A poroelastic model for near-field underwater noise caused by offshore monopile driving

Impact monopile driving in the offshore wind industry generates extremely high sound pressure levels in the water. Previous studies have simplified the seabed as either an acoustic medium or an elastic medium, which may not be appropriate for certain offshore environments. In this paper, the seabed is modelled as a poroelastic medium using Biot's theory, which can consider pile driving noise in soils with partially drained conditions. For saturated clays and silts, the sound pressure level in the poroelastic model can be approximated by the elastic model using the same soil P-wave and S-wave speeds. For saturated sands, the noise level in the poroelastic model is generally lower than that in the elastic seabed model. The new noise attenuation mechanism is due to the relative displacement between the pore fluid and the soil skeleton. The effects of several key parameters in the poroelastic model, such as the soil permeability, soil shear modulus and degree of soil saturation, are analysed in detail. Generally, the noise level is higher in stiffer, fully saturated soils with lower permeability. The noise level in unsaturated soil is always lower due to the compression of small air bubbles in the soil. The contributions of the pseudo-Scholte wave and Biot's slow compressional wave are also discussed. In addition, a parametric study of pile radius and pile embedment depth is highlighted. The proposed model is a useful addition to the existing simplified models.