Research On The Mechanism And Control Of The Underwater Acoustic Radiation From Offshore Pile Driving

Recent years experience a great increase in ocean engineering projects,which brings benefits for human but poses new threats to the ocean environment.Increased ocean noise caused by these activities has been getting increasing concern worldwide.Offshore pile driving noise is one of the most severe categories of underwater noise.A hammer-driven pipe pile vibrates and radiates wide-band sound pressure into the seawater,which would have adverse effects on ocean animals,such as fishes,dolphins and whales.High-level pile driving noise would cause restlessness in animals,interfere with their communication,damage their hearing systems,and even kill them.It is of great significance for both ocean development and animal protection to investigate the mechanical mechanism,modeling,and noise-control approaches for offshore pile driving.The research on the offshore pile driving noise just started in several years.There are still a number of problems which require more in-depth studies and analysis.The imperfections of the existing methodologies limit the scope of application.The existing noise-control schemes have not been widely accpted and applied due to their requirement in amount of work,long construction period and great investment.Exploring an economical,effective,and conveient approach is quite necessary.Supported by National Natural Science Foundation of China(Grant No.11272208),this work explores corresponding solutions for the existing problems in this field.The mechanism of the structural wave propagating in the pile system,the interaction between the hammer,pile and soil,and the coupling responses of the pile vibration and sound pressure are investigated by combining theoretically modeling,numerical simulations and experimental tests.The main contents and the corresponding conclusions are summarized as follows:(1).The mechanism of the structural wave propagating in the pile system and the interaction between the hammer,pile and soil are investigated.A one-dimensional non-linear finite difference model for wave propagation analysis is established,with which the significance of rebounds,re-stricking impacts,and the non-linearity of the cushion material on pile-head stress and soil penetration depth(SPD)is revealed.It is found that the inclusion of the soil resistance is significant for the evaluation of the pile-head stress.No further soil penetration depth can be achieved by the re-striking impacts caused by the anvil.The cushion significantly dampens the peak of the pile-head stress and the high-frequency components of the stress.The rebounds of the ram and anvil reduce the energy imparted to the pile and hence reduce the SPD.(2).An semi-analytical methodology based on Hamilton principle is proposed to investigate the vibration and radiated sound of the pile,whereby the shell vibration is solved by a variational equation.The theoreticall research reveals a number of characteristics of pile driving noise,such as the regularities of spatial distribution,propagation,attenuation,and dispersion effect.The comparison bwtween the theoretical results and the experimental data validates the effectiveness of the theoretical model in predicting offshore pile driving noise.On the frequency spectrum curve of the sound pressure,there is an obvious drop-off around the ring frequency of the pile.Both the propagating modes and evanescent modes contributes to the sound pressure,and the latter donimates in the low frequency range.As the frequency increases,more sound wave is reflected and less is refracted on the water-soil interface.(3).An experiment for underwater pile driving noise is designed,in which the pile-head force and underwater sound pressure are collected when a mini pile is driven.The experiment data show several characteristics of pile driving noise,such as frequency spectrum,regularities of spatial distribution and attenuation.It is found that the first resonant frequency of the sound pressure depends on the first-order natual frequency of the axial vibration of the pile.The underwater sound pressure has obvious dependence on the depth of the observed position.The decay curve of the sound pressure with distance indicates that the sound wave can effectively pass through the water-soil interface.The tested sound results validate the presented theoretical model.(4).The effects of the cushion and anvil on pile driving noise are investigated and a noise-control scheme based on the selection of cushion parameters is proposed.Firstly,we investigate the effects of the cushion and anvil on the waveforms,peaks and frequency response of the sound pressure,and reveal the noise-reducing mechanism from the point of energy transfer.Then the effects of varied cushion parameters and anvil mass on the peak sound pressure level(SPL)and sound exposure level(SEL)are investugated.Based on the analysis,the mounting position and optimal parameters for the cushion are recommended.The sharply rising edge of the pile-head stress causes intense high-frequency radial vibration on the pile wall and strong sound pressure peaks.The presence of the cushion significantly dampens the stress and eliminates the shrpaly rising edge.The cushion dapens the radial vibration on the pile mainly by increasing the time for the energy transfer from the hammer to the pile.Without a significant loss SPD,a cushion with a propper stiffness reduces the SPL and SEL markedly.A large restitution is beneficial to reducing sound radiation and increasing SPD.