Submitted to: Submitted by: Peter Oliver ChevronTexaco Ltd. Ian Buchanan TotalFinaElf Exploration UK PLC Dr J Nedwell Subacoustech Ltd Chase Mill Winchester Road Bishop s Waltham Hampshire SO32 1AH Graham Jackson DSTL Tel: +44 (0) 1489 891849 Fax: +44 (0) 8700 513060 Graeme Cobb Department of Trade and Industry Debbie Tucker Shell U.K. Exploration and Production Ltd. email: subacoustech@subacoustech.com website: A review of measurements of underwater man-made noise carried out by Subacoustech Ltd, 1993 2003 by J.R. Nedwell and B. Edwards Subacoustech Report ref: 534R0109 29 September 2004 Approved for release:...
Contents 1 Introduction... 1 2 Measuring noise... 2 2.1 Units for measuring noise... 2 2.2 The decibel scale for underwater applications... 2 2.3 Parameters for estimating noise... 2 2.3.1 Source Level... 3 2.3.2 Transmission Loss... 3 2.3.2.1 Estimates of Transmission Loss... 3 2.4 Effects on mammals... 4 2.4.1 Units for measuring the effect of underwater sound; the db ht (Species)... 4 3 Noise sources studied and reports referenced... 5 4 Sound propagation and seismic surveying... 6 4.1 Introduction... 6 4.2 Seismic surveys in the Poole and Weymouth areas... 6 4.2.1 Introduction... 6 4.2.2 The June 1994 measurements, in Poole Harbour and Poole Bay... 6 4.2.2.1 Introduction... 6 4.2.2.2 Poole Harbour tests... 6 4.2.2.2.1 Sound sources... 6 4.2.2.2.2 Measurement details... 6 4.2.2.2.3 Results and conclusions... 7 4.2.2.2.4 Background noise levels... 9 4.2.2.3 Poole Bay tests... 9 4.2.2.3.1 Sound source and measurement locations... 9 4.2.2.3.2 Results and conclusions... 9 4.2.3 The July 1994 measurements, in Weymouth Bay and Poole Harbour and Poole Bay...10 4.2.3.1 Introduction...10 4.2.3.2 Measurement locations...10 4.2.3.3 Sources...10 4.2.3.4 Measurement details...11 4.2.3.5 Results...12 4.2.3.6 Background noise in Weymouth Bay...17 4.2.4 The October/November 1994 measurements, in Poole Harbour and Poole Bay.17 Document ref: 534R0109 i
4.2.4.1 Introduction...17 4.2.4.2 Measurement locations...17 4.2.4.3 Source...17 4.2.4.4 Measurement details...17 4.2.4.5 Results...20 4.3 Measurements in the Wadden Sea...22 4.4 Measurements in the North Sea...23 4.4.1 Introduction...23 4.4.2 Details of measurements...23 4.4.3 Results...26 4.4.3.1 Unweighted results...26 4.4.3.2 Weighted results...30 4.4.4 'Soft start' procedures...39 4.4.5 Conclusions...43 5 Drilling noise... 44 5.1 Introduction...44 5.2 Measurements around Jack Bates...44 5.2.1 Introduction...44 5.2.2 Measurement details...44 5.2.3 Results...44 5.2.3.1 Introduction...44 5.2.3.2 Overview of Results...45 5.2.3.2.1 Background noise measurements...45 5.2.3.2.2 Drilling noise measurements...46 5.2.3.2.3 Dynamic Positioning Thruster noise measurements...46 5.2.3.2.4 A comparison of noise levels for the Jack Bates background, with drilling taking place and with DP thrusters operating...47 5.2.3.3 Effects on mammals...49 5.2.3.4 Conclusions...50 5.3 Measurements around West Navion...51 5.3.1 Introduction...51 5.3.2 Measurement details...51 5.3.3 Results...52 5.3.3.1 Results in conventional units...52 5.3.3.2 Results in db ht (Species) units...57 5.4 Conclusions....58 Document ref: 534R0109 ii
6 Pipe laying and other construction noise... 59 6.1 Introduction...59 6.2 The location of the measurements...59 6.3 Details of the measurements...59 6.4 Pipe laying noise measurements...60 6.4.1 Details of data captures...60 6.4.1.1 Measurements taken in Yell Sound...60 6.4.1.2 Pipeline laying in deep water...62 6.4.2 The results...63 6.4.2.1 Introduction...63 6.4.2.1.1 General features of the Solitaire data...63 6.4.2.1.2 Conclusions drawn from the measurements taken of the Solitaire in Yell Sound...68 6.4.2.2 Measurements of pipe laying in deep water...69 6.4.2.2.1 Features of the Solitaire measurements...69 6.5 Trenching noise...73 6.6 Rock placing noise...75 6.6.1 Introduction...75 6.6.2 Measurements in Yell Sound...75 7 Piling 81 7.1 Introduction...81 7.2 Measurements taken in the North Sea...81 7.2.1 Pile driving in the Schiehallion field...81 7.2.2 Results locations...81 7.2.3 Results for pile driving in deep water...82 7.2.4 Discussion of results...83 7.3 Measurements taken at Littlehampton...86 7.3.1 Outline of work...86 7.3.2 Impact driving...89 7.3.3 Vibrodriving...91 7.3.4 Effect on selected species...97 7.3.5 Conclusions...97 7.4 Measurements at Town Quay, Southampton...99 7.4.1 Introduction...99 7.4.2 Location of pile driving and measurement positions...100 Document ref: 534R0109 iii
7.4.3 Instrumentation and measurement procedure...102 7.4.4 Analysis of piling noise measurements and results...103 7.4.4.1 Vibrodriver...103 7.4.4.2 Impact driver...104 7.4.4.2.1 Unweighted results...104 7.4.4.2.2 Results as db ht levels...105 8 Use of explosives... 108 8.1 Introduction...108 8.2 Methods of defining levels of blast...108 8.3 Measurements in Singapore...109 8.3.1 Introduction...109 8.3.2 First set of measurements...109 8.3.3 Second set of measurements...111 8.4 Wellhead removal in the North Sea...117 8.4.1 Introduction...117 8.4.2 Measurement procedure...118 8.4.3 Results...118 8.5 Measurements in Spain...122 8.5.1 Introduction...122 8.5.2 Measurement procedure...122 8.5.3 Results...123 9 Miscellaneous... 125 9.1 Measurements around a storage vessel in deep water in the North Sea...125 9.1.1 Introduction...125 9.1.2 Results locations...126 9.1.3 Results...126 10 References... 130 Appendices... 131 Appendix 1. Record of changes....131 Document ref: 534R0109 iv
1 Introduction Subacoustech has been a company offering specialist research and consultancy into underwater noise and its effects since 1992. During that period the company has developed advanced techniques for measuring and interpreting underwater noise, and has used them to take measurements of underwater noise and provide interpretations of them for a wide range of organizations spanning the offshore construction industry, oil and gas producers, the underwater blasting industry and defence. Its database of measurements of underwater noise currently runs to some 300 Gbytes of information and many thousands of individual recordings of hundreds of sources of noise. Research and consultancy companies such as Subacoustech operate under conditions of commercial confidentiality, and hence much of the information that is gleaned in the course of such work remains grey literature, with at most a limited circulation of reports and, at least, the information remaining confidential to the client. This is often contrary to the best interests of the industry, where free exchange of information can lead to benefits to all. The purpose of this report is to draw together much of the information Subacoustech has, and to provide a summary and interpretation of this, such that the information can be readily used. Document ref: 534R0109 1
A review of measurements of man-made noise carried out by Subacoustech Ltd 2 Measuring noise 2.1 Units for measuring noise Underwater sound is usually expressed using the logarithmic decibel scale. Underwater sound is conventionally presented in decibels referenced to 1 micropascal, i.e. as db re 1 µpa, and this convention has been adhered to in this report. 2.2 The decibel scale for underwater applications The fundamental unit of sound pressure is the Newton per square metre, or Pascal. However, in expressing underwater acoustic phenomena it is convenient to express the sound pressure through the use of a logarithmic scale termed the Sound Pressure Level. There are two reasons for this. First, there is a very wide range of sound pressures measured underwater, from around 0.0000001 Pa in quiet sea to say 10000000 Pa for an explosive blast. The use of a logarithmic scale compresses the range so that it can be easily described (in this example, from 0 db to 260 db re 1 µpa). Second, many of the mechanisms affecting sound underwater cause loss of sound at a constant rate when it is expressed on the db scale. The Sound Pressure Level, or SPL, is defined as: SPL = 20 log P P ref where P is the sound pressure to be expressed on the scale and P ref is the reference pressure, which for underwater applications is 1 µpa. For instance, a pressure of 1 Pa would be expressed as an SPL of 120 db re 1 µpa. 2.3 Parameters for estimating noise In order to provide an objective and quantitative assessment of degree of any environmental effect it is necessary to estimate the sound level as a function of range. To estimate the sound level as a function of the distance from the source, and hence the range within which there may be an effect of the sound, it is necessary to know the level of sound generated by the source and the rate at which the sound decays as it propagates away from the source. These two parameters are: the Source Level, i.e. level of sound generated by the source, and the Transmission Loss, i.e. the rate at which sound from the source is attenuated as it propagates. These two parameters allow the sound level at all points in the water to be specified, and in the current state of knowledge are best measured at sea, although it is in principle possible to estimate the transmission loss using numerical models. Usually these data have to be extrapolated to situations other than those in which the noise was measured; the usual method of modelling the level is from the expression: SPL = SL TL where SPL is the sound pressure level in db, SL is the source level in db, and TL is the transmission loss in db. In all cases the reference value is 1 Pa. Ways of calculating or estimating SL and TL are considered below. If the level of sound at which a given effect of the sound is known, an estimate may be made of the range within which there will be an effect. Document ref: 534R0109 2
A review of measurements of man-made noise carried out by Subacoustech Ltd 2.3.1 Source Level The Source Level of a source is defined as the "effective" level of sound at a nominal distance of 1m, expressed in db re 1 Pa. However, the assumptions behind this simple definition warrant careful explanation. It is normal to measure the sound pressure in the far field, at sufficient distance from the source that the field has "settled down", and to use this pressure to estimate the apparent (or effective) level at a nominal 1m from the source. The apparent level may bear no relation to the actual level. A measurement of the apparent level can be accomplished by assuming inverse dependence of pressure on the range, R, from the noise source, or by extrapolating the far field pressure. For instance, if measurements were made in the range 100m to 10000m in the example shown in Fig. 2.1, the apparent level would, as illustrated by the extrapolation, be much higher than the actual level. There is in general no reliable way of predicting the noise level from sources of man-made noise, and hence it is normal to directly measure the Source Level where a requirement exists to estimate far-field levels. 2.3.2 Transmission Loss Transmission in the ocean has probably been the subject of more interest than any other topic in underwater communication, since it is the parameter that is the least predictable and the least capable of being influenced. The sound from a source can travel through the water both directly and by means of multiple bounces between the surface and seabed. Sound may also travel sideways through the rocks of the seabed, re-emerging back into the water at a distance. Refraction and absorption further distort the impulse, leading to a complex wave arriving at a distant point which may bear little resemblance to the wave in the vicinity of the source. Finally, sound may be carried with little loss to great distance by being trapped in sound channels. Predicting the level of sound from a source is therefore extremely difficult, and use is generally made of simple models or empirical data based on measurements for its estimation. 2.3.2.1 Estimates of Transmission Loss Transmission Loss, or TL, is a measure of the rate at which sound energy is lost, and is defined as TL = 20 log P 0 P R where P 0 is the pressure at a point at 1m from the source, and P R is the pressure at range R away from it. The usual method of modelling the Transmssion Loss is from the expression TL = N log ( R) αr Fig. 2.1. Illustration of variation of sound level with distance from source. Document ref: 534R0109 3
where R is the range from the source in metres and N and are coefficients relating to geometric spreading of the sound and absorption of the sound respectively. High values of N and relate to rapid attenuation of the sound and limited area of environmental effect, and low values to the converse. For ranges of less than 10 km the linear attenuation term can in general be ignored; a value of N of 20, corresponding to spherical spreading of the sound according to the inverse square law, is often assumed. 2.4 Effects on mammals 2.4.1 Units for measuring the effect of underwater sound; the db ht (Species) The authors have developed a generic db scale which enables better estimates of the effects of sound on marine species to be made. The measure of a species' ability to perceive sound is the audiogram, which presents the lowest level of sound, or threshold, at which a species can hear as a function of frequency. The audiogram thus represents the filter characteristics of the animal's hearing; examples are shown below. Levels of sound lower than the hearing threshold defined in the audiogram of a species cannot be perceived by that species; the degree of perception of the sound relates to the amount it is above the threshold. Hearing threshold (db, re. 1µPa) 160 140 120 100 80 60 40 20 0 10 100 1000 10000 100000 Frequency (Hz) Dab Cod Harbour porpoise Harbour hair seal Killer whale Salmon Fig. 2.2. Audiograms of various species. In the db ht (Species) a frequency dependent filter is used to weight the sound. The suffix ht relates to the fact that the sound is weighted by the hearing threshold of the species. The db ht (Species) level is estimated by passing the sound through a filter that mimics the hearing ability of the species, and measuring the level of sound after the filter; the level expressed in this scale is different for each species and corresponds to the perception of the sound by the species. In effect, the scale may be thought of as a db scale where the species hearing threshold is used as the reference unit. The benefit of this approach is that it enables a single number (the db ht (Species)) to describe the effects of the sound on that species. Usually effective noise levels of sources measured in db ht (Species) are much lower than the unweighted levels, both because the sound will contain frequency components that the species cannot detect, and also because most marine species have high thresholds of perception of (are insensitive to) sound. Document ref: 534R0109 4
3 Noise sources studied and reports referenced The various categories that have been used to classify the work that Subacoustech has carried out are: 1 sound propagation and seismic surveying; 2 drilling noise; 3 noise during pipe laying and other construction activities not treated under other headings; 4 noise due to pile driving; 5 noise due to use of explosives; 6 other sources not falling within the above. Most studies fell within just one of the above categories, but some have parts that fell within more than one category. As an example, the report on noise due to work in the Magnus Field in the North Sea (Nedwell, J.R, et al (2002)) has measurements which fall within categories 2, 3, 4 and 6. In such a case the parts of the report that deal with each category of source have been referred to in the appropriate category. Document ref: 534R0109 5
