Danish sustainable offshore
decommissioning
Decommissioning of an oil rig in the Ekofisk oil field
A risk assessment
March 2013
orth Sea
Danish sustainable offshore
decommissioning
Decommissioning of an oil rig in the Ekofisk oil field
A risk assessment
March 2013
Prepared for Offshore Center Denmark
Offshore Center Danmark
Danish Sustainable Offshore
Decommissioning
Decommissioning of an oil rig in the Ekofisk oil field
A risk assessment
March 2013
Authors:
Frank Thomsen, DHI
Henriette Bitsch Schack, DHI
Publisher
Offshore Center Danmark
Dokvej 3
6700 Esbjerg
Tel. + 45 36 97 36 70
www.offshorecenter.dk
This is a published document where copyright rests
with the Offshore Center Denmark and consortium
members. All rights reserved.
Information contained in this document are owned
by the aforementioned parties and supplied without
liability for errors or omissions.
No part may be reproduced or used except as permitted by contract or other written permission.
Contents
1 Introduction...................................................................................................................................................1 2 Introductionintounderwatersound.....................................................................................................1 3 3.1 3.2 3.3 3.4 Potentialnoiselevelsfromdecommissioningandestimatedbackgroundnoiselevels.......2 Explosives/cuttingusingexplosives...........................................................................................................................2 Drilling......................................................................................................................................................................................3 Shipping...............................................................................................................................................................................4 Backgroundnoise................................................................................................................................................................5 4 4.1 4.2 4.3 5 Underwaterhearinginharbourporpoises,whitebeakeddolphins,killerwhales,
Northernminkewhales,AtlanticherringandAtlanticcod............................................................6 Underwaterhearingandsoundproductioninharbourporpoises,whitebeakeddolphinsand
killerwhales...........................................................................................................................................................................6 Hearingandsoundproductioninnorthernminkewhales...............................................................................9 HearingandsoundproductioninAtlanticherringandAtlanticcod............................................................9 5.1 5.2 5.3 5.4 6 Estimationofpotentialeffectsofdecommissioningnoiseandtheresultingimpactzones
foronharbourporpoise,whitebeakeddolphin,killerwhale,minkewhale,Atlantic
herringandAtlanticcod..........................................................................................................................11 Definitionsofzonesofimpact.....................................................................................................................................11 Impactzonesforexplosives.........................................................................................................................................14 Impactzonesfordrilling................................................................................................................................................14 Impactzonesforshipping.............................................................................................................................................15 7 OverallConclusion.....................................................................................................................................19 8 References....................................................................................................................................................20 Occurrenceofharbourporpoise,whitebeakeddolphin,killerwhale,minkewhale,
AtlanticherringandAtlanticcodinthecentralNorthSea...........................................................15 Tabels
Table5.1 Table5.2 Table5.3 CriteriausedforcalculatingthezonesofimpactforPTS/TTS,avoidancebehaviourandmaskinginthesix
marineanimalsinvestigated:Harbourporpoise,white‐beakeddolphin,killerwhale,northernminkewhale,
Atlanticherring,andAtlanticcod..................................................................................................................................................13 Zonesofimpacttodrillingnoise,definingzonesofPTS/TTS,avoidancebehaviourandmaskinginthe
harbourporpoise,white‐beakeddolphin,killerwhale,northernminkewhale,Atlanticherring,andAtlantic
cod.ThedrillingnoiseusedisfromRichardsonetal.1995...............................................................................................14 Zonesofimpacttoshippingnoise,definingzonesofPTS/TTS,avoidancebehaviourandmaskinginthe
harbourporpoise,white‐beakeddolphin,killerwhale,northernminkewhale,Atlanticherring,andAtlantic
cod.TheshippingnoiseusedinfromArvensonandVendittis2000.............................................................................15 Rambøll_Decommissioning_Report_31-08-12
i
Figures
Figure3.1 Peaksoundpressurelevelsat1mandpeakfrequencyofexplosionsof0.5,2and20kgTNT.Modifiedfrom
Richardsonetal.1995...........................................................................................................................................................................3 Figure3.2 Sourcelevelsfromtwodifferentdrillingshipsin1/3octavebands.ModifiedfromRichardsonetal.1995..4 Figure3.3 Noisefromfourdifferentvesseltypesin1/3octavebands.Tug/bargeandsupplyshipnoiseismodified
fromRichardsonetal.1995.MerchantshipnoiseismodifiedfromArvesonandVendittis2000,and
containershipismodifiedfromMcKennaetal.2012.Recordingdistanceback‐calculatedto1m......................5 Figure3.4Backgroundnoisein1/3octavebandlevels.ThedarkbluelineistheWenzcurveforhighshippingnoiseanda
windspeed4‐6m/s,modifiedfromWenz1962.Theorangelineisnoisemeasuredattheportof
Rotterdam,modifiedfromDreschleretal.2009.Thelightbluelineandtheredlinearetheminimumand
maximumvalues,respectively,ofnoisemeasurementsmadeclosetotheEkofiskareausingsonobuoys,
modifiedfrom(Liddell2011).............................................................................................................................................................6 Figure4.1 AudiogramsforharbourporpoisesmodifiedfromKasteleinetal.2002,Andersen1970andPopovetal.
