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 (Figure 1) were used to store the extracted fuel and transport it to shallow depths for recovery.
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 below. 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.
After the 2003 season, in which the batch extraction method was successfully tested, it was decided to proceed with the shuttles scheme approach to the extraction of the fuel trapped in the wreck of the tanker Prestige.
The challenge, with most operations taking place at depths around 4,000 m, required non-conventional engineering solutions to be first devised and then developed, giving rise to a new range of specialized tools and procedures.
The planned procedure required the design of a rigid shuttle that would receive the fuel extracted from the wreck and transport it to shallow waters, where a riser assembly would latch into it to pump the fuel out into a tanker, acting as a Floating Storage and Offload (FSO) facility.
For sea-worthiness and ease of construction, the shuttles have a cylindrical body with both ends tapped by cones. Soon it was identified that the reinforcement of the shuttle should be external to provide a clear and effective internal flow area for the fuel to fill up the shuttle as quickly as possible, as this would increase the efficiency of the cycles.
The shuttles were designed for a 300 m3 net payload.