Report of the Round Table Session
Racca, R.1*, Robinson, S.2*, MacGillivray, A.1, Erbe, C.3, Wood, M.4, Pace, F.5, de Jong, C.6, Theobald, P.7, and Holmberg, A.8
1 JASCO Applied Sciences, Victoria BC, Canada
2 NPL, UK
3 Centre for Marine Science & Technology, Curtin University, Perth, Western Australia
4 JASCO Applied Sciences, Droxford, Ha, UK and University of Southampton, UK
5 Baker Consultants Marine Ltd, Matlock, Db, UK
6 TNO, Sonar and acoustics, The Netherlands
7 National Physics Laboratory, UK
8 Swedish Defence Research Agency, Stockholm, Sweden
This report can be referenced as:
Racca, R., Robinson, S., MacGillivray, A., Erbe, C., Wood, M., Pace, F., de Jong, C., Theobald, P., and Holmberg, A. (2015). Report of the Pile Driving Session, Oceanoise2015, Vilanova i la Geltrú, Barcelona, Spain, 10-15 May. (Editors Michel André & Peter Sigray). Retrieved from http://oceanoise2015.com
Marine Pile Driving and other Impulsive Sources
There were a number of thematic topics which came out of the discussion. The first relates to modelling of sound generation mechanisms within aquatic pile driving. It was agreed that there has been significant progress in the last few years with understanding the mechanisms through which sound is generated during the pile driving process. Pile driving is a unique underwater sound source, distributed in both water column and the substrate. The sound generation is influenced by factors such as the hammer energy and use of a cushion, the pile dimensions and material properties, the properties of the seabed, the pile penetration depth and the water depth at the pile position. Empirical models based only on measured data have not provided sufficient detail to enable accurate predictions because the measurements made are not under controlled conditions and too many factors may influence the results. Simulations enabled by physical models, typically involving finite element analysis, have shown the nature of the sound field in the immediate vicinity of the pile, and the predictions have in some cases been validated by experiment. The pulse of energy travelling down the pile radiates sound into the water with wave fronts at relatively shallow angles to the pile axis, the angles being dependent on the wave speed in the water and pile and being typically around 15°. Initially, models struggled to account for all the losses inherent in the process, but most recently models have been developed which take account of the energy used to move the pile into the sediment and the energy lost in frictional slippage as the pile penetrates the seabed. Recent efforts to compare the results of modelling using different models showed relatively good agreement between approaches. It was agreed that if the recent advances in modelling continue, in the near future it will be possible to predict the acoustic field accurately in the pile vicinity with such models. Coupling the output of such a model with an appropriate acoustic propagation model will enable the prediction of the sound field at some distance from the pile.
The use of simpler source models was also discussed, and it was agreed that these were desirable for use in noise mapping applications, where a complex model for every source is not practicable. The need to be able to assess the contribution of large-scale pile driving construction activity to noise registers and noise maps of regional seas was raised, this being a particular concern for European seas where the Marine Strategy Framework Directive places specific requirements on European member states.
It was noted that most of the modelling that has been undertaken is for steel monopiles. There is less understanding of the radiation mechanisms for other types of pile (e.g. concrete, and wood) and for techniques such as sheet piling, and vibro-piling.
The measurement of pile driving noise was also the subject of discussion. The work to write a new standard for measuring underwater sound radiated by marine pile driving within the International Organisation for Standardisation (ISO) was noted. The standard has built upon guidance issued by national bodies in countries such as Germany, The Netherlands, UK and USA. The new standard (ISO 18406) should be published in 2016 and will provide guidance on a common methodology for measurement of pile driving noise both for offshore applications such as windfarms, and inshore applications such as bridge foundations in harbours and estuaries. Inevitably, slightly different methodologies are needed for offshore waters and the shallower inshore waters, but the procedures have many common aspects. The standard will provide guidance on the appropriate acoustic metrics to report, which are those relevant to a biophysical effect and include sound exposure level, sound pressure level and peak sound pressure level, both for individual acoustic pulses and for sequences of multiple pulses. Guidance is also given on the auxiliary data, which should be reported alongside the acoustic parameters, such as pile dimensions, hammer energy, water depth, and measurement locations.
Most attention has focused on piling in relatively shallow water where the depth is less than 100 m, for inshore and near-offshore construction applications. Pile driving is increasingly being carried out using submerged hammers at deeper offshore locations for the construction and fixing of deep water oil and gas structures (for example for FPSO’s, or seabed jacket structures). Radiated noise levels and sound generation mechanisms may differ under these circumstances, and there is a lack of measured data for such scenarios.
