Hydrotechnik Lübeck has a long construction record for different applications of air in water, and has participated in all offshore sound mitigation tests carried out in German waters. What follows is an abstract of the report of the test done in winter 2010 with 300kg mines.
Controlled underwater explosions were used to clear mines from the Second World War still present in the Baltic Sea. These explosions create shockwaves of considerable amplitude. In order to protect the environment of the Baltic, and to ensure that porpoises in particular suffer as little harm as possible from these shockwaves, investigations are being carried out into the use of air bubble barriers for the damping of shockwaves.
The measurements carried out in February 2010 took place about four kilometers off the coast of Heidkate, near Kiel, Germany. This involved placing a special nozzle pipe at a water depth of 12m, following a 70-meter-radius semicircle. Compressors operating on a floating platform supplied the nozzle pipe with air. The air escaped from the nozzles to create the barrier of bubbles.
Pressure sensors and hydrophones were placed on the inside of the semicircular blast pipe (45m from the site of the explosion, position P1), and onboard the measuring vessel (105m from the site of the explosion, position P3) at different depths (2m to 10m). This allowed measurement of both the full force of the explosions and of the explosions as damped by the air bubble barrier. Divers from the KRD (bomb disposal service) planted in the middle of the semicircle, for each measurement operation, an individual Type-C anchored naval mine with an explosive charge of 300kg of ‘gun cotton 39’. Once the barrier of air bubbles had accumulated, the mine was detonated.
The pressure readings are shown in Figure 1. The zero timepoints for individual readings have been deliberately merged to make the graph easier to read. The curve for Mine 5 indicates, shortly after the peak reading, the expected failure of the sensors due to the explosion. The three measurement readings shown were taken at position P1, at a distance of 45m from the point of the explosion and at a depth of 4m.
Peak pressures of between 7.4MPa and 9.4MPa were recorded for all three explosions, taken at all depths. The margin of deviation thus lies within the source level at 2dB and, in the centre, about 2dB above the bibliographical reference readings. The signals at position P3, at a distance of 105m from the point of the explosion are shown in Figure 2. The shockwave from a detonation remains only in the case of Mine 4. In the other two cases, with the fully formed bubble barrier, the peak has almost disappeared. The zero time-point was likewise deliberately selected here for ease of representation.
The investigation concerned the damping by an air bubble barrier of pressure signals from the explosion of three old anchored naval mines, each loaded with an explosive charge of 300kg of gun cotton. It was possible, with a fully formed bubble barrier, to damp pressure peaks by 16dB to 19 dB, and even a partially formed bubble barrier was able to damp a pressure peak by 6dB. In a spectral sense, and at frequencies of higher than 500Hz, it was possible to reckon with damping of at least 5dB from the fully formed barrier in comparison to its partially formed counterpart, with a drop in the equivalent continuous sound level – in these cases, of 7dB to 8dB.
The shock-damping effect of an air bubble barrier was successfully demonstrated in this experiment, including for shockwaves emanating from the underwater activation of large explosive charges.