It is well recognized that combating oil spills in coastal waters is of great communal interest. This has led to continuous efforts to improve oil spill mitigation strategies. The preferred spill mitigation strategy often uses oil booms to contain and concentrate floating oil, prior to oil skimmer recovery. Despite improvements over the decades, limitations of tow speed and operation under real world conditions remain serious.
Conventional Oil Boom Leakage
A well-designed oil boom should be flexible to conform to wave motions, yet sufficiently rigid to retain as much oil as possible. Designs range from small, lightweight booms for manual harbor deployment, to large, robust booms for open sea use that need a crane and sizeable vessels to handle and deploy.
Important oil boom failure modes include overtopping where oil passes over the boom, and leakage where oil passes under the boom, surfacing as downstream oil patches. Boom overtopping mechanisms include overfilling, wave splashover, and boom-diving due to high towing speed. Boom leakage includes overfilling, frontal slick droplet injection (entrainment) from the confined pool, and boom-surfing due to opposing currents and/or winds blowing in the tow direction.
Oil pool thickness at the boom increases as more oil is collected and/or as the boom is towed faster. Overfilling often occurs with waves and current-induced boom diving and splashover. Leakage occurs when oil droplets are injected sufficiently deep to pass under the oil boom and is highly sensitive to towing speed. Once the towing velocity exceeds the boom's critical towing speed, the frontal wave ‘breaks’ on a large scale, increasing oil entrainment dramatically. Instability development and thus droplet injection increase strongly with towing speed.
Leakage also occurs for acute angle flows at boom-segment junctions from vortices formation. These vortices can inject oil into the water that then underpasses the boom. Acute angle boom flows occur on the leading boom segments that steer oil towards the apex.
Bubble Oil Boom Background
In 1971, the U.S. Coast Guard tested a pneumatic boom for oil spill control, otherwise known as a bubble oil boom (BOB) (US Coast Guard. Heavy-duty oil containment systems: pneumatic barrier system. Report 714102/A/094, US Coast Guard Office of Research and Development, Contract DOT-CG-00-490-A to Oil Containment Division, Wilson Industries, 1971). A pneumatic boom generally is formed from a long, submerged air pipe with a series of holes along its length, typically at the upper generatrix. The curtain of bubbles rise in a sheet that drives an upwelling flow, which at the water surface is converted (by continuity) into an outwelling, which is the oil-blocking barrier. The Coast Guard study concluded that BOBs only were useful for low currents such as harbors, where current commercial BOBs are found. A number of other attempts at using BOBs have been made, such as in U.S. Pat. Nos. 3,491,023, 3,744,254, and more recently U.S. Patent Publication No. 2011/0303613. However, to date there are no large-scale commercial BOB systems available outside harbors due a number of drawbacks with previous designs.