1986..............................................................................................................................................................................................................7 Figure4.2 Audiogramforwhitebeakeddolphin(Lagenorhynchusalbirostris)modifiedfromNachtigalletal.2008...8 Figure4.3 Audiogramoftwokillerwhales(Orcinusorca)modifiedfromSzymanskietal.1999............................................9 Figure4.4 AudiogramforAtlanticherringmodifiedfromEnger1967..............................................................................................10 Figure4.5 AudiogramsforAtlanticcod(Gadusmorhua)modifiedfromBuerkle1967,ChapmanandHawkins1973
andOffutt1974.......................................................................................................................................................................................11 Figure5.1 Detectiondistanceoflowdrill‐shipnoisebyaharbourporpoise...................................................................................14 Figure5.2 Detectiondistanceofshippingnoisefromalargemerchantshipbyawhite‐beakeddolphin............................15 Figure6.1 SightingsofharbourporpoiseduringtheshipboardandaerialsurveysoftheSCANS‐IIsurvey.From
Hammondetal.2006...........................................................................................................................................................................16 Figure6.2 Averageallyeardistributionbasedon26harbourporpoisestaggedinSkagen.FromTeilmannetal.2008.17 Figure6.3 Distributionofkillerwhalesbasedonsightingsmadebetween1970‐2006.FromFooteetal.2007)............18 ii
Rambøll_Decommissioning_Report_31-08-12
1 Introduction
Decommissioningofcurrentlyexistingoffshoreoil‐rigsisexpectedtobecomemorefrequentin
theyearstocome,asoldrigsmustberemovedorreplacedbynewones.IntheEkofiskoilfield
intheNorthSeasuchdecommissioningactivitiesarealsoexpectedinthefuture.Thefollowing
sections present predictions of the sound produced by three different activities (explo‐
sions/cuttingusingexplosives,drillingandshipping)associatedwithdecommissioningofanoil
rig, based on previously published data from such activities. The relationship between these
soundlevelsandtheresponseofthemarinemammalsandfishknowntoinhabitthesewatersis
establishedregardingdetection,masking,behaviouralresponseandhearingloss(temporaryor
permanent)forsomekeyspecies.Theresultsarecomparedtotheknowndistributionofmarine
mammalsintheNorthSea.Anestimateofthenumberofindividualspresumablyexposedtothe
impactsismade.Basedonthesefindingstheoverallestimatedeffectsonmarinemammalsand
fishfromthedecommissioningofanoilrigintheareaisdiscussed.
Four species of cetaceans are regularly observed in the Northern North Sea. Three are odon‐
tocetes:theharbourporpoise(Phocoenaphocoena),thewhitebeakeddolphin(Lagenorhynchus
albirostris),andthekillerwhale(Orcinusorca).Theyareallknownfortheiruseofecholocation.
ThefourthspeciesistheNorthernminkewhale(Balaenopteraacutorostrata),abaleenwhale.
Theareaisalsohometoawidevarietyoffishspecies.Twocommerciallyimportantspeciesof
fish,theAtlanticherring(Clupeaharengus)andtheAtlanticcod(Gadusmorhua),willbeusedas
exampleswhendiscussingtheeffectsofdecommissioningactivitiesonfishinthearea.
2
Introduction into underwater sound
Forabetterunderstandingofthetechnicaltermsusedinthisreport,thissectionwillprovidea
shortintroductionintounderwatersound.
Soundinwaterisatravellingwaveinwhichparticlesofthemediumarealternatelyforcedto‐
getherandthenapart.Thesoundcanbemeasuredasachangeinpressurewithinthemedium,
which acts in all directions, described as the sound pressure. The unit for pressure is Pascal
(Newtonpersquaremetre).
Eachsoundwavehasapressurecomponent(inPascals)andaparticlemotioncomponentindi‐
catingthedisplacement(metres),thevelocity(metrespersecond)andtheacceleration(metres
persecond2)ofthemoleculesinthesoundwave.Dependingontheirreceptormechanisms,ma‐
rinelifeissensitivetoeitherpressureorparticlemotionorboth.Thepressurecanbemeasured
withapressuresensitivedevicesuchasahydrophone(anunderwatermicrophone).
Duetothewiderangeofpressuresandintensitiesandtakingthehearingofmarinelifeintoac‐
count,itiscustomarytodescribetheseusingalogarithmicscale.Themostgenerallyusedloga‐
rithmicscalefordescribingsoundisthedecibelscale(dB).
Thesoundpressurelevel(SPL)ofasoundisgivenindecibels(dB)by:
SPL(indB)=20log10(P/P0)
wherePisthemeasuredpressurelevelandP0isthereferencepressure.Thereferencepressure
in underwater acoustics isdefined as 1micropascal (µPa). Asthe dB value isgiven on a loga‐
rithmicscale,doublingthepressureofasoundleadstoa6dBincreaseinsoundpressurelevel.
Rambøll_Decommissioning_Report_31-08-12
1
1
Asthereferencepressureformeasurementsinairis20µPa,andwaterandairdifferacoustical‐
ly,thedBlevelsforsoundinwaterandinaircannotbecompareddirectly.
Mostterrestrialanimals,aswellasthemarinemammalsdiscussedinthefollowing,aresensi‐
tivetosoundpressure.Fishandmanyinvertebratesontheotherhandarealsosensitivetothe
localparticlemotionofthesoundfield.
3
Potentialnoiselevelsfromdecommissioningandestimated
backgroundnoiselevels
Accordingtotheinformationsuppliedbytheclient,thereareavarietyofactivitiesinvolvedin
thedecommissioningprocess.Fromtheinitialdescription,thelikelynoisegeneratingactivities
whichwillhavetobeassessedareshipping(toandfromthesiteandduringthedecommission‐
ingworks),cuttinganddrilling.However,thereisnoinformationoncuttingsound.Thefollow‐
ing sections will therefore concentrate on noise from three types of activities associated with
decommissioning1)explosions,whichalsoincludesexplosivesusedforcutting,2)shippingac‐
tivitiesofvesselsofdifferentsizes,and3)drillingfromdrill‐shipsandjack‐upplatforms.
Wethinkthattheinclusionofexplosivescanbeofbenefittotheclientevenifitisnotexplicitly
plannedtouseit,asthismethodofdecommissioningisinuseinotherareas(seeOSPAR2009
forareview).
3.1
Explosives/cutting using explosives
The frequency content of explosions is characterized by being broad band. The signals are of
shortdurationandhaveaveryshortrisetime.Closetothesourcetheshockwavecancausetis‐
suedamage.Theveryshortrisetimecausestheformationofairbubblesandinthetissue,this
tears the cells apart (Elsayed 1997). Explosions from TNT have higher peak frequency the
smallerthechargeis,andthesourcelevelisdirectlyrelatedtothesizeofthechargeused(Rich‐
ardsonetal.1995).Cuttingtechniquesusingshockwavefocusingreducethechargesizeneces‐
sary,thereby reducingtheresulting sound pressure(Continental Shelf Associates, Inc. 2004).
Thepeaksourcelevelsandmajorfrequencycontentfromthreedifferentchargesizes(Richard‐
sonetal.1995)areshowninfigure3.1.
