Slick trick -The Engineer

12 December 2011 By Andrew Czyzewski The now famous ’X-Prize’ series of global incentivised competitions is a way of bringing out the very best in human endeavour and technological achievement. The inaugural prize was awarded for the first private organisation to…

12 December 2011

By Andrew Czyzewski

The now famous ’X-Prize’ series of global incentivised competitions is a way of bringing out the very best in human endeavour and technological achievement. The inaugural prize was awarded for the first private organisation to launch a reusable manned spacecraft, while follow-ups included a competition to quickly and cheaply decode the human genome, in an effort to bring the medical benefits of modern genetics to the masses.

So it’s perhaps strange to see the most recently awarded prize essentially acknowledging one of our more shameful environmental failings as a society.

The Wendy Schmidt Oil Cleanup X Challenge put up $1m (£600,000m) to initiate a generation of solutions to speed up the clean-up of seawater surface oil that results from spillages from ocean platforms, tankers and other sources.

Of the 350 entrants, 10 finalist teams demonstrated their clean-up systems during rigorous field testing over a 10-week period this summer at the National Oil Spill Response Research & Renewable Energy Test Facility (Ohmsett) in Leonardo, New Jersey.

The industry-standard removal rate for oil skimmers is around 3,400 litres of oil per minute. The X-Prize set the bar high, asking teams to collect more than 9,400 litres of oil per minute at 70 per cent efficiency or greater, meaning that only 30 per cent of the oily mix in the recovery tanks was water.

The team that won – Elastec/American Marine – gathered at a rate of 17,677 litres per minute, with an efficiency of 89.5 per cent, using technology based on channelling oily water into four rows of 16 giant spinning grooved discs that effectively cling to the oil and skim it off.

The runner-up prize of $300,000 went to a Norwegian Team, NOFI, which used a V-shaped flexible boom to capture 10,266 litres per minute at an efficiency of 83 per cent. None of the other teams achieved the competition minimum recovery rate, so the $100,000 third prize was not awarded.

Clearly the winner was a highly effective system based on a robust, active, mechanistic approach. But NOFI’s passive approach, which takes advantage of differential flow speeds, is claimed to excel in harsh conditions that were not tested at Ohmsett and, as such, has found itself to be a popular choice for industry.

’The beginning of the company was in traditional fisheries and some of the technology we use today comes from that part of the business, because equipment that works 365 days in the Arctic will work in other places in the world,’ said Dag Nilsen, NOFI’s research and development manager.

Oil booms are generally used on medium-sized spills from tankers or in harbours that, after drifting with the current for some hours, normally split up into long and narrow slicks. Booms act to concentrate these oil slicks into thicker surface layers so that skimmers and vacuums can be used more effectively.

The devices generally comprise a float that is air filled or made of rigid foam where the ’freeboard’ is the height measured from the water line to the top of the float section. The skirt is the continuous portion below the float, which is designed to contain the oil that tends to pass under the boom.

A ballast weight is sometimes added to the bottom of the skirt to maintain the barrier in a position perpendicular to the surface of the water and, in some cases, to provide the tension member for the boom. Larger booms are reeled up on dedicated supply vessels and inflated where needed in the ocean, while smaller booms for harbours can be deployed from simple palettes.

’The main challenge for a conventional oil boom is actually relative movement to the sea,’ Nilsen said.’When you move an oil boom in the sea, you build up the oil layer on the one side and, hopefully, there is clean water on the other side, but the speed limits are extremely small. When you pass the speed limit, that oil goes under the skirt and that is probably one of the main reasons why oil-spill-recovery operations fail.’

These limits are around 0.4-0.7 knots for fresh oil before the light fractions have evaporated into the atmosphere and only 0.3 knots for oils with higher viscosities.

’Out in the field, steady coastal currents are generally up to 1 knot and then you have tidal currents going out of inlets that are often more than 1 knot,’ Nilsen said.

’On top of that, you have wind-driven currents, which go quite deep into the water column and are roughly two to four per cent of the wind speed – so even moderate wind can create a challenge for conventional oil booms.’

NOFI set about designing a boom system that could operate in current tow speeds of around 3-5 knots with its Current Buster series. Its inflatable devices vary in size for different applications and consist of two sweep booms that are shaped to form a smoothly tapered chamber, which precedes an expanded section with a submerged, flexible floor acting as a collection chamber.

The drop in velocity of the surface water/oil on entering the collection chamber causes the oil to collect at the surface, while water escapes from vents through the floor.

The device is designed to connect to a sweep boom and can be towed or anchored in a flowing current. A lower membrane and inflatable cross-members are used to maintain the shape of the sweep boom at higher current speeds so that flow remains laminar both inside and outside of the chamber, allowing a vessel to operate directly alongside to recover oil using a pump or skimmer. By means of this separation settling technique, the unit contains a thick layer of calm oil, thus providing a skimmer or pump with almost pure oil and giving good recovery rates.

NOFI is continually honing its Current Buster technology, but it has already proved successful in the field with earlier iterations, for example in the ’Windy Bay’ diesel spill of 2001 in Alaska and the 2004 ’Vicuna’ heavy fuel oil spill in Brazil.

Notably, in the 2004 ’Rockness’ oil spill near Bergen in Norway, Current Buster alone contained and controlled more than 50 per cent of the day’s recorded total recovery. It also showed flexibility in moving from one drifting slick to another without losing any oil.

’We want to work further in developing the technology and there are several challenges related to really thick oil that has been on the surface for a day or two, mixing with all kinds of debris, which we call “peanut-butter-like oil”. So [we want to make] pumps that can handle this,’ Nilsen said.

The Engineer