Modelling and prediction of acoustic disturbances from off-shore piling

• Main Author: Wood, Michael Alan
• Other Authors: Humphrey, Victor
• Published: University of Southampton 2016
• Subjects: 627
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.720160

This work presents details of a number of approaches used to elucidate the noise generating mechanisms involved in pile-driving through multiple modelling techniques. Principally, the work is divided into four sections: fluid-sediment analysis, recording analysis, elastic-sediment analysis, and wave equation analysis of piles. In the fluid-sediment analysis section, finite-element models were used to investigate the effect of varying the hammer cushion compliance on the radiated noise of a pile in situ. The compliance of the cushion affected the frequency response of the forcing function with softer cushions having reduced energy at higher frequencies than harder cushions. Using these forcing functions as inputs to the finite-element model showed similar changes in the frequency domain of the radiated noise, illustrating the linear nature of the finite-element model. The results from a similar finite-element model were submitted for the COMPILE piling noise modelling benchmark meeting to be compared against others' contribution. In order to take into account damping in the sediment, the benchmark model description employed a loss factor in the embedded section of the pile. The results from all parties generated consistent results, with predicted SELs within 2 dB and Lp;0-p within 3 dB. Also considered in the section is a propagation model based on normal mode analysis. The radiated noise from piling propagates at predictable angles determined by the relative wave speeds in the pile and in the water. As normal mode decomposition reduces the field into individual modes propagating at distinct angles, great modelling efficiencies can be made by limiting analysis to a reduced number of modes near the expected angle of propagation particularly at higher frequencies. The recording of a large section of a piling sequence is analysed to provide a comparison against modelling techniques. The recordings showed that the rate of decay exhibited in the recording required a non-linear energy loss mechanism to be present in the system. Of the metrics recorded it was found that the pile set, the increased penetration per strike, had the greatest correlation to the radiated noise. On examination of the fluid-element models, the pile set was found to increase over time with no indication of settling. This led to investigations focussing on the effects of a non-fluid sediment both on the propagation of noise and the direct coupling with the pile itself. Finite-element models including an elastic sediment allows for the propagation of shear waves through the sediment. As the pile is directly coupled to the sediment, the shear waves are generated much more readily than for a source in the water column. Sediments often exhibit an increase in shear speed based on a power law. This causes refraction of shear waves towards the water-sediment; trapping the energy near the interface allows for efficient propagation of the shear waves. The finite-element models also show that the acoustic pressure near the interface can be comparable to the compressive waves from the pile through the water column. As the propagation of these waves is much slower than the compressive waves the acoustic pressure decays evanescently with increasing distance from the interface. Inspecting the effect of the non-fluid sediment on the pile motion led to an investigation into wave equation analysis of piles models. These are finite-difference models that are used in civil engineering to determine, among other things, the ultimate capacity of the pile. These are time-domain finite-difference models that model the pulse within the pile following impact. Using these results, the radial expansion of the pile can be determined from which, when coupled to an acoustic model, an acoustic output may be generated. The coupled models with non-linear sediment damping components demonstrated behaviour not realised in the finite-element models, and ultimately the influence the sediment can have on the pile motion and the radiated noise.