2
2
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280
20 kg TNT
2 kg TNT
0.5 kg TNT
278
dB re 1 µPa peak
276
274
272
270
268
266 0
10
Figure 3.1
1
10
Frequency(Hz)
10
2
Peak sound pressure levels at 1 m and peak frequency of explosions of 0.5, 2 and 20 kg TNT. Modified from
Richardson et al. 1995
These source level pressure measurements also include the pressure from the initial shock
wave.Closetothesourcethispressuredropsoffrapidlyasmuchoftheinitialenergyislostdue
to heat dissipation, andatsmaller depths, energy isalso lost by pressure releaseatthe water
surface(Richardsonetal.1995).
3.2
Drilling
Noisefromdrillingoperationsdependslargelyontheplatformusedfordrilling.Drill‐shipspro‐
ducethehighestnoiselevels,whereasnoisefrombottomfoundeddrillingrigssuchasjack‐up
rigsislikelylowinbothsourcelevelsandfrequencycontent(<1.2kHz;Richardsonetal.1995).
Noisefromtwodrill‐shipsisshowninFigure3.2,andisusedinthefollowingastheworstcase
scenariofordrillingnoise,asthenoisemostlikelywillnotexceedtheselevels.
Rambøll_Decommissioning_Report_31-08-12
3
3
180
175
dB re 1 µPa
170
165
160
155
150
145 1
10
Figure 3.2
10
2
Frequency (Hz)
10
3
10
4
Source levels from two different drilling ships in 1/3 octave bands. Modified from Richardson et al. 1995
Thespectralenergyofthenoisefromthetwodrilling‐vesselsismainlyfoundbelow1kHz,and
anyeffectsonanimalsmaythereforebemostpronouncedinanimalswithapredominantlylow
frequencyhearing.
3.3
Shipping
Shippingintheworld’soceanshasincreasedsincethemid‐twentiethcentury(NRC2003),and
assuchshippingnoiseisthemostintensivelystudiednoisesourceoftheonesdiscussedhere.In
Figure3.3noisefromfourdifferenttypesofvesselsispresented.Theyrepresentdifferentsize‐
classes,andareshipsexpectedtobeusedinadecommissioningoperation.
4
4
Rambøll_Decommissioning_Report_31-08-12
200
Tug/Barge
Supply ship
Merchant ship
Container ship
dB re 1 µPa 1/3 oct
190
180
170
160
150
140 1
10
Figure 3.3
10
2
3
10
Frequency (Hz)
10
4
10
5
Noise from four different vessel types in 1/3 octave bands. Tug/barge and supply ship noise is modified
from Richardson et al. 1995.Merchant ship noise is modified from Arveson and Vendittis 2000, and
container ship is modified from McKenna et al. 2012.Recording distance back-calculated to 1 m
ThevesselsconsideredinFigure3.3producenoisewithenergycontentprimarilybelow1kHz.
However,itisimportanttonotethatthereisstillconsiderableenergyatfrequenciesalsoabove
1kHz.Cetaceanhearingismoreacuteathigherfrequencies,andthereforethehigh‐frequency
componentsofthevesselnoiseareofparticularinterestwhendiscussingtheeffectonthesean‐
imals. Measurements have shown that the noise generated is not radiated omnidirectionally
fromtheship(ArvesonandVendittis2000).Moresoundenergywillberadiatedfromthestern
than the bow(McKenna etal. 2012). However, in the following omnidirectionalradiation pat‐
ternsareassumedasaconservativemeasure,astheradiationpatternisdifferentfordifferent
vessels.
3.4
Background noise
The assumed background noise in the Ekofisk area is presented in Figure 3.4. The noise from
Wenz(1962)includesnoiseatwindconditionsof4‐6m/s(BeaufortSeaState1)andcoversan
extensivefrequencyrangefromverylowfrequencies(1Hz)dominatedbyseismicbackground
noise, to very high frequencies (500 kHz) dominated by noise from molecular agitation. The
rangefrom10Hzto1kHzisdominatedbyshippingnoise,butastheshippingnoiselevelshave
increasedsince1962,noisemeasurementsmadejustoutsideRotterdamharbour(Dreschleret
al.2009)areusedasamorerealisticlevelforshippingnoise.Thevalidityoftheselevelsissup‐
portedbydatacollectedclosetotheEkofiskareabytheBritishRoyalNavy(Liddell2011).
5
Rambøll_Decommissioning_Report_31-08-12
5
130
Wenz
Drescler
Liddell min
Liddell max
120
110
dB re 1 µPa 1/3 oct
100
90
80
70
60
50
40 0
10
10
1
10
2
3
10
Frequency (Hz)
10
4
10
5
10
6
Figure 3.4
Background noise in 1/3 octave band levels. The dark blue line is the Wenz curve for high shipping noise
and a wind speed 4-6 m/s, modified from Wenz 1962.The orange line is noise measured at the port of Rotterdam,
modified from Dreschler et al. 2009. The light blue line and the red line are the minimum and maximum values,
respectively, of noise measurements made close to the Ekofisk area using sonobuoys, modified from (Liddell 2011)
4
Underwater hearing in harbour porpoises, white beaked
dolphins, killer whales, Northern minke whales, Atlantic
herring and Atlantic cod
Inthissection,thestateofknowledgeabouttheunderwaterhearingcapabilitiesofthefourma‐
rine mammal species and two of the fish species occurring in the Ekofisk area is briefly re‐
viewed.
4.1
Underwater hearing and sound production in harbour porpoises, white
beaked dolphins and killer whales
Asmentionedabovemarinemammalsdetectthepressurecomponentofthesoundfield(Supin
etal.,2001;Finneranetal.,2002).
Harbourporpoiseuseecholocationtonavigateandforage(Verfussetal.,2005,Verfussetal.
2009).Theyemitintenseultrasonicclickswithafrequencycontentcentredaround130kHzand
peaktoppeaksourcesoundpressurelevelsaround200dBre1µPa(Villadsgaardetal.2007).
Harbourporpoisesalsoseemtouseecholocationclicksforcommunication,butatsignificantly
lowersoundpressurelevels(140‐160dBre1µPa;Clausenetal.2010).Hearingisthekeymo‐
dality for harbour porpoises for most aspects of their life, and their hearing capabilities have
beeninvestigatedinseveralstudies(Figure4.1;Andersen1970,Popovetal.1986,Kasteleinet
al.2002).Inadditiontothehearingthresholds,harbourporpoisehearingbecomesincreasingly
directionalwithhigherfrequencies.Thisdirectionalityimprovestheirecholocationcapabilities
bymakingthemlesssusceptibletobackgroundnoiseandclutterechoes(i.e.returningechoes
fromotherobjectsthantheintendedtarget;Kasteleinetal.2005).
