The noise radiated by marine piling for the construction of offshore wind farms

The most commonly used method for the installation of offshore wind turbine foundations in the shallow coastal waters in the UK is pile driving. The construction technique consists of large steel piles being driven up to around 30 m into the seabed using powerful hydraulic hammers, in water depths up to around 25 to 30 m. It is a source of high-amplitude impulsive sound and has the potential for impact on marine life. Methodologies developed for the measurement of underwater sound radiated from marine piling, for the estimation of source level are described in this paper. Data are presented for piles of 5.2 m in diameter driven by hammers with typical strike energies of up to 1350 kJ. Data were recorded as a function of range from the source using vessel-deployed hydrophones, with the data then used to estimate the energy source level. In addition, fixed acoustic buoys were used to record the entire piling sequence, including soft-start period. The dependencies of the radiated noise on the physical parameters of the piling operation are discussed, along with limitations and knowledge gaps

Effectiveness of exclusion zones and soft-starts as mitigation strategies for minimizing acoustic impact from underwater noise sources

Several recent studies have suggested that cumulative Sound Exposure Level (SEL) is an important metric for assessment of the impact of exposure to anthropogenic sound sources by various marine receptors including marine mammals and fish. This metric allows the cumulative exposure of an animal to a sound field for an extended period to be assessed against a predefined impact criteria. Two widely used mitigation strategies used by both industrial and military users to reduce potential impact on marine receptors are exclusions zones (zones from source where received levels exceed a certain threshold), a source is either not started or stopped if receptors are detected within this zone and ‘soft-starts’ or ‘ramp-ups’ (lower energy levels at the commencement of a noise source allowing a receptor to move out of the area). This paper discusses the relative effectiveness of these methodologies in terms of a cumulative exposure impact criteria. Cumulative exposure examples are given including typical marine piling operations for wind-farm construction and sub-surface piling operations for various receptor models, including, static, fleeing and transiting animals

Theoretical comparison of cumulative sound exposure estimates from jacket and tripod foundation construction

Foundation pile driving during offshore construction has led to increasing concerns regarding radiated noise and its effects on the marine fauna (receptors). In the case of many static offshore developments two commonly used foundation techniques are tripod-constructions involving installation of a series of smaller diameter piles surrounding a central structure and mono-piles using a single larger diameter pile. Pile installation itself may involve sequences of percussive piling at different hammer energies, vibro-piling (more rapid, lower level vibrations) and drilling. In some cases all three techniques are used on a single pile installation. The spectral characteristics, as well as duration and level of the total radiated energy from these techniques can vary significantly and may result in different Sound Exposure Levels (SEL) experienced by marine fauna. This paper theoretically explores the potential difference in total SEL for various receptor scenarios for each of these techniques using available source characteristics data. The total sound SEL’s for each scenario are compared and model sensitivities identified

Environmental dependence of underwater sound propagation resulting from percussive pile driving [abstract]

Environmental dependence of underwater sound propagation resulting from percussive pile driving [abstract

Theoretical determination of the long term contributions to ambient noise levels from offshore wind farm construction

Marine pile driving during offshore wind farm construction may increase the anthropogenic component of ocean ambient noise and has led to increasing concerns regarding its effects on the marine fauna (receptors)1. In the case of many static offshore developments two commonly used foundation techniques are tripod and jacket constructions involving installation of a series of smaller diameter piles surrounding a central structure and mono-piles using a single larger diameter pile. Pile installation itself may involve sequences of percussive piling at different hammer energies, vibro-piling (more rapid, lower level vibrations) and drilling. In some cases all three techniques are used on a single pile installation, with the construction phase lasting several hours for each pile, and with perhaps 50 - 100 turbine supports in a typical windfarm development

Measurement and characterisation of radiated underwater sound from a 3.6 MW monopile wind turbine

This paper describes underwater sound pressure measurements obtained in close proximity (50 m) to two individual wind turbines, over a 21-day period, capturing the full range of turbine operating conditions. The sound radiated into the water was characterised by a number of tonal components, which are thought to primarily originate from the gearbox for the bandwidth measured. The main signal associated with the turbine operation had a mean-square sound pressure spectral density level which peaked at 126 dB re 1 lPa2 Hz 1 at 162 Hz. Other tonal components were also present, notably at frequencies between about 20 and 330 Hz, albeit at lower amplitudes. The measured sound characteristics, both in terms of frequency and amplitude, were shown to vary with wind speed. The sound pressure level increased with wind speed up to an average value of 128 dB re 1 lPa at a wind speed of about 10 ms 1, and then showed a general decrease. Overall, differences in the mean-square sound pressure spectral density level of over 20 dB were observed across the operational envelope of the turbine

