In November 2002, the Prestige oil tanker sank in 3,850 meters of water depth offshore Spain carrying some 77,000 tons of heavy fuel. During the sinking, and for a period of several months, a large amount of fuel was spilt and ended up mainly in the shores of Galicia (Northwest Spain). The wreck continued to leak fuel at slowly decreasing rates.
Repsol YPF, though in no way connected to the vessel or its cargo, was appointed by the Spanish government to recover the fuel remaining inside the wreck. Later Repsol YPF selected Sonsub as the main contractor.
The assignment faced daunting problems of depth, pressure and an extremely viscous product, which prevented the use of conventional extraction techniques and pumping. New tools and techniques were developed, many of which had never been used before, at least in the context of ultra deep waters:
Logging tools were used to accurately determine the remaining fuel volume in the wreck tanks.
Large diameter holes were perforated on the Prestige deck and double gate valves installed to extract the fuel.
Large volume (300 m3) aluminium shuttles were used to transport the fuel to shallow depths.
An innovative core-flow technique was applied to transfer the extracted fuel from the shuttles to a FSO.
Complementary bioremediation techniques were successfully devised and applied.
A project overview and details of other aspects of the project are described in separate papers, see References.
The Repsol YPF Prestige Recovery Project has won the prestigious "Energy Engineering Project of the Year" award at the Platts Global Energy Awards for 2004.
The fuel extraction process can be summarized as follows:
The drainage of the wreck's tanks is done using dual gate, 700 mm diameter, extraction valves, which are attached to the wreck's deck by a special ROVoperated module that also performs the hot tapping operation.
Water inlet casings are installed into each tank connecting the bottom of the tank (water zone) to the sea, to allow water entry when fuel is being extracted.
The shuttle is connected to the valve and winched down to approximately 1 m above it.
The extraction valve is opened by a ROV and the fuel flows into the shuttle.
The flow into the shuttle causes its buoyancy to increase. When the buoyancy reaches a predetermined level, a fuse installed in the extraction valve breaks and the EXV upper gate is pulled closed by the shuttle's buoyancy. Flow can also be stopped at any moment by closing the lower gate (ROV).