6
6
Rambøll_Decommissioning_Report_31-08-12
120
110
100
dB re 1µPa
90
80
70
60
50
40
30 2
10
Figure 4.1
10
3
4
10
Frequency (Hz)
10
5
10
6
Audiograms for harbour porpoises modified from Kastelein et al. 2002, Andersen 1970 and Popov et al.
1986
White‐beaked dolphins also use echolocation to navigate and forage. They emit very broad‐
bandecholocationclicksofveryshortduration,withafrequencycentredaround120kHz.Some
clicksmayhaveabimodalfrequencystructurewithasecondarypeakat250kHz(Rasmussen
andMiller2004).Thepeaktopeaksourcelevelsoftheecholocationclickshavebeenfoundto
beashighas219dBre1µPa(Rasmussenetal.2002).Unlikeporpoisesdolphinsuseavarietyof
different sounds for communication. They use clicks, but also sound with significantly lower
frequencies known as whistles and pulsed calls. Lowering the frequency content causes the
soundtobeemittedmoreomnidirectionally(LammersandAu2003)whichcouldbeadvanta‐
geouswhencommunicatingisagroup.Whitebeakeddolphinwhistleshaveafundamentalfre‐
quencyaround10kHz(RasmussenandMiller2004),butcontainssignificantenergyat>50kHz
(Rasmussenetal.2006).Sourcelevelsofwhistlesarehighlyvariable,dependingonanumberof
factorssuchasbackgroundnoiselevel(Scheifeleetal.2004),buttherangeofthemeasurewhis‐
tlesis118‐167dBre1µParms(Rasmussenetal.2006).Thehearingofthewhitebeakeddol‐
phinhasbeeninvestigatedusingABRbyNachtigalletal.(2008)andtheaudiogramispresented
inFigure4.2.Forthosedolphinswerethehearinghasbeenthoroughlyinvestigated,itisevident
thatlikeporpoisesthehearingbecomesincreasinglydirectionalwithincreasingfrequency(Au
andMoore1984).
Rambøll_Decommissioning_Report_31-08-12
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7
140
130
120
dB re 1µPa
110
100
90
80
70
60
50
40 4
10
Figure 4.2
5
10
Frequency (Hz)
10
6
Audiogram for white beaked dolphin (Lagenorhynchus albirostris) modified from Nachtigall et al. 2008
Killerwhalesarefoundthroughouttheworld’soceans.Theirhearingsensitivityhasbeenin‐
vestigatedbySzymanskietal.(1999;Figure4.3).Theirecholocationsignalsareverybroadband
infrequencyandhaveashortdurationcomparedtoharbourporpoiseclicks,butaslightlylong‐
erdurationthanotherdolphinclicks.Theclicksofkillerwhaleshaveabimodalfrequencystruc‐
ture,buthaveacentrefrequencyaround50kHzwhichisabout anoctavelowerthanwhatis
found in other delphinid species investigated. Source levels of the clicks are usually between
195and210dBre1µPabuthavebeenmeasureashighas224dBre1µPapp(Auetal.2004,
Eskesenetal.2011).Killerwhalesareseparatedintoatleastthreedistinct“ecotypes”basedon
dietarypreference(Hermanetal.2005).ThekillerwhalesintheNorthSeamostlikelyfeedon
herringandotherfishspecies(SimiläandUgarte1993),howevertheareamayoccasionallybe
visitedbymarinemammaleatingindividuals.Fisheatingkillerwhalesuseecholocationtofind
theirprey(Barrett‐Lennardetal.1996),butkillerwhalesinIcelandicandNorwegianwatersal‐
so use a type of low frequency “herding” calls when feeding schools of herring (Simon et al.
2006).Thecallhasapeakfrequencyof683Hz,andasourcelevelsbetween169and192dBre1
µPa(Simonetal.2006).Marinemammaleatingkillerwhalesontheotherhandareveryquiet
andlocatetheirpreybylisteningforsoundsgeneratedbytheprey(Barrett‐Lennardetal.1996;
Deeckeetal.2002).Thisdiscrepancyinforagingbehaviourbetweenfish‐andmammaleating
killer whales creates differences in their susceptibility to masking. Like other dolphins, killer
whalescommunicateusingwhistlesandcalls.Thefrequencyrangeofwhistlesisbetween500
Hzand10kHz(Thomsenetal.2001),butthesourcelevelsofthewhistleareveryvariable.Holt
etal.(2011)measuredsourcelevelsofcallsbetween135and175dBre1µPa,andfoundaposi‐
tiverelationshipbetweensourcelevelandbackgroundnoise,alsoknownastheLombardeffect.
8
Rambøll_Decommissioning_Report_31-08-12
8
120
110
100
dB re 1µPa
90
80
70
60
50
40
30 3
10
Figure 4.3
4
10
Frequency (Hz)
10
5
Audiogram of two killer whales (Orcinus orca) modified from Szymanski et al. 1999
4.2 Hearing and sound production in northern minke whales
Thehearinginbaleenwhaleshasyettobeinvestigated.However,anatomicalstudiesofthein‐
nerearinthenorthernrightwhale(Eubalaenaglacialis)suggestthatthisspecieshasahearing
rangeof10Hzto22kHz(Parksetal.2007).Thisstudyistheonlystudytodirectlyinferthe
complete hearing range of any baleen whale. However, an anatomical study of the northern
minkewhaleearsuggestesthattheyhaveahearingrangeofbestfrequenciesbetween100Hz
and30kHz(Tubellietal.2012).Baleenwhalesareknowntoproducelowfrequency,high in‐
tensitycallsforcommunication(Širovićetal.2007).Communicationsoundsfromthenorthern
minke whale have been described for apopulation living in thewater offthe Australiancoast
(Gedamkeetal.2001)andhaveafrequencyrangeof50Hzto9.4kHzandbroadbandsource
levelsbetween150to165dBre1µPa.
