The Wandoo B platform is the first gravity base production platform to be installed on Western Australia's North-West Shelf. The platform, located about 75km offshore in 54m of water, consists of a Concrete Gravity Substructure (CGS) supporting a two level integrated deck. The CGS has closed cells providing 400,000 barrels of oil storage, and open cells containing solid ballast.
This paper describes the investigations carried out to determine group conditions at the proposed site, and presents the results of conventional and special laboratory testing undertaken to derive geotechnical design parameters for the carbonate materials present at the site.
Ground conditions dictate that foundation stability under environmental loading is governed by sliding. The procedures used to assess the amount of solid ballast required to provide sliding stability under storm loading are described, together with details of the underbase drainage system developed to aid dissipation of excess porewater pressures generated by cyclic shear loading.
The Wandoo oilfield is located in the Carnarvon Basin, some 75km north-west of Dampier on Western Australia's North-West (See Figure 1). Prior to 1994 production was via the Wandoo A facility which comprised a braced monopod steel jacket, process facilities mounted on an adjacent jack-up, and pipeline offloading via a CALM buoy. In September 1994 the Wandoo Alliance was established to evaluate a range of concepts for upgrading the production capacity of the Wandoo oilfield. The outcome of these screening and evaluation studies was that the Wandoo B facilities should comprise a Concrete Gravity Structure (CGS) supporting a two-level integrated deck providing processing facilities for 12,000 barrels per day throughput and 40,000 barrels per day oil production. Oil export would be by tanker loaded via dual pipelines connecting to a refurbished CALM buoy. An artists impression of the Wandoo B installation is shown on Figure 2 The CGS is located in 54m of water and comprises a rectangular base caisson114m x 69m in plan, 17m high, with four 11m diameter shafts rising 69m above the caisson roof. The caisson provides 400,000 barrels of oil storage with production wells drilled through one of the shafts. The caisson also has 16 open cells providing storage capacity for up 22,750m3 of solid ballast.
Based upon reservoir considerations, and the proximity of the exiting Wandoo A and CALM facilities which were to remain operational throughout the Wandoo B development phase, possible locations for the Wandoo B facility were indentified. Existing site investigation data indicated that ground conditions within the potential area would consist of a shallow depth of carbonate sand (up to about 4m) overlying a substantial thickness of calcarenite. It was also know from previous investigations that calcarenite outcrops were likely. Using existing geophysical survey data an area for the CGS approximately 500mx 500m was identified where the seabed was relatively flat and it was anticipated that there would be continuous sand covering to the calcarenite.
The first phase of the investigation was a detailed geophysical survey of the selected area. The survey consisted of a grid of east-west primary survey line