Modelling of offshore impact piling acoustics by use of wave equation analysis

Abstract

The rapid expansion of UK wind farms to meet the 2020 energy targets will see a large increase in the number of piling operations in UK waters. It is widely recognised that the piling process generates high levels of noise that can propagate to large distances. However, the sound generation mechanism has yet to be fully elucidated; although finite-element models have provided insight to the conical wavefront produced in the water column, little attention has been applied to the exact nature of the pile interaction with the sediment. This work considers a pile modelled in situ using a Wave Equation Analysis of Piles (WEAP) program. WEAP programs are commonly used in the design process to determine the driveability of a pile. The model comprises of a finite-difference code that calculates the stress wave as it propagates through the pile. This allows the radial velocity to be calculated as a function of time and position along the pile. The radial velocity then provides a source function that may be weakly-coupled to an acoustic model. The advantage of modelling the pile using this technique is that the pile-sediment interface is more accurately treated than in many finite-element based approaches. Additionally, the theory has been well established and used as a standard approach in the industry for many years. The acoustic model comprises an axially-symmetric ring-source model such that both ingoing and outgoing acoustic waves are considered. The results show wavefronts emanating from the pile at the expected angles due to the relative sound speeds of the pile and the water. As the model includes the energy lost in penetration, the attenuation of the travelling pulse on reflection stems from the sediment properties. Additionally, the received pressure at a point can be modelled much more rapidly than using typical finite-element techniques

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ORCID for V.F. Humphrey: orcid.org/0000-0002-3580-5373

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