A comprehensive new study of oil’s behaviour in various sea conditions has resulted in significant advances in oil spill recovery using boom-style systems. Put into practice in DESMI ‘s Speed-Sweep Oil Spill Response (OSR) solution, the new approach speeds up oil recovery, increases encounter rates, covers a wider area in a shorter time and provides new flexibility in the choice of vessels to do the job.
When oil spills into the sea, time is of the essence. Two tasks are of immediate importance: preventing further spillage and recovering spilled oil from the surface before it is able to spread too widely or sink below the surface.
Just how quickly and effectively a spill can be addressed depends on a number of factors, including the size of the spill, the sea conditions, and the vessel or vessels used. Just as important, however, is the efficiency and usability of the oil recovery equipment itself. And this is precisely where recent advances promise to make a crucial difference.
Wanted: better solutions
Today, oil spills are commonly recovered using a boom system, which surrounds floating oil to prevent it spreading over the water surface, and increases its thickness to enable recovery. These systems use a variety of methodologies to recover oil from the sea. Some, for example, skim the top layer of water into a pool equipped with an outlet for extracting oil from the surface. Another approach currently under development is based on fishing industry technology and uses a system of nets to guide oil into a funnel where it is collected in a special recovery system. To date, however, these approaches haven’t managed to deliver the level and speed of recovery required to handle serious spill situations – especially in bad weather conditions.
Beyond the basic size and length of the boom, the most important parameter is the OSR system’s ability to contain oil. And key to this ability is the way the boom moves in relation to the water upon which it rests.
Most conventional boom systems are limited to operational speeds of 0.7-1.0 knots, above which, the oil slips under the boom itself from a bow wave effect called entrainment. This results in the recovery vessels to operate at a snail’s pace, often clutching the gear box in and out while still having large areas of sea to cover.
The choice of vessel or vessels to operate the boom is a key parameter – from cost, speed and, not least, safety points of view. One common operating model requires up to three ships, one or two to tow the containment boom in sweep configuration, and yet another to pump away the oil gathered. However, operating multiple vessels, particularly if they lack the superior maneuverability of modern ships, can be a significant challenge. In fact, many OSR operations have been poorly executed due to the perhaps inevitable accidents that occur when you have vessels 200m apart and having to make constant turns. A string of minor disasters such as broken booms and tow lines encountering around propellers has made single-ship solutions, where the boom is operated on one side of the vessel using a jib arm or Ro-Vane (water kite), a much more desirable option.
A well-designed boom needs to strike a balance between the flexibility needed to follow wave movements and the rigidity required to keep hold of as much oil as possible. Oil is likely to escape whenever the boom skirt, for example, bridges two wave crests – or when the structure sinks momentarily, allowing over-the-top drainage.
In recent times, a small number of boom systems have been introduced that are able to move more quickly and in higher seas. But their overall performance has left much to be desired. To close the performance gap then, would require the development of a new type of OSR system that:
- Enables much higher operational speeds and encounter rates
- Can be operated by a single vessel rather than two or three
- More resilient to wave and wind conditions
- Longer operational life and high resistance to UV
- Robust and damage resistant
- Can be easily cleaned, maintained and repaired
- Stows in a compact manner
In 2010, dissatisfied with multiple aspects of currently available solutions for OSR at sea, leading OSR solution provider DESMI’s R&D team began a project to revisit the underlying assumptions with regard to sea conditions, materials, angles and more. In short, a complete review of the real-life operating conditions that could guide a new approach to designing OSR containment booms.
In the rugged and harsh environment of the offshore, DESMI is a market leader in boom recovery systems, burn kits and storage and skimming and recovery systems for oil spill response. The company has a long history in the area, and has supplied systems to a long list of maritime authorities, navies and industry OSR groups.
The company’s R&D team conducted in depth research into the characteristics of oil behaviour in a variety of sea conditions and discovered useful new insights into the movements of oil on water. They were able to address the challenges through multiple cycles of development and trials. This included computational fluid dynamics (CFD) and a variety of test models and speed censors.
Effectiveness and durability
The project’s design ambitions went still further, with the design team not just focused on an effective oil recovery mechanism, but also aiming to achieve a system that would last longer and be more resistant to general damage than systems already in use. The answer, it turned out, was to use the Ro-Boom model but in an entirely new configuration. Ro-Boom is known worldwide as a robust, heavy-duty rubber boom system ideal for all offshore operations but equipped with specially developed Kevlar screens coated with polyurethane, could operate at speeds well beyond the conventional. This created a more rugged and faster collection system than other, more conventional OSR booms, going against other manufacturers’ principles of OSR design.
A well-designed boom needs to strike a balance between the flexibility needed to follow wave movements and the rigidity required to hold of as much oil as possible. Oil is likely to escape whenever the boom skirt, for example, bridges two wave crests – or when the structure sinks momentarily, allowing over-the-top drainage. So does the extra ruggedness of the DESMI solution impact its flexibility? And does this, therefore, increase the likelihood of running into similar problems? Not according to DESMI’s R&D Manager for Oil Spill Response, Lars Boldt Rasmussen.
