New approach makes faster and efficient work of oil spills
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 greater flexibility in the choice of vessels to carry out the work.
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 spreads too widely or sinks 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(s) 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 from spreading across the water’s surface and increases its viscosity 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 an OSR system’s ability to contain oil. And key to this functionailty is the way the boom moves in relation to the water upon which it floats.
Most conventional boom systems are limited to operational speeds of 0.7-1.0 knots, above which the oil slips under the boom from a bow wave effect called entrainment. This requires recovery vessels to operate at a snail’s pace, frequently straining the gear box by moving in and out of gear while having to cover large areas of sea.
The choice of vessel(s) from which to operate the boom is a critical parameter – from the point of view of cost, speed and, not least, safety. One common operating model requires up to three ships: one or two to tow the containment boom in sweep configuration, and another to pump away the oil collected. 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 somewhat inevitable accidents that occur when vessels are only 200 metres apart and have to frequently make turns. A string of minor disasters such as broken booms and tow lines wrapping around propellers has made single-ship solutions, where the boom is operated on one side of the vessel using a jib arm or Ro-Kite (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 retain as much oil as possible. Oil is likely to escape whenever the boom skirt 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 can be operated at faster speeds and in higher seas. But their overall performance has left much to be desired. To close the performance gap then, requires 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
- Is more resilient to wave and wind conditions
- Has a longer operational life and high UV resistance
- Is robust and damage-resistant
- Can be easily cleaned, maintained and repaired
- Stows compactly
In 2010, dissatisfied with multiple aspects of currently available solutions for OSR at sea, an R&D team from leading OSR solution provider DESMI began a project to revisit the underlying assumptions with regard to factors such as sea conditions, materials and angles. In short, they undertook a complete review of the real-life operating conditions that could guide a new approach to designing OSR containment booms.
For the rugged and harsh offshore environment, DESMI is a market leader in boom recovery systems and burn kits, as well as storage, skimming and recovery systems for oil spill response. The company has considerable experience in the area, and has supplied systems to a long list of maritime authorities, navies and industry OSR groups.
DESMI’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 iterative cycles of trial and development. This included employing computational fluid dynamics (CFD) and a variety of test models and speed censors.
Effectiveness and durability
The project’s design ambitions went even further, with the design team not only focusing 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 the systems already in use. The answer, it turned out, was to use the DESMI Ro-Boom model, but with an entirely new configuration. Ro-Boom is renowned worldwide as a robust, heavy-duty rubber boom system ideal for all types of offshore operations. It is equipped with specially developed Kevlar screens coated with polyurethane and can operate at speeds well above the norm. Going against other manufacturers’ principles of OSR design, Ro-Boom was used to create a more rugged and faster collection system than other, more conventional OSR booms.
Recalling that a well-designed boom needs to strike a balance between flexibility and rigidity to avoid oil escaping when the boom skirt bridges two wave crests or when the structure sinks momentarily, it might be asked whether the extra ruggedness of the DESMI solution impacts its flexibility – and whether this increases the likelihood of encountering problems. DESMI, explains why the new solution maintains the right balance:
“We focused on the twin aspects of effectiveness and durability,” DESMI says. “Think of a car tire compared with an ordinary inflatable beach ball, where our products are equivalent to the car tire, which is produced using 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 continue to operate at a good level if a puncture occurs, whereas conventional designs will quickly 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 4.5 metres, 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 position 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. The boom has to be flexible horizontally, but not vertically. We still use non-metallic fiber rods much like sail pins to keep the screens vertical 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 carefully in order to decrease both oil and water velocity.
DESMI calls its system ‘Speed-Sweep’ because it offers the industry a more efficient collection of oil at greater speeds than have previously been possible.
The company describes Speed-Sweep’s structure as a “system with individual buoyancy chambers tapered at either end to the tow sets”. DESMI’s technical experts consider this to be the ideal guide boom for the sweeping system. The three Kevlar screens, which are strategically 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, the surface water and oil speed can be slowed by up to 80 percent, allowing concentration of the oil and better 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 used to begin recovery. Collection can continue even while the sweep system is moving forward. An option also exists for building in a skimmer or pump at the apex which can be operated by the lead vessel. The collection system can either be connected to a Ro-Boom containment boom or operate as an independent collection unit. Towing of the Speed-Sweep system can be shared between two vessels or carried out by a single vessel using a jib arm or Ro-Kite (water kite). According to DESMI, no head wave phenomena or planing have been observed.
DESMI has identified other advantages of Speed-Sweep’s design, too. For example, the heavy-duty system’s screens can be connected to existing boom systems to increase their efficiency. This can also extend an existing system’s lifetime by several years. So far, this particular capability is limited to DESMI 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 Speed-Sweep, and it is therefore less sensitive to poor weather and other disturbances.
Another new attribute is the recent development of an integrated water kite for Speed-Sweep – The Ro-Kite.
“Our smallest system, the 1500, is designed for wave heights of no more than 1.5 meters,” says DESMI. “So it’s ideal for spills that occur close to shore and for wind speeds of up to 8 m/sec. At the other end of the scale, for offshore conditions 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 at wave heights of 2.5-3 metres with good results. So we can offer products that span the full range of oil spill recovery conditions.” Thorough testing of this approach has been executed in collaboration with the Norwegian Clean Seas Association for Operating Companies (NOFO).
Creating a superior product is important, but it may not be enough to reap the full benefits of this new approach – at least with regard to protecting marine and coastal environments and the sea life that depends on them. An additional challenge must be overcome: the advanced age and outdated design of many of the products already purchased and held in storage by local authorities, coastguards and naval forces around the world, or in some cases, umbrella response organizations that represent multiple oil companies. These systems can be as old as thirty years.
“Of course, a thirty year-old system may not be a bad solution if all that’s required is containing the oil spill to prevent it from reaching the shoreline while you wait for someone to come and take it away,” says DESMI.
Progressive equipment owners, however, consider efficiency. Using a more efficient system to resolve an oil spill in half the time has significant advantages. Equipment owners don’t have to divert their vessels away from day-to-day tasks for long – or charter other vessels. They can offer better environmental protection by preventing much of the oil from sinking to the bottom of the sea (as will occur with any oil spill, given enough time). And they can minimise negative publicity by completing the job more quickly.
“We decided it was high time to upgrade Speed-Sweep. Our new design will be far more effective and cost-efficient on the day it needs to be deployed.”