4 Sound propagation and seismic surveying 4.1 Introduction Three series of detailed measurements have been undertaken by Subacoustech of sound generated by seismic surveys. The first, in coastal water, was in 1994 in the Poole and Weymouth area; the second, in very shallow estuarine mudflats, was in 1996 in the Wadden Sea (Meldorfer Bucht); and the third, in deep offshore water, was in 1998 in the North Sea. 4.2 Seismic surveys in the Poole and Weymouth areas 4.2.1 Introduction In the latter half of 1994 Subacoustech took sound pressure measurements in Poole Harbour and Bay and Weymouth Bay as part of oil industry seismic surveys in these areas. The measurements were taken in three periods, viz. mid-june 1994, mid-july 1994 and mid- October/November 1994. The first set of measurements was a preliminary study prior to the main survey, using explosives and a single airgun, taken in Poole Harbour and Poole Bay. The second set used a single airgun, and measurements were taken in the three locations. The third set used a 20 airgun array, and measurements were taken in Poole Harbour and Poole Bay. 4.2.2 The June 1994 measurements, in Poole Harbour and Poole Bay 4.2.2.1 Introduction These measurements were taken to establish the levels of sound that would be emitted into the water by buried explosive charges and by an airgun deployed in shallow water, and to determine if they would be likely to affect fish or wildlife. Most of the trials were carried out at Redhorn Quay, on the southwest side of Poole Harbour, with the charges being buried on land close to the water s edge, and the airgun being deployed in the water from a pontoon. The measurements were taken in adjacent shallow water areas of Poole Harbour. Additionally, tests of the airgun alone were made along a transect in Poole Bay. The locations at which the tests were conducted in Poole Harbour are shown in Fig. 4.1, and those at which they were conducted in Poole Bay are shown in Fig. 4.2. 4.2.2.2 Poole Harbour tests 4.2.2.2.1 Sound sources For the explosives, charge weights varied between 200g and 1kg. The charges were buried at depths of between 1m and 20m in pre-drilled holes, with the holes backfilled with bentonite or gravel. Details of the charges and their depths are given in Table 4.1. In Fig. 4.1 depths relative to MLWS are marked; all firings took place when the water level was at or above the mid-tide level. For the airgun, a Bolt Model 1900B 40cu. in. unit was used, suspended 1m below the water surface. At this depth, operated at a pressure of 1500psi, the source level was measured to be 191dB re 1 Pa; operated at a pressure of 2000psi, the source level was measured to be 193.5dB re 1 Pa. 4.2.2.2.2 Measurement details Sound measurements were made at two locations, as indicated in Fig. 4.1. At the location nearer to the source a B&K Type 8105 hydrophone was lowered over the side of a 6m dory, En Fête, to within 0.5m of the seabed. The hydrophone was connected to a B&K Type 2635 charge amplifier, the output of which was recorded on a Sony TCD-D7 DAT recorder. At the location further from the source a self-contained rig, which was lowered to the seabed, was used. The rig comprised a hyperbaric chamber fitted with a B&K Type 8103 hydrophone, and containing a Subacoustech Type SAV010 low-noise instrumentation amplifier and a DAT recorder. Document ref: 534R0109 6
Peak SPL values were obtained from the recordings later in the laboratory, from pressure~time histories, using a Rockland System 90 Signal Processing Workstation. 4.2.2.2.3 Results and conclusions Fig. 4.1. Poole Harbour test locations The measured peak SPLs for the explosives tests is given in Table 4.1. At the nearer measurement position they ranged from 146.2dB to 192.1dB re 1 Pa, and at the further position they ranged from 142dB to 146.6dB re 1 Pa. At the further distance some values could not be measured as the ambient noise (approximately 125dB re 1 Pa) was dominant. From the results it was concluded that the transmission loss was about 50logR (R in metres), which could have been due to the waves travelling predominantly in dispersive or absorptive sediments, with the sound wave having been diffracted out as a waterborne head wave. For Document ref: 534R0109 7
Fig. 4.2. Poole Bay test locations the larger charges (0.8 to 1kg) it was considered that the range of possible disturbance to fish would have extended to 750m from the shot hole. This was based on data on the reaction threshold of cod, the most sensitive species; their reaction threshold to impulsive sources is 160dB re 1 Pa. The authors noted that when the first charge was detonated a shoal of fish, probably grey mullet, jumped out of the water within a few hundred metres range of the hole. For divers, for which the threshold for bodily injury is taken to be 225dB, for hearing damage to be 190dB, and for startle to be 155dB (all re 1 Pa), it was considered that a standoff range of 2km would be needed to avoid startle. Sound emitted by the airgun was not detectable at either of the measuring locations. Document ref: 534R0109 8
Table 4.1. Details of explosive charges used, and SPLs measured charge receiver 1 (boat based) receiver 2 (hyperbaric rig) wt (kg) depth depth range SPL (db depth range SPL (db (m) (m) (m) re 1 Pa) (m) (m) re 1 Pa) 0.6? 2 470 173.9 5 1900 145.4 0.6? 2 470 192.1 5 1900 146.6 0.6? 2 470 172.6 5 1900 150.9 1.0? 2 470 171.6 5 1900 142.0 0.6 6 2 470 146.2 5 1900 n/d 0.6 2 2 470 163.1 5 1900 n/d 0.6 6 2 470 168.0 5 1900 n/d 0.4 11 2 470 172.2 5 1900 n/d 0.8 15 2 470 172.2 5 1900 n/d 0.6 3 2 470 171.1 5 1900 n/d 0.2 3 2 470 169.3 5 1900 n/d n/d = nothing detectable 4.2.2.2.4 Background noise levels Some measurements were taken of the noise due to boat and ship activity in Poole Harbour, and the results are given in Table 4.2. The values ranged from 137 to 143dB re 1 Pa. A measurement taken in calm conditions and when there were no boats within several hundred metres of the measuring station gave a level of around 125dB re 1 Pa, while outside the harbour in the more exposed shallow waters of Hook Sands it rose to 131dB re 1 Pa. Table 4.2. Background noise measurements in Poole Harbour Source of noise SPL (db re 1 Pa) 13ft. pleasure cruiser, 50HP outboard motor, passing within 50m 144 Harley's Pleasure Vessel, 50ft, passing within 50m 143 20ft cabin cruiser, 30HP outboard engine, passing within 50m 137 Background noise, East Hook buoy, Kooh Sands, no motor vessels within 500m, sea state 2 to 3 131 Background noise in Redhorn Lake, Poole Harbour, no motor vessels within 500m, calm weather 125 4.2.2.3 Poole Bay tests 4.2.2.3.1 Sound source and measurement locations The sound source was a Bolt Model 1900B 40cu. in. airgun, suspended 1m below the water surface, and operated at a pressure of 1500psi. It stayed at a fixed location, and sound measurements were taken at a range of distances from it along a transect, as indicated in Fig. 4.2. Two or three shots were fired at each of the measurement ranges. The measuring equipment was the same as that used in the Poole Harbour tests, i.e. the hydrophone was hung from the dory with recordings taken of its signal. Additionally, the hyperbaric chamber rig was deployed at the East Hook Buoy. Measurements were made in the morning at around low water, and repeated in the afternoon at around high water. 4.2.2.3.2 Results and conclusions Transmission Loss for the two tide states is plotted in Fig. 4.3. For the low water condition the water depths at both the source and receiver locations were 15m for all ranges except 6.7km, for which the source depth was 15m and the receiver depth 7m. For the high water condition the two depths were 16.5m for all ranges for both the source and receiver locations. Document ref: 534R0109 9
It was found that the decay rate was approximately 21logR (R in metres). The threshold range for fish disturbance in this case was estimated to be 6km for the most sensitive species, and the range for diver startle was estimated to be10km. Fig. 4.3. Transmission loss values for measurements in Poole Bay 4.2.3 The July 1994 measurements, in Weymouth Bay and Poole Harbour and Poole Bay 4.2.3.1 Introduction Source and receiver positions were selected to cover as wide a range of conditions as possible within the geographic range covered. Poole Harbour provided representation of channel and tidal creek/saltmarsh configurations, whilst Poole Bay and Weymouth Bay provided areas of moderately deep water (approximately 20m) shallowing to beaches and banks, with various substrate types. 4.2.3.2 Measurement locations Figs. 4.4 to 4.6 show respectively the locations of the transects used in Weymouth Bay, Poole Bay and Poole Harbour. For each transect the source was kept at a fixed position and measurements taken at a series of ranges from it, usually at distances of 250, 500, 1000, 2000 and 4000m, but occasionally closer in or further away. These measurements were taken from the dory; in Poole Bay additional measurements were taken using the hyperbaric chamber. Positions were determined using the GPS. Water depth at the source point was measured using a plumb line; water depth at measurement points was obtained from Admiralty charts and corrected relative to the source point using the plumb line data. 4.2.3.3 Sources Most measurements used a Bolt Model 1900B 40 cu. in. airgun as the source. It was operated at either 1500 or 2000psi., and was suspended below the water surface at a depth of 1m. The nominal source levels for these operating pressures were 217.5 and 219dB re 1 Pa @ 1m Document ref: 534R0109 10
Fig. 4.4. Measurement locations in Weymouth Bay. respectively. On most occasions five airgun shots were fired for each measurement point, but sometimes more if detection was poor. Additionally, higher source level pyrotechnic charges were used to obtain data in cases where the airgun impulse was not detectable over the longer ranges tested. Charge weights of 25g of perchlorate-based explosives were deployed over the side of the boat at a depth of approximately 5m. The source level for these charges was measured in the dock at Fawley Aquatic Research Laboratories to be 226.9dB re 1 Pa @ 1m. 4.2.3.4 Measurement details Recordings of the sounds were made either with a hydrophone and its associated equipment, deployed from the dory En Fête, or with a self-contained rig housed in a hyperbaric chamber which was lowered to the seabed. For the measurements from the dory, a B&K Type 8105 hydrophone was lowered over the side to within 0.5m of the seabed. The phone was connected to a B&K Type 2635 charge amplifier, the output of which was recorded on a Sony TCD-D7 DAT recorder. The hyperbaric chamber was fitted with a B&K Type 8103 hydrophone, and contained a Subacoustech Type SAV010 low-noise instrumentation amplifier and a DAT recorder. Peak SPL values were obtained from the recordings later in the laboratory, from pressure~time histories, using a Rockland System 90 Signal Processing Workstation. Document ref: 534R0109 11
Fig. 4.5. Measurement locations in Poole Bay. 4.2.3.5 Results The Transmission Loss values for Weymouth Bay are given in Figs. 4.7. For all these measurements the airgun was operated at 2000psi. The water depths at the source and receiver locations are given in Table 4.3. At most receiver positions the water was relatively deep (>20m), while the water depth at the source position was around 10m. The TL fell mostly around 20logR (R in metres), with some tendency for the TL to increase towards 25logR and 30logR at the furthest ranges. Document ref: 534R0109 12
Fig. 4.6. Measurement locations in Poole Harbour. Document ref: 534R0109 13
Fig. 4.7. Transmission loss variation with range, for airgun releases in Weymouth Bay. ident transect 1 transect 2 transect 3 transect 4 transect 5 Table 4.3. Water depths during Weymouth Bay measurements range (m) 240 500 1000 2000 4000 5000 depth at source (m) 10.5 10.5 10.5 10.5 10.5 10.5 depth at receiver (m) 20.5 20.5 21.5 21.5 23.0 26.5 depth at source (m) 10.0 10.0 10.0 10.0 10.0 10.0 depth at receiver (m) 21.0 24.0 24.0 24.0 24.0 24.0 depth at source (m) 9.5 9.5 9.5 9.5 9.5 9.5 depth at receiver (m) 24.5 24.5 24.5 24.5 24.5 21.0 depth at source (m) 9.0 9.0 9.0 9.0 9.0 9.0 depth at receiver (m) 24.0 24.0 24.0 24.0 25.0 27.0 depth at source (m) 10.0 10.0 10.0 10.0 10.0 10.0 depth at receiver (m) 24.0 24.0 24.0 22.0 22.0 14.0 The TL values for Poole Bay are given in Figs. 4.8. For these the airgun was operated at both 1500 and 2000psi. Airgun data for source point 2 were not useable and pyrotechnic data were used instead (Fig. 4.9). Water depths at both source and receiver positions were relatively deep (around 20m). For the airgun data, the TL values were similar for both pressures. Document ref: 534R0109 14
Fig. 4.8. Transmission loss variation with distance, for airgun releases in Poole Bay. Fig. 4.9. Transmission loss variation with distance, for pyrotechnic and airgun sources in Poole Bay. Document ref: 534R0109 15
ident transect 6 transect 7 transect 8 pyro Table 4.4. Water depths during Poole Bay measurements range (m) 250 500 1000 2000 4000 water depth at source (m) 20.0 20.0 20.0 20.0 20.0 water depth at receiver (m) 20.0 20.0 20.0 20.0 19.9 water depth at source (m) 20.0 20.0 20.0 20.0 20.0 water depth at receiver (m) 20.0 20.0 20.0 19.7 19.1 water depth at source (m) 20.0 20.0 20.0 20.0 20.0 water depth at receiver (m) 20.0 20.0 19.2 19.2 19.1 water depth at source (m) 3.6 3.6 3.6 water depth at receiver (m) 3.6 6.5 10.5 The TL values for Poole Harbour are given in Figs. 4.10. Again, the airgun was operated at both 1500 and 2000psi. Water depths were considerably shallower, with that at the source being between 2 and 5m, and that at the receiver between 1.7 and 6.7m. Again TL varied between 20logR and 25logR. Fig. 4.10. Transmission loss variation with distance, for airgun releases in Poole Harbour. Document ref: 534R0109 16
4.2.3.6 Background noise in Weymouth Bay On the day that the measurements were taken in Weymouth Bay a naval exercise was taking place, and occasionally frigates passed within a few hundred metres of a receiver point. Little noise was apparent and the ambient SPLs were substantially lower than in the Poole area, ranging from 92 to 105dB re 1 Pa. Some fishing vessels passing close to the receivers were more clearly detectable, giving levels of up to 130dB re 1 Pa at 100m distance. Sea state was 1 to 2 during the measurement period. Table 4.5 gives some background SPLs that were measured. Table 4.5. Background noise levels in Weymouth Bay. Source of noise SPL (db re 1 Pa) 27ft fishing boat, passing within 100m 130 (peak) Naval frigate, passing within 300m 109 (peak) Background noise measured with no vessels within 2000m 92 to 105 4.2.4 The October/November 1994 measurements, in Poole Harbour and Poole Bay 4.2.4.1 Introduction A full seismic survey in Poole Harbour and Poole Bay was carried out in October and November 1994 using a 15 airgun array as the source, and the opportunity was taken to obtain SPL measurements during this work. Measurements were made in Poole Bay on two days (14 October and 10 November), and in Poole Harbour on one day (18 October). 4.2.4.2 Measurement locations The measuring locations used on the three days are shown in Fig. 4.11 (Poole Bay) and Fig. 4.12 (Poole Harbour). On the two earlier dates measurements were made at relatively close range (<1km) as the fishing vessels used were slow and it was not feasible to travel further from the source within the 25min during which the seismic shooting took place. On the third date measurements were started at 12km offshore from the source and continued to be taken as the receiver vessel closed to within 6km of the source. A microcomputer-based satellite navigation system based on the recording vessel was used to take position fixes for each recording. The time and position data for each impulse were later matched with corresponding data from the seismic survey so that distances from the source could be calculated. 4.2.4.3 Source The underwater sound impulses were generated by an in-line array of 15 sleeve airguns, of total volume 480cu. in., deployed from the survey vessel Modulo 5. The nominal source level of this array was 237dB re 1 Pa @ 1m. The normal deployment depth was 2m below the water surface, but the array could be raised to a minimum of 60cm below the surface for shallow water operation. The horizontal output profile of this array was known to be directional, with minimum radiation along the array axis (i.e. along the tow-line) and maximum radiation at right-angles to the tow-line, so all measurements were made close to perpendicular to the tow-line. 4.2.4.4 Measurement details Recordings of the underwater sound were made with a hydrophone and associated equipment deployed from a fishing vessel. A B&K Type 8105 hydrophone was used; it was lowered to within 0.5m of the seabed. The phone was connected to a B&K Type 2635 charge amplifier, whose output was recorded on a Sony TCD-D7 DAT recorder. Peak SPL values were obtained from the recordings later in the laboratory, from pressure~time histories, using a Rockland System 90 Signal Processing Workstation. Document ref: 534R0109 17
