New approach makes faster and efficient work of oil spills

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Courtesy of DESMI A/S

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.

Fast response

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.

Gathering frustrations

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

Intensive study

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.

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