Why use ultrasound to test for watertightness?

Proper definitions of watertightness are crucial for proper maintenance of seals.

Watertightness and airtightness could be defined mathematically, using this formula for approximating the flow rate Q through an aperture: 

Q = the flow rate of fluid through the aperture (where the term ‘fluid’ here is taken to mean an ideal liquid or gas)
A = is the area of the leak aperture
g = the acceleration due to gravity acting on the fluid, and
h = the head of fluid.

This equation (which contains simplifications) is used to demonstrate how since no two meeting surfaces in a seal can ever truly be perfectly aligned, a leak aperture will always exist which will allow leaking of some rate under a head of water. Watertightness is hence defined as a maximum water leakage allowance per minute.
High pressure watertight doors are designed for pressures in excess of 100' (30.5m) (this is equivalent to to a pressure of 299 kPa/ or 3 bar). Typical applications include protection for ship power plants, high-pressure chambers, hydro-electric plants and diving/decompression chambers which are often found on oil platforms and support vessels. In these sensitive areas a small 1m2 watertight door is often expected to be able to resist the weight of 30 tonnes of water without buckling or allowing the protected compartment to flood, and as such the requirement for frequent and accurate preventative maintenance on these seals cannot be overstated.

Ultrasonic Testing of seals

Ultrasonic Technology is being rapidly adopted as a safe, easy and accurate method of identifying seal leak sites and has been quickly adopted by the Royal Navy to quickly identify any incorrectly installed MCT's. UT equipment has now been in service for 3 years aboard HMS Albion and HMS Bulwark, and has also recently been adopted this year by the Indian Navy, and Indian Coast Guard.

Different models of equipment vary, but in general UT equipment consists of two main units; a generator and a receiver. The generator produces an ultrasonic modulated tone, usually at a round 40kHz which is positioned on one side of the seal. The receiver is then used by the operator from the far side of the closed seal. If at any point the seal is imperfect, the ultrasonic signal will be able to pass though the seal through the leak, which can then be detected by the receiver. Some models of ultrasonic watertight compartment doors testing are capable of detecting leak apertures as small as 0.06±0.02mm in size. UT testing seals can provide the sensitivity to detect the smallest leaks, and can give two different types of readout scale: linear and decibel. The linear scale provides an intuitive measurement of the leak size, whereas the decibel scale allows for comparison of standards set by international classification societies. Use of UT is far more efficient that the methods described above, taking less time, requiring no clean up and is used in a portable, light-weight model for ease of use. Due to its convenience, UT tests can be conducted more frequently and can contribute to safety management and preventative maintenance procedures on board. Moreover, the accuracy of this method is unrivalled, and leak sites can be identified and specifically located quickly for the operator. Furthermore, ultrasonic indicators use safe and green technology; it that does not violate any environmental codes, which also means there are no IATA transportation restrictions

C.S.P. Hunter, G.H.C. Hunter, C.M.E. Hunter, S. Watson, J. O’Connor, ‘2016 sees hatch cover watertight integrity testing extended to 24/7 continuous monitoring&2020 look ahead to vessel system
Integration and shore-based data management’, RINA Smart Ships Conference, 2015

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