When the crude oil tanker Erika foundered 60 km offshore Norm-andy in December 1999, more than 400 km of France's western coastline suffered varying degrees of environmental pollution. The situation would have been worse, but for a complex recovery operation between April and September last year, which involved pumping 11,000 tons of oil from cargo tanks trapped in the wreck, in 120 meters water depth.
TotalFinaElf, which had chartered the tanker, created an Atlantic Coast Task Force for the salvage operation. This included personnel from its operations in the North Sea and offshore Tierra del Fuego and a specially appointed consortium of Coflexip Stena Offshore and Stolt Offshore, charged with neutralizing and then recovering the heavy fuel remaining in two sections of the wreck, which had drifted 10 km apart. Their solutions were described in a recent edition of TotalFinaElf's house magazine "Energies."
Operations
Pumping could not begin before mid-May, due to heavy swells that prevail in this region during the winter months. Another problem was that temperatures of 10° C on the seabed added to the oil's viscosity. The solution was to adapt processes used to extract heavy crude from Venezuela's Orinoco Belt.
To avoid further accidental pollution, biodegradable rapeseed methyl ester was chosen as the fluidizing agent. Due to the structural weakness of the tanker's two halves, the team also had to be careful to maintain a constant internal hull pressure when replacing the fuel-oil with equivalent volumes of seawater.
This would not be easy, given the hull's structural weakness. For the same reason, vibration from machinery in contact with the hull would have to be avoided. This meant developing a purpose-designed pumping system using hydrostatic pressure.
Following modeling work on the pumping process at TFE's CERT research center in Normandy, a pilot was built that could pump oil at one-hundredth of the rate anticipated for the Erika. This was used to demonstrate the validity of the planned procedures.
At the end of May 2000, the anti-pollution vessel British Shield arrived on location, equipped with floating booms and skimmer pumps, accompanied by the standby tug Anglian Duke for the deployment. They were soon joined by the Seaway Kestrel and the CSO Constructor, which would prepare the two halves of the wreck for pumping while also serving as a base for divers and inspection ROVs.
Early in June, the Crystal Ocean - normally a well test vessel, but now modified for pumping duty - also arrived followed by the tanker Melide, which would store the recovered oil. The vessel would subsequently be assisted by the Zeus tug supplied by Maersk.
Subsea work
Initially, a Triton XL ROV was used to survey the wreck sections. Then throughout June, CSO and Stolt divers drilled holes, attached pumping valves, installed mixing and pumping equipment, and connected these units via flexible hoses. Three teams of three men worked in saturation mode at seabed pressure during the entire diving operation.
They undertook six-hour shifts, also spending a further two hours maneuvering their diving bell. Their remaining hours were passed inside a pressure chamber on the mother ship. Altogether, they spent 24 days in saturation and a further four in decompression.
While pumping valves were being attached to each half of the wreck, a hot-tap technique was applied to stop the leaking oil. Once a valve was secured and water-tight, a drill was mounted on the valve, which then bored through the hull section beneath to pump out the oil. When pumping was completed, the valve was sealed and the drilling device was purged to avoid further leaks into the sea. The drilled hole was then hermetically sealed by the closed valve. Altogether 26 valves were installed on the hull's fore section and a further 40 on the aft section. Preparations for pumping were concluded by early July.
Pumping
The Crystal Ocean took on board nearly 6,000 cu meters of fuel oil, where it was heated from its seabed temperature of 10° C up to 60° C to achieve required fluidization. Then it was pumped through an underwater pipeline for storage on the Melide, stationed 1 km distant. The diving support vessels (DSV) used dynamic positioning to remain close to the pumping vessels. By July 31, over 10,800 tons of oil had been pumped out of the wreck, so there was no longer a risk of large-scale pollution. Smaller oil quantities of oil, still remaining in the cargo tanks' trapped compartments, were eased out using the rapeseed agent. The oil was displaced downwards and out of the compartments where it was trapped and then recovered using suction pipes. Finally, the rapeseed agent was also pumped back out.
Altogether, 40 personnel onshore and 300 offshore were mobilized for this operation. According to TFE, every phase complied with criteria agreed with the French government, and was also supervised by government representatives onboard the pumping vessels.
