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This paper presents two textile-based environmental technologies that are intended mainly for coastal environment. Oil-spill booms are used for the containment and control of floating oil pollution. Flexible containers are used to store water in case of seasonal scarcity.
(Published on December 2005 – JEC Magazine #21)
BY FRÉDÉRIC MUTTIN, EIGSI (INDUSTRIAL SYSTEMS ENGINEERING SCHOOL), LA ROCHELLE, FRANCE
Both technologies use coated technical fabrics. Because the structures are subject to extreme stress from external forces such as sea-current, wave and wind action, high strength fabrics are used. Commonly used fabrics for booms are – in order of increasing strength and costs – polyester, nylon and aramid. The coatings are PVC or polyester. Flexible containers are made from polyester and polyamide fibres, and PVC, TPU or Alcryn coatings.
The size of a floating boom is a function of where it is to be used – along a coast, in a harbour or in deep-sea environments. A boom used along a coast, for example, would have a 0.75m-high underwater barrier skirt and a floating tube diameter of 0.55m.
We know that the longitudinal tension borne by a boom can be as much as 32,000 daN, requiring reinforcement in the form of a longitudinal chain and strap. The chain weighs 12kg/m and also serves as a ballast. The main practical limit on the efficiency of a boom is the speed of the sea current. A current in excess of 0.7 knots (0.35m/s) that is perpendicular to the boom can cause an oil leakage. Because of this, the oil spill contingency plan should be designed to install the boom at a specific angle to the sea current as a function of sea-current velocity so as to reduce boom tension and deviate the oil.
A typical oil-spill boom is secured by means of mooring lines that attach a series of floating 2m3 boxes to 5-ton blocks on the seabed. Depending on the oil viscosity and its emulsion in sea water, polyamide or polyethylene nets are sometimes used with booms for oil containment and recovery.
The operating principle for a boom consists in reducing the surface-fluid velocity to stop the floating pollution.
When the sea current becomes stronger, however, vortexes appear on both sides of the boom. These can capture oil droplets, which are swept under the boom by a strong current. The downstream vortex can fling the oil droplets back against the boom. High waves can also cause oil leakage.
A typical floating container can be 27m long, 14m wide and 4m deep, with a water storage volume of 1,000m3. Different types of water can be stored, depending on whether it is to be used for irrigation, industry or human consumption. Storage may be seasonal.
Small islands and coastal areas with high constraints on the use of available land are more likely to be concerned by this type of application.
Due to the complex interaction between sea waves, the fluid stored and the flexible structure, we still do not have an accurate model of the forces acting on a flexible floating container.
It is possible to distinguish between the stresses applied when the container is anchored and when the container is moved by a tug boat, however.
In the second case, a specific risk analysis for manoeuvring the container must be carried out.
Managing the water in a flexible floating container is a three-step process.
First, the container is filled with water at a shoreline site with a water surplus. The container is then towed to a protected area, such as a harbour or a quay, to serve as a reserve water supply. In response to a specific demand for water, the flexible container is towed to the coastal location in question and water is pumped to the end-user. It should be noted that reprocessed water can be used, depending on its purification stage and on the available water treatment system. (This could be based on air flow injection, for example.) Reprocessed waste water is a major alternative source of water.
Figure 1 shows hydrodynamic testing of oil containment by a boom. The scale reduction is 1/15. With strong currents, oil leakage is observed.
Figure 2 shows the main stress computed for a 30m-long boom section. A high level of stress can be observed in the floating part and on the bottom of the skirt, where a chain is located.
Figure 3 shows a flexible floating container for subsurface water storage. The volume (1,000 m3) is enough to provide 100 people with 100 litres per day for 100 days.
Our research project on boom modelling is part of the French research and technology Ritmer network on accidental maritime pollution, and is funded by the French Ministry of Ecology and Sustainable Development.
Our partners are CEDRE (Brest), La Rochelle University, CETMEF (Brest), and LNHE (EDF Chatou).
Our investigations into integrated water resource management using floating balloons in coastal areas are carried out at the European level, and also in the USA and Japan.