147
1
INTRODUCTION
Mooring area generally refers to the place where a
ship, boat, or aircraft may be moored, but in the
contextofthispapertheexpressionmooringarea(or
nauticalanchoragearea)willbelimitedtoadefined
area within which smaller vessels and boats will be
moored to mooring buoys.
Mooring buoy is a buoy
that floats and is secured with a heavy weight (or
special anchor) to the sea bottom. Mooring area for
smallervesselsandboats,especiallytouristones,are
normallylocatedoutsideportsandharbours,inareas
with heavier traffic i.e., in areas where there is an
increased demand for mooring places, and the
capacitiesofportsandharboursarenotsufficientfor
the reception, or there are not any available at all.
Unlike ordinary anchorages, they require some
investment, although on a much smaller scale than
harbours, wharves, marinas, and other similarly
placesdesignedtoreceiveand
accommodateships.In
comparison to anchorages, they are also much safer
forusersandenvironmentallymoreacceptable.They
are especially popular in tourist attractive areas
outside the cities, if the area is to be given in
concession.The problemsthat ariseare the result of
the very nature of their
usage. Namely concession
holders and other serviceproviders want to achieve
as bigger economic capacity utilization as possible,
anditis,fromthebeginning,incompleteopposition
to the safety and enviro nmental protection factors.
Therefore,acompromiseshouldbereachedregarding
the size and the shape of the mooring area, the
distance from the shore and other installations, the
number of buoys within the area and the distance
betweenthem,theminimaldepth,thetypesofbuoy
anchoring, etc. Another problem is the inconsistent
practice andthe lack of legalregulations.Two main
challengesemerge:whereandhowtochoosean
area
to define it as a mooring/buoys area, and then the
selection of the field, its shape, and the number of
buoyswithin.Thechosenareamustbe economically
justified,oftenevenmotivatedbylocalpolitics,andat
the same time it should take into consideration that
reasonable level of safety
is achieved. Reasonable
safety here means an acceptablelevel of risk,taking
intoaccountthe nature and severity of the potential
damageandthelikelihoodthatthedamagewilloccur
duringthevesselʹsarrival/departureorstayatberth.
Mooring Area and Mooring Buoys Plan
Z.Lušić&D.Pušić
UniversityofSplit,Split,Croatia
ABSTRACT:Developingtourismandgrowingnumberoftouristandothersmallervesselsandpersonalboats
requirealargernumberofberths,outsidededicatedports,andharbours.Vesselscanalwaysanchor,butfor
ecological,andcommercialreasonsmooringareaswithinstalledmooringbuoysarebetterandmore
efficient
solutions.However,choosingmooringareas,theirsize,numberofbuoys,etc.comparedtochoosinganchorage
areas,isamuchmorecomplicatedprocedure,evenmoresofornothavingaunifiedformnorpractice.This
paperanalysessomeofthemainissuesintheselectionofmooringsitesandfields,
focusingonthefactorsthat
should be considered in the planning process. As a result of the carried‐out analysis some basic
recommendationswillbegivenconcerningthechoiceofmooringfieldshape,swingradiuscalculation,distance
fromtheshore,nearbyvessels,andotherobstacles.
http://www.transnav.eu
the International Journal
on Marine Navigation
and Safety of Sea Transportation
Volume 17
Number 1
March 2023
DOI:10.12716/1001.17.01.15
148
Oncetheareaisselected,thefieldselectionandbuoys
anchoringfollow.Thelatterbeingthemainsubjectof
thispaper,withinwhichanoverviewoftheexisting
recommendationsandpracticeswillbegiven,aswell
as corresponding guidelines of how to standardise
and optimisethe selection of mooringarea
size and
shape,includingthelayoutofbuoyswithinthefield.
Aspecialattentionwillbegiventothedetermination
ofswingradius,i.e.theareaaroundthebuoy(anchor)
thatshouldbefreeofallothervessels(orobstacles).
