The Yolla A Platform incorporates a new foundation concept, defined as an un-ballasted raft foundation, and was successfully installed in the Bass Strait of Australia in 2004. Soil conditions at the site comprise a complex sequence of normally consolidated calcareous silt, calcareous sand and calcareous clay, and required extensive soil interpretation to support the design process. The foundation is subject to extreme combinations of environmental loading and was designed using a combination of analytical and numerical design tools. Soil preloading using underbase suction was incorporated in the design to facilitate increases in foundation capacity and to limit post installation long term foundation displacements. Skirt penetration and subsequent suction preloading were issues requiring close attention during detailed design, as well as extensive monitoring and interpretation both during and after installation. This paper presents a general overview of the design and installation processes, and includes a summary of observations made during foundation installation.


The Yolla A Platform was installed in the central Bass Strait (Figure 1a) of Australia in March 2004, as part of the BassGas Project for client/operator Origin Energy and venture partners AWE, CalEnergy and Wandoo (note that Wandoo has since sold its share in the platform to a new venture partner). The main contractor for the construction and installation of offshore platform was Clough Engineering Ltd, and the substructure design was undertaken by Arup Energy.

The Yolla A Platform is a self-installing steel gravity base platform comprising three primary components: the foundation base, the jacket and the deck (Figure 1c). The platform uses the buoyancy of the deck to provide floating stability for the entire platform during installation, with the deck and base initially clamped together during wet tow to the platform site (see Figure 1b, which shows the deck on top of base prior to departing the fabrication yard in Batam, Indonesia). Once on site, a jacking system is used to

  • lower and install the base on to the seabed and

  • raise the deck clear of the water after the base is installed.

Once the deck is in the final position, it is welded to the jacket.

The Yolla A Platform base comprises a 50m x 50m skirted foundation, with skirts penetrating 5.4m into a seabed comprising primarily soft to very soft calcareous silt. The skirts subdivide the base into 12 individual compartments, and installation is achieved using ‘suction’ within the skirt compartments.

This paper outlines two separate aspects of the project, namely foundation design and foundation installation.

Foundation Design

Water depth at the Yolla A Platform site is approximately 80m. The foundation design is novel in many aspects, including:

  • The Yolla A foundation is believed to be the first foundation of this type (with deep skirts and no ballast) to be installed in calcareous soil

  • Consolidation was incorporated in the design to ensure adequate foundation stability

  • Underbase suction was used to for base installation, which had not previously been undertaken at this scale in calcareous soils

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