Big solution for small vessels
A number of demonstrations have shown how DESMI’s antipollution devices can deal effectively with a number of perplexing problems – including a dearth of vessels on hand to help deal with a spill.
Happily, the innovation works more smoothly than the name: the ‘Ro-Kite Single Vessel Speed-Sweep’ has been developed for areas where a fast response is critical but there are limits to both the number and capability of the vessels that can be called on.
As Ole Warncke of DESMI explained, oil recovery booms need to be held open in order to trap the oil but in many cases only a single vessel is on duty, not two. So the company developed a really novel solution: a floating water kite based on a combination of trawl door and ram-air parachute to replace the need for a second ship to hold the other end of the boom.
It works because the Ro-Kite floats on the surface with an angled vane dropping down into the water below. The effect under tow is to move the kite obliquely through the water, at an angle to the direction of the mothership. This, explains Mr Warncke, allows just one vessel to open the mouth of the boom, forming a deep ‘J’ or ‘U’ shape.
While the principle may be straightforward, the engineering details are not – the Ro-Kite’s drag has to be limited so it will keep the shape open without putting too much strain on the power of the deployment vessel.
Secondly, Mr Warncke explained the other big problem when collecting oil spills is that oil very quickly flattens out on the water to quite a thin layer “even if you’re talking about crude oil so thick it is like chewing gum”.
He explained skimmers can usually only collect about 0.5m2 per second as the operation has a limit of 1.5 knots “otherwise even if your boom reached two meters below the surface, oil will still get sucked under the boom through the effect of entrainment”.
However, the DESMI Speed-Sweep has “found a way to cheat physics”, Mr Warncke told MJ. The idea is to string two or three nets across the mouth of the boom at regular intervals and this slows down the relative surface water speed at the apex of the formation, keeping it to roughly 0.7 knots.
He added that for efficiency’s sake DESMI doesn’t rely on hanging a device held over the side of the ship on a crane arm, but has developed the system so that either a brush or weir skimmer is integrated into the apex of the boom, hydraulic pumps link extraction hoses to the reception tanks onboard the deployment vessel.
Overall, the Speed-Sweep allows operations to run at a much faster rate of 3 knots, without losing any of the oil under the boom.
To underscore the solution’s versatility it has been demonstrated using a range of vessels of opportunity, small and large. In fact a sea trial at the Shahid Rajaee Complex in Bandar Abbas demonstrated the system could safely hold a 300m thick oil slick equivalent to 20t of oil in its apex – while still being pulled by a small, 25m multicat harbour workboat.
It’s got a big audience: DESMI says that since demonstrating the system’s potential in a number of international sea trials it’s been “inundated” with interest - from both customers and spectators.
By Stevie Knight
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