NEW METHODS IN IMPACT PILE DRIVING NOISE ATTENUATION

Transcription

1 NEW METHODS IN IMPACT PILE DRIVING NOISE ATTENUATION Per G. Reinhall a, John T. Dardis II a a University of Washington, Mechanical Engineering, Stevens Way, Box , Seattle, WA 98195, United States John T. Dardis II, University of Washington, Mechanical Engineering, Stevens Way, Box , Seattle, WA 98195, United States, Fax: , tdardis@u.washington.edu Abstract: Under water noise from impact piling reaches peak sound pressure levels on the order of ~103 Pa at range 3000 m, ~104 Pa at range 60 m and ~105 Pa at range 10 m. These peak pressures have deleterious effects on underwater fauna resulting in regulatory agencies placing limitations on offshore impact pile driving. In this paper, we review current noise attenuation methods and discuss their limited success. Our inspection of the mechanics behind impact pile driving finds the rise time in the pile s propagating stress wave responsible for peak acoustic pressures. This knowledge guides us to alternate methods to achieve noise reduction. Of the possible methods, two stand out as economically feasible, structurally sound and high implementable. It is these two methods, the double pile and the zero Poisson s ratio (ZPR) pile, that are presented in detail. Both finite element analysis and sub-scale testing promise greater than a 20 db reduction in peak sound pressure levels and full scale testing is in preparation. Noise reduction at these levels will have a profound positive effect on the industry of marine piling. Keywords: Double pile, Impact, Marine, Piling, Sound pressure level, Under water noise, Zero Poisson s ratio (ZPR) 271

2 1. INTRODUCTION Impact pile driving has been associated with extremely high sound pressure levels [1]. Underwater anthropogenic noise has widespread effects on marine fauna [2]. In the Northwest region of the United States much of the marine fauna being affected is listed under the Endangered Species Act (ESA) or the Marine Mammals Protection Act (MMA). Regulatory agencies in the United States; National Oceanic and Atmospheric Administration (NOAA) and the United States Fish and Wildlife Service (USFWS), have instituted underwater noise regulations to ensure these species are protected. Southall et al. provides detailed explanation and background surrounding the development of these noise limits [3]. In order to meet these underwater noise regulations many organisations are conducting research to understand pile driving noise and its propagation [4, 5, 6, 7]. A brief review of this work is shown. From the review, two new methods of attenuation are introduced. 2. NOISE GENERATED BY IMPACT PILE DRIVING Underwater noise is typically described by three metrics and expressed in decibels (db); peak sound pressure level (Lpeak), root mean square sound pressure level (RMS), and sound exposure level (SEL) [1]. After reviewing each of the metrics it can be seen that a reduction in Lpeak results in a reduction in the other metrics. Therefore this discussion concentrates on the reduction of Lpeak. Sound pressure associated with pile driving is created by a propagating elastic compression wave that travels down the pile from the pile head to the pile toe. This compression wave is set up by the interaction between the pile driving hammer and the pile head. Associated with the compression wave is local pile expansion. The expansion, or diameter increase, is related to the Poisson s ratio of the pile material. This expansion of the pile displaces the local fluid and creates an acoustic wave front which then propagates downward and away from the pile. As the compression wave reaches the pile toe it is reflected and begins propagating upward. The acoustic wave front associated with the upward traveling elastic wave propagates upward and away from the pile. A detailed explanation of this phenomenon can be found in [4]. Through axisymmetric finite element analysis, we also show this phenomenon, see Fig. 1. Peak sound pressure at the pile wall is directly related to the velocity of pile expansion. This velocity can be found from hammer energy, hammer geometry, pile geometry, and pile material properties. 272

3 UA2014-2nd International Conference and Exhibition on Underwater Acoustics Fig.1: Acoustic radiation from an axisymmetric finite element pile model. 3. ANALYSIS OF CURRENT NOISE ATTENUATION METHODS Several authors have summarized the current research in pile driving attenuation [8, 9]. They have outlined the effectiveness of bubble curtains, cofferdams, isolation casings, encapsulated bubbles, pile cushions, soft start methods, and others. At first glance it is easy to see that there is wide variation in the effectiveness among the different methods. However, as a general rule, sound reductions greater than 10 db cannot be reliably predicted [9] Water column attenuation methods As shown in [4], even perfect attenuation systems that only occupy the water column fail to capture the upward traveling acoustic wave fronts. Although these wave fronts are attenuated by the soil surrounding the pile, partially reflected at the soil-water interface, and cylindrically spread until they reach the hydrophone, they still contain a significant amount of energy. The amount of energy is heavily dependent on the soil attenuation properties. At farther distances from the pile, changes in acoustic energy are attributed to bathymetry, bottom reflection loss, and additional spreading Hammer adjustment methods The use of pile cushions to reduce acoustic Lpeak is known in industry. Pile cushions reduce the rise time in pile stress by delaying the onset of force into the pile. This reduced rise time results in reduced fluid particle velocities, and reduced Lpeak. The reduced rise time can also reduce pile driving efficiency, possibly increasing the acoustic SEL. 273

