Wandoo B is a concrete Gravity Base Structure (GBS) and is the main production facility for the Wandoo field offshore NW Australia. It was installed in 1997 with a design life of 20 years. The structural assessments discussed in this paper are part of a comprehensive life extension project encompassing wells, subsea systems, marine and safety systems, topsides facilities and structures to demonstrate fitness for service through the end of field life (EOFL).

The challenge was to demonstrate compliance efficiently and effectively for a large structure with a range of materials (steel, reinforced concrete (RC)) and operations supported (oil storage, drilling, production) under increased loading criteria compared to the original design. There is comprehensive industry guidance for assessing existing steel jacket structures, but far less for a concrete GBS such as Wandoo B. Demonstrating compliance required a combination of computer model results, project-specific tools to check reinforced concrete sections, and engineering judgement to define how much damage constitutes failure.

A number of global and local structural models were developed to assess the linear and nonlinear performance of the reinforced concrete and steel structure. A phased approach was employed using basic, conservative approaches in initial phases to demonstrate code compliance, and progressing to more advanced, less conservative approaches for those components under higher stress.

Developing models that more accurately simulate the behavior of the different structural components and materials was a large part of the project scope, particularly for the nonlinear behavior of the reinforced concrete and the interface connections between the steel and reinforced concrete structures. It was inefficient to develop a detailed steel and reinforced concrete solid model of the large GBS shafts and base, so an equivalent shell model was developed and tested to determine the global behavior and onset of damage. This equivalent model aimed to predict behavior accurately for metocean and seismic loads under material tension and compression. Local detailed models were then developed including a constitutive model of reinforced concrete and used to define the extent of the damage and predict where failure would occur.

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