4.3 Hearing and sound production in Atlantic herring and Atlantic cod
Contrarytomarinemammals,fisharehighlysensitivetothelocalparticlemotionofthesound
field,andcannotnecessarilydetectpressure(Kalmijn,1988;SandandKarlsen,2000).Regard‐
lessofthepresenceofair‐filledstructures,theadequatestimulusforthefishauditorysystemat
frequenciesbelowahundredHzistheparticlemotion(Kalmijn,1988;Karlsenetal.,2004).At
higherfrequenciesapressurewaveimpingingonaswimbladdercausesanincreaseinthepar‐
ticlemotionstimulatingtheinnerearandthesoundpressurebecomesthedominantstimulus.
Fishwithswimbladdersthushaveanincreasedhearingsensitivityathigherfrequencies(Sand
andEnger,1973;FayandPopper,1974).TheAtlanticcodpossessesaswim‐bladder,buthasno
specialcouplingbetweentheswim‐bladderandtheinnerear.IntheAtlanticherringtheswim‐
bladderextendstotheinnerear,wheretheyaredirectlyconnected(Blaxteretal.1981).Parti‐
cle motion measurements of noise sources are very rare and are not available for this report.
Pressuremeasurementsarethereforeusedwhendiscussingtheimpactonfish.
ThehearingofAtlanticherringhasbeeninvestigatedbyEnger(1967),andthehearingofAtlan‐
ticcodbyBuerkle(1967),ChapmanandHawkins(1973)andOffutt(1974).Theaudiogramsof
Rambøll_Decommissioning_Report_31-08-12
9
9
these species are shown in Figure 4.4 and 4.5. The thresholds of the audiograms are given in
unitsofsoundpressure.
Bothherringandcodproducesoundforcommunication,butinverydifferentways.TheAtlantic
herringproducessoundbyreleasingairbubblesfromtheanalduct(WahlbergandWesterberg
2003,Wilsonetal.2004).Thiscreatesapulsedchirpconsistingofaseriesofpulseswithcen‐
troidfrequenciesrangingfrom3to5.1kHzandasourcelevelrangingfrom55to90dBre1µPa
rms(WahlbergandWesterberg2003).Atlanticcodproducessoundbycontractingmusclesas‐
sociatedwiththeswim‐bladder,thusvibratingtheswim‐bladderwalls.Aspartoftheirmating
behaviourAtlanticcodproduces“grunts”.Thesegruntshavefrequencieswithintherangeof50
to120Hz(FinstadandNordeide2004).Thesourcelevelofthesecallsisnotknown.Atlanticcod
has also been documented to produce a click sound associated with anti‐predator behaviour.
Thesesoundshaveapeakfrequencyof5.95kHzandasourcelevelof153dBre1µPa(Heikeet
al.2004).
140
130
dB re 1 µPa
120
110
100
90
80
70 1
10
Figure 4.4
10
10
2
Frequency (Hz)
10
3
10
4
Audiogram for Atlantic herring modified from Enger 1967
Rambøll_Decommissioning_Report_31-08-12
10
140
130
120
dB re 1µPa
110
100
90
80
70
60 1
10
Figure 4.5
5
2
10
Frequency (Hz)
10
3
Audiograms for Atlantic cod (Gadus morhua) modified from Buerkle 1967, Chapman and Hawkins 1973
and Offutt 1974
Estimation of potential effects of decommissioning noise and
the resulting impact zones for on harbour porpoise, white
beaked dolphin, killer whale, minke whale, Atlantic herring
and Atlantic cod
Impacts are assessed based on the predicted sound levels presented in a previous chapter, as
wellaspublishedinformationonthecriteriaforbehaviouralandphysiologicalresponsetoun‐
derwaternoiseinthefourcetaceanspeciesandtwofishspecies.Noisepropagationinthewater
columnisassumedtofollowgeometricalspreadingof15log(r)andfrequencydependentab‐
sorption.Thegeometricalspreadingdependsonwaterdepth,bottomsubstrateandnoisefre‐
quency,andisthereforedifferentfromplacetoplace.However,15log(r)seemstobeagood
approximationinmanysituations(Shapiroetal.2009).Thecriteriaused,arebasedonrelative‐
ly few studies, and were conducted on a very limited number of individuals and species. The
conclusionsshouldthereforebeviewedbearingthisinmind.
5.1
Definitions of zones of impact
Ingeneral,theeffectofnoiseonmarinemammalscanbedividedintofourbroadcategoriesthat
largelydependontheindividual’sproximitytothesoundsource:
1)
Detection
2)
Masking
3)
Behaviouralchanges
4)
Physicaldamages
Itisimportanttorealisethatthelimitsofeachzoneofimpactarenotsharp,andthatthereisa
largeoverlapbetweenthedifferentzones.Furthermore,behaviouralchanges,maskingandde‐
tectioncriticallydependonthebackgroundnoiselevelandallimpactsdependontheage,sex
Rambøll_Decommissioning_Report_31-08-12
11
11
andgeneralphysiologicalandbehaviouralstatesoftheanimals(Popovetal.2011;Southallet
al.2007).
Detection
The detection threshold of a noise source can be obtained by comparing the absolute hearing
thresholdobtainedinlow‐noiselaboratoryexperimentswiththe1/3octavenoise(seechapter
2).Ifthebackgroundnoiselevelisabovethehearingthresholdofthespeciesinquestion,the
detectiondistancedependsonthehearingabilitiesundernoisyconditions.Here,detectionofa
sound will be possible approximately out to a distance where the received level matches the
ambientnoiselevelinthe1/3octavefrequencybandinquestion.Thehearingthresholdsforthe
animalsstudiedhereareshowninchapter3.
Masking
Thezoneofmaskingisdefinedbytherangeatwhichsoundlevelsfromthenoisesourcearere‐
ceivedabovethresholdwithinthecriticalbandcentredonthesignal(Frisketal.2003).Inother
words,significantmaskingstartswhenthe1/3octavebandsoundlevelofthemaskingsound
equals the ambient noise in the frequency of the signal. It should be noted, however, that for
noisewithspectralpeaks,thisdefinitionisnotcompletelycorrectasthemaskingpowerofcon‐
tinuouspuretonesissignificantlylowerthanthatofbroadbandnoise(Madsenetal.2006).
