In general, reducing the diameter of a pile reduces the required hammer energy to drive the pile which in turn will result in a reduction in emitted noise. Nehls et al. (2007) present a comparison of methods for estimating corrections to pile source noise strengths in order to correct for the pile diameter. Although there is no definitive method of making this correction (the actual noise levels depends not only on the pile diameter but
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also on the properties of the sediment, pile penetration depth, pile driving energy etc.), a quadratic relation between pile diameter and noise emission can be assumed27.
When a pile is driven it is normal engineering practice to start with a reduced hammer energy and to ramp up the energy until full power is reached. Because the noise emitted by the pile is related to the hammer energy, this ramp up procedure can be used over a longer period of time (gradually building up the energy incrementally) so that the first few hammer strikes produce a lower level of noise and give the marine mammal a chance to leave the area upon hearing the first few strikes. Such a process is known as a “soft start”, which is different to a “slow start”, in which the time interval between the first few strikes is increased to allow the mammal to vacate the area before a more rapid hammer blow rate is used (slow starts are neither within the scope of this study, not a commonly applied technique in MMMPs). In the UK, the current piling mitigation protocol (JNCC, 2010b) state that the soft start duration should be a period of not less than 20 minutes. It is understood, based on discussions with pile installation engineers during the course of this project, that the ramp up of energy required for engineering reasons is for a shorter period of time (5-15 minutes), and sometimes uses a higher initial hammer blow energy, than the “soft start” protocol required to reduce risk of injury to marine mammals. Although progress has been made on tailoring soft starts, through the development of bespoke detailed soft start procedures at some OWF sites, there is a lack of guidance on exactly what constitutes a soft start. This lack of clear guidance means that there is some uncertainty about the required initial reduction in hammer blow energy and the likely benefits of soft start procedures.
As shown in below soft starts have been used on all OWF projects to date within the UK with the exception of a small number of piles where technical issues have led to non-conformances with MMMP (e.g. due to set up). Although there is also no engineering or practicality evidence to suggest that a 20 minute soft start will not be viable for R3 and STW sites, the larger piles sizes proposed for use in these developments may in fact require soft starts of several hours to work as effective mitigation. Whether it is feasible to soft start for several hours is as yet unclear, and should be further explored.
The effect of soft starts on noise emission has been considered in a number of studies (e.g. Robinson et al., 2009, Robinson et al., 2013, Lepper, 2007, Lepper et al., 2012, Bailey et al., 2010). Example time histories are shown in Figure K.4. The figures show that typical reductions in noise due to soft start are in the range 5 – 10 dB.
27It should be noted that no empirical data is available for piles larger than approximately 5 m diameter to the knowledge of the authors. Therefore, any extrapolation of existing data could result in large scale inaccuracies in estimating the noise emission from very large piles.
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(a)
(b)
Figure K.4 Measured sound level time histories during soft start (a) SEL by Robinson et al., 2013, (b) Peak to peak by Robinson et al., 2009
As discussed previously the effectiveness of soft start is dependent upon many factors, not least the hammer blow energy. The relationship between hammer blow energy and noise emitted by the pile is shown in K.5.
Figure (a) appears to show a much greater reduction in initial hammer energy compared to final hammer energy than (b) and therefore also shows a much larger reduction in sound. As can be seen in Figure (c), the relationship between hammer blow energy and noise appears to be fairly straightforward such that the halving the hammer blow energy results in a 3 dB reduction in sound and a tenfold reduction in energy results in a 10 dB reduction in sound. In order for soft start procedures to be effective in mitigating potential for injury to marine mammals, it is therefore important that piling protocols are designed with as low a hammer energy as possible for as long as possible, preferably starting with at least a tenfold reduction in hammer energy and not increasing the energy too rapidly but instead steadily and gradually over the entire soft start time.
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(a)
(b)
(c)
Figure K.5 Relationship between hammer blow energy and sound level measured during soft start (a) Peak to peak by Lepper, 2007, (b) Source level by Bailey et al., 2010 and (c) acoustic
pulse energy by Lepper, 2007.
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The efficacy of soft start procedures relies heavily on the assumption that a marine mammal will be able to locate the initial sound and will react in the desired way and move away from the source to avoid exposure.
This is based on common sense reasoning, but there is no evidence that soft starts always result in the desired effect.
Because using soft start as a mitigation method relies on using the initial sound to “disturb” the marine mammal, it is important to consider whether soft starts constitute acceptable disturbance in accordance with policy requirements. Of course, disturbance would have occurred anyway had no soft start been used so there is no additional impact from soft start, other than perhaps the additional time taken to drive each pile. It is considered that 15 minutes additional piling time (compared to a minimum 5 minutes reduced energy piling for a “standard start”) would be of minor consequence in terms of disturbance especially when the potential benefits of reducing the likelihood of injury are taken into account (injury being a more severe impact than disturbance, albeit that disturbance occurs over a much wider area). These same considerations would apply to the use of ADDs (i.e. that the use of ADDs as a mitigation measure for injury relies on the effect of disturbance which would be over a slightly longer period than the piling activities alone).
It should be noted that soft start procedures will not eradicate all possibility of injury occurring. In particular, it is possible that for very large piles and hammer energies injury could occur during the very first hammer blow over a fairly wide area even during soft start. However, soft start will still reduce the injury range compared to no soft start and therefore this “unintended consequence” is not an additional consequence of the soft start, but rather a question of efficacy.