Ultrasonic antifouling

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Ultrasonic antifouling is a technology that uses high frequency sound (ultrasound) to prevent or reduce biofouling on underwater structures, surfaces, and medium. Ultrasound is just high frequency sound (which humans can not hear). Ultrasound has the same physical properties as human-audible sound. The method has two primary forms: sub-cavitation intensity and cavitation intensity. Sub-cavitation methods create high frequency vibrations, whilst cavitation methods cause more destructive microscopic pressure changes. Both methods are shown to inhibit or prevent biofouling by algae and other single-celled organisms.

History[edit]

This term comprises at least two topics: ultrasonic (ultrasound) and antifouling (biofouling):

  1. Ultrasound has been known about since 1794 when an Italian physiologist and biologist named Lazzarro Spallanzani discovered that bats navigate in the dark through the reflection of high frequency sounds. [1] Ultrasonic antifouling is believed to have been discovered by the US Navy in the 1950s[citation needed]. During sonar tests on submarines, it is said that the areas surrounding the sonar transducers were cleaner of fouling than the rest of the hull[citation needed].
  2. Antifouling (the removal of biofouling) has been a desire of sailors for what feels like forever. "Coating technology has been applied to ships and vessels since very ancient times, either to protect the wood from shipworms or to prevent fouling. The first materials to be used were natural products like waxes, tar or asphalt. Later on, copper and lead sheathings were introduced by the Phoenicians and Carthaginians."[2] The Cutty Sark is one example of such copper sheathing, available to view in Greenwich, England.

Theory[edit]

Ultrasound[edit]

Range of sound frequencies including audible and inaudible sound

Ultrasound (ultrasonic) is just sound at a high frequency such that human's can not normally hear it. There is nothing different about ultrasound compared to sound than its frequency. As we know, sound has a frequency (low to high) and an intensity (quiet to loud).

Ultrasound is familiar in many household and industries as a technique to: clean fine jewellery, weld rubber, treat absesses, and perhaps most famous of all, sonography permitting the observation of a fetus in the womb. All these techniques rely on a physical interaction of sound with the mediums through which the sound travels. In maritime application, ultrasound is the key ingredient in sonar; sonar relies on ultrasound.

Biofouling starts with biofilms[edit]

Biofouling starts small; it's a small issue which could simply be wiped away with a soft cloth, but quickly worsens as the habitat created by one organism permits, attracts, or otherwise leads to another "The fouling process starts from the moment the surface is immersed in water and takes place in three main stages: formation of a conditioning film, microfouling and macrofouling"[2] "The combination of the conditioning film and the slime of living and dead bacteria cells generates the first stage of microfouling, so-called the primary film."[2]

Ultrasonic Antifouling[edit]

In the references here, there are at least two views about sound used in ultrasonic antifouling:

  1. Cavitation intensity power antifouling: The ultrasound is of such a high intensity that the water boils with cavitation. The biofilm and organisms are annihilated. This type has been shown to remove established marine biofouling. With these high intensities of ultrasound there can be concerns as to the effect on the hull, not just the water and biofilm touching the hull. 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] Such systems are more appropriate where power consumption, and the surfaces-to-be-protected can tolerate highly destructive cavitation.
  2. Sub-cavitation intensity power antifouling: The ultrasound causes vibration in the surfaces (e.g. hull, propeller shafts, rudders, sea chests, water coolers) to which the transducer is attached. At lower intensity levels, the rapid vibrations, creates small movements of the surrounding water, making it extremely difficult for marine life to firmly attach to a surface, and even to exist and function. This type works best to maintain a clean hull; such as inhibiting the settlement of barnacle cyprids.[5] The disturbance of this first stage "biofilm" with even a subtle intensity of ultrasound reduces or removes the entire sequence of upper stages which are the cause of so much trouble for the boat owner. For sailing yachts, where power is a limited resource, especially when not connected to shore power, such low power systems are more appropriate.

The effectiveness of different frequencies and intensities (or power) of ultrasonic waves on various marine life, such as barnacles,[5] mussels and algae.

Commercial ultrasonic systems have been used to control algal blooms in ponds, harbours and reservoirs.[6] 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.

Components of an Ultrasonic Antifouling System[edit]

There are two main components of an ultrasonic antifouling system:

Installed ultrasonic transducer
  1. Transducer, or in human audible terms, a speaker. A speaker, like a transducer, is just a device that takes an electrical control signal and vibrates the medium in which it is located. Thus, a speaker normally vibrates the air in order to propagate sound such as music or voices. The term transducer is used in more engineering based discussion, and in this case since the medium is water (sea water) then the vibration from the transducer propagates through the water. The transducer is in such firm and direct contact with the hull or other surfaces that it causes them to propagate the (ultra)sound. Hull materials such as concrete and wood do not provide good antifouling since they contain many voids which dissipate and absorb the sound. "The transducers are directly fixed to the inside of the boat hull" with a firm bond to the inside surface of the boat hull or attached to other items such as propeller shafts, sea chests, pipework etc., such that the ultrasound radiates from the surface of the item to be protected from fouling.
  2. Control box, or in music terms the sound source and amplifier, provides the signals and power to each transducer. A control box might control multiple transducers with either the same signal or a variation, depending on the control box, "Each system includes a Control Box with status LEDs and 1 to 4 Transducer units". The control box is positioned in any convenient location such that power is easily connected and future access is possible.

Between the two is often standard single-core 'satellite' cable.



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
  • Propeller Shafts and Propellers

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[citation needed]

Limitations[edit]

Effectiveness[edit]

Ultrasonic anti-fouling may be considered a preventive measure to deter marine growth from a surface. There is so much latitude in the installation of an ultrasonic antifouling system from:

  • the frequencies used,
  • the intensity of the ultrasound,
  • the location of each transducer,
  • the water temperature and salinity, as well as
  • the type of organisms in the water,

that some trial and error might be required before complete antifouling is achieved.

Hull Materials[edit]

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.

References[edit]

  1. ^ "The History of Ultrasound". Ultrasound Schools Guide. Retrieved 20 January 2021.
  2. ^ a b c "Non-toxic, non-biocide-release antifouling coatings based on molecular structure design for marine applications". The Royal Society of Chemistry. 2015. Retrieved 20 January 2021.
  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. ^ 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.
  6. ^ "LG Sonic Tholen harbour"