Technique for the calibration of hydrophones in the frequency range 10 to 600 kHz using a heterodyne interferometer and an acoustically compliant membrane

A technique for the calibration of hydrophones using an optical method is presented. In the method, a measurement is made of the acoustic particle velocity in the field of a transducer by use of a thin plastic pellicle that is used to reflect the optical beam of a laser vibrometer, the pellicle being acoustically transparent at the frequency of interest. The hydrophone under test is then substituted for the pellicle, and the hydrophone response to the known acoustic field is measured. A commercially available laser vibrometer is used to undertake the calibrations, and results are presented over a frequency range from 10 to 600 kHz. A comparison is made with the method of three-transducer spherical-wave reciprocity, with agreement of better than 0.5 dB over the majority of the frequency range. The pellicle used is in the form of a narrow strip of thin Mylar©, and a discussion is given of the effect of the properties of the pellicle on the measurement results. The initial results presented here show that the method has the potential to form the basis of a primary standard method, with the calibration traceable to standards of length measurement through the wavelength of the laser light

The contribution to anthropogenic noise from marine aggregate extraction operation in UK waters

As of 2009, there were 75 licensed areas within UK waters for marine aggregate extraction. Each year, around 20 million tonnes of sand and gravel are extracted from these areas for use in the construction and building industry1. Extraction of marine aggregate has the potential to generate noise, and if at sufficient levels, this could have a negative impact on marine species in or around the dredging area. However, measurement of the noise generated during marine aggregate extraction has been limited, particularly in UK waters. The most extensive measurements were undertaken in the Beaufort Sea during oil exploration in the 1980s2,3. Other measurements around Sakhalin Island have been reported in the literature, which were compared by Ainslie et al4 to other vessels including the Overseas Harriette5. This paper presents the results of underwater noise measurements for six different dredgers measured in three locations around the UK, with aggregate type varying from sand to coarse gravel. From the measurements of radiated noise for dredgers under normal operation an estimate is made of the long term contributions to ambient noise levels from typical dredgers under normal operation, the contribution to the overall ambient noise budget, and the cumulative Sound Exposure Level for receptors in the vicinity

Measurement of underwater noise arising from marine aggregate dredging operations

This is the final report for project MALSF MEPF 09/P108, funded by the Aggregate Levy Sustainability Fund, the aim of which is to provide data for the typical underwater radiated noise levels from marine aggregate dredgers in the UK fleet during normal operations. The work is aligned with the stated aims of the ALSFMEPF to reduce the environmental footprint of marine extraction of aggregates, and follows directly from the key knowledge gap identified in the initial scoping study conducted in MEPF Project 08/P21 [Thomsen et al, 2009]. The key finding of the study is the noise output of dredging vessels is similar to a ‘noisy merchant vessel’ and is substantially quieter in terms of acoustic energy output than some other anthropogenic noise sources such as seismic airguns and marine pile driving. This project has been an extensive study of the noise generated by the UK’s fleet of trailing suction hopper dredgers during marine aggregate extraction. The objectives of the work were (i) to develop a suitable methodology for measuring underwater noise radiated by dredgers, (ii) undertake measurements on UK dredgers at up to four sites and report, whilst disseminating the results to the wider stakeholder community. In the report, data is presented for 6 vessels, measured across 3 different areas around the UK’s coast

The measurement of underwater noise radiated by dredging vessels during aggregate extraction operations

The total marine aggregate extracted from the seabed in UK waters can exceed 20 million tonnes each year, and there is a need to understand the noise generated during the extraction process in order to evaluate any potential impact on the marine environment. For aggregate extraction, the type of vessel used is a trailing suction hopper dredger, which lowers a drag head and suction pipe to the sea floor to extract the sand or gravel, depositing it in a hopper on the vessel, whilst returning unwanted material and water over the side of the vessel. There are a number of potential noise generation mechanisms during this type of dredging activity. This paper presents the results of underwater noise measurements for six different dredgers measured in three locations around the UK, with aggregate type varying from sand to coarse gravel. One vessel was measured in two different areas with different aggregate types. The methodology used to derive the source level for the dredgers is described, and the results of an investigation undertaken into the origin of the radiated noise is given. Measurements were made at frequencies up to 100 kHz, with limited data obtained up to 200 kHz. Noise levels are shown for the same dredger under different operational modes, illustrating that the noise output level is partially dependent upon the mode of operation and the aggregate type being extracted