“We’ve been focused on the twin aspects of effectiveness and durability,” he says. “Think of a car tire in contrast to an ordinary inflatable beach ball, where our products are equivalent to the car tire, which is produced in a very tough, rubber-based material. Clearly, the material used for the ball has lower puncture resistance and will wear out sooner. The traditional Ro-Boom is one of the most robust products around, so we can keep operating at a good level if a puncture should occur whereas conventional designs will soon lose functionality if they’re punctured.
“We’ve conducted many open sea and tank tests both with and without oil, and have learned a great deal about the essential principles. Using a material that’s more like a car tire wall instead of a beach ball does mean the boom is stiffer, but we’ve countered this by restricting the distance between the air chambers to no more than 4.5m, which provides a very good wave response. There’s also a good buoyancy-to-ballast relationship – buoyancy is provided by the air chambers and ballast from the skirt and chain. And we’ve made sure that the main boom has the stiffness needed to maintain an optimum positioning on the water. In bigger waves, which typically exhibit a lot of turbulence and high-velocity circulating currents, a rigid design is necessary to avoid oil being lost underneath the structure – you have to be flexible horizontally but not vertically. We still use non metallic fiber rods much like sail pins to keep the screens standing vertically in the water, and the materials we’ve chosen elsewhere provide plenty of flexibility in the horizontal direction.”
In addition, the solution incorporates floats on the system’s screens to ensure good wave response. And the screens are positioned very carefully in order to break down both oil and water velocity.
DESMI is calling the system, ‘The Speed-Sweep’, which offers industry more efficient collection of oil at greater speeds than have previously been possible.
The company’s materials describe the Speed-Sweep’s structure as a “system with individual buoyancy chambers tapered down at either end to the tow sets”, which the company’s technical experts consider to be the ideal guide boom for the sweeping system. The three Kevlar screens, which are specifically sized and positioned, have foam-filled circular floats used to interrupt the speed of the oil, allowing it to be collected at the apex of the system. With this design, surface water and oil can be slowed by as much as 80 percent, enabling the oil to concentrate ready for collection. Put simply, the Speed-Sweep system can be operated at up to three knots without oil escaping.
Once the pollutant has been collected at the apex, a skimmer can be located here and recovery can begin, continuing even while the sweep system is moving forward. An option also exists to have a built in skimmer or pump at the apex which can be operated by the mother vessel. collection system can either be connected to a Ro-Boom containment boom or operate as an independent collection unit. The Speed Sweep system can be towed either between two vessels or operated by a single vessel using a jib arm or Ro-Vane (water kite). According to DESMI, no head wave phenomena or planning have been observed.
DESMI has identified another upside too, from the Speed-Sweep design: the heavy-duty system’s screens can be connected to existing boom systems to increase their efficiencies. Doing so can provide new efficiency and extend an existing system’s lifetime by several years. Thus far, this capability is limited to DESMI‘s own boom systems.
Yet another feature is the fact that DESMI’s system can adopt an in-line skimmer solution. In many of today’s systems, vessel operators have to work with a skimmer operated from a third ship, hanging it over the side and into the recovery pool or apex. In-line-integrated designs are part of the sweep, and is, therefore, less sensitive to weather and other disturbances. Another new feature is the very recent development of an integrated paravane for the Speed-Sweep – The Ro-vane.
“Our smallest system, the 1500, is designed for wave heights no higher than 1.5 metres,” says Lars. “So it’s ideal for nearshore spills with wind speeds up to 8m/sec. At the other end of the scale, for offshore in the North Sea, we offer larger systems with bigger main booms and screens. They’re basically the same design, but feature longer, deeper screens that enable recovery teams to work in wave heights of 2.5-3 m with good results. So we can stretch from one end of oil spill recovery to the other.” Long series of testing this approach have been executed in collaboration with the “Norwegian Clean Seas Association for Operating Companies” (NOFO).
Building a better product may not be all that’s necessary to bring the advantages of this new approach in protecting marine and coastal environments and the life that depends on them. There is an additional challenge: the advanced age and outdated designs of many of the products already purchased and held in storage by local authorities, coastguards or naval forces around the world, and in some cases, umbrella response organizations that represent a number of oil companies. These systems may be as old as thirty years.
“Of course, a thirty year-old system may, of course, not be a bad solution if all that’s required is to surround the oil spill and prevent it reaching a beach, while you wait for someone to come and take it away,” says Lars Boldt Rasmussen.
Progressive equipment owners, however, have efficiency in mind. Resolving an oil spill in half the time with a more efficient system has many advantages. They don’t, for example, have to take their own vessels away from normal tasks for so long – or charter others. They can offer better environmental protection by preventing too much of the oil sinking to the bottom of the sea (as any oil spill will, sooner or later). And they can minimize negative media exposure by doing the job more quickly.
“It’s high time to upgrade the Speed-Sweep that will be far more effective and cost-efficient the day they need to be deployed.”