There was also extensive discussion of the impact assessment of pile driving noise on marine fauna. This includes both acute effects in the vicinity of individual pile driving events, where the effects can be physiological in nature, and behavioural effects at more remote ranges from the source where displacement from habitats may be a significant effect. The latter population-scale effects are likely to be more important in the long term and may be exacerbated by the cumulative effect of many construction activities taking place simultaneously (or in close sequence) in the same region of ocean. Pile driving in relatively enclosed areas such as river estuaries may have the risk of causing barrier effects to migrating species.
Much of the work to investigate the impact has focused on marine mammals, and it was agreed that although knowledge and understanding has increased in recent years, there are still knowledge gaps. Nevertheless, the state of the art in injury and behaviour thresholds will evolve as more evidence is provided by the results of research.
However, there is an urgent need to attain a better understanding of the impact of pile driving on species sensitive to sound particle velocity and seabed vibration, including species of fish, crustaceans and invertebrates. This is a particular concern for bottom-dwelling creatures where the particle motion and substrate vibration induced by the pile driving may have a significant effect. There are several aspects to this which require further research. Firstly, the technology to measure acoustic field quantities such as sound particle velocity are not commonly available, though it is recognised that there are a number of promising recent developments in sensor technology in this area (and there may also be scenarios where predictions made from sound pressure measurements may suffice). Secondly, there is a lack of measured data for the typical values of particle velocity and seabed vibration that are generated by pile driving activity (nor are there values for typical background levels of these quantities). Finally, more research is required to understand the levels which cause impact in the sensitive species, both acute physiological impact and behavioural or chronic impact. The subject of sound particle velocity and seabed vibration and its effect on sensitive species was recognised as a major knowledge gap.
There was also some discussion of mitigation techniques. These can include the use of exclusion zones with monitoring by passive acoustic monitoring and marine mammal observers, and even the use of acoustic deterrent devices. However, there has been a recent focus on barrier methods which attempt to reduce the source output using a variety of techniques. These include bubble curtains, and various designs of sleeves and screens, often using air or gas-filled cavities to reduce the output amplitude. These have shown considerable promise in prototype testing, with amplitude reductions of up to 20 dB reported, and it is possible that such techniques will offer practicable mitigation solutions in the next few years. However, there are still some doubts over the potential for sound to propagate trough the sediment and by-pass the mitigation screens.
The session also covered explosive sources. The discussion recognised that explosive sources have been studied for far longer than pile driving and there is a substantial historical data set on which to base any evaluation of likely acoustic output levels and impacts. However, it was also noted that standardisation of measurement protocols is desirable.
It is clear that significant progress has been made in understanding the physics of marine pile driving noise, and how to assess impact on marine species. The progress so far has been prompted largely by the requirements of regulators. However, there are still many unanswered questions, and knowledge gaps. The following are among the key questions that emerged from the panel discussion at OceaNoise 2015.
- In the light of the recent progress with physical modelling of sound generation mechanisms, how soon will it possible to accurately make sufficiently accurate predictions of the sound field generated by pile driving including the dependencies on key input parameters?
- Is it desirable to have simpler source models which may be used (for example) in noise mapping applications, and if so, what form should these take?
- What are the physical radiation mechanisms for vibro-piling and sheet piling?
- The work to produce an ISO standard for pile driving measurements is recognised, but what about suitable protocols for deep-water submerged piling (and indeed, vibro-piling and sheet piling)?
- What are the most appropriate thresholds for physiological and behavioural impact on key species of marine mammals? Should specific indicator species be chosen on which to base an assessment?
- How should the large-scale population effects such as displacement and habitat loss be assessed? How should the cumulative effect of many simultaneous construction activities be assessed?
- What are the best sensors to use for sound particle velocity and seabed vibration measurements, and what measurement protocols should be adopted? There is an urgent need to improve knowledge of typical levels produced by pile driving.
- What levels of sound particle velocity and seabed vibration cause impact in the sensitive species of fish, crustacean and invertebrates, including acute physiological impact and behavioural or chronic impact?
- What are the most effective and cost-efficient mitigation techniques for pile driving, and what procedure do we use to measure the effectiveness?
- Is the standardisation of measurement protocols for explosive sources needed and if so how should this be achieved?