Previous research on this topic is quite limited;
manuals, guides,
recommendations, and real‐world
examples predominate. Regarding the selection of
mooringareas,andanchoragesingeneral,itisworth
highlighting the work [Pušić, Lušić, 2022], guides
dealing with the design of harbours, approach
channels and basins in general [Pianq, 2014], [ROM,
2007],[Technicalstandards,2002.]andtheguidelines
for
mooringsGLNobleDenton[TechnicalStandards,
2016]. Methods for mooring small vessels and
recommendationsforfieldinstallationcanbefoundin
[Mooring buoy Planning Guide, 2005],
[Quartermaster,2013], andanumber ofguidesfrom
individual port authorities [Anchorage area, 2019],
[Moorings, 2015], [Falmouth Haven, 2022] or
generally in guides from
the Internet [McVinney,
2022],[Mooringbasics,2022],[Mooringbasics,2021].
MooringsinSplit‐DalmatiaCounty(SDC)areusedfor
analysisoftheformofmooringfields andexamples
frompractice[Račićetall,2019],[ActionPlan,2013].
2
ANCHORAGELOCATIONSELECTION
According to their purpose and shape, anchorage
locations can be divided into two main types: those
for accommodation of bigger vessels and those for
smallerones,bearinginmindthatnauticalanchorage
areas are a special category for mooring of smaller
boatsontoanchoredbuoyswithinaspecially
defined
area(mooringarea).Anchoragesforbiggervesselsare
usuallylocatedattheentrances,ornearharboursand
otherpopulatedplaces,butalsowherethereisaneed
to wait for any reason. They are generally made in
simpler geometric shapes and follow, more or less,
sameinstallationlogic.On
theotherhand,anchorages
for smaller vessels, or simply nautical anchorages,
including the area of mooring buoys, are
characterized by numerous differences, such as the
choiceoflocation,shapeandsize,capacity,anchoring
techniques,purpose,etc.Regardlessoftheabove,any
anchorage planning should start from general rules
and standards, and
good practice, for both, large or
small vessels anchorages. Some of the general
recommendationsindefininganchorageareasareas
follows:
they must be of a sufficient size to allow
movementfromanyobstacle,
they have to provide as safe as possible shelter
from the influence of external factors, primarily
fromwaves,wind,andcurrents,
thebathymetryshouldberelativelyflatandclear
ofanyobstructions,
thetypeofseabedshouldbeconsidered,
theyshouldnot interferewiththe trafficof other
vessels,
they should be sufficiently distanced from other
installationsandobjects,
the distance between moored vessels should be
suchthattheirswingradiidonotoverlap,etc.
AccordingtoPIANC[PIANC,2014]thedesignof
an anchorage, in general, mainly depends on the
followingfactors:
size, dimensions, and characteristics of the
vessel(s),
typeofoperationsexpectedtobeundertaken,
durationforwhichthevessel(s)willstayatanchor,
siteʹs general configuration and availability of
spaceformanoeuvring,
arrangementasageneralanchorage area orhave
definedanchoragepositions,
number of defined anchoring points to be
providedatthesite,
marine environment in the area and operational
limitingconditions,
siteʹs physical characteristics and, in particular,
depth and shapeof the seabed andthe ability of
theseabedmaterialforanchorholding,
availabilityofpollutioncombatingresources,etc.
According to Anchorage Area Design and
Management Guideline (2019) the key elements in
anchorage(mooringarea)designare:
anchoragelocation(waterdepth,holdingground,
weather, port layout and infrastructure, other
waterway users, vicinity of populous areas,
communicationstoshoresidefacilities,etc),
anchoragesizeandlayout,
anchorageuse,
environmental considerations (environmental
assessment, disturbance to seabed from anchor
drop and chain drag, management of emissions,
pollutants or wastes, aesthetic value, marine pest
Introduction, conservation‐dependent species,
localheritagevalues).
Considering all the above‐mentioned criteria,
designprocessshouldbeclearandeasilycarriedout.
However a larger number of problems
arise in
practice, especially with nautical anchorage areas.
Theseareasareusuallygiven inconcession,andthe
concession holders, together with local authorities,
make economic cost‐effectiveness and profit their
priorities,whichregularlyresultsinbypassingsafety
andenvironmentalfactorsinwhateverwayitcanbe
done.