4 The soft start method also reduces pile noise. To practice the soft start method a pile is initially driven with low hammer energy. As the pile is driven further into the soil, the hammer energy is increased as necessary to achieve soil penetration. The soft start method is intended to be a warning mechanism for fauna so that they can vacate the area before maximum hammer energy is reached Summary Water column attenuation methods are dependent on both the effectiveness of the attenuation system and on the soil properties near the pile. Hammer adjustment methods may not reduce noise through the entire pile driving cycle and may increase acoustic SEL. 5. NEW ATTENUATION METHODS Due to the ineffectiveness in existing attenuation methods two new methods are suggested. One is to divorce the pile to water interaction along the entire length of the pile, a double pile. The other is to create a pile that does not displace radially when compressed, a zero Poisson s ratio pile Double Pile A double pile is constructed by inserting one smaller diameter pile inside another larger diameter pile, such that the two piles are concentric to one another. The volume between the two piles is sealed from water intrusion and thus full of air. At one end, the pile toe, the two piles are joined to each with a special cutting shoe and flexible connection. This connection is designed such that full force of the hammer from each pile strike is available for driving the pile toe into the soil. As the inner pile toe moves downward into the soil, force is simultaneously applied to the toe of the outer pile thus pulling it into the soil. The force applied to the outer pile is done so through the flexible connection which reduces the rise time of the stress state in the outer pile and acoustic Lpeak created. A pile of this design eliminates all inconsistences in noise attenuation performance and after pile installation the inner pile may or may not be removed and reused. 274

5 UA2014-2nd International Conference and Exhibition on Underwater Acoustics Fig.3: Schematic of double pile system Results from finite element analysis and subscale testing are provided in Fig. 4 and Fig. 5 respectively. Both methods suggest a 20dB reduction in peak sound pressure levels. Fig.4: Acoustic radiation from an axisymmetric finite element double pile model. 275

6 UA2014-2nd International Conference and Exhibition on Underwater Acoustics Single Pile 0.8 Pressure Amplitude Time Double Pile 0.8 Pressure Amplitude Time Fig.5: Acoustic pressure plot of the double pile design vs. typical single pile Zero Poisson s Ratio Pile One method for constructing a ZPR pile is to use composite fiber winding techniques. This construction technique allows the angle of the composite fibers to be adjusted on a ply by ply basis. The resulting structure has a modulus and Poisson s ratio that can be adjusted in each of the three primary directions; x, y, z. Both closed form and finite element methods can be used to determine the fiber angle necessary in each ply to create a ZPR structure. Fig. 6, shows a finite element simulation of a ZPR pile constructed using these methods. Again the analysis shows a 20dB reduction in Lpeak. Fig.6: Acoustic radiation from an axisymmetric finite element ZPR pile model Summary The double pile and ZPR pile eliminate the pile wall as a noise source. The remaining noise source in the system is related to the pile toe advancing downward into the soil. The movement of the pile toe cannot be eliminated, thus the associated noise must remain. 276

7 However, by using soft start methods the Lpeak generated by the pile toe can be regulated. As the pile penetrates the soil, the acoustic waves generated at the pile toe are attenuated by more and more soil, helping to offset the increasing hammer energy necessary for soil penetration. It should also be noted that, as in [4], the finite element analysis shown models the soil as a fluid with equivalent soil wave speed. Modelled as such, the soil does not resist pile penetration. This results in artificially high pile toe velocities, hence higher acoustic pressure. 6. CONCLUSION Current noise attenuation methods have been reviewed and analysed. The source of their ineffectiveness was highlighted. Two new pile systems were introduced. Both systems eliminate the pile wall as a sound source. The remaining noise generated by the two systems can be regulated by monitoring hammer energy. The key advantage to these pile systems is that the noise created by each system will be consistent from day to day and site to site. 7. ACKNOWLEDGEMENTS The authors would like to express gratitude to the Washington State Department of Transportation and the program manager, Rhonda Brooks. REFERENCES [1] WSDOT, C A/0/BA_PileDiameterNoiseLevels.pdf [2] University of Rhode Island, Discovery of Sound in the Sea website, Pile-Driving References, Additional Links, and Resources, Retrieved from edlinks [3] Southall, B. L., et al, Aquatic Mammals, Thomas, J. A., Document and Publication Services, Western Illinois University, pp.1-121, [4] Reinhall P. G. & Dahl, P. H., Underwater Mach wave radiation from impact pile driving: Theroy and observation, Journal of the Acoustic Society of America, volume (130), pp. 1209, [5] BORA, Predicting Underwater Noise due to Offshore Pile Driving, Retrieved from [6] WSDOT, [7] Caltrans, [8] Koschinski, S., Ludemann, K., Development of Noise Mitigation Measures in Offshoe Wind Farm Construction, Commissioned by the Federal Agency for Nature Conservation, pp , [9] ICF Jones & Stokes and Illingworth & Rodkin, Inc., CalTrans Report: Final Technical Guidance for Assessment & Mitigation of the Hydroacoustic Effects of Pile Driving on Fish, California Dept. of Transportation, pp.1-298,

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