Analytical model for the airborne sound pressure waveform radiated when an offshore steel pipe pile is driven with an impact hammer

An analytical model has been developed for the airborne sound pressure waveform radiated when an offshore steel pipe pile is struck by a short hammer. The model is based on the coupled equations of motion for axial and radial vibration of a thin shell, and yields frequency-dependent phase velocity and attenuation of these vibrations. A harmonic solution is obtained for the radiated sound pressure, using Junger & Feit’s “Transform formulation of the pressure field of cylindrical radiators”. The model is applied to a measurement of a 10tonne hammer with cushion driving a pile 91 cm in diameter and 38 m long, in water 17 m deep. The height of the pile face above the water surface is estimated to have decreased from approximately 15 to 3 m during its driving. SPL (Leq) is examined at the measurement range (15 m) and microphone height (1.5 m). The present model predicts SPL would have ranged between 104 and 109 dB re 20 μPa as the pile face descended. The measured SPL data had an average of 103 dB and a maximum of 111 dB re 20 μPa. Possible reasons for the smaller spread of the model predictions are discussed.