FPSO [Floating Production Storage and Offloading] structures have been accepted as a sustainable economic solution for deepwater development projects. Short to medium length (typically 15 to 25m) large diameter driven piles are often used to anchor FPSOs. The loading in such piles during a storm can be resolved into two components:

  • Lateral load, which is one-way cyclic;

  • Tensile (upward) load, which is typically only a few percentage of the lateral load.

The greatest uncertainty in the analysis is the load carrying capacity of the pile, since the cyclic storm loading results in progressive degradation of the soil (sand or clay) supporting the pile. Thus understanding the degradation of the supporting oil is critical, for a safe, economic design. This paper thus hastwo aims:

  • (a) to propose criteria and considerations for design of such piles;

  • (b) to set out simple modifications in the p-y formulation that will provide a safe working envelope for the full range of ground conditions likely to be encountered at different sites.

A parallel is also drawn to the approach routinely used by the geotechnical earthquake engineering profession, and reported centrifuge tests have been used to validate the proposed modification.

The energy challenge, deepwater oil exploration and the FPSO

It is predicted that the world will need almost 60% more energy in 2030 than in 2002, and fossil fuels will still meet most of this need (IEA, 2005). We shall still depend on petroleum for 90% of our transport needs including aviation, land and sea transport. Hydrocarbons will also be used for agriculture, for electricity generation and for the feedstock of the chemical industry. A prime reason for this enhanced energy demand is the economic growth of developing countries such as China and India. Meeting this energy need is one of the greatest engineering challenges of the next decades. Though research on alternative sources of sustainable energy such as wind, wave and solar power are continuing, specific challenges in the oil and gas exploration areas need to be addressed, as these will still be a major source of energy.

Much of the world's oil and gas comes from offshore locations such as the North Sea, Middle East, Gulf of Mexico and West Africa. Offshore oil and gas production started with platforms consisting of fixed structures resting on the seabed. Over the years, these have evolved into sophisticated modern steel or concrete structures, which may be located in deep, hostile waters. These fixed structures are typically suitable for water depths up to about 300 m, but the tallest fixed platform is Shell's Bullwinkle structure, located in 412 m of water in the Gulf of Mexico, (Offshore Magazine 2005).

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