The measurement of the underwater radiated noise from marine piling including characterisation of a "soft start" period

Underwater radiated noise is often an unintended by-product of offshore activities, and the increasing levels of man-made sounds in the ocean (whether deliberately generated or not) have led to concern over marine noise pollution and its effect on marine life. A significant source of impulsive underwater noise is marine piling where a pile is driven into the sea-bed using a hydraulic hammer. Such a technique is typically used to position piles in relatively shallow water for construction of offshore windfarms, bridge supports, and offshore structures associated with the oil and gas industry. To mitigate the effects of the noise generated, the piling sequence is often begun with a gradually increasing energy level, this procedure being termed a "soft start". This paper describes work undertaken to monitor the underwater acoustic radiated noise during offshore marine piling. The measurements made include full characterisation of the soft start period at the beginning of the piling sequence, where the hammer energy was gradually increased from 10% to 100% of the final energy level. Detailed measurements of underwater noise levels were made throughout the entire piling sequence (a little over 4 hours) at two locations corresponding to ranges of 57 m and 1,850 m. The soft start sequence lasted for approximately 68 minutes, during which time the hammer energy was incrementally increased from 80 kJ to 800 kJ.