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Concrete In Australia : September 2008
OFFSHORE STRUCTURES REPORT Australia and the Asia Pacific region, are experiencing a period of sustained growth which is expected to continue for many years. Support infrastructure and services, such as the AMC Common User Facilities and the Henderson services providers continue to develop as a result of the petroleum and minerals resources boom. There is a clear case for project developers to consider the AMC and other Western Australian sites, such as Bunbury, for fabricating and manufacturing CGS/concrete fl oater/storage vessels as well as other key resource project development items. This report is a summary of studies Constructing the caisson substructure of the Wandoo CGS platform at Bunbury, in the mid 1990s. PHOTO: WANDOO ALLIANCE shipping or fl oating them to remote areas through port facilities such as the AMC. Studies sponsored by the Concrete Offshore Structures Industry (COSI) Group, made up of Industry, Government Departments and Cement and Concrete Association representatives, have shown that concrete structures, even larger than the Wandoo B CGS, could be constructed in Western Australia. For large structures, unique soil conditions and water depths on and adjacent to the North West Shelf impose structural requirements such as long skirts on the underside of the structure. These requirements may be adequately addressed during construction by using advanced construction techniques. Float out of the structure from the casting basins and through the channels and harbour may be accomplished using either additional dredging of tow out channels or by using air cushions to support the structures. The latter technique (known as “air-lift”) has been studied extensively and documented in various reports available to industry. These reports demonstrate the adequacy of existing knowledge in air-lift techniques for optimising the capacity of existing and potential casting basin and harbour facilities while maximising the benefi ts to both oil and gas project developers and local industry operators. Published literature has also shown that using air-lift during tow-out can alleviate a critical construction bottleneck and result in significant fi nancial savings by reducing the base footprint. This report, prepared on behalf of the COSI, presents to industry the current state of knowledge in this area so that oil and gas/minerals resource project developers can be assisted in selecting concrete structures as the preferred option. It clearly summarises the results of COSI studies directed at developing technical solutions for resolving fl oat-out bottlenecks, a key solution being the use of air cushion support or air-lift techniques. The marine, resource and offshore industries, both in Western 38 Concrete in Australia Vol 34 No 3 undertaken on behalf of COSI Committee and provides an overview of applications and technologies related to the construction and development of large concrete structures, associated with resources industry in WA. Within this context, the report is directed at: • Concrete Gravity Structures for oil and gas production and processing such as the Wandoo B • Extending the Wandoo precast concrete methods to other marine, mining, and resource infrastructure projects. An extensive literature overview and elaboration of previous studies are presented in this report. CGS for oil and gas production and processing The number of oil and gas discoveries in Western Australia and the Northern Territory has increased over the recent years, and there is a predicted demand for a range of offshore platforms to be installed in soft sea bed conditions, typically found in deeper waters. Piled steel structures have suffered from installation problems, and Concrete Gravity Structures The Wandoo CGS during tow-out from its casting basin in Bunbury. PHOTO: WANDOO ALLIANCE