Harbourporpoises,whitebeakeddolphinsandkillerwhalesrelyheavilyonacousticsignalsfor
allaspectsofforagingandnavigation.Harbourporpoisesuseacousticsignalsduringe.g.sexual
displays and in communication between the mother and the calf, and white beaked dolphins,
killerwhalesandNorthernminkewhalesuseawidevarietyofacousticsignalsduringsocialbe‐
haviour,sexualdisplaysandforcoordinatinggrouphuntingbehaviour.Maskingofanyofthese
signals may have serious consequences for the overall fitness of the animal. Especially for
Northernminkewhales,whitebeakeddolphinsandkillerwhalesmaskingmaybeaproblem,as
their communication signals have much lower peak frequencies, and mammal eating killer
whalesmaybeparticularlyvulnerabletomaskingofpreysoundsignatures.
Atlantic herring and Atlantic cod may also be vulnerable to masking as their communication
soundsareverylowinfrequency.Forfishhowever,theremaybeanadditionalproblem.Many
fish species migrate over considerable distances and may rely on acoustic cues from the sur‐
roundingenvironment(vanOpzeelandandSlabbekoorn2012).Increasednoisecouldpotential‐
lymakeitmoredifficultforfishtofindvitalareassuchasspawninggrounds,thusnegativelyaf‐
fectlocalpopulations.
Behaviour
Behaviourisinherentlydifficulttoevaluate.Behaviouralchangesrangefromverystrongreac‐
tions,suchaspanicorflight,tomoremoderatereactionswheretheanimalmayorientitselfto‐
wards the sound or move slowly away. However, the animals’ reaction may vary greatly de‐
pending on season, behavioural state, age, sex, as well as the intensity, frequency and time
structureofthesoundcausingbehaviouralchanges.
According to Southall et al. 2007, all recorded exposures exceeding 140 dB re 1 µPa induced
profoundandsustainedavoidanceinwildporpoises.Thisvaluehasrecentlybeencorroborated
instudieslookingatthebehaviouralimpactsofpiledrivingandseismicsurveysinwildandcap‐
tive harbour porpoises (Brandt et al. 2011; Tougaard et al. 2009; Lucke et al. 2009;). For the
studiesinvestigatingwildporpoisesdoubtshavebeenraisedastowhethertheanimalsactually
vacatetheareaorwhethertheyremainintheareabutkeepsilent.AstudybyTougaardetal.
(2012)showsthattheanimalsdoindeedleavethearea.The140dBcriterioncouldthereforebe
usedtoestimatethezoneofavoidance.
Asimilarcriterionisalsoassumedforkillerwhalesasthisissupportedbyavoidancebehaviour
inkillerwhalesinresponsetoacousticharassmentdevices(MortonandSymonds2002).There
seemstobesomeinterspeciesvariationregardingwhatsoundpressurelevelswillelicitbehav‐
ioural avoidance (Barry et al. 2012, Palka and Hammond 2001). However, as no studies have
documented avoidance behaviour in the white beaked dolphin, the criterion for behavioural
avoidanceinkillerwhalesisadoptedhere.Northernminkewhaleshavebeenshowntoavoida
12
12
Rambøll_Decommissioning_Report_31-08-12
vesselatsoundpressurelevelsof120dBre1µPa(PalkaandHammond2001).Asimilarcrite‐
rionisproposedbySouthalletal.(2007).
Fish behaviour in response to noise is not well known. Sound pressure levels that may deter
some species, mayattractothers. One study by Thomsenet al. (2012) does, however, demon‐
strate avoidance behaviour in the Atlantic cod, when exposed to play‐back pile‐driving noise.
Levelsthatcausedavoidancebehaviourwerebetween140and161dBre1µPa.ForbothAtlan‐
ticcodandAtlanticherringacriterionof140dBre1µPawillbeusedasaworstcasescenario.
Physicaldamages
Physicaldamagestothehearingapparatusleadtopermanentchangesintheanimals’detection
threshold (permanent threshold shift, PTS). This can be caused by the destruction of sensory
cellsintheinnerear,orbymetabolicexhaustionofsensorycells,supportcellsorevenauditory
nervecells.Hearinglossisusuallyonlytemporary(temporarythresholdshift,TTS)andthean‐
imalwillregainitsoriginaldetectionabilitiesafterarecoveryperiod,butduringprolongedex‐
posures,wheretheearisexposedtoTTSinducingsoundpressurelevelsbeforeithashadtime
torecover,TTSmaybuild,andaTTSof50ormorewilloftenresultinpermanentdamage(Ket‐
ten2012).ForPTSandTTSthesoundintensityisanimportantfactorforthedegreeofhearing
loss,asisthefrequency,theexposureduration,andthelengthoftherecoverytime(Popovetal.
2011).
PTS has not been investigated in any cetaceans due to their conservational status. A study by
Luckeetal.(2009)measuredTTSintheharbourporpoisewhenexposedtoasinglesoundpulse
fromanairgunarray.TheTTSlimitwasatapeaktopeaksoundpressurelevelof200dBre1
µPa(TTS=6dB,recoveryofhearingafter>4h).ATTSof6dBwillhalfthedistanceoverwhich
ananimalcandetectasoundssource.ArecentstudybyPopovetal.(2011)hasinvestigatedTTS
foranotherPhocoenoidspecies,theYangtzefinlessporpoise(Neophocaenaphocaenoidesasiae‐
orientalis). When exposed to prolonged noise (30 min) between 32 and 128 kHz, they found
TTStooccuratsoundpressurelevelsaslowas140dBre1µPaandthatthelowerthefrequen‐
cy,thestrongerwasthenoiseeffect.
TTSinkillerwhalesandwhitebeakeddolphinshasnotbeenmeasured.Inthefollowinglevels
are therefore based on what has been measured in bottlenose dolphins (Tursiops truncatus).
Southalletal.(2007)proposesacriterionof230dBre1µPaforsinglepulsesounds.Forpro‐
longednoiseexposures(30min)Nachtigalletal.(2004)measureda5dBTTSatlevelsof160
dBre1µPa.The160dBcriterionwillbeusedasaworstcasescenario.
Hearing has never been measured in a baleen whale, therefore anymeasures of either PTS or
TTShavenotbeenmeasuredeither.Southalletal.2007proposesaTTScriterionof230dBfor
singlepulseexposuresandinthefollowingthe160dBcriterionforbottlenosedolphinswillbe
adoptedfornorthernminkewhales.