3
SIZEANDSHAPEOFANCHORAGEAREA
Anchorages,areaswhereshipsdroptheiranchorsand
anchortothebottomtoresistmovement,areusually
located off harbours, channels, popular destinations
where ships wait to enter, or where there are no
developedmoorings,oranywherethereisaneedfor
shelter. Anchorage areas
can beʹʹgeneral anchorage
areasʹʹ orʹʹseries of designated anchorages for
differentsizeshipsʹʹ.Ageneralanchorageareaisused
wheretherearenotsomanyvesselsandwherethere
isenoughspaceforanchorage.Ontheotherhand,a
seriesofdesignatedanchoragesisusedinareaswhere
largernumberofvesselsisexpected,wherethespace
is limited and where there is a need to reduce the
negativeimpactofanchorageontheseabed.
149
A series of designated anchorages will alwaysbe
clearly marked, each individual anchorage site, as
well as the outer border of the whole area. General
anchorages can, but do not have to, have marked
outer borders. If the borders exist, simpler shapes
prevail (circles, square rectangles, parallelograms,
rhombus, etc), for
example see approach to the
Europort [16]. General anchorages without clearly
markedouterborderswillbewhereincomingvessels
arerare,asareserveforotheranchorages,as places
forshelter,andwherethereisageneralpossibilityto
anchor.
ThePIANC[Pianc,2014]providesgoodguidance
onhowto
determinethesizeofindividualanchorage.
Determination is based on the calculations of an
anchorage circle of a determined radius (swing
radius) based on a vessel anchoring roughly at the
centre, an allowance for length of anchor chain
deployed based on predominate water depth, tides,
weatherconditions,lengthofthevessel
andasafety
margin.
When anchorage (mooring) areas are given into
concession,theyarealwaysdefinedbytheirexternal
boundaries, and since they are always located in a
relativelylimitedareawithhighertrafficintensity,the
sizeandshapeselectionofthenauticalanchorage,i.e.,
mooring area, will be more
complex compared to
conventionalanchorages.Iffieldareasareanalysedin
relationtotheavailablespace,thevicinityofthecoast,
theshape,etc.,agreatdiversity,butalsoasignificant
deviation from already given recommendations, can
benoticed(Figure1)[Račićetall,2019].
Figure1. Diversity of anchorage area shapes (E Coast of
Adriatic)
In defining the external boundary of the
anchorage, as well as the external boundary of the
mooringarea,thekeycriterionshouldbethesizeof
the expected vessels and their swing radii, i.e.,
definingeveryindividualanchorageasithasalready
been described for theʹʹseries of designated
anchoragesʹʹ.
But common practice is that external
boundariesofthemooringareasaredefinedwithout
a detailed accommodation plan [Račić et all, 2019].
Outerborderandmaximumspaceofa mooringarea
are derived from general recommendations, and
concessionholders areenabledtochoosethevessels
accommodation plan at their
own discretion,
includingthetotalnumber of buoyswithin the area
and the distance between them. All this results in a
greater diversity of area shapes, and tendency of
concession holders to install as many buoys as
possible to gain profit at the expense of safety.
Namely, it has become a
practice to accommodate
bigger boats than the ones the anchorage had been
designed for (by concession holders` requests), in a
way to circumvent recommendations on minimum
buoy distances. There are a number of examples
where buoy distances are only a few metres larger
than the moored boats, e.g., the East
coast of the
Adriatic (Figure 2) [Danielis Yachting, 2022], the
English coast [Falmouth Haven, 2022], etc. In the
study“NauticalTourismDevelopmentPlanforSplit‐
DalmatiaCounty”[ActionPlan,2013],aproposalfor
nautical anchorages for dozens of bays of SDC was
given,anditisquiteclear,fromtheplan,
thatcriteria
ofsafemutualdistancebetweenmooredboatscannot
bemetaccordingtothenumberofplannedvesselson
thegivenarea.Swingradiiarenotpredetermined,as
mentioned in Section 4 of the paper, and it can be
concluded, from the area and accommodation
capacities ratio, that the
swing radius is
approximately 1 meter larger than the length of the
largestexpectedvessel,providedthattheswingradii
do notoverlap. Some other conceptual solutions are
onthistrack,too[Račićetall,2019].Itisobviousthat
commonpracticeincludesplacingbuoysatadistance
whichis
slightlylargerthanthevessellength,butalso
planningmooringareasinawaythattheswingradii
partlyoverlap.
