When ships sink, as well as the loss of property (and very possibly life), there’s the danger of environmental damage. An oil tanker breaking up is a disaster, but even a cargo ship going down can mean oil leaking from fuel bunkers. Double Inverted Funnel for Intervention on Shipwrecks (DIFIS) is an EU project coordinated by Maritime Research Institute Netherlands (MARIN) that uses a passive system to catch oil as it leaks out of a wreck on the ocean floor.
In 2002, the damaged oil tanker Prestige carrying 77,000 tonnes of heavy fuel oil broke apart and sank in a storm off Cape Finisterre. The authorities launched heroic efforts to seal the leaks using a submersible and extracting the oil inside using “shuttle bags,” but 60 percent of the oil still escaped into the ocean over a period of 22 months. Thousands of miles of coastline in Spain and Portugal were contaminated and the local fishing industry was badly hit.
It was this incident and others like it that were the inspiration behind DIFIS. According to the developers, DIFIS is a more efficient and cost effective way of recovering oil and reducing environmental damage. It avoids having to patch a wreck, pump out oil, or bring it to the surface where rough seas can turn it into an unmanageable brown froth. The system also has the advantage of being applicable to all types of wrecks, with the main criteria that the oil hasn’t had a chance to interact with seawater and remains buoyant.
The DIFIS system is a light, flexible structure that can be quickly deployed and is completely passive, so it can remain in place until as much of the escaping oil as possible is collected. The design starts with a funnel or dome that’s a sort of upside-down fabric umbrella. When a ship goes down, a remotely operated vehicle is sent down to scout the area and report back on bottom terrain, soil, and currents. A surface ship then uses cranes to position a ring of concrete mooring blocks around the wreck. The dome, which is connected by cables to each block, is then lowered over the wreck and flotation balloons open it up.
Next, a flexible tube called a riser is installed in sections. The bottom section is attached to the dome and others are attached above it using steel connecting collars. Along the outside of the collars are a system of cables to maintain tension.
At the top of the riser is a buffer bell that sits about 50 m (164 ft) under the sea surface. This is open at the bottom to allow a collecting tube to be inserted by surface ships. The whole thing works very simply. It sits over the wreck and as the oil escapes, the dome channels it toward the riser, which conveys it to the buffer bell. The bell stores the oil and as it does so, the oil provides partial buoyancy and helps keep the whole structure in shape.
The DIFIS system requires no human operator, and is capable of recovering oil at depths up to 4,000 m (13,000 ft). It needs only periodic visits, weather permitting, for inspections and offloading the collected oil.
The system has been subjected to computer modelling, and tank and basin tests, and, according to the developers, has shown no unexpected behavior.The results of the DIFIS project were published in the Proceedings of the Twentieth (2010) International Offshore and Polar Engineering Conference (PDF)