Ultrasonic antifouling

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Ultrasonic antifouling is a technology that helps reduce fouling on underwater structures, through using small-scale acoustic cavitation to destroy, denature and discourage attachment of algae and other single-celled organisms.

History[edit]

The technology is believed to have been created by the US Navy in the 1950s: during sonar tests on submarines, it was discovered that the areas surrounding the sonar transducers were cleaner than the rest of the hull. Since then, research has been conducted into the effectiveness of different frequencies and intensities of ultrasonic waves on various marine life, such as barnacles,[1] mussels and algae.

Commercial ultrasonic systems have been used to control algal blooms in ponds, harbours and reservoirs.[2] In controlling the algae, the first stage in the fouling sequence is halted, acting as a prevention, rather than a cure as with traditional anti-fouling paint.

Theory[edit]

Acoustic cavitation[3] can be predicted theoretically through the calculation of acoustic pressure and where this pressure is low enough, the liquid can reach its vaporisation pressure. This results in localised vaporisation of the liquid, forming small bubbles; these collapse quickly and with tremendous energy and turbulence, generating heat on the order of 5000K and pressures of the order of several atmospheres.[4] At lower intensity levels, this rapid pressure change is enough to create small movements of the surrounding water, making it extremely difficult for marine life to firmly attach to a surface. Research has also shown that low-intensity ultrasound has an effect on behavior of some species without inducing cavitation, such as inhibiting the settlement of barnacle cyprids.[1]


Modern usage[edit]

Modern commercial systems are available in a wide range of powers and installation variants, however all use similar ceramic piezoelectric transducer as the ultrasound source. There are dedicated systems for:

  • Pool cleaning (to reduce chemicals necessary to prevent algae blooms)
  • Ship hull protection (to prevent fouling, increasing speed and reducing fuel costs)
  • Heat exchanger protection (to extend operational cycles between cleaning)
  • Seachest / water intakes (to prevent blockages by marine growth)
  • Fuel tank protection (to stop algae growth and prevent diesel contamination)
  • Protection of offshore structures (such as wind farms, oil & gas installations etc.)
  • HVAC Cooling Towers to reduce or eliminate chemical dosing treatment

However, most of these systems are controlled by fairly simple variable-frequency drive units, which run random frequencies in the ultrasonic spectrum of 20–45 kHz over an operational cycle. Intelligent systems will target specific frequencies, as well as manage power consumption, protect batteries & power supplies and come with various other features and options such as remote monitoring, alarm systems and daylight sensors. Several companies have patents on their intelligent systems.[5][6][7]

Some systems will actually automatically calibrate after installation and target the ideal frequency range for the substrate to maximize effectiveness.[8][9]

Limitations[edit]

Ultrasonic anti-fouling should not be considered a complete replacement for traditional anti-fouling paints; instead, it should be viewed as a preventative measure to deter marine growth from a surface. Most companies will suggest applying a coat of ‘hard’ antifouling paint.

Ultrasonic systems cannot work on wooden-hulled vessels, or vessels made from ferro-cement. Vessels with foam or wooden cored composite hulls will require modification to the hull in the specific locations that the transducers are to be installed. Ultrasonic systems will work with reduced effectiveness on vibration isolated fittings, such as sterndrives. This is because the hull must pass the ultrasound waves from the transducer located inside the hull through to the water, and these materials act to dampen the amplitude of ultrasonic waves.

Applications[edit]

Sites and structures using ultrasonic anti-fouling include:

  • HVAC Cooling Tower full scale trial results[10]
  • Tholen Harbour, the Netherlands[11]
  • Longham lakes, UK[12]
  • MV Nova Cura[13]
  • 3x MOD Mk IV LCVP landing craft trial[14]
  • Kippari magazine test boat[15]
  • Moody 36 ‘Dalriada’[16]
  • Alloy Yachts ‘Mondango 3’[17]

References[edit]

  1. ^ a b Guo, S. F.; Lee, H. P.; Chaw, K. C.; Miklas, J.; Teo, S. L. M.; Dickinson, G. H.; Birch, W. R.; Khoo, B. C. (2011). "Effect of ultrasound on cyprids and juvenile barnacles". Biofouling. 27 (2): 185. doi:10.1080/08927014.2010.551535. PMID 21271409.
  2. ^ "LG Sonic Tholen harbour"
  3. ^ "Acoustic Cavitation Explained - H2oBioSonic"
  4. ^ Environmental Health Perspectives, Vol 64, pp. 233-252, 1985. "Free radical generation by ultrasound in aqueous and nonaqueous solutions. P. Riesz, D. Berdahl, and CL Christman
  5. ^ "Ultrasonic Antifouling - CleanAHull.com.au"
  6. ^ "CMS Marine"
  7. ^ "Ultrasonic Works"
  8. ^ "CleanAHull.com"
  9. ^ "HullSonic.com.au – Ultrasonic Antifouling"
  10. ^ "H2oBioSonic HVAC Trial Results, Joshua David"
  11. ^ "LG Sonic Tholen Harbour"
  12. ^ "LG Sonic reservoir test"
  13. ^ "USAF projects page"
  14. ^ "Ultrasonic Antifouling LTD testimonials page"
  15. ^ "CMS Marine magazine review"
  16. ^ "CMS Magazine review"
  17. ^ "Ultrasonic Antifouling Testimonials - CleanAHull.com"