Fig. 4.11. Measurement locations in Poole Bay. Document ref: 534R0109 18
Fig. 4.12. Measurement locations in Poole Harbour. Document ref: 534R0109 19
4.2.4.5 Results Several hundred measurements were taken, of which a small proportion were considered unuseable. Most difficulty was found with the Poole Harbour data, as the choppy water conditions prevailing when the measurements were made caused excessive noise from the slap of water against the hull of the receiver boat. The results obtained in Poole Bay on 14 October are given in Fig. 4.13. The data accord well with TL varying as 20logR (R in metres). Fig. 4.13. Variation of sound pressure level with distance, for airgun survey in Poole Bay on 14 October 1994. The results obtained in Poole Bay on 10 November are given in Fig. 4.14. It was found that, over distances of 6 to 10.4km from the source, the TL varied nearly as 20logR. However, at 12km from the source there was a marked drop in peak SPL. Possible explanations for this were considered to be (i) that the water at the largest ranges has a substantial swell, and it was possible that that was forcing bubbles into the water, with a consequent higher attenuation of the sound propagating in the direct sound path (Urick (1983)); (ii) that the sound was being deflected away from the measurement point by a sound channel at a higher or lower depth. The phenomenon of downslope conversion (where the sound is captured from surface paths in shallow water and carried out to deep water in a sound channel) has been noted in military applications, and could have been leading to the effects seen in the measurements. The results obtained in Poole Harbour on 18 October are given in Fig. 4.15. The recordings made here yielded a characteristic double impulse for each airgun emission. The first, and lower, level was the rockborne headwave, and the second was the direct waterborne wave, which had a peak pressure some 10 to 15dB higher than the rockborne wave. Both wave showed a 20logR dependence. Document ref: 534R0109 20
Fig. 4.14. Variation of sound pressure level with distance, for airgun survey in Poole Bay on 10 November 1994. Fig. 4.15. Variation of sound pressure level with distance, for airgun survey in Poole Harbour on 18 October 1994. Document ref: 534R0109 21
4.3 Measurements in the Wadden Sea A seismic survey was conducted in the Wadden Sea in October 1996, to provide information about the consequential impact of airgun activity on local marine life. Monitoring of underwater noise was conducted by Subacoustech from a vessel as the survey was in progress. The Wadden Sea is a shallow coastal sea along the western and northern coasts of Denmark, Germany and the Netherlands. Towards the North Sea it is bounded by 17 large, inhabited barrier islands as well as numerous small, uninhabited sand banks and tiny islands. The boundary towards the mainland is characterised by the presence of salt marshes, sea walls and some pleistocene cliffs. Three large rivers, the Ems, Weser and Elbe, as well as a number of smaller rivers, have estuaries opening into the Wadden Sea. The tidal range within the Sea and the adjacent estuaries varies between 1 and 4m. During ebb vast areas of tidal flat emerge, and normally about two thirds of the area is exposed during low tide. The barrier islands included, the Wadden Sea area occupies about 10,000km², thus being the largest estuarine area in Europe. The Meldorfer Bucht seismic survey involved firing an airgun array, consisting of 15 Bolt airguns of total capacity 480cu. inches, in very shallow water, typically of 1 or 2m depth. Concerns had been expressed over possible environmental effects on wildlife in the vicinity of the survey, as the shallow waters may harbour a high density and rich variety of species. As the Meldorfer Bucht area lies within the Wadden Sea National Park, and little information was available concerning airgun sound propagation in shallow waters, it was considered important to investigate and monitor the sound pressure levels in the water surrounding the airgun, and to interpret the findings in the light of any possible environmental effects. Monitoring was conducted by Subacoustech as the survey was underway, and recordings were made over a wide variety of conditions and ranges. A total of 564 recordings were analysed, and the data were assessed in terms of peak pressure levels. The results from the analysis are summarised in Fig. 4.16, which presents peak Fig. 4.16. Variation of peak pressure level with range. Document ref: 534R0109 22
pressure as a function of distance from the source. The figure also indicates some specific levels of interest, ranging from ambient noise levels through to the level at which injuries to fish might be expected to occur. It was concluded that, on a peak pressure basis and by taking the lowest estimate of avoidance threshold for cetaceans, an avoidance reaction might have occurred up to 3km from the airgun array. That was where the level of noise from the array had reduced to that of background noise. For flatfish and swimbladdered fish, again taking the lowest avoidance threshold, the range of disturbance would have been up to 1.1km from the airgun array. 4.4 Measurements in the North Sea 4.4.1 Introduction Subacoustech Ltd undertook measurements of the noise radiated during a 3D seismic survey in blocks 14/14a of the North Sea during the summer of 1998. The purpose of the measurements was to provide good-quality recordings of seismic noise, and to analyse them in units relevant to environmental effects. The measurements were taken between 26 th July and 18 th August, and the results were presented in Nedwell et al (1999). 4.4.2 Details of measurements The water depth in this area is about 100m. The survey was conducted using the boat SRV Veritas Viking. A total of 6 seismic hydrophone streamers (for obtaining measurements for the seismic study itself) were towed behind the seismic boat, covering a width of 600m. The seismic boat followed a pattern of east-west lines, turning in a line change manoeuvre at each end on a 2.5km radius arc. Two seismic noise sources were used, each consisting of an array of 18 tuned and clustered Bolt airguns, totaling 3955 cu. inches, as illustrated in Fig. 4.17. The port and starboard arrays were fired alternately in what is termed a flip flop arrangement. Fig. 4.17. Details of airgun arrays. Document ref: 534R0109 23
Noise measurements were conducted from the survey guard boat Northstar on an opportunity basis. Due to considerations of safety it was not possible to make recordings closer than 1.4km from the seismic boat. Measurements were made up to a maximum range of 12km. Fig. 4.18 shows the positions, in relation to the source array, at which sound measurements were taken. It will be noted that very few measurements were taken in the region behind the source boat to 45 either side of the boat's axis. In order to minimise the self-noise from the guard boat during recording the main engine of the boat was turned off and the boat allowed to drift as the seismic boat travelled past. The point of closest approach of the seismic boat to the guard boat was as the guard boat was on beam (90 to the direction of travel of the seismic boat). For each recording, bearing and distance to the survey vessel were noted. Figs. 4.18 to 4.20 give schematic illustrations of the positions at which all the recordings were made. Each line on the illustrations represents the position of the hydrophone relative to the position of each seismic discharge. In the latter two figures red lines represent the measurements made at 5m depth, green lines those at 10m depth, and blue those at 20m depth. In all, recordings were taken at 783 measuring points, with two simultaneous measurements at two depths at each point; this includes background noise measurements. Thus a total of 1586 individual recordings of underwater sound were made. Measurements were made using Brüel & Kjær Type 8105 hydrophones, with their signals conditioned with B&K Type 2635 charge amplifiers. Recordings were made at depths of 5, 10 and 20m. Only two hydrophones were used at one time, so at each recording position the signals at 5m depth and one other depth were recorded simultaneously. In order to preserve a wide bandwidth and high signal-tonoise ratio, signals from the conditioning amplifiers were directly digitised with a National Instruments analogue-to-digital acquisition card (Type AT-MIO-16E-2) and stored by direct x 10 4 1 0.5 Range (m) 0-0.5-1 1 0.5 0 Range (m) -0.5-1 x 10 4 Fig. 4.18. A diagram showing the positions at which measurements were made, relative to the airgun array (plan view). Document ref: 534R0109 24
Measurement depth (m) 0-5 -10-15 -20-1 -0.5 0 x 10 4 0.5 1-1 Range (m) -0.5 0 0.5 Range (m) 1 x 10 4 Fig. 4.19. A diagram illustrating the positions at which measurements were made, relative to the airgun array (isometric view from starboard bow). Measurement depth (m) 0-5 -10-15 -20 1 x 10 4 0.5 0-0.5 Range (m) -1-1 1 0.5 0 x 10 4-0.5 Range (m) Fig. 4.20. A diagram illustrating the positions at which measurements were made, relative to the airgun array (isometric view from starboard stern quarter). Document ref: 534R0109 25
memory access on the hard disc of a PC. The data were digitised at 190ksamples/sec, and hence the useable frequency range of the recordings was from 10Hz to 95kHz. Due to the high data acquisition rate data were continually written to a buffer; an incoming seismic pulse acted as a trigger which initiated the writing of data to the disk, including a period prior to the reception of the pulse. 4.4.3 Results During analysis, every record was inspected, both visually as a time history and audibly using a high quality loudspeaker, for imperfections in the data. A total of 920 recordings were rejected, some because of spurious noise of electrical origin, and some as a result of a hydrophone subsequently being suspected of being faulty. In addition, the sea state was found to have a large influence on the quality of the recordings. Because of the large bandwidth it was not always possible to eliminate false triggers arising from the signals caused by bobbing of the guard vessel or from the sea crashing against its hull. Therefore, some data were rejected because of extraneous noise. This quality control procedure left a total of 596 recordings, including background noise measurements, which were considered to be of acceptable quality, and which were used in the results presented. The results were presented as peak pressure levels, both unweighted (linear) and weighted to yield levels in db ha (species). 4.4.3.1 Unweighted results Fig. 4.21 illustrates a typical result, at a depth of 5m, for the variation of peak sound pressure level with range. The range extends from about 1.4km to 14km. Four sets of data, taken on four separate days of seismic surveying, are presented. Four general features may be seen. 1. The data at the shortest range in each set of data tend to be higher in level than the general trend in level of the rest of the data. This is due to directivity of the array. Fig. 4.22 shows the same data, but as an isometric plot, with the peak SPL as a function of both the range and the bearing between the axis of the array (i.e. its direction of travel) and the measurement point. It may be seen that the higher levels correspond to the point of closest approach of the seismic array to the measurement point, which is as the seismic boat passes with the measurement point on beam, i.e. at 90 to the direction of travel. It may be noted that there is a directional effect, with the levels typically higher than the general trend by about 10dB within an arc spanning about 20. This result, and similar values of directivity, have been noted by other workers. 2. If the peaks caused by the directional effects noted above are ignored, there is a regular and uniform reduction in the peak SPL with range, as would be expected from simple physical considerations. 3. The results could be modelled reasonably well by a simple N log(r) curve. In this case, the best fit was given by N=-25.35. An extrapolation to give the Source Level gave a value of 262.9dB re 1µPa @ 1m. Measured source levels of seismic arrays are typically around 240-255dB re 1µPa @ 1m, so the result obtained was rather high. It was considered likely that it was a result of error in estimating the value of N due to the propagation in the immediate vicinity of the source differing from that measured at greater ranges. 4. The results showed a scatter of about 10dB around the best fit line. It was noted that where the source boat approached the measurement point, and then passed further away again, the measurements during approach differed from those at the same range during moving away. This difference was apparently not systematic, and might have been caused by either spatial or temporal inhomogeneities of the sea. Document ref: 534R0109 26
200 190 Peak SPL in db(lin), re. 1µPa 180 170 160 150 140 29/7/98 30/7/98 08/8/98 14/8/98 N=-25.35 SL=262.9 Range (m) 10 4 Fig. 4.21. Variation of peak sound pressure level with range, for 5m depth. Peak SPL in db(lin), re. 1µPa 200 180 160 140 29/7/98 30/7/98 08/8/98 14/8/98 400 200 10 4 Range (m) 0 Angle to source boat bow ( ) Fig. 4.22. Variation of peak sound pressure level with range and location relative to the source, at 5m depth (isometric view). Document ref: 534R0109 27
Results for depths of 10m and 20m are presented in Figs. 4.23 to 4.26. 200 Peak SPL db(lin), re. 1µPa 190 180 170 160 29/7/98 30/7/98 08/8/98 N=-24.56 SL=262.1 150 Range (m) 10 4 Fig. 4.23. Variation of peak sound pressure level with range, for 10m depth. Peak SPL db(lin), re. 1µPa 200 180 160 140 29/7/98 30/7/98 08/8/98 400 200 10 4 Range (m) 0 Angle to source boat bow ( ) Fig. 4.24. Variation of peak sound pressure level with range and location relative to the source, at 10m depth (isometric view). Document ref: 534R0109 28
190 Peak SPL in db(lin), re. 1µPa 185 180 175 170 09/8/98 10/8/98 13/8/98 14/8/98 N=-14.13 SL=226.5 165 Range (m) 10 4 Fig. 4.25. Variation of peak sound pressure level with range, for 20m depth. Peak SPL db(lin), re. 1µPa 200 180 160 09/8/98 10/8/98 13/8/98 14/8/98 400 200 10 4 Range (m) 0 Angle to source boat bow ( ) Fig. 4.26. Variation of peak sound pressure level with range and location relative to the source, at 20m depth (isometric view) Document ref: 534R0109 29
