The 1987-built Tricolor was lost following a collision with the container ship Kariba onDecember 14, 2002, in thick fog, some 20 miles off the coast of Dunkirk. The Norwegian flagged Tricolor was en-route to Southampton from Antwerp, laden with nearly 3,000 luxury cars. There were also some 2,000 tonnes of bunkers on board. The Tricolor suffered damage so severe that she went down in less than half an hour. Thankfully, there were no injuries and all crew members were evacuated to safety.

The ship came to rest on her port side, lying just under the surface of the water, at a position within one of the world's busiest shipping lanes. Her location and Winter weather conditions proved major challenges. On December 16 the general cargo vessel Nicola grounded on the wreck. The Nicola was refloated by two tugs, but two subsequent collisions occurred with different vessels.

The first phase of the salvage response - the recovery of the Tricolor 's bunkers - was completed over a two-month period. SMIT deployed 'hot-tap' subsea pollutant recovery technology for the task, which commenced on December 21 with the arrival of the 3,200 tonnes lift capacity sheerlegs Asian Hercules II from Rotterdam. Fuel oil was pumped from the Tricolor by means of a ship-to-ship transfer and stored on board the Asian Hercules II in two dedicated reception tanks.

The large crane vessel also provided a safe work platform for the divers inspecting and monitoring the wreck's condition.

Subsequently, recovered oil was transferred to the Diving Support Vessel (DSV) Normand Flower , which discharged the Tricolor 's bunkers at Rotterdam.

Access to the Tricolor 's bunker tanks was gained by installing pumping stations in the vessel's hull by means of the 'hot-tap' procedure. Oil removal was achieved by making two penetrations in each tank (the upper and lower stations).

Oil was pumped to the surface via the upper station while, simultaneously, seawater entered the tank via the lower station, replacing the recovered oil.

Over 1,600 tonnes of fuel oil was recovered, despite prolonged periods of severe weather (winds of up to Force 9 and swell of up to 5m). Around five per cent of the vessel's bunkers remain within the hull, for recovery during the wreck removal.

As a precaution against collision (with the wreck) during the pump-out, five Cardinal buoys were placed around the site (one fitted with a Racon).

Several guard vessels also patrolled the area of the wreck, while regular information was broadcast on VHF.

Work on the main wreck removal commenced in April, following the award of the contract to the consortium.

The salvage team includes an offshore workforce of around 100 salvage personnel and divers, supported by an onshore technical team. The plan to remove the 15,536dwt wreck requires the hull to be cut into nine sections. Two sheerlegs will work in tandem to lift the large sections onto barges, for transportation to a disposal facility at Zeebrugge, Belgium.

Beyond the two sheerlegs - the Rambiz and Asian Hercules II -the salvage assets mobilised includes a fleet of 10 vessels - tugs, anchorhandlers, DSVs, communications vessels, transport barges and jack-up rigs. The hull will be cut using a variant of SMIT's innovative cutting system - used successfully during the recovery of the Russian nuclear submarine Kursk in October 2001.

The main removal operation began with a fresh inspection of the wreck by divers, to check hull condition and the location of the remaining oil in the bunker tanks. The divers then identified the eight locations for cutting.

The nine sections will be cut by the abrasive wire cutting system used to sever the Kursk's bow.

Hans van Rooij, Managing Director of SMIT Salvage, says:

'The Kursk 's bow was severely damaged and had to be cut from the rest of the hull to ensure a safe lift. The Kursk variant of this system consisted of a cutting wire connected up to two hydraulic cylinders positioned on the seabed, to each side of the submarine's hull. The cylinders produced a sawing motion and the wire, equipped with special bushes coated in abrasive material, cut through the hull.

'Over the last 18 months we have refined the cutting system, increasing the tensile strength of the wire and its abrasive properties, to enhance efficiency and reliability. In the case of the Tricolor, the seabed hydraulic cylinders will be replaced with a surface-based winch system.

The net effect is the same - the winches provide the sawing motion. The great advantage of working in relatively shallow water is that the winches are readily accessible, while the cylinders had to be repositioned by ROV on the seabed for each cut.'

The cutting phase is due to commence this month (June).

Each severed section will be lifted, complete with cargo, onto the barges for transportation to Zeebrugge. Once the cutting wire is positioned, each cut will be completed within 24/48 hours, assuming favourable weather.

The cutting system is one of many recent innovations from SMIT's active research and development division. Among the most important developments are the company's subsea pollutant location and recovery technologies (POLREC and POLSCAN). These systems provide a total solution for the recovery of oils and chemicals from wrecks.

The first variant of POLREC (Pollutant Recovery Service) was developed in association with pumping specialists Frank Mohn Flatoy of Norway over a decade ago. This diverless system is designed to recover liquid pollutants from wrecks.

It features a hot-tap unit and a powerful pump (Frank Mohn Flatoy's Remote-Operated Offloading System, or ROLS).

Following systems development, a Superdeep POLREC variant can now be deployed in water depths of up to 4,000m. Pump out of pollutants can be achieved at extreme depth by adding a dilutant (such as rape seed oil) to improve oil viscosity and increase flow.

POLSCAN is an automated wreck survey system. It was developed to provide an accurate, versatile and cost-effective method for identifying those wrecks posing an active threat to the marine environment.

This non-intrusive system can be deployed in shallow waters by divers or beyond the 50m saturation zone with the addition of an ROV. It utilises a neutron backscattering (NBS) device to assess the volume of water/oil/ chemicals/gas within fuel tanks and cargo spaces. The NBS detects changes in hydrogen density in different liquids, identifies the various interfaces and provides accurate data on the position/volume of pollutants within wrecks.

POLSCAN was first tested in mid-2002 during the trial survey of a container vessel at Rotterdam. Hans van Rooij adds: 'POLSCAN enables governmental authorities to develop cost-effective marine environment remediation plans.

The system identifies those wrecks that present an active threat and allows resources to be allocated accordingly.'

MJ Information No: 18367