PTS and TTS in fish has been investigated fish anatomically after exposure to pile‐driving
sounds,andacriterionof206dBre1µPahasbeensuggestedasthesoundpressurelevelonset
ofobservablephysicaldamagesintheear(Halvorsenetal.2011).
Criteriaforallthezonesofimpactforthedifferentanimalsareshownintable5.1.
Table 5.1
Criteria used for calculating the zones of impact for PTS/TTS, avoidance behaviour and masking in the six
marine animals investigated: Harbour porpoise, white-beaked dolphin, killer whale, northern minke whale,
Atlantic herring, and Atlantic cod
Impactzonecriteria
Harbourpor‐
White‐
Killer
Northern
poise
beaked
whale
minke
dolphin
(dBre1µPa)
206/‐
206/‐
Behaviour
160‐140
160‐140
range)
5.1kHz
120Hz
(dBre1µPa)
Masking(frequency
Rambøll_Decommissioning_Report_31-08-12
Atlanticcod
herring
PTS/TTS
Atlantic
3‐
50‐
whale
200/140
230/160
230/160
110‐
500Hz‐
10‐50
140
150kHz
140
10kHz
140
kHz
230/‐
120
50Hz‐
9.4kHz
13
13
5.2 Impact zones for explosives
The sound pressure source level for an explosion using a charge size of 0.5kg TNT is roughly
270dBre1µPa(seechapter2;Richardsonetal.1995).Asthesoundpropagationfromexplo‐
sives is very complex, the simplified calculations, assuming frequency dependent absorption
andgeometricalspreadingof15log(r),cannotgiveanaccurateestimateofthezonesofimpact.
However,detectionrangeswillmostlikelybebeyond50km.MarinemammalPTS/TTSdistanc‐
eshavebeenmeasuredbyDosSantosetal.2010toablastofsimilarsoundpressurelevel,and
werebeyond1km.Theyalsomeasuredsoundpressurelevelshigherthanthecriteriausedhere
atdistancesofmorethan2km.Injuryinfishfromblastinghasbeendocumentedtodistancesof
100mfromtheblastsitewithmostfishbeingfoundwithin50m(ContinentalShelfAssociates,
Inc.2004),thisisalsoinlinewithwhatwasobservedbyDosSantosetal.2010.
5.3 Impact zones for drilling
Thedrillingimpactzoneiscalculatedasarangebetweenthehighandlowdrillingnoises(see
chapter2;Richardsonetal.1995).AnexampleisgiveninFigure5.1,whichshowstheharbour
porpoisedetectiondistanceofthelowdrillingnoise.Allcalculatedrangesfordrillingimpactare
collectedinTable5.2.
dB re 1 uPa rms
200
Background
Audiogram
Decom noise
150
100
50
0 0
10
Detection distance (km)
10
10
10
10
10
1
10
2
10
3
10
4
10
5
10
6
2
0
-2
-4
10
1
2
10
Frequency (Hz)
10
3
Figure 5.1
Detection distance of low drill-ship noise by a harbour porpoise
Table 5.2
Zones of impact to drilling noise, defining zones of PTS/TTS, avoidance behaviour and masking in the
harbour porpoise, white-beaked dolphin, killer whale, northern minke whale, Atlantic herring, and Atlantic
cod. The drilling noise used is from Richardson et al. 1995
Impactzone
Atlantic
Atlanticcod
herring
Harbour
White‐
porpoise
beakeddol‐
‐/50‐300m
‐/3‐12m
Killerwhale
Northern
minkewhale
phin
PTS/TTS
Behaviour
Masking
Detection
‐
5–50m/
10–300m
‐
4‐>10km
‐
5–50m/
10–300m
4‐>10km
4‐>10km
50‐300m
50‐300m
4‐>10km
4‐>10km
‐
‐
‐/3‐12m
50–300m
3‐>10km
3‐>10km
‐
2–6km
4‐>10km
4‐>10km
14
14
Rambøll_Decommissioning_Report_31-08-12
5.4
Impact zones for shipping
Theimpactofshippingisverydependentonthetypeofshipemployed.Theimpactzonescalcu‐
lated here are based on the noise from a large merchant ship (see chapter 2; Arvenson and
Vendittis 2000). In Figure 5.2 an example is given of the white‐beaked dolphin detection dis‐
tance of shipping noise. All calculated ranges for shipping noise impact are collected in Table
5.3.
dB re 1 uPa rms
200
Background
Audiogram
Decom noise
150
100
50
0 0
10
Detection distance (km)
10
10
10
10
10
1
10
2
10
3
10
4
10
5
10
6
2
0
-2
-4
10
1
10
2
3
10
Frequency (Hz)
10
4
10
5
Figure 5.2
Detection distance of shipping noise from a large merchant ship by a white-beaked dolphin
Table 5.3
Zones of impact to shipping noise, defining zones of PTS /TTS, avoidance behaviour and masking in the
harbour porpoise, white-beaked dolphin, killer whale, northern minke whale, Atlantic herring, and Atlantic
cod. The shipping noise used in from Arvenson and Vendittis 2000
Impactzone
Atlanticher‐
Atlanticcod
Harbour
White‐
porpoise
beakeddol‐
‐
‐/1km
‐/35m
>10km
>10km
ring
Killerwhale
Northern
minkewhale
phin
PTS/TTS
Behaviour
6
Masking
Detection
‐
50m–1km
2km
4.5‐>10km
50m–1km
>10km
1km
‐
1km
>10km
>10km
‐/35m
1km
>10km
>10km
‐
>10km
>10km
>10km
Occurrence of harbour porpoise, white beaked dolphin,
killer whale, northern minke whale, Atlantic herring and
Atlantic cod in the central North Sea
Harbour porpoises, white‐beaked dolphins, killer whales and Northern minke whales are all
protected species under the European Commission’sHabitat Directive’s Appendix IV. Harbour
porpoises, white‐beaked dolphins and killer whales are also of concern in the ASCOBANS
agreementundertheBonnConvention,withtheharbourporpoiseasthe“flagship”species.