4.4.3.2 Weighted results db ha (species) values were calculated for catfish, cod, dab, the harbour hair seal, the harbour porpoise and the killer whale. Table 4.6 gives the species names for the animals, and includes the method by which the audiograms were obtained, and Fig.4.27 shows the audiograms that were used in the calculation process. Table 4.6. Species for which db(species) levels calculated, and the manner of obtaining their audiograms. Species Electrophysiological Behavioural Catfish (Ictalurus nebulosus) Cod (Gadus morhua) Dab (Limanda limanda) Harbour hair seal (Phoca vitulina) Harbour porpoise (Phocoena phocoena) Killer whale (Orcinus orca) 140 Hearing threshold in db, re. 1µPa 120 100 80 60 40 Cod Dab Harbour hair seal Killer whale Harbour porpoise 20 10 1 10 2 10 3 10 4 10 5 Frequency (Hz) Fig. 4.27. The audiograms that were used to generate db ha (species) levels. Fig. 4.28 shows a typical seismic waveform, with no weighting, and Fig. 4.29 the associated spectrum. Figs. 4.30 to 4.34 show the same waveform after it had been passed through filters representative of the five species. Document ref: 534R0109 30
Fig. 4.28. A typical unweighted pressure time history for an airgun impulse, measured at 10m depth and 3000m range. Fig. 4.29. The spectrum associated with the unweighted pressure time history presented in Fig. 4.28. Document ref: 534R0109 31
Fig. 4.30. The time history of Fig. 4.28 weighted according to the cod audiogram. Fig. 4.31. The time history of Fig. 4.28 weighted according to the dab audiogram Document ref: 534R0109 32
Fig. 4.32. The time history of Fig. 4.28 weighted according to the harbour hair seal audiogram. Fig. 4.33. The time history of Fig. 4.28 weighted according to the killer whale audiogram. Document ref: 534R0109 33
Fig. 4.34. The time history of Fig. 4.28 weighted according to the harbour porpoise audiogram. It may be seen that for the two fish species (Figs. 4.30 and 4.31) the waveform was not much changed by the filtering process. This was to be expected, since the majority of the energy from the seismic source was at frequencies at which the fish could hear. The process of filtering the data to represent fish hearing consequently removed little of the sound energy. The level of the signal was significantly reduced, however, as a consequence of the threshold of hearing of the two fish species being relatively high. This can be seen clearly in Fig. 4.30, which illustrates the results for the cod. It is even more noticeable in Fig. 4.31, which illustrates the results for the dab, a flatfish which is relatively insensitive to sound. The results for the harbour hair seal, in Fig. 4.32, were significantly different from those for fish. The level of the seismic signal was significantly reduced, since little of its energy was at frequencies at which the seal could hear. There was background noise at a fairly constant level, and it can be seen that the signal was not greatly above the level of background noise. The results for the killer whale (Fig. 4.33) were very similar, although the seismic signal was in this case rather higher than the level of background noise. Similar comments apply to the results for the harbour porpoise (Fig. 4.34). For each species every sound record was used to calculate the peak sound level in db ha (species) as a function of range; this was done for the three depths at which measurements were made. Fig. 4.35 shows the result for the cod. Comparison with Fig. 4.21, the unweighted variation, shows that the results were very similar. This was to be expected, since most of the acoustic energy generated by the airgun array was at frequencies which cod can hear. The rate at which the sound decayed (N=27.8) was also similar to that of the unweighted case. However, the overall level, and consequently the effective Source Level, was rather lower, since the hearing process of cod is poor. Document ref: 534R0109 34
130 Peak SPL in dbha(gadhus morhua), re. 1µPa 120 110 100 90 80 70 29/7/98 30/7/98 08/8/98 14/8/98 N=-27.76 SL=195.1 60 Range (m) 10 4 Fig. 4.35. Variation of peak sound pressure level with range for the cod, in db ha (Gadhus morhua) re 1µPa, at 5m depth. Fig. 4.36 presents the results for the harbour porpoise, at 5m depth. Peak SPL in dbha(phocoena phocoena), re. 1µPa 120 110 100 90 80 70 60 29/7/98 30/7/98 08/8/98 14/8/98 N=-30.06 SL=203.6 Range (m) 10 4 Fig. 4.36. Variation of peak sound pressure level with range for the harbour porpoise, in db ha (Phocoena phocoena) re 1µPa, at 5m depth. Document ref: 534R0109 35
There were significant differences between the results for the porpoise and the cod. Firstly, the levels were rather low. This was because the majority of the sound energy was at frequencies where the porpoise is not sensitive. Secondly, the variability of the data was much higher than for the cod. This was probably because the results were dominated by the high frequency sound that the harbour porpoise can hear; high frequency sound is readily diffracted by local small scale inhomogeneities of the water, leading to the variability of the data. The db ha (Phocoena phocoena) level did not show the increase in level where the source was on beam (at 90 to the direction of travel); this might have been expected as the seismic array was only directional at low frequencies. The results of db ha (species) calculations for the cod and the porpoise at 10m depth are given in Figs. 4.37 and 4.38 respectively, and for the same species at 20m depth in Figs. 4.39 and 4.40 respectively. The results for all the calculations is given in Table 4.7. species Table 4.7. Source Levels and decay rates for selected species, allowing for their hearing responses. measurement depth (m) 5 10 20 S.L. N S.L. N S.L. N frequency range of audiogram (Hz) most sensitive frequency (Hz) db ht level unweighted 262.9 25.4 262.1 24.6 226.5 14.1 catfish 201.5 27.3 192.8 24.6 154.1 13.6 50 10k 800 80 cod 195.1 27.8 183.4 24.3 141.7 12.4 60-500 200 71 dab 166.1 25.9 161.6 23.6 129.8 14.5 30-250 100 90 harbour hair seal 183.5 29.2 166.0 26.0 134.8 17.9 1k 100k 32k 63 harbour porpoise 203.6 30.1 175.7 23.8 144.8 16.0 1k 100k 32k 45 killer whale 209.5 30.7 216.5 34.6 158.9 19.9 500 32k 16k 34 homo sapiens 186.1 26.8 180.6 25.5 145.3 15.5 25 16k 800 67 The results showed that Source Level and Transmission Loss reduced as depth increased; this was particularly apparent at 20m depth, where the Source Level was about 50dB lower than at 5m, and the Transmission Loss factor (N) was12 to 15 compared with 25 to 30 at 5m. This result was of great interest, since the actual Source Level had to be the same for all three depths. It implied that the airgun source was able to couple better into sound waves travelling at the water surface than into sound waves travelling deeper down. It also implied that the waves nearer the surface were more rapidly attenuated. However, it was difficult to generalise the results. For the three fish species the Transmission Loss values were relatively similar, but the effective Source Levels were very species dependent, varying from 201dB ha (Ictalurus nebulosus) for the catfish to 166dB ha (Limanda limanda) re 1µPa @ 1m for the dab at 5m depth. This agreed with the marked differences in hearing sensitivity found amongst the species. The catfish is a hearing specialist, and the relatively high levels for it result from its sensitivity to the sound. By comparison, the dab lacks a swimbladder and therefore is insensitive to sound; the effective source levels are thus relatively low. The cod is intermediate in its hearing ability, and the corresponding db ha (Gadus morhua) levels are intermediate between those of the other two species. Similar comments apply to the mammals. The Transmission Loss was similar for the three species, because their hearing ability is similar in frequency terms. The effective Source Levels were somewhat different, with the effective levels highest for the killer whale, which has the most sensitive hearing. Document ref: 534R0109 36
130 Peak SPL in dbha(gadus morhua), re. 1µPa 120 110 100 90 80 70 29/7/98 30/7/98 08/8/98 N=-24.28 SL=183.4 Range (m) 10 4 Fig. 4.37. Variation of peak sound pressure level with range for the cod, in db ha (Gadhus morhua) re 1µPa, at 10m depth. Peak SPL in dbha(phocoena phocoena), re. 1µPa 110 105 100 95 90 85 80 75 29/7/98 30/7/98 08/8/98 N=-23.82 SL=175.7 Range (m) 10 4 Fig. 4.38. Variation of peak sound pressure level with range for the harbour porpoise, in db ha (Phocoena phocoena) re 1µPa, at 10m depth. Document ref: 534R0109 37
120 Peak SPL in dbha(gadus morhua), re. 1µPa 110 100 90 80 70 09/8/98 10/8/98 13/8/98 14/8/98 N=-12.42 SL=141.7 Range (m) 10 4 Fig. 4.39. Variation of peak sound pressure level with range for the cod, in db ha (Gadhus morhua) re 1µPa, at 20m depth. Peak SPL in dbha(phocoena phocoena), re. 1µPa 105 100 95 90 85 80 75 09/8/98 10/8/98 13/8/98 14/8/98 N=-15.99 SL=144.8 Range (m) 10 4 Fig. 4.40. Variation of peak sound pressure level with range for the harbour porpoise, in db ha (Phocoena phocoena) re 1µPa, at 20m depth. Document ref: 534R0109 38
4.4.4 'Soft start' procedures To minimise environmental impact it is normal operating procedure when starting firing of the airgun array to use a 'soft start' procedure. In this the number of airguns discharged is gradually increased from one up to the whole array over a period of a few minutes. The aim is to minimise environmental effects by allowing animals that might be affected to leave the area. Often the volumes of the airguns in an array are not the same, and it is normal to start by discharging the airgun having the smallest volume first on its own. The volume of air discharged is increased from this least value up to the maximum volume that can be discharged in roughly equal increments. A number of recordings of soft starts were made. In order to remove both the range and array angle dependence, measurements were conducted by stationing the guard boat at the centre of the semi-circular path of the seismic boat during its line change manoeuvre. Drift of the guard vessel proved to be a significant factor when analysing these recordings the radius of the circles described by the seismic vessel was in the region of 2.5km, and drift of over 1km of the guard boat during a recording of a soft start was not uncommon. Figs. 4.41 and 4.42 illustrate the results for soft starts at depths of 5m and 20m respectively; the data presented in the figures in unweighted. The figures show the SPL at a range of 2.5km as a function of the volume of the compressed air discharged by the airgun. It may be seen that there is a fairly consistent relationship between the total volume V discharged and the level of sound. Each of the measurements has been fitted to a law of the form: SPL = M log (V) + V 0 where M and V 0 are constants, V is the total volume of compressed air discharged and SPL is the resulting sound pressure level. For the 5m depth results M has a value of 8.4, giving the relationship P = k V 2.4 where P is the sound pressure and k is a constant. This shows that the soft start procedure achieves its objective of gradually raising the sound pressure as the array is started. Figs. 4.43 and 4.44 illustrate the levels during soft starts weighted for cod, in db ha (Gadus morhua) units, at two depths. The results are very similar to the unweighted ones, although there is rather more scatter in the results at the higher volumes than is the case for the unweighted data. Figs. 4.45 and 4.46 show the results of a similar calculation for the harbour porpoise. There was a very significant scatter in the results both at 5m and 20m depth. It could not be determined from these results whether this arose from variability in propagation or from variability in the characteristics of the airgun array Document ref: 534R0109 39
Peak SPL in db(lin), re. 1µPa 190 185 180 175 170 165 11/8/98 number 1 11/8/98 number 2 11/8/98 number 3 16/8/98 number 1 16/8/98 number 2 M =8.388 V 0 =145.5 160 155 10 2 10 3 10 4 Airgun volume (cubic inches) Fig. 4.41. Peak SPL measured at 5m depth vs compressed air volume discharged. Peak SPL in db(lin), re. 1µPa 190 185 180 175 170 165 11/8/98 number 1 11/8/98 number 2 11/8/98 number 3 14/8/98 number 3 14/8/98 number 4 16/8/98 number 1 16/8/98 number 2 M =11.01 V 0 =138.4 160 155 10 2 10 3 10 4 Airgun volume (cubic inches) Fig. 4.42. Peak SPL measured at 20m depth vs compressed air volume discharged. Document ref: 534R0109 40
Peak SPL in dbha(gadus morhua), re. 1µPa 120 115 110 105 100 95 90 85 80 11/8/98 number 1 11/8/98 number 2 11/8/98 number 3 16/8/98 number 1 16/8/98 number 2 M =10.18 V 0 =64.03 10 2 10 3 10 4 Airgun volume (cubic inches) Fig. 4.43. Peak SPL for cod, measured at 5m depth, vs compressed air volume discharged. Peak SPL in dbha(gadus morhua), re. 1µPa 120 115 110 105 100 95 90 85 80 11/8/98 number 1 11/8/98 number 2 11/8/98 number 3 14/8/98 number 3 14/8/98 number 4 16/8/98 number 1 16/8/98 number 2 M =11.7 V 0 =60.76 10 2 10 3 10 4 Airgun volume (cubic inches) Fig. 4.44. Peak SPL for cod, measured at 20m depth, vs compressed air volume discharged. Document ref: 534R0109 41
Peak SPL in dbha(phocoena phocoena), re. 1µPa 105 100 95 90 85 80 75 70 65 60 11/8/98 number 1 11/8/98 number 2 11/8/98 number 3 16/8/98 number 1 16/8/98 number 2 M =12.85 V 0 =38.81 10 2 10 3 10 4 Airgun volume (cubic inches) Fig. 4.45. Peak SPL for the harbour porpoise, measured at 5m depth, vs compressed air volume discharged. Peak SPL in dbha(phocoena phocoena), re. 1µPa 80 75 70 65 60 55 11/8/98 number 1 11/8/98 number 2 11/8/98 number 3 14/8/98 number 3 14/8/98 number 4 16/8/98 number 1 16/8/98 number 2 M =6.372 V 0 =47.19 10 2 10 3 10 4 Airgun volume (cubic inches) Fig. 4.46. Peak SPL for the harbour porpoise, measured at 20m depth, vs compressed air volume discharged. Document ref: 534R0109 42
4.4.5 Conclusions The airgun array was found to be directional at low frequencies, with the levels typically higher than the general trend by about 10dB within an arc about ±10 from the perpendicular to the array. A simple N log (R) curve gave a reasonable model for the variation of SPL with range. The effective Source Level, N, was found to be rather higher than expected, possibly due to local acoustical effects near the array, and the scatter of the results could have been caused by spatial or temporal inhomogeneities of the sea. The db ha (species) approach was used to estimate the possible effects on selected species. This showed that values differed markedly between species, showing that the unweighted peak pressure level of a seismic signal has no general biological validity. The soft start procedure achieved its objective of gradually raising the sound pressure level during the start of the firing of the array. Document ref: 534R0109 43