Figure2.Examplesofanchoring(SDC).[DanielisYachting,
2022],[GoogleEarth]
The intention of concession holders to
accommodate as many vessels as possible is
completelyunderstandable,buttheauthoritieswhich
approve conceptual designs, and the ones that
supervise their implementation should ensure the
applicationofregulationsandprofessionalstandards.
If the area is too small for single buoy mooring,
multiple anchor moorings
technique can be applied
(Figures 3 and 4). This technique allows the
accommodationofalargernumberofvesselsperarea
unit,butitisalsomuchmoredemandingintermsof
installation, manoeuvring, external factors influence
etc.
4
DISTANCEBETWEENBUOYS
When determining the distance between buoys, it is
crucial to determine the type of mooring and then
calculate the corresponding swing radius with the
150
appropriate reserve. Mooringat buoys canbe swing
mooring or multi‐anchor mooring. Swing mooring
(single‐point mooring) is the most common type of
mooring.Inthistypeofmooring,avesselisattached
to a single anchored buoy and swings in a circle
aroundthatanchor.Theswingradius
dependsonthe
length of the boat, theanchor/mooringline, and the
depth of the water. Swing moorings should be
designedsothattheswingcircles(areaofinfluence)
ofeachvesseldonotoverlap.However,insomeareas
wheresmallervesselsofsimilardesignsdominate,it
occurs that the
swing circles overlap to increase the
number of vessels in the available mooring area
[Moorings, 2015]. There are several options for
multiplemoorings,themostpopularbeingʺForeand
Aftʹʹ.Inthismooring,anchorsaresetforeandaftto
fixthepositionofthevessel.Trotmooring(Figure4)
is almost identical to fore and aft mooring; it is set
away from shore and is intended for smaller boats
[Mooring basics, 2021]. Considering that mooring at
one buoy is the simplest and most commonly used
wayofmooring,butalsothebasisofallotherwaysof
mooring, a
somewhat more detailed analysis of
determining the swing radius of the vessel, i.e., the
areawhichissupposedtobefree ofallotherobjects,
willbegivenbellow.
Figure3.ForeandAftmooring
Figure4.Trootmooring
The swing radius of a vessel depends on several
factorsandthemostimportantonesareasfollows:
seadepth,
sealevelchanges,
vessellength(L)
vesselmooringlinelength(Sml)
anchorlinelength(AL)
Sea level has its medium, reference value
(hydrographic zero or chart date), but also its
extremes, i.e., extreme high tide (EHT) and extreme
low tide (ELT). Consequently, when taking into
accounttheseadepth,thedepthatextremehightide
(DEHT) and at extreme low tide (DELT) should be
known.
Length of vessel mooring line primarily
depends on freeboard at the bow i.e., the vertical
distancebetweenthebowrollerandthesurfaceofthe
waterandthechosenoptimalanglefromthemooring
buoy attachment to the stem head. Vessel mooring
line can be approximately definedas 2.5m height
of
freeboard [Mooring basics, 2022] or simply rounded
upat3m[Quartermaster,2013].Anchorlinelengthis
infunctionofdepthandanchoringtype.Inorderto
defineanchorlinelength,itisnecessarytoknowthe
Scope, i.e., ratio of anchor line length and water
depth.Forstandardanchoringthe
scopecanreach5
andmore,butincaseofbuoyanchoringitisusually
between 3 and 1. For example, in case of concrete
anchors, scope isbetween 2.5and 1,and forHelical
andsimilarembeddedanchors(screwpiles)between
1.5 and 1 [Quartermaster, 2013]. These last ones
provide the most secure fastening systemin muddy
bottom sediments and minimize the risk of vessels
dragging or breaking away under adverse weather
conditionsandwillbetakenasreferentvalueintext
further on. For simpler calculations it can be taken
thattheanchorlinelengthforembeddedanchorsis
at
least 3 m more than the sea depth during extreme
hightide[MooringbuoyPlanningGuide,2005].
