The tanker Prestige, carrying 77,000 Tm of heavy fuel, sank in 3,850 m of water 153 nautical miles off the north coast of Spain in November 2002. The ship broke apart and for several months afterward large quantities of spilt fuel washed ashore in Galicia and elsewhere. Repsol YPF, although in no way connected to the vessel or its cargo, was appointed by the Spanish government to recover the fuel remaining in the wreck. Repsol YPF selected Sonsub as its main contractor. This paper describes the data acquisition campaign carried out in 2003 to establish a solid and safe base for intervention on the wreck. The campaign included geotechnical data, detailed wreck survey, extensive marine current measurement and metocean studies. Determination of the actual amounts of fuel left in the wreck and its exact distribution was essential for planning the fuel extraction campaign. The solution selected by Repsol YPF for measuring the fuel levels consisted of adapting two downhole logging tools:

  1. A high energy neutron interaction sensor and

  2. A differential pressure sensor-both used in the oil industry.

Adapting the tools to 4,000 m open water operations, handling the tools with ROVs, and measuring fuel levels both through the hull of the wreck and by their introduction into central tanks presented several challenges. The first measurement indicated the actual fuel volume remaining in the tanks was some 14,000 – 15,000 Tm instead of the 37,000 Tm previously believed

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