of the module is hollow, allowing larger species and divers to pass through. The steel moulds for the modules were fabricated at PMG’s marine precinct facility in Townsville, with the concrete being poured at PMG’s pre-cast facility. The Fish Grotto consisted of 30t of mild steel SHS and Plate including vertical chains and mesh infills for further habitat creation and an increased surface area.
The 180ft ‘PMG 183W’ barge was mobilised to site from the PMG facility with a Kobelco CKE1800 180T Crawler Crane, utilising the ‘PMG Pride’ tug which also served as accommodation for the installation crew. The Fish Grotto and Arrowhead modules were installed over 6 days with a break in the middle due to challenging weather conditions.
The crane was fitted with Trimble GPS for positioning of modules to an accuracy of <1m in a sea state of 1m.
The modules were installed in a “Y” arrangement which agreed with the DPI to work best running parallel with the East Australian Current (EAC).
- 1 x mild steel Fish Grotto 10m tall with a 14m dia. octagonal base; Innovative features built in that sustain stability and productivity.
- After placement of the Grotto on the seabed a further 96t of concrete block was added to provide an increased volume to the structure and adding weight to prevent movement during a 1 in 100 year ARI storm event.
- 32 x high mass concrete Arrowhead modules were placed on the seabed; Each 4m x 4m x 5m module weighed 17t; Fibrecon 100% recycled fibres were used to provide omnidirectional strength with minimal steel reinforcing being used, resulting in a 95% reduction in carbon footprint.
- 4 types of concrete modules where designed: louvered, textured, inlaid with pipes and standard module.
- Current mould configuration allows voids to be changed for every subsequent project.
- Modules where placed in groups of 4 with each type of module represented in a cluster.