5 Drilling noise 5.1 Introduction Drilling for oil at sea may introduce waterborne noise from numerous sources. Floating drill rigs are usually massive structures, of the order of 100m along each side. For stability reasons heavy plant, such as engines, are mounted on the superstructure and sited in the legs, which are submerged in the water. Noise resulting from the drilling operation may include: Machinery noise, such as that from the drill s drive machinery, including drilling noise, engine and exhaust noise, and from the generators and other hotel plant used on the rig. Noise and vibration from the grinding of rock in the seabed, which can either radiate directly from the drill bit through the rock into the water, or can conduct upwards through the drill shaft, radiating into the surrounding water. Noise from communication and positioning systems, such as submarine warning beacons and Doppler type flow meters. These are generally single frequency sources. Noise from dynamic positioning thrusters, used to position the drill ship. Subacoustech has taken measurements of drilling noise in two projects, viz> i. around the 'Jack Bates' rig, northwest of the Shetlands in September 2000, and ii. around the 'West Navion' rig, west of the Hebrides in May 2001. 5.2 Measurements around Jack Bates 5.2.1 Introduction Measurements were made of the noise radiated during drilling from the Jack Bates semisubmersible rig while drilling in deep water northwest of the Shetlands, during September 2000. The work was reported on in Nedwell, J.R, et al. (2001a). Measurements were made from the drill rig both during drilling, and when the drill was not in use. The measurements were made on an opportunity basis, and over as wide a range of conditions as possible within the constraints of operation. 5.2.2 Measurement details Measurements of underwater noise from the Jack Bates semi-submersible drilling rig were taken between 5th and 7th September 2000 by a hydrophone suspended from the drill rig, at a depth of 100m below it. While it was difficult to relate these measurements directly to the level of sound that would have arisen far from the rig, they were extremely valuable in that they for the first time identified the dominant noise characteristics and sources on a drill rig. All the results presented were taken using a B&K Type 8106 hydrophone. This hydrophone incorporates a charge amplifier which, when powered by a Subacoustech Type PS7 power supply, gave a wide dynamic range and very low electronic noise. The signals from the hydrophones were digitised using a National Instruments Type DAQCard-AI-16E-4 A/D converter of 12 bit resolution. The converter was driven from a Toshiba Satellite 1620 CDS laptop, and a sample rate of 262ksamples per second was used. 5.2.3 Results 5.2.3.1 Introduction A total of 25 sets of data were successfully recorded. Each data set comprised about 130 seconds of data, with the frequency range of the recording being from 1 Hz to 125 khz. Document ref: 534R0109 44
The overview of the results given below provides detailed interpretations of three typical sets of data for three main operating conditions. After that a comparison of these is presented. Full detailed sets of data are to be found in the original report. 5.2.3.2 Overview of Results 5.2.3.2.1 Background noise measurements This section presents information on the level of background noise from the Jack Bates, with the hotel plant operating but with no drilling taking place or thrusters operating. Fig. 5.1. A typical waterfall spectrum of the noise, when drilling was not taking place and the thrusters were not being operated. Fig. 5.1 shows a typical waterfall spectrum of the noise recorded in a 130 second period from the Jack Bates, for the case of no drilling and without thrusters operating. The figure presents the Power Spectral Density (in db re 1 µpa 2 /Hz) of the noise in one second segments. The measurement was made at a depth of 100m below the rig at a distance of about 40m horizontally from the drill shaft. Several features are noteworthy. First, the level of the spectrum of the sound was relatively constant with time. Second, there was a high level of very low frequency sound, below 10Hz. It was believed that this corresponded to hydrodynamic noise caused by surface movement, and was not true noise. Third, there was a broad peak in the spectra at about 10Hz. The source of this peak was not known; it varied in level somewhat in the recordings taken and hence it was possible that it was caused by rotational noise from machinery. Document ref: 534R0109 45
Fourth, tonals could be seen at several unrelated frequencies ranging from about 20Hz to 600Hz. These were thought to correspond to machinery noise. The machinery noise could be heard in the background on the time history. There was also noise from production water striking the sea surface. It was interesting to note that the submarine positioning and location beacons could also clearly be heard, which indicated that the recording was of good quality. 5.2.3.2.2 Drilling noise measurements Fig. 5.2 shows a typical waterfall spectrum of the noise recorded in a 130 second period while the rig was drilling, but without the thrusters operating. Fig. 5.2. A typical waterfall spectrum of the noise when drilling was taking place but the thrusters were not working. It may be seen that the level of sound at the lowest frequencies below 10Hz was unchanged from the case where there was no drilling. However, the level of sound in the band from 20Hz up to 1kHz was significantly elevated over that for no drilling, and displayed tonal components which were about 20-30dB higher than the level with no drilling. It was thought that these tonals might have corresponded to natural frequencies of the drill shaft, excited by the drilling machinery on the rig or by the action of cutting at the seabed. The noise from the drilling could be clearly heard on the recording. 5.2.3.2.3 Dynamic Positioning Thruster noise measurements Fig. 5.3 shows a typical waterfall spectrum of the noise recorded in a 130 second for the case of no drilling, but with the thrusters operating. Document ref: 534R0109 46
Fig. 5.3. A typical waterfall spectrum of the noise for the case of no drilling but with the thrusters operating. The blade pass rate of the thruster blades could be clearly heard as a periodic fluctuation in low frequency broadband noise. It may be seen that the thrusters caused an elevation of the low frequency sound from about 2 or 3Hz up to about 30Hz. It may also be seen that due to variations in the inflow to the thrusters, or in the level of thrust generated by the thrusters, the level of sound in this band was much less constant than for the preceding cases, with the level varying by 10dB or more between successive 1 second segments. 5.2.3.2.4 A comparison of noise levels for the Jack Bates background, with drilling taking place and with DP thrusters operating Fig. 5.4 compares the three conditions. The green plot indicates the level with the plant only operating, the red plot indicates the level while drilling was taking place, and the blue plot indicates the level with the Dynamic Positioning thrusters operating. Also included in the figures, as the pink plot, is the spectrum obtained from measurements taken during another project (Nedwell, J.R., & Needham, K, (2001)), when a hydrophone was located 100m below a 50 metre guard vessel to obtain typical sound levels, that is, at the same depth and position that the noise of the Jack Bates was recorded. The two plots are thus directly comparable. In Fig. 5.4(a) the PSD is shown as a function of frequency over five decades from 1Hz to 100kHz. In Fig. 5.7(b) the same spectrum has been plotted up to 1kHz. Document ref: 534R0109 47
Fig. 5.4(a). The full frequency range of the spectrum. Fig. 5.4(b). The low frequency part of the spectrum. Fig. 5.4. Comparison of the noise generated by the various conditions. It may be seen that, when drilling was taking place, the level of sound was elevated by about 10-20dB in the range from 20Hz to 500Hz. Also there were clear tonals, for instance at about 130, 200, 350 and 600Hz, which probably resulted from resonant frequencies of the drill shaft being excited by cutting forces from below or driving forces from above. These tonals could also be heard on the sound files of the drilling noise. Document ref: 534R0109 48
The level of low frequency noise below 60Hz when the thrusters were in use was roughly comparable to the noise generated by the guard boat. Above 60Hz the level was higher than the guard boat when the thrusters were in use, and similar when they were not in use, but the rig was drilling. 5.2.3.3 Effects on mammals For each measurement of rig noise an average spectrum over the duration of the measurement was calculated and used as the input to the db ht (Species) calculation procedure. The results of the calculations by the procedure are given in the Table 5.1. Table 5.1. db ht (Species) values for each measurement for various species. Species Run Date & time No. Killer Harbour Harbour Human Catfish Cod Dab Salmon whale hair seal porpoise diver 1 5/9/2000, 16-23-29 73.6 63.2 42.2 38.0 92.8 71.9 88.4 62.9 2 5/9/2000, 16-26-35 73.2 64.2 43.4 38.8 92.1 70.7 86.9 61.3 3 5/9/2000, 16-27-53 71.6 62.0 42.1 36.6 92.3 69.7 85.7 60.1 4 5/9/2000, 16-29-41 71.4 62.3 41.9 36.9 92.1 69.5 85.3 59.3 5 5/9/2000, 16-31-03 70.7 61.3 41.7 36.3 92.1 69.8 85.7 59.0 6 5/9/2000, 16-33-20 74.6 65.4 43.4 39.2 92.7 70.8 87.0 63.1 7 5/9/2000, 16-35-33 74.2 63.9 42.7 38.1 92.3 70.5 86.6 63.6 8 5/9/2000, 16-37-47 70.4 60.6 40.8 35.6 92.3 69.9 85.8 59.2 9 5/9/2000, 16-43-11 73.2 63.9 43.5 38.5 92.6 70.9 87.1 61.5 10 5/9/2000, 16-45-42 72.0 63.3 42.6 37.8 92.2 70.5 86.6 59.6 11 5/9/2000, 16-46-54 76.2 67.7 45.7 41.8 93.3 71.4 87.6 64.0 12 5/9/2000, 21-04-15 67.0 54.2 35.4 28.9 88.5 69.2 85.9 56.5 13 5/9/2000, 21-06-29 66.9 53.9 35.5 28.8 88.7 69.1 85.9 56.3 14 5/9/2000, 21-08-41 65.6 53.5 35.4 29.0 88.8 69.5 86.2 54.9 15 5/9/2000, 21-10-53 64.8 53.0 35.5 29.0 88.7 69.1 85.6 54.0 16 5/9/2000, 21-14-11 66.0 53.4 35.4 28.9 88.7 69.5 86.0 55.3 17 5/9/2000, 21-18-24 64.5 52.3 35.4 28.7 88.2 68.9 85.3 53.6 18 6/9/2000, 15-56-09 60.4 44.6 23.6 18.4 86.1 67.5 84.4 51.4 19 6/9/2000, 15-59-02 60.1 44.6 23.8 18.5 86.2 66.4 83.2 50.9 20 6/9/2000, 16-01-19 60.9 46.9 32.8 25.4 86.3 65.0 81.4 50.9 21 6/9/2000, 16-08-12 61.3 46.7 30.2 23.9 87.4 73.0 90.3 51.8 22 7/9/2000, 13-38-41 82.6 73.5 54.9 49.9 83.1 63.4 80.2 70.4 23 7/9/2000, 13-42-14 66.1 52.0 32.7 26.7 86.2 64.9 81.2 56.8 24 7/9/2000, 13-45-50 80.5 69.2 48.3 43.9 93.3 70.7 88.1 69.9 25 7/9/2000, 13-48-01 78.3 64.6 43.5 38.0 91.5 69.3 86.5 68.5 In Table 5.3 the results are presented from the perspective of different rig operating conditions. In it are given the average values of db ht (Species) for a given operating condition, the average being over all measurements for that condition. Again, the values for different species are given. Additionally, the linear (unweighted) level is included. There are several interesting features to the results. First, in general, the levels when drilling was taking place were slightly higher than those when no drilling was being undertaken. The species having the highest perceived noise level was the killer whale, where a level of 91.1 db ht (Orcinus orca) was recorded while drilling was being undertaken. The perceived level for the harbour porpoise (86.3 db ht (Phoecaena phoecaena)) in similar conditions was slightly lower. The remaining values ranged down to 40.2 db ht (Limanda limanda) for the dab. Document ref: 534R0109 49
Table 5.2. Averages of db ht (Species) values for different rig operating conditions. Killer Harbour Harbour Human Linear Catfish Cod Dab Salmon whale hair seal porpoise Diver Drilling 176.3 70.3 59.9 40.2 34.8 91.1 70.1 86.3 59.1 Not drilling Not drilling, no thrusters Not drilling, thrusters 177.5 68.8 55.3 36.2 30.6 87.5 67.5 84.4 58.8 159.3 70.0 57.4 39.3 33.5 85.6 67.1 83.9 59.7 188.4 68.0 54.0 34.4 28.8 88.7 67.8 84.7 58.3 The dynamic range of human hearing, from the threshold of hearing to the threshold of auditory damage for long term exposure, is about 90dB. If the assumption is made that other species will have similar dynamic ranges, then all of these values, with the possible exception of that for the killer whale, are well within this dynamic range. Consequently, it is unlikely that auditory injury of any of these species would be caused by exposure at the range of measurement. Nevertheless, there was a strong possibility that these levels would be sufficient to cause some species to avoid the close proximity of the drill ship. 5.2.3.4 Conclusions Measurements of the noise radiated from the Jack Bates semi-submersible drill rig while working in deep water northwest of the Shetlands were made from the rig both while drilling was taking place and when the drill was not in use. They were recorded at a depth of 100m directly beneath the rig. The main conclusions drawn were: 1. When the rig was not drilling the level of the spectrum of the sound was relatively constant with time. There was a broad peak in the spectrum at about 10Hz, possibly caused by rotational noise from machinery. Tonals were noted at several unrelated frequencies ranging from about 20Hz to 600Hz, also possibly resulting from machinery noise. There was also noise from production water striking the water surface. The submarine positioning and location beacons could clearly be heard. 2. When the rig was drilling the level of sound at the lowest frequencies, below 10Hz, was unchanged. In the range from 20Hz to 500Hz the level of sound was elevated by about 10 to 20dB. There were also clear tonals at about 130, 200, 350 and 600 Hz, probably resulting from resonant frequencies of the drill shaft being excited by cutting forces from below or driving forces from above. These tonals could clearly be heard, and resulted in the sound having the timbre of arriving from "down a mineshaft". 3. The use of the Dynamic Positioning thrusters caused a significant elevation of the low frequency sound from about 2 or 3Hz up to about 30Hz. The blade pass rate of the thruster blades could clearly be heard as a periodic fluctuation in low frequency broadband noise. 4. The level of low frequency noise below 60Hz when the thrusters were in use was roughly comparable to the noise generated by a 50 metre guard boat measured in an identical way. Above 60Hz, the level was higher than the guard boat when the thrusters were in use, and similar in level when they were not in use but the rig was drilling. 5. Average values of db ht (Species) were calculated to indicate the perceived level for a given operating condition. The species having the highest perceived noise level was the killer Document ref: 534R0109 50
whale (91.1 db ht (Orcinus orca)); the level for the harbour porpoise was slightly lower (86.3 db ht (Phoecaena phoecaena)). The remaining values ranged down to 40.2 db ht (Limanda limanda) for the dab. 6. It was thought unlikely that auditory injury of any of these species would be caused by exposure at the range of measurement, but some species might avoid the close proximity of the drill ship. 5.3 Measurements around West Navion 5.3.1 Introduction Measurements were undertaken of the underwater noise radiated during normal operations from the drill ship West Navion during May 2001 when it was drilling in deepwater west of the Hebrides. A photograph of the West Navion, which is 250m long, is shown in Fig. 5.5. The work was reported on in Nedwell, J.R. & Needham, K, (2001). Measurements were taken on an opportunity basis along six transects radiating from the drill ship. The measurements were made from a dedicated survey boat (the Rasmus), both during periods when the drill ship was drilling, and during other operations. In general, the measurements were taken over a very wide frequency band (1 Hz 100 khz), to include not only the range in which small cetaceans vocalise and hear, say from 1 khz to 100 khz, but also the range over which fish can hear, from say 10 Hz to a few hundred Hz. Fig. 5.5. The West Navion 5.3.2 Measurement details On the first transect sound measurements were taken at nominal ranges of 500, 1000, 2000, 5000 and 10,000 m. For the subsequent five transects measurements were also taken at a range of 7400 m. At each of the measurement locations measurements were taken at depths of 50, 100 and 200 m. Document ref: 534R0109 51
