ABSTRACT:

This paper provides an overview of the tools and techniques developed to recover the fuel from the wreck of the tanker Prestige, which sank 153 nautical miles off the northwestern coast of Spain in November 2002 in 3,850 meters of water whilst carrying some 77,000 Tm of heavy fuel. The ship broke apart and large quantities of spilt fuel washed ashore in Galicia. Repsol YPF, though in no way connected to the vessel or its cargo, was appointed by the Spanish government to recover the fuel remaining in the wreck. Later Repsol YPF selected Sonsub as the main contractor. The assignment faced daunting problems of depth, pressure and an extremely viscous product (3 to 4 million centipoises at depth) that prevented the use of conventional extraction techniques and pumping. Thus, the project required the development of a large array of new tools and techniques, many of which had never been used before, at least in the context of ultradeep waters. A novel batch extraction method, that included carrying out large diameter perforations on the deck of the wreck and using shuttles to transfer the extracted fuel to a FSO, was selected. A data acquisition campaign, including extensive laboratory and simulation work and an extraction test, was carried out during the weather window of summer 2003, whilst the main extraction and complementary bio-remediation works were successfully completed in summer 2004. Fluid flow computations showed that maximum diameter perforations were vital to efficient fuel extraction from the wreck's tanks. This required the design and construction of a 700 mm hot tapping tool and double-gate extraction valves. The fuel extracted was transported to the surface using purpose-built 300 m3 aluminium shuttles equipped with buoyancy as well as sophisticated product and pressure compensation valves. This paper also describes the transferral of the fuel from the shuttles to a Floating Storage and Offloading tanker (FSO) through a short riser despite of a fuel viscosity of about 500,000 centipoises. The innovative core-flow pumping technique (oil enveloped in water) employed and the riser assembly are also described here. The rigorous testing of tools and techniques and the challenges this project encountered are also described in the paper. The Prestige Recovery Project has won Repsol YPF the prestigious Energy Engineering Project of the Year at the Platts Global Energy Awards for 2004

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