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.
Due both to very high viscosity of the fuel and to the wreck depth, the neutralization project required the design and use of novel techniques and tools, including a new batch extraction method involving aluminium shuttle tanks to extract the fuel from the wreck and taking it up to some 60 m depth before offloading to a FSO trough a purpose designed flexible riser.
This paper describes an innovative application of the coreflow technique, which was required to reduce frictional losses within the flexible riser while pumping the highly viscous fuel oil from the extraction shuttles to the FSO.
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 and offloading process can be summarized as follows:
The drainage of the wreck's tanks is done using extraction valves attached to the wreck's deck, in which 700 mm diameter hot tapping has been carried out.
The shuttle is connected to the valve, the extraction valve is opened and the fuel flows into the shuttle.
Once the shuttle is filled, the valve closes automatically, the shuttle is disconnected and raised to a depth around 60 m.
A pump-equipped flexible riser is connected to the top of the shuttle, and the fuel is offloaded to a FSO using the core-flow technique, described in this article.
When empty, the shuttle is lowered again to the wreck and the cycle recommences.
The rheological characteristics of the heavy fuel at the wreck's conditions were determined at the Huelva (Spain) University specialized rheological laboratories. The main results are shown in Figures 1 and 2.
These experiments showed that the fuel remained fluid at the wreck conditions (pressure 383 Bar, temperature 3 °C), and that it exhibited a clear thixiotropic and pseudoplastic behaviour, with apparent viscosity at low shear rates as high as 18 million cP, that dropped to some 3 - 4 million cP at shear rates consistent with the extraction regime expected.