A Bruel & Kjaer Type 8106 hydrophone was used for all the measurements. It had a weight attached to it and was hung over the side rail of the survey boat. It was connected to a Subacoustech amplifier, the output of which was fed to a National Instruments DAQCard Type 6062E inserted into a Sony laptop computer. The A-to-D conversion was controlled by a program written using the NI LabView application, and was at a sample rate of 262 ksamples/sec. When measurements were taken all machinery on the survey boat was shut down, apart from the radar system, which was used to determine the distance of the boat from the West Navion. Consequently the survey boat drifted while measurements were being taken, and was to the stern of the West Navion. The positions of the boat from the drill ship for each measurement location are illustrated in Fig. 5.6 56.88 North (Degrees) 56.86 56.84 56.82 56.8 56.78 56.76 Run 1 Run 2 Run 3 Run 4 Run 5 Run 6 56.74 9.85 9.8 9.75 9.7 9.65 West (Degrees) Fig. 5.6. The positions of the monitoring vessel during recording 5.3.3 Results 5.3.3.1 Results in conventional units The spectra of the measured noise are presented in the three graphs of Fig. 5.7, each graph being for a particular hydrophone depth. In each figure the six spectra shown are average spectra, at the noted range, over all measurement runs. The measurements, and the conclusions that were drawn from them, divided into three bands of frequency for all the results. Band 1; Surface noise. Within the lowest frequency band, from 1 Hz to about 40 Hz, it could be seen that the noise was not dependent on the range from the drill ship, but decreased slightly with increasing depth. The noise in this region was associated with the water surface and not with the drill ship, and was dominated by pseudo-noise, or pressure changes caused by the passage of waves over the hydrophone position. The changes in level were associated with varying weather states. Thus, at all positions, including those at the closest range, there was no significant component of noise from the drill ship. Band 2; Drill ship noise. The second frequency band covers the range from 40 Hz to about 600 Hz. Within this band the noise level decreased significantly with the range from the drill ship for the measurements taken at the closer ranges, i.e. 500 m, 1 km and 2 km. Therefore, at those ranges, it was concluded that there was a significant contribution to the noise from the drill ship. At the greater ranges of 5, 7.4 and 10 km there was no significant variation in level and hence it was concluded that at those ranges the noise from the West Navion had fallen to the level of the background noise. Document ref: 534R0109 52
A review of measurements of man-made noise carried out by Subacoustech Ltd Average PSD across all runs, taken at 50m depth 140 PSD (db, re. (1µPa)²/Hz) 120 100 500m 1000m 80 2000m 5000m 60 7400m 10,000m 40 20 0 1 10 100 1000 10000 100000 Frequency (Hz) Fig. 5.7(a). 50m depth. Average PSD across all runs, taken at 100m depth 140 PSD (db, re. (1µPa)²/Hz) 120 100 500m 1000m 2000m 5000m 7400m 10,000m 80 60 40 20 0 1 10 100 1000 Frequency (Hz) Fig. 5.7(b). 100m depth. Document ref: 534R0109 53 10000 100000
Average PSD across all runs, taken at 200m depth 140 120 PSD (db, re. (1µPa)²/Hz) 100 80 60 40 500m 1000m 2000m 5000m 7400m 10,000m 20 0 1 10 100 1000 10000 100000 Frequency (Hz) Fig. 5.7(c). 200m depth. Fig. 5.7. Averaged spectra at various ranges, for 3 depths. Band 3; High frequencies. The level in the third frequency band, from 600 Hz to 100 khz, showed a significant and consistent decrease in level both with increasing frequency and with range. It therefore indicated that noise from the drill ship was generally of much lower level than in Band 2 but was dominating over a greater range, perhaps to 10 km or more. However, as a note of caution, it was considered possible that due to the rapid decrease in level with frequency this could have arisen in some cases from leakage of the high levels in the second band to the third band during the spectral estimation process, i.e. it might have been an artefact of the analysis process 1. It was therefore not possible to state categorically whether in all cases the estimated noise level was an accurate representation of the true noise level or not. In many cases, however, there was a complex structure to the spectrum which indicated that the level indicated was real, since that would not have occurred in the case of leakage. It may be seen that the results for the three depths were generally similar. In the surface noise band (Band 1), there was a significant variability of level of the recordings at 50m depth, which was probably due to the state of the local surface at the time of the recording. This was less noticeable in the recordings at 100m depth, and at 200m depth the level was lower and fairly constant. In Band 2, it was interesting to note that there was evidence of a sound channel at 100m and 200m, at a frequency of about 100 200 Hz, in which the noise propagated with relatively low loss. 1 In estimating the spectrum of noise, segments of data are individually converted into spectra before averaging them to provide a statistically significant estimate of the noise level. This process causes an apparent and unavoidable leakage of energy from frequencies at which there are high levels of noise into any adjacent frequencies in which there are lower levels. Consequently, areas of high level have skirts or sidebands surrounding them in which the level may result from leakage rather than from actual noise. Document ref: 534R0109 54
Fig. 5.8 presents the result for Run 6 at a depth of 50 m as an isometric plot. The features of the sound field noted previously can clearly be seen. First, in Band 1 (frequencies below 40 Hz), there was no significant variation of level with range; this region was dominated by surface noise. In Band 2 the influence of the West Navion as a noise source could clearly be seen at the closer ranges, where the level decreased significantly. Fig. 5.8. Isometric plot of sound levels of field, for Run 6. There was an interesting anomaly for the measurement at 7.4 km. It was seen that there was a bump in the spectrum at about 10 khz. This was actually due to one or more sperm whales in the area vocalising; the high level was due to echolocation clicks from the whales. The clicks could clearly be heard on the recording, and were identified as sperm whales by an expert in cetacean vocalisations 2. An analysis in more detail of a section of one of the recordings taken in this run is given in Fig. 5.8, which shows the spectra in 1 / 10 th-second segments as a function of time over a period of 40 seconds. In Fig. 5.9(a) the spectra cover the frequency range from 1 Hz to 100 khz. Figs. 5.9(b) and 5.9(c) are respectively for the frequency ranges 1 Hz to 1 khz, and 100 Hz to 100kHz. In Fig. 5.9(b) there is a noticeable line of peaks in the spectra at about 200 Hz. There is a periodicity in time to this line, which led to the conclusion that this part of the noise was due to the operation of a seismic airgun. In Fig. 5.9(c) there is a line of peaks in the spectra at about 10 khz; these peaks are due to the clicks of the whales. 2 Dr Jonathan Gordon of Ecologic. Document ref: 534R0109 55
Fig. 5.9(a) The full frequency range. Fig. 5.9(b) The lower end of the frequency range, including airgun noise. Document ref: 534R0109 56
Fig. 5.9(c) The upper end the frequency range, including whale clicks. Fig. 5.9. Detailed analysis of a portion of the recording taken during Run 6. 5.3.3.2 Results in db ht (Species) units Fig. 5.10 illustrates the measured levels from the drill ship in db ht (Species) units. The levels are plotted for seven species of fish and marine mammals as a function of the range from the drill ship. The results have been averaged over all six of the transects, and also over the three depths, at which measurements were taken. It may be seen that the general form of the levels was fairly similar from species to species. Near to the source, at ranges of 500 m to 2 km, all of the db ht (Species) levels showed an increase, indicating that for all species there was a contribution to their perceived noise field from the drill ship noise. As the range increased the level settled down to a relatively constant value; this was the level of background noise in the perception scale of the species. It is reasonable to assume that where the source falls to the background level there is no possibility of any environmental effect. In some cases, such as the harbour seal and killer whale, there was evidence of a contribution from the drill ship noise to a greater range, of perhaps 5 km or more. The perceived levels, however, vary from species to species. In the case of the killer whale the level at 500 m was 85 db ht (Orcinus orca). In other words, the level of the noise was 85 db above the killer whale s threshold level. However, the background noise level was also relatively high, at about 55 db. This arose largely as a result of the killer whale s relatively sensitive hearing. By comparison, the results for the harbour seal indicated a maximum level of about 60 db ht (Phoca vitulina) and a perceived background level of about 35 db ht (Phoca Document ref: 534R0109 57
90 80 Perceived level of noise (dbha) 70 60 50 40 30 20 Catfish Cod Dab Harbour porpoise Harbour seal Killer whale Salmon 10 0 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 vitulina). This difference probably arose from the harbour seal s relatively insensitive hearing, which probably arises in turn from its adaptation for noisy littoral waters. Several perceived levels have been included for fish; these were included for interest rather than because of any likelihood that there would be populations near the West Navion. It may be seen that the levels were in general rather low; this resulted from the fact that the noise from the drill ship was predominantly above the frequencies at which fish hear best. As an exception to this, the results for the catfish were relatively high; however this fish is a hearing specialist which has a hearing ability which is both acute and broadband. 5.4 Conclusions. Range (m) Fig. 5.10. Variation of perceived noise level with range for various species. The noise divided into three bands of frequency for all the results. Within the lowest frequency band, from 1 Hz to about 40 Hz, the noise measured was dominated by pseudonoise, or pressure changes caused by the passage of waves over the hydrophone position, with no significant component of noise from the drill ship. Within the second frequency band, from 40 Hz to about 600 Hz, there was a significant contribution of noise from the drill ship at ranges of 2 km and less. At the greater ranges of 5, 7.4 and 10 km the noise from the West Navion had fallen to the level of the background noise. The level in the third frequency band, from 600 Hz to 100 khz, showed a significant and consistent decrease in level both with increasing frequency and with range. Noise from the drill ship was therefore generally of much lower level than in Band 2 but was dominating over a greater range, perhaps to 10 km or more. There was some evidence of a sound channel at 100 m and 200 m depth and at a frequency of about 100 200 Hz, in which the noise propagated with relatively low loss. For the ranges and frequencies at which drill ship noise was evident there was perhaps a slightly higher level at 200 m depth than at the other depths. The ship West Navion could be approximated as a broadband noise source in the range 100 Hz to 400 Hz, having a Source Level of about 195 db re 1 µpa and a Transmission Loss modelled as 23 log R, where R is the range in metres. These values are typical of other measurements made by the authors. Document ref: 534R0109 58
6 Pipe laying and other construction noise 6.1 Introduction As part of the development of the Magnus oilfield in the North Sea west of the Shetland Islands two pipelines were laid from the Sullom Voe oil terminal on the Shetland mainland through Yell Sound and into the deep water to the west of the Shetlands. Measurements were made of the underwater noise caused by the various activities that took place during the operation, viz. trenching, pipe laying, rock placing, pile driving, and supplying materiel to an FPSO (Floating Production, Storage and Offloading) vessel. The piling and FPSO measurements are considered in Sections 7 and 9 respectively, while the trenching, pipe laying and rock placing measurements are considered in this section. The work was reported on in Nedwell et al (2002). 6.2 The location of the measurements A large part of the activities measurements were taken in deep water in the open sea to the west of the Shetland Islands (the Shiehallion Field), but the measurements of pipeline laying noise were taken both there and in Yell Sound, which is a navigable deepwater channel which lies in the north of Shetland, between Yell Island and the Shetland mainland. The two locations are indicated in Fig. 6.1, while the Yell Sound area is shown in a little more detail in the inset to that figure and in much more detail in Fig. 6.2. 6.3 Details of the measurements The pipelines were laid by the Solitaire, a purpose-built pipeline laying vessel. The Solitaire is the largest pipelay vessel in the world, having a pipe-carrying capacity of 15,000 tonnes. Pipeline can be laid at up to 8 kms per day in depths of over 1500m. Manoeuvering of the vessel during pipelaying is done by dynamic positioning. The trenching work was done by the Trenchsetter, a deep water trenching vessel. The rock placing was done by the Rollingstone, a dedicated rock placement vessel which can accurately place gravel and/or rock material in a controlled manner up to a water depth of 600m. The measurements were taken at a number of distances from the source being studied, ranging from 200m to 10km, at depths varying between 1m and 200m. The measurements were made using a hydrophone lowered over the side of a small vessel detailed for the purpose. While measurements were in progress this vessel s engines and ancillary machinery were turned off to minimise the possibility of contamination of the measurements by the self-noise of the vessel. The measurement programme was conducted at various times between May and August 2001. The procedure adopted for taking the measurements was for the instrumentation boat to go to the starting point for the capture, and for it to turn off its engine and ancillary machinery, whereupon the hydrophone was lowered over the side of the boat to the required depth and data capture was initiated. 64 seconds of data were captured at a rate of 262ksamples/sec. Once this had been done the hydrophone was raised or lowered as necessary to the next capture depth, and the capture procedure repeated. Once all the data at a given nominal distance from the source had been captured the hydrophone was raised out of the water, and the instrumentation boat was powered up and moved to the next measurement point starting location, where the capture procedure was repeated. The boat s engines and machinery were turned off for the duration of the data capture to minimise the possibility of its noise contaminating the signal being sensed by the hydrophone. Document ref: 534R0109 59
As a result of its not being under power while data was being captured the boat drifted, so the distance to the source was monitored using the boat s radar display. Its location was also Approximate position of the Solitaire when deep water measurements taken Approximate location of deep water background noise measurements Fig. 6.1. The track of the pipeline and the positions of the measurement locations noted from a GPS display. 6.4 Pipe laying noise measurements 6.4.1 Details of data captures 6.4.1.1 Measurements taken in Yell Sound Four measurement runs were made to the north of the Solitaire, at distances between 0.2km and 10km, on the 8 th and 10 th May. Measurements were made at depths between 1m and 50m. Two runs were also made to the west of the Solitaire, also on the 8 th and 10 th May, at distances between 0.2km and 3.7km, at similar depths. Details are given in Table 6.1. Document ref: 534R0109 60
Fig. 6.2. Yell Sound, showing the track of the pipeline Document ref: 534R0109 61
