Navigation path

Decrease textIncrease textDividerPrint versionRSSDivider

DIFIS
Double Inverted Funnel for Intervention on Shipwrecks

The scope of the DIFIS project is the study, design (including costing, planning, deployment procedures, etc.) and validation of an EU reference method for the prompt and cost-effective intervention and remediation of tanker wrecks, dealing with eventual leaks and recuperating the fuel trapped in their tanks, even at considerable depths. The proposed method will be of general applicability so long as the trapped pollutant does not dissolve and is of lower density than seawater.

Tags: Water

Background

A great number of wrecks exist in the sea or on the ocean bed all over the world, many of them having a quantity of hydrocarbons trapped in their tanks (cargo and/or fuel). Each one of these wrecks constitutes a more or less serious threat for the environment in the shorter or longer term.

Maritime disasters leading to major environmental pollution happen almost regularly every two to three years: Amoco-Cadiz in 1978, Tanio in 1980, Aegean Sea in 1992, etc. In December 1999, the sinking of the tanker Erika caused a major pollution on the coasts of Brittany and triggered several measures aiming at the prevention of similar maritime catastrophes. Several proposals were made on the prompt mitigation of the results of such accidents, by treating the released hydrocarbons with intervention on the wreck to seal the leaks or pumping the trapped hydrocarbons out. However, few of these proposals went further than the conceptual state; none of them was anywhere close to a practical intervention system.

On 19 November 2002, the tanker Prestige, loaded with 77 000 tonnes of heavy fuel oil, broke apart and sunk 133 miles off Cape Finisterre. At the beginning of December 2002, the Prestige wreck was leaking as much as 125 tonnes of oil every day. Although many of the 20 leaks were stopped by the submersible Nautilus of Ifremer, 14 months after the accident the wreck was still leaking about 350 kg daily. An intervention method through special ‘shuttle bags’ was developed and tested by the Spanish company Repsol YPF to extract the oil from the wreck. At the end of the process, almost two years after the accident, less than 15% of the original fuel load of Prestige had been recuperated; 25% had leaked before its sinking, while 60% of its original load had slowly dispersed in the ocean during the 22 months it took to plan and implement the intervention.

The Prestige case puts in evidence, among other things, the lack of tools, systems and methodologies for the prompt intervention on shipwrecks in order to confine the pollution and thus eliminate the source of the pollution threat.

Objectives

The DIFIS system will consist of a light, quickly deployable, flexible structure that should stay in place until all the tanks of the wreck are emptied and the pollution threat eliminated. The concept, on which the proposed method is based, is shown in the included illustrations.

The envisaged solution relies on gravity forces to channel the flux of spilt fuel towards the surface. However, instead of channelling the flux directly to the surface, where the recovery operation would be greatly affected by adverse weather conditions, the fuel-water mixture will be channelled to a buffer reservoir/separator some 30-50 m below the sea surface.

DIFIS system dome covering a wreck at the sea bottom
DIFIS system dome covering a wreck at the sea bottom
Fivos Andritsos (JRC)

Description of work

The DIFIS system is very innovative and most of its components, as well as the deployment and procedures, must be validated and optimised with experimentation as well as advanced modelling techniques and tools prior to proceeding to their detailed design.

The leaking fuel is collected by a kind of inverted funnel, consisting of a fabric dome solidly anchored around and completely covering the wreck. The collected fuel is channelled, along with seawater, through a long, flexible riser tube (typical diameter: 1.5-2 meters) into a second inverted funnel, or buffer bell, close to the sea surface (30-50 m). The buffer bell acts like a separator and reservoir. It is made from steel, having a capacity of several hundred tonnes (typically 1 000 tonnes or more). Fuel occupies the upper part of the buffer bell while heavier seawater is forced out from the open bottom. The buffer bell also has the function of a terminal buoy, which keeps the whole riser line in tension. The reservoir, into which the spilt fuel is channelled, is provided with standard equipment through which shuttle vessels can recuperate the fuel rapidly, using standard offshore loading equipment and procedures.

The principal issue regarding DIFIS has to do with the behaviour of such a wide and long tube in various configurations and patterns of sea currents. The prime factor for specifying the number and strength of anchors, tube and cables will be the pattern of the expected sea currents, and the resulting static and dynamically induced pull. This is the major issue on the feasibility and costing of the system. Another not so trivial issue is caused by the possible vibration modes induced by the currents (VIV, vortex-induced vibrations), various instability modes, possible flexing or buckling of the riser tube which in the proposed configuration presents almost no torsion resistance.

All these and other issues that will inevitably rise will be dealt with by the extensive modelling and experimentation activities planned within the project duration.

Results

The DIFIS solution is innovative both as a technical solution and as a method to tackle both the containment (short term) and the elimination of the pollution threat (long term) at the same time. There have been concepts and solution proposed before which had one or more characteristics and/or functionalities common with DIFIS. However, none of these solutions has progressed, to our knowledge, further than an abstract concept. The few methods that have actually been applied differ drastically from DIFIS.

DIFIS system buffer bell for collecting the oil from the wreck
DIFIS system buffer bell for collecting the oil from the wreck
Fivos Andritsos (JRC)

Back