A parametric study of the air transport of big bubble curtains

Abstract

An increased interest in the use of renewable energy, mainly accelerated by the growing awareness of the problem of global warming, has been observed over the last decades. For the installation of the popular monopile and other support structure types, high-energy pile driving is used in around 90% of the cases. The noise emitted by the pile driving forms a serious threat for marine mammals. To mitigate the underwater noise, air-bubble curtain systems are used in the offshore industry. A better understanding is required of the working principle of air-bubble curtain systems in practice to gain more insights into the air transport through the hose-nozzles (perforated pipes).
For this study a physical medium scale pneumatic model of the air-bubble curtain system was designed based on data available from the offshore industry. Tests were performed both in dry and wet conditions which approximate those that are seen in the industrial practice. The role of several geometrical and operational parameters of the pneumatic system, like the inlet pressure, inlet flow rate, internal hose diameter, nozzle diameter, nozzle spacing, static water pressure, on the operational conditions along the hose-nozzle is explored. The influence of the volumetric flow rate along the hose-nozzle on the pressure development along the hose-nozzle is examined. The examined correlation between air flow temperature and air flow rate at the inlet of the compressed air supply and the pressure development over the hose-nozzle forms valuable input for a future numerical model that includes the temperature of the inlet flow rate. The test setup with configurations of reduced lengths in dry conditions is used to experimentally determine the discharge coefficients of a range of different nozzles. Here, the limitations of the test setup are carefully considered and a comparison with numerical results provided by compressible flow theory is followed.