Table 6.1. Details of measurement runs during pipeline laying in Yell Sound. Nominal distance Sound source Run between source and Hydrophone depths i.d. recording station (m) Comments (km) 06 0.5, 1, 2, 5, 7.4, 10 1, 5, 10, 25, 50 at all to the north of Solitaire distances 07 0.2, 0.5, 1, 2, 5, 7.4, 10 1, 5, 10, 25, 50 at all to the north of Solitaire distances 09 0.2, 0.5, 1, 2, 5, 7.4, 10 1, 5, 10, 25, 50 at all to the north of Solitaire Solitaire distances 10 0.2, 0.5, 1, 2, 5, 7.4, 10 1, 5, 10, 25, 50 at all to the north of Solitaire distances 08 0.2, 0.5, 1, 2.4, 3.7 1, 5, 10, 25, 50 at all distances to the west of Solitaire 11 0.2, 0.5, 1, 2.4, 3.7 1, 5, 10, 25, 50 at all distances to the west of Solitaire 6.4.1.2 Pipeline laying in deep water Eight measurement runs were made around the Solitaire, on the 25 th and 26 th June, while it was pipeline laying in deep water approximately 20nm west of the Shetland Islands, as indicated in Fig. 6.1. For some of the runs various supply vessels were alongside or close to the Solitaire offloading pipe to it. Details of the various runs are given in Table 6.2. Sound source Solitaire Table 6.2. Details of measurement runs during pipeline laying in deep water. Nominal distance Run Hydrophone between source Comments i.d. depths (m) and recording station (km) 28 29 30 34 35 32 0.5, 1, 2, 5, 7.4, 10 0.5, 1, 2, 5, 7.4, 10 0.5, 1, 2, 5, 7.4, 10 10, 25, 50, 100 at all distances 10, 25, 50, 100 at all distances 10, 25, 50, 100 at all distances 0.5, 1, 2, 5, 10 10, 25, 50, 100 at all distances 0.5, 1, 2, 5, 7.4, 10 0.5, 1, 2, 5, 7.4, 10 10, 25, 50, 100 at all distances 10, 25, 50, 100 at all distances From port beam of Solitaire. Supply vessel Norman Carrier on port side of Solitaire. From port beam of Solitaire. Supply vessels Norman Carrier on port side, and Highland Star on stbd side, of Solitaire. From port beam of Solitaire. Supply vessels Norman Carrier and Highland Warrior on port side, and Highland Star on stbd side, of Solitaire. From port beam of Solitaire. Supply vessel Stream Truck on stbd side of Solitaire. From port beam of Solitaire. Supply vessel Stream Truck on stbd. side of Solitaire. From port bow of Solitaire. Supply vessels Stream Truck on port side, and Highland Star on stbd. side, of Solitaire. Document ref: 534R0109 62
Table 6.2 (contd.). Details of measurement runs during pipeline laying in deep water. Nominal distance Run Hydrophone between source Comments i.d. depths (m) and recording station (km) Sound source Solitaire 31 33 0.5, 1, 2, 5, 7.4, 10 0.5, 1, 2, 5, 7.4, 10 10, 25, 50, 100 at all distances 10, 25, 50, 100 at all distances From stbd. beam of Solitaire. Supply vessels Norman Carrier on port side, and Highland Star and Highland Warrior on stbd side, of Solitaire. From stbd. bow of Solitaire. Supply vessels Stream Truck on port side, and Highland Star on stbd. side, of Solitaire. 6.4.2 The results 6.4.2.1 Introduction The time histories that were recorded were frequency analysed to obtain spectra using a program written using the National Instruments LabVIEW application. However, early analysis showed that some recordings included some extraneous noise, such as that due to the background noise of the hydrophone cable slapping against the side of the boat from which the recordings had been taken. Accordingly, all the recordings were listened to prior to a full analysis being undertaken, and where there was any doubt as to the quality of the recording it was excluded from further analysis. 6.4.2.1.1 General features of the Solitaire data Fig. 6.3 illustrates results for the noise from the Solitaire in Yell Sound. The first plot of each pair is an isometric plot, showing the Power Spectral Density of the noise in db re 1 µpa 2 /Hz as a function of the frequency in Hz and the range in metres. The second plot of each pair is the same as the first, but is viewed from above, i.e. it is effectively a contour plot of the sound field. Each pair of figures presents the sound field at a particular depth; the depths range from 1m to a maximum of 50m for the measurements taken; the latter depth was chosen because the bottom of Yell Sound was typically at about 60 to 70m. Document ref: 534R0109 63
Fig. 6.3(a) at 1m depth PSD (db re 1µPa 2 /Hz) PSD (db re 1µPa 2 /Hz) Fig. 6.3(b) at 5m depth Document ref: 534R0109 64
Fig. 6.3(c) at 10m depth PSD (db re 1µPa 2 /Hz) PSD (db re 1µPa 2 /Hz) Fig. 6.3(d) at 25m depth Document ref: 534R0109 65
PSD (db re 1µPa 2 /Hz) Fig. 6.3 (e) at 50m depth Fig. 6.3. Sound fields, at 5 depths, to the north of Solitaire, when operating in Yell Sound. [Run 07]. The above results are plotted more conventionally for four depths in Fig. 6.4. Fig. 6.4 (a) at 5m depth Document ref: 534R0109 66
Fig. 6.4 (b) at 10m depth Fig. 6.4 (c) at 25m depth Fig. 6.4 (d) at 50m depth Fig. 6.4. Spectra for measurements to the north of the Solitaire in Yell Sound. [Run 07]. Document ref: 534R0109 67
It may be noted that the data fall into three frequency ranges: Surface noise; 1 Hz 20 Hz. At frequencies below about 20 Hz the noise was found to be generally dominated by surface-generated noise, which probably arose from wavebreak, entrained bubbles, etc. Consequently, the level of noise was dependent on the wind conditions at the time of the measurement, and did not show any consistent variation with range. The level was generally slightly higher for the measurements near the surface than for the measurements at greater depths. Vessel noise; 20 Hz 50 khz. At frequencies of 20 Hz to about 50 khz the noise was generally dominated by vessel noise for measurements in proximity to the vessels. In most cases, for measurements presented in the report taken within about 1 km of the vessels, vessel noise generally dominated in this frequency range. Background noise; 50 khz 100 khz. At frequencies of about 50 khz and above the noise level was relatively constant; this band was dominated by background noise. It was noted, however, that in some instances within both band 2 and band 3 there were lines of constant frequency at higher levels than the surrounding noise; these were thought to be due to sonar and acoustic positioning equipment. 6.4.2.1.2 Conclusions drawn from the measurements taken of the Solitaire in Yell Sound Significant features of the results for the Solitaire in Yell Sound were: 1. The noise level in band 1, at frequencies of 1 20 Hz, was independent of the distance of the measurement from the vessel. Indeed, in some instances the level was actually highest at the greatest range. This was a consequence of the noise being dominated by surfacegenerated noise; the high level in a measurement at 10 km was a consequence of the waves being strongest at the time the measurements were taken. As might also be expected, the effects of surface noise were most evident in the recordings at the shallowest depth, and particularly in the measurement at 1m. For measurements made where there was significant surface noise it could be seen that the surface noise dominated over a somewhat wider frequency range, to perhaps 100 Hz for the measurement taken at 10 km in the shallower measurements. 2. At the greatest ranges measured, from perhaps 5 km to 10 km, the noise level was relatively constant in bands 2 and 3. It was thought that, in this region, the noise was generally dominated by background noise. 3. For all of the measurements of the Solitaire in Yell Sound the vessel noise dominated in bands 2 and 3 for the measurements closest to the vessel. The vessel noise peaked at about 200 Hz, and showed no particular spectral characteristics; audibly the noise was found to be a low frequency rumble or rushing. The noise was thought to be caused by the dynamic positioning thrusters of the vessel, as there was audible evidence of blade pass rate noise. 4. While vessel noise clearly dominated near the vessel, the results indicated a significant degree of variability of propagation of the noise. In one case, such as for Run 08, at a range of about 1 km there were noise hot spots of relatively high level. In other cases, such as some of the results at 50m depth, there was apparently no noise contribution. The changes in propagation were not related to any climatic or geophysical parameters, and hence were probably due to differences in propagation caused by local inhomogeneities in the acoustic properties of the seawater. It was thought most probable that patchiness of temperature caused by turbulent mixing was the reason for this effect. Document ref: 534R0109 68
In summary, the significant conclusion that was drawn from these observations was that the noise propagation from the vessel was dominated by the random variations in propagation caused by local inhomogeneities rather than by geophysical parameters, such as the bathymetric profile. This conclusion was of further significance in that it indicated that it was unlikely that the noise field could be directly estimated by means of an acoustical model, since all acoustical models require an a-priori knowledge of the detailed structure of the sound speed as a function of range and depth. In the case of random fluctuations, this will of course be unknown 3 This conclusion was reinforced by comparison of the results obtained to the north of the Solitaire, that is, roughly along the main axis of Yell Sound, with those which obtained to the west of the Solitaire, that is, across the main axis of the Sound. It was seen that there was no systematic difference between the two sets of results, which reinforced the preceding conclusion. 6.4.2.2 Measurements of pipe laying in deep water 6.4.2.2.1 Features of the Solitaire measurements Fig. 6.5 shows typical results for the case of the Solitaire operating in deep water to the west of the Shetland Islands. The measurements show the power spectral density of the noise as a function of range, for depths of 10, 25, 50, 100 and 200 metres. PSD (db re 1µPa 2 /Hz) Fig. 6.5 (a) at 10m depth 3 It may be noted, however, that some work has been undertaken on estimating the variability of Transmission Loss directly from the statistics of the variability of the sound speed; this has been reported on, for instance, in Nedwell, J R, Needham, K and Ward, P D. Holographic estimation of propagation in surface scattering ducts. Subacoustech report reference: 223R0310, February 1999. Document ref: 534R0109 69
Fig. 6.5 (b) at 25m depth PSD (db re 1µPa 2 /Hz) PSD (db re 1µPa 2 /Hz) Fig. 6.5 (c) at 50m depth Document ref: 534R0109 70
PSD (db re 1µPa 2 /Hz) Fig. 6.5 (d) at 100m depth Note: Norman Carrier alongside port side of Solitaire. Fig. 6.5. Sound fields, at 4 depths, to the port beam of Solitaire, when operating in deep water west of the Shetland Islands. [Run 28]. The above results are plotted below more conventionally in Fig. 6.6. Fig. 6.6(a) at 10m depth Document ref: 534R0109 71
Fig. 6.6 (b) at 25m depth Fig. 6.6 (c) at 50m depth Fig. 6.6 (d) at 100m depth Fig. 6.6. Spectra for measurements around the Solitaire in deep water. [Run 28]. The measurements divided into similar frequency bands to the Yell Sound results. For all of the results the level of the surface-generated noise in band 1, from 1 Hz to about 20 Hz, was Document ref: 534R0109 72
as expected, independent of the distance from the vessel and dependent only on the local surface conditions at the time of the measurement. Near to the vessel it was found that the level in bands 2 and 3 increased, indicating a contribution from vessel noise. The noise levels recorded were in general slightly higher than those measured in Yell Sound, typically by about 10dB at the closest ranges. It was also found that the propagation of noise was more efficient than in Yell Sound, with a relatively high level of vessel noise even at the greatest ranges at which measurements were made, of 10km. A variation of propagation with depth was also noted. In one case, for a depth of 25 metres, there was evidence of a sound channel. There was very low attenuation, with the noise from the Solitaire propagating with little change in level and dominating over background noise at the greatest distance measured of 10km. Several of the measurements of the Solitaire were made with other vessels in the vicinity. In one instan the Norman Carrier and Highland Warrior were alongside the port side of the Solitaire, and the Highland Star was alongside the starboard side. There was some indication that the level of noise was increased by the presence of these vessels, but in the absence of measurements of the noise from each vessel individually it was not possible to quantify these effects. 6.5 Trenching noise Fig. 6.7 shows a typical result of measurements of the noise from the Trenchsetter, a deep water trenching vessel which supports the activities of the Solitaire and other similar vessels. The Trenchsetter is 130m long, is equipped with three lifting cranes or frames, and utilizes a dynamic positioning system with two drives. In general, the noise levels were comparable in level with the levels of noise generated by the Solitaire, and the noise from the vessel in band 2 and the lower end of band 3 dominated over the entire range of distances measured. PSD (db re 1µPa 2 /Hz) Fig. 6.7(a) at 25m depth Document ref: 534R0109 73
Fig. 6.7 (b) at 50m depth PSD (db re 1µPa 2 /Hz) PSD (db re 1µPa 2 /Hz) Fig. 6.7 (c) at 100m depth Document ref: 534R0109 74
PSD (db re 1µPa 2 /Hz) Fig. 6.7 (d) at 200m depth Fig. 6.7. Sound fields, at 4 depths, in port bow quarter of Trenchsetter, while trenching in the Schiehallion field. [Run 19]. It was considered noteworthy that there was significant tonal noise, with two distinct peaks present at about 20 khz. It was clear that the Trenchsetter was the source of this noise, since it decayed steadily with distance from the vessel. It was thought likely that the source was the acoustic positioning equipment used by the vessel. 6.6 Rock placing noise 6.6.1 Introduction The Fall Pipe Vessel Rollingstone is a dedicated rock placement vessel which can accurately place gravel and/or rock material in a controlled manner up to a water depth of 600m. The vessel is equipped with a fall pipe, which is deployed through a moonpool in the centre part of the vessel, and which guides the material down to the sea bottom. A ROV monitors the bottom of the fall pipe. The fall pipe consists of steel/hdpe pipe sections, allowing the length of the pipe to be adapted to the water depth. On site the fall pipe is deployed and a feeder controls the rate at which material is unloaded onto the central conveyor belt, which transports the material into the fall pipe. The vessel is positioned by DGPS and/or radio positioning, which are used as input for the dynamic positioning system of the vessel, which controls two main variable pitch propellers and rudders, two bow thrusters and two azimuth thrusters. 6.6.2 Measurements in Yell Sound Fig. 6.8 presents a typical result for the Rollingstone in Yell Sound.. Document ref: 534R0109 75
Fig. 6.8(a) at 1m depth PSD (db re 1µPa 2 /Hz) PSD (db re 1µPa 2 /Hz) Fig. 6.8 (b) at 5m depth Document ref: 534R0109 76
Fig. 6.8 (c) at 10m depth PSD (db re 1µPa 2 /Hz) PSD (db re 1µPa 2 /Hz) Fig. 6.8 (d) at 25m depth Fig. 6.8. Sound fields, at 5 depths, to the north of Rollingstone, in Yell Sound, while rock placing was taking place. [Run 12]. Features similar to those noted in the Solitaire set of measurements may be seen. In band 1, from 1 to 20 Hz, the noise was, as previously, dominated by surface-generated noise. Vessel noise dominated for the measurements made at closer ranges in bands 2 and 3. The vessel noise was, however, possibly more prominent than in the case of the Solitaire, and appeared to dominate to somewhat greater ranges, in some cases to the maximum range measured of 10 km. Document ref: 534R0109 77
Fig. 6.9 illustrates similar measurements to those in Fig. 6.8, but while rock placement was not taking place. Within the variability of the measurements there was no evidence that the rock placement was contributing to the noise level. Fig. 6.9(a) at 1m depth PSD (db re 1µPa 2 /Hz) PSD (db re 1µPa 2 /Hz) Note: Rock not being placed during this run. Fig. 6.9(b) at 5m depth Document ref: 534R0109 78