TheharbourporpoiseisaverycommoncetaceaninthecentralNorthSea,andhasbeenthe
subjectofintenseinvestigation.Large‐scalevisualandacousticsurveysofthespecieswerecon‐
Rambøll_Decommissioning_Report_31-08-12
15
15
ductedallthroughEuropeanwatersinthesummersof1994and2005(Figure6.1;Hammondet
al.2002,2006;Teilmannetal.2008).TheestimatedpopulationsizeinthecentralNorthSeais
approx.59,000individuals(Hammondetal.2006).Datacollectedfromsatellite‐taggedindivid‐
ualsintheNorthSeaindicatetheareaaroundandnorthofSkagenouttoadepthofaround200
metersaswellastheinnershelfoftheDanishpartofSkagerrakwithwaterdepthsof10to50
metersasimportanthabitatsforharbourporpoisesareasacrossseasons(Figure6.2;Teilmann
etal.2008).
Figure 6.1
16
Sightings of harbour porpoise during the shipboard and aerial surveys of the SCANS-II survey. From
Hammond et al. 2006
16
Rambøll_Decommissioning_Report_31-08-12
Figure 6.2
Average all year distribution based on 26 harbour porpoises tagged in Skagen. From Teilmann et al. 2008
For white‐beaked dolphins and Northern minke whales the same detailed studies are not
available. However, population sizes of white‐beaked dolphins and Northern minke whales in
theentireNorthSeawereestimatedbyHammondetal.(2006)withapprox.10,500and8,400
individualsrespectively.
Killer whale abundance estimates are not available for the central North Sea, but Figure 6.3
showssightingsofkillerwhalesintheareabetween1970and2006(Footeetal.2007).Killer
whales are mostly found in the central North Sea between September and December, where
theyarethoughttofollowthemigrationoftheNorthSeamackerel(Luqueetal.2006).
17
Rambøll_Decommissioning_Report_31-08-12
17
Figure 6.3
Distribution of killer whales based on sightings made between1970-2006. From Foote et al. 2007)
AtlanticherringandAtlanticcodpopulationestimatesinthecentralNorthSeaarenotavaila‐
ble,butspawninggroundshavebeenidentifiedforbothspeciesandareshowninFigure6.4and
6.5.
Figure 6.4
18
Spawning grounds of autumn- and spring-spawning herring in the North Sea and djacent waters.
Orange denotes autumn spawning herring and yellow denotes spring spawning herring. Circles
denote locations of spring-spawning herring in fjords. From Dickey-Collas et al. 2010
Rambøll_Decommissioning_Report_31-08-12
18
Figure 6.5
7
The distribution of stage I cod eggs from a 2004 ichthyo-plankton survey, indicating cod spawning areas in
the North Sea. The area of the solid circles is proportional to the daily production of cod eggs. Crosses
indicate where a plankton sample was collected but no stage I cod eggs were found. From Fox et al. 2008
Overall Conclusion
Therearefourmainsourcesofnoisegeneratedduringoil‐rigdecommissioning,threeofwhich
have been described above. These are 1) explosives, 2) drilling, and 3) shipping. The fourth
noisesourceiscutting,forwhichnonoiserecordingsareavailable.
In the Ekofisk oil field, where decommissioning will occur, four species of cetaceans are com‐
monlyfound:theharbourporpoise,thewhite‐beakeddolphin,thekillerwhaleandthenorthern
minkewhale.Furthermore,twospeciesoffishcommoninthisareawerechosenasmodelfish
species,duetotheirhighcommercialvalue,theAtlanticherringandtheAtlanticcod.
19
Rambøll_Decommissioning_Report_31-08-12
19
Explosivesareexpectedtohavefarreachingeffectsonallspeciesinvestigated,causingphysical
injuries within a distance of at least 500 meters. For cetaceans, behavioural disturbances are
expectedtooccurseveralkilometresawayfromtheblastingsite.Fortheothernoisesources,ef‐
fects are not expected to be as dramatic. TTS of marine mammals are most likely to occur at
closeranges(<300m)andafterprolongedexposures(>30minutes).Fordrillingusingajack‐up
drillingrig,behaviouralchangesareexpectedtooccuratdistancesatlessthan300mformost
ofthemarinemammalspecies.Thenorthernminkewhalemay,however,showsadversebehav‐
iouratgreaterdistances(~6km).Duetothehighspectralnoiselevelsofthelowerfrequencies
forthenoisesourcesdescribedhere,maskingofcommunicationsignalsisquitepossibleforall
species,excepttheharbourporpoise,withmaskingeffectsatdistancesofmorethan10kmfrom
thesource.Thebigunknowninthisriskassessmentiscutting,andthereforefurtherinvestiga‐
tionsshouldincludetheacquisitionofnoisedetailsfromcuttingactivities.
Decommissioningofasingleoil‐rigcouldpotentiallyaffectalargenumberofanimalsinthecen‐
tralNorthSea,dependingonthedecommissioningmethodutilized.Theuseofexplosivescould
potentiallycausephysicalinjuryinhundredsofmarinemammalsandbehaviouraldisturbances
inseveralthousand,andthenumberoffishaffectedneartheblastsitecouldpotentiallybemil‐
lions.Itwillmostlikelyalsoaffectnearbyspawningsitesforcodthroughbehaviouralchanges
andmasking,ascodusesoundduringspawning.Shippinganddrillingwillnothaveasfarreach‐
ing effects, but the number of marine mammals affected behaviourally may still be relatively
high.Shippingmayalsohaveanegativemaskingeffectatcodspawningsites.
Mitigationeffortsshouldfocusontheimmediatevicinityofthesites,employingmethodsusing
marineobserversandacousticmonitoringofthemarinemammalssounds.Thiscanbedoneus‐
ing click detectors (PODs) which automatically register high frequency orientation sounds
(=clicks).Furthermoreonecanuseacousticdeterrentorharassmentdevices.Theseareinstru‐
mentsthatplaybacksoundsthatareaversivetomarinemammals.Theycanbeusedtokeepce‐
taceansoutoftheareaofinjury.
Dependingonthenumberofrigstobedecommissionedsimultaneously,thecumulativeeffect
maycausesignificantlylargerareastobeaffected,anditcouldresultinprolongeddisplacement
effectsaswell.Theevaluationofthecumulativeeffectswillmostlikelybepartofanyenviron‐
mentalimpactassessmentstobeprepared,oncethenumber,locationandtimingofdecommis‐
sionedrigsaredetermined.
20
20
Rambøll_Decommissioning_Report_31-08-12
8
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