Fig. 6.9(c) at 10m depth PSD (db re 1µPa 2 /Hz) PSD (db re 1µPa 2 /Hz) Note: Rock not being placed during this run. Fig. 6.9(d) at 25m depth Document ref: 534R0109 79
PSD (db re 1µPa 2 /Hz) Note: Rock not being placed during this run. Fig. 6.9(e) at 50m depth Fig. 6.9. Sound fields, at 5 depths, to the north of Rollingstone, in Yell Sound, while not placing rock. [Run 15]. A noticeable feature of these results, which was not present in the results of the Solitaire, was that in some of the measurements there was a significant tonal component to the sound; this could be seen clearly for instance in Fig. 6.9, with strong tones at about 28 Hz, 42 Hz, 52 Hz and 70 Hz. The tonals were evident in both the measurements taken during rock placement and in those taken when no rock placement was taking place, and consequently were not directly associated with rock placement. The records of ship activity were not adequate to ascertain what the cause of the tonals was, but it would seem likely that rotating machinery was the source. Document ref: 534R0109 80
7 Piling 7.1 Introduction Subacoustech has taken measurements of noise due to piling operations at three sites, viz.: i. in the North Sea, in the Schiehallion field; ii. iii. in the River Arun at Littlehampton; and adjacent to a quayside at Town Quay in Southampton. 7.2 Measurements taken in the North Sea 7.2.1 Pile driving in the Schiehallion field Four measurement runs were made on the 6 th August to obtain data while pile driving was taking place for the locating of a manifold on the seabed at the position of the Schiehallion. Fig. 6.1 showed the location of the work. Only a limited amount of data was captured, as the duration of the piling operation was too short to permit the instrumentation boat to go to all the distances and depths desired. Details of the data obtained are given in Table 7.1. Sound source pile driver Table 7.1. Details of measurement runs during pile driving in deep water. Nominal distance between Run Hydrophone depths source and i.d. (m) recording station Comments (km) 36 0.5, 1 10, 25, 50, 100, 150 at both distances 37 0.5, 1 10, 25, 50, 100, 150 at both distances 38 2, 5 10, 25, 50, 100, 150 at both distances Airgun noise detectable. 39 5, 7.4 10, 25, 50, 100, 150 at both distances Airgun noise detectable. 7.2.2 Results locations The locations at which useful data was obtained is given in Table 7.2. Run i.d. 36 37 Table 7.2. Locations at which results obtained from measurements. nominal distance (km) depth 0.5 1 2 5 (m) actual distance (km) 7.4 10 10 0.426 1.000 N.A. N.A. N.A. N.A. 25 0.417 0.949 N.A. N.A. N.A. N.A. 50 0.407 0.898 N.A. N.A. N.A. N.A. 100 0.398 0.847 N.A. N.A. N.A. N.A. 150 0.389 0.796 N.A. N.A. N.A. N.A. 10 0.593 0.889 N.A. N.A. N.A. N.A. 25 0.532 0.829 N.A. N.A. N.A. N.A. 50 0.472 0.769 N.A. N.A. N.A. N.A. 100 0.412 0.708 N.A. N.A. N.A. N.A. 150 0.352 0.648 N.A. N.A. N.A. N.A. Document ref: 534R0109 81
Table 7.2 (contd.). Locations at which results obtained from measurements. nominal distance (km) depth 0.5 1 2 5 7.4 10 (m) Run i.d. 38 39 actual distance (km) 10 N.A. N.A. 2.130 5.093 N.A. N.A. 25 N.A. N.A. 2.056 5.056 N.A. N.A. 50 N.A. N.A. 1.982 5.019 N.A. N.A. 100 N.A. N.A. 1.908 4.982 N.A. N.A. 150 N.A. N.A. 1.833 4.945 N.A. N.A. 10 N.A. N.A. N.A. 5.167 7.408 N.A. 25 N.A. N.A. N.A. 5.139 7.385 N.A. 50 N.A. N.A. N.A. 5.112 N.A. N.A. 100 N.A. N.A. N.A. 5.084 N.A. N.A. 150 N.A. N.A. N.A. 5.056 N.A. N.A. 7.2.3 Results for pile driving in deep water Measurements were made of the noise created during piling in water of approximately 180m depth. The location of the operation was approximately 16 km east of the Schiehallion. The work was carried out from the surface vessel Seaway Eagle, which is a supply vessel which is 142m long and has a gross tonnage of 9556t. Fig. 7.1, which presents the results of Run 37, illustrates a typical measurement of pile driving noise, in this case recorded at a distance of 1km from the piling and at a depth of 100m. The figure presents the pressure~time history of the noise in Pa as a function of time in seconds. Run 37; measurement location:- nominal distance of 1km; depth of 100m Fig. 7.1. Typical time history for pile driving operation in the Scheihallion field. Document ref: 534R0109 82
It may be seen that the noise was of high level. It was characterized by short impulsive events as the pile driver struck the pile; the impulses occurred at intervals of about 1½ sec. The peak pressures varied somewhat from impact to impact, from about 200 Pa to about 500 Pa; the average was about 300 Pa, or about 170 db re 1 µpa. Fig. 7.2 illustrates the statistical properties of the peak pressures of the individual impulses as a function of the distance of the measurement from the pile driver. The peak pressures have been expressed as peak to-peak values. In addition to the average value of the peak-to-peak pressure, error bars giving two standard deviations of the peak pressure are given. The figure shows the results obtained at five depths. 7.2.4 Discussion of results It may be seen that the peak-to-peak pressures of the impulses were, within the variability of the results, independent of the depth of the measurement. The peak pressure was roughly proportional to (range) -1.23, corresponding to a Transmission Loss of about 24 log (R), where R is the range in metres. This was rather more rapid attenuation than is given by spherical spreading of the noise, which leads to a Transmission Loss of 20 log (R). The corresponding effective Source Level of the pile driving was about 246 db re 1 µpa @ 1 metre. This level was high, and compares with levels that are encountered during, for instance, underwater blasting. The authors remarked that they had recently taken measurements of noise from pile driving during a blasting operation using large charges of explosive in bedrock underwater, and the pile driving and the explosions had been found to be similar in level. It was remarked that it was not generally recognised that pile driving generates high levels of noise, but it was the only non-explosive source of noise that had, in the authors experience, caused fish kill (although it was pointed out that this was only for fish within a few metres of the piling). d 10m 10000 pk-pk pressure (Pa) 1000 100 Run 36 (0.5) Run 36 (1) Run 37 (0.5) Run 37 (1) Run 38 (2) Run 38 (5) Run 39 (5) Run 39 (7.4) 10 0.1 1 10 distance (km) Fig. 7.2(a) at 10m depth Document ref: 534R0109 83
d 25m 10000 pk-pk pressure (Pa) 1000 100 Run 36 (0.5) Run 36 (1) Run 37 (0.5) Run 37 (1) Run 38 (2) Run 38 (5) Run 39 (5) Run 39 (7.4) 10 0.1 1 10 distance (km) Fig. 7.2(b) at 25m depth d 50m 10000 pk-pk pressure (Pa) 1000 100 Run 36 (0.5) Run 36 (1) Run 37 (0.5) Run 37 (1) Run 38 (2) Run 38 (5) Run 39 (5) 10 0.1 1 10 distance (km) Fig. 7.2(c) at 50m depth Document ref: 534R0109 84
d 100m 10000 pk-pk pressure (Pa) 1000 100 Run 36 (0.5) Run 36 (1) Run 37 (0.5) Run 37 (1) Run 38 (2) Run 38 (5) Run 39 (5) 10 0.1 1 10 distance (km) Fig. 7.2(d) at 100m depth d 150m 10000 pk-pk pressure (Pa) 1000 100 Run 36 (0.5) Run 36 (1) Run 37 (0.5) Run 37 (1) Run 38 (2) Run 38 (5) Run 39 (5) 10 0.1 1 10 distance (km) Fig. 7.2(e) at 150m depth Fig. 7.2. Variation, at 5 depths, of pk-pk pressures with distance for pile driving operation in the Schiehallion field. Document ref: 534R0109 85
7.3 Measurements taken at Littlehampton 7.3.1 Outline of work In April and May 2002 some measurements were taken of noise generated during pile driving operations along the margin of the River Arun as part of a project undertaken by David Wilson Homes Ltd. to construct dwellings at County Wharf, Littlehampton. The results were reported in Nedwell and Edwards (2002a). The two sorts of piling undertaken on the site were impact piling and vibropiling. County Wharf is located near the mouth of the River Arun, on the eastern bank of the river and on the western edge of Littlehampton. The location of the site is illustrated in Fig 7.3, which is a sketch map of Littlehampton in the region where the piling operation was carried out. In total 41 piles were driven at the site. Fig. 7.4 is a sketch giving the location of the individual piles on the site. Fig. 7.3. Sketch map showing location of pile driving site in Littlehampton Document ref: 534R0109 86
Fig. 7.4. Sketch showing general layout of piles, and measurement locations for particular driving operations. Document ref: 534R0109 87
Sound measurements were initially taken on the County Wharf site, adjacent to the pile driving. The levels recorded were well above background, so a second position on a pontoon at the Yacht Club, on the opposite bank of the river directly facing the site, was added. The pontoon was at a distance of about 80m from the position of the piling. The levels were also found to be high at this position, so a third set of measurements was taken at a position on a footbridge over the River Arun, about 650m upstream from the position of the piling. The position of each measurement and the pile which was being driven when the measurements were taken are indicated in Fig. 7.4, with PX denoting the pile and MX the associated measurement location. When working on the County Wharf site the hydrophone was deployed by being thrown out into the river on a heaving line; the line had a large anchor weight attached to it with a buoy to locate the hydrophone at a constant distance below the surface. The anchor weight was dragged back after deployment such that the hydrophone was located 2m away from the existing quayside wall (the piles were being placed in front of this wall). At the Yacht Club pontoon the hydrophone was hung in the water from the pontoon; similarly, for the measurements taken from the upstream bridge, the hydrophone was hung from the bridge into the water. The distance between the piling and measurement locations was measured using a hand-held GPS receiver and display. Piling was not conducted on a continuous basis at the site because various difficulties were encountered by the driving company. Nevertheless, a total of 13 sets of measurements was taken, each comprising typically 5 minutes of recorded data, and several sets of data directly digitised onto a computer. Table 7.3 lists the details of the various measurements that were made. Measurement i.d. Table 7.3. Details of piles driven and the measurement locations. Identifying number of Distance between pile the pile that was being and measurement driven location (m) Method of driving P1/M1 4 7.5 Impact P2/M2 6 22.5 Impact P3/M3 5 42 Impact P4/M4 6 80 Impact P5/M5 7 80 Impact P6/M6 20 24 Vibro P7/M7 9 16 Vibro P8/M8 11 80 Vibro P9/M9 33 82 Vibro P10/M10 33 82 Impact P11/M11 32 82 Impact P12/M12 31 652 Impact P13/M13 30 652 Impact The hydrophones used were either a Brüel & Kjær Type 8103 or a Brüel & Kjær Type 8105. The hydrophone being used was connected to a Brüel & Kjær Type 2635 Charge Amplifier to condition the signal. For the measurements on the site and on the pontoon the output signal from the charge amplifier was fed simultaneously to a Sony TCD-D8 DAT recorder and to an analogue-to-digital converter card which was inserted in a PCMCIA slot in a Sony laptop computer. The computer was running a data acquisition and analysis program written using the National Instruments LabVIEW application. For most measurements on the site and at the Yacht Club the hydrophone was attached to the rope such that it was 1m below the water surface. In one case, however, the hydrophone had to be suspended only 0.2m below the water surface. Document ref: 534R0109 88
7.3.2 Impact driving A typical pressure~time history, in this instance taken on the County Wharf site, for the impact driver case, is shown in Fig. 7.5. Part of the time history obtained from the footbridge is shown in Fig. 7.6(a). The impacts could be distinctly heard on the recording, but are buried in the general noise at this location. The tide was going out when the recording was made, and was running quite strongly. The signal has been high-pass filtered at 200Hz, and the resulting signal is shown in Fig. 7.6(b). The parts due to the impacts can be identified in this signal, as can be seen in Fig. 7.6(c). Fig. 7.5. A typical pressure~time history for the impact driver. Measurement taken on County Wharf site. Fig. 7.6(a). A typical pressure~time history for the impact river, taken at the footbridge. Unfiltered signal. Document ref: 534R0109 89
A review of measurements of man-made noise carried out by Subacoustech Ltd Fig. 7.6(b). A typical pressure~time history for the impact driver, taken at the footbridge. Signal has been high-pass filtered at 200Hz. Fig. 7.6(c). A typical pressure~time history for the impact driver, taken at the footbridge. Detail of part of Fig. 7.6(b), to show section due to impact driving. The data captured were displayed in the LabVIEW program and the peak pressures, both positive and negative, determined by inspection of the graph. For each location the average values so determined were expressed as peak levels, defined as peak level = 20 log average peak pressure 6 1 10 db re 1µ Pa where the peak pressure is in Pa. Document ref: 534R0109 90
The peak levels are plotted in Fig. 7.7. It may be seen that three of the measurements, made on the County Wharf site, were well below the general trend. It was thought possible that this was a result of the shielding of the measurements by existing piles on the site. Impact driving, pk pressures 200 190 pk level (db re 1microPa) 180 170 160 Wilson Homes site Arun Y C Bridge 150 140 0 100 200 300 400 500 600 700 800 900 1000 distance (m) Fig. 7.7. Variation of peak pressure level with distance for impact driving. An attempt to generalise the results in the usual formulation of SPL = SL N g log(r), where N g is a factor to allow for geometric losses, showed that the measurements did not fit well to a model of that sort. It appeared that the losses were mainly due to absorption; consequently a reasonable fit was given by SPL = SL N a (R) where the Source Level was about 192 db re 1 µpa, and the Transmission Loss rate N a was about 0.07 db per metre. 7.3.3 Vibrodriving A typical pressure~time history obtained on the County Wharf site for the vibro driven pile case is shown in Fig. 7.8(a), and it can be seen that it had a triangular appearance to it. A time history obtained from the yacht club pontoon is shown in Fig. 7.9(a), and in this case the signal appeared random since the low frequencies were masked by higher frequency background noise. Document ref: 534R0109 91
Fig. 7.8(a). Pressure~time history for vibro driving. Fig. 7.8(b). Sound spectrum for the above case. Top frequency of plot 100kHz. Document ref: 534R0109 92
Fig. 7.8(c). Sound spectrum for the above case. Top frequency of plot 10kHz. Fig. 7.8(d). Sound spectrum for the above case. Top frequency of plot 500Hz. Fig. 7.8. Results for measurement at County Wharf site. For the vibro drive cases sections of the captured time histories were frequency analysed using the LabVIEW program to obtain the spectra of the sound signal. Figs. 7.8(b), (c) and (d), which respectively have top frequencies of 100kHz, 10kHz and 500 Hz, are the spectra obtained from the data which was used in part to give Fig. 7.8(a). From Fig. 7.8(d) it can be seen that the dominant frequency, that at which the driving was taking place, was about 27 Hz. Figs. 7.9(b), (c) and (d) are the spectra obtained from the data which is presented in Document ref: 534R0109 93
Fig. 7.9(a). Again there was a strong component at around 27Hz, but the levels were generally higher in the mid frequencies. Fig. 7.9(a). Pressure~time history for vibro driving. Fig. 7.9(b). Sound spectrum for the above case. Top frequency of plot 100kHz. Document ref: 534R0109 94