Figure5a)showsastandardmooringsetup(fora
singleswingingmooring)thatconsistsof2lengthsof
chain,heavygroundchainonthebottom,connected
toalighterchainon
top.Bottomchainlengthshould
be1.5timesthemaximumheightofwater(i.e.,spring
high tide) [Mooringbasics, 2022]. Figure 5 b) shows
modernembeddedhelicalanchorsolution.
Figure5. Standard mooring and embedded helical anchor
withamid‐linefloat
Figure6.Swingradius
Anexampleof howtocalculate swingradius for
vessels of different lengths, for the eastern Adriatic
coast area, will be given hereafter. Sea level on the
eastern part of the Adriatic oscillates within 80 cm
[Tide tables, 2015). However, hydro‐meteorological
conditionsshouldbetakenintoaccount,primarilyair
pressure
and wind, which can additionally increase
sea level, up to 0.8 m in the central Adriatic
[Peljar/Pilot,1999]. Although extremely rarely, levels
over 2 m have been recorded [HHI news, 1999].
Accordingly, in the following calculations, a 1.5 m
highervaluefromthelevelgiveninthenauticalchart
will
betakenforextremehightidelevels,and0.5m
less for extreme low tide. Also, the length of the
anchor line will be increased for about three metres
151
more than the extreme high tide, and three more
metres for the mooring line, too. It is assumed that
modernembeddedanchorsareused.
Thus, for the above conditions, and for smaller
depths, it can be said that the swing radius is
approximatelytwicethelengthoftheship(Table
1).
Accordingly, the distance between the two adjacent
buoys, i.e., anchor pins, should be four times the
lengthofthevessel,ifthevesselsareidentical.Since
most boats intended for nautical anchorages are
between 10 and 20 m, this means that the distance
betweenbuoysshouldbeminimumfrom
40mto80
m.Inpractice,asalreadydescribed,buoys,i.e.,anchor
pinsareroughlyatthesedistances,howevertheboats
that use them are larger than expected and thus,
swingradiiofadjacentboatsregularlyoverlap.Oneof
the recommendations in the manuals says that it is
necessary to
ensureʹʹsufficient swing room between
boats, a minimum distance of 130 feet, between
anchorpinsforboatsupto65‐feetinlength.Insome
areasspacinghasbeenincreasedto200feetbetween
anchor pinsʹʹ [Mooring buoy guide, 2005]. Another
goodexampleisProposedbuoyfieldforDickton,for
scope1‐1,5,whichproposesswingradiiof78ftto91
ftforboatswithlengthof30ft[Quartermaster,2013].
So,forboatswithlengthupto20m,thedistancesare
about40to60m,whichisnotenoughifswingradii
overlappingforthelargestvessels
istobeavoided.
Ifitisassumedthatallthevesselsintheassociated
mooringareaareofthesametypeandsimilarsize,in
case of a single swinging mooring, they should all
behaveinthesameway,i.e.,theeffectoftheexternal
factorsshouldmoreor
lesspointtheminthesameor
approximately the same direction. Moreover, with
changesinthedirectionoftheresultvectorofexternal
factors, all vessels should also rotate symmetrically.
So,intheory,theyallrotateinthesamedirection.In
this case, it could be considered acceptable for the
swing
radii of adjacent vessels to partly overlap
[Moorings, 2015], it is sufficient to ensure that the
adjacent buoy does not enter the swing circle. To
makethishappen,awholerangeofconditionsshould
bemet,amongwhich:
all vessels are approximately the same type and
size,
vesselsaregenerallyof smallersizes,
the mooring site is well sheltered from the
influenceoftheopensea,
currentsareminimal,
sealeveloscillationsareminimal,
shorterstayatmooring,e.g.,dayvisits,overnight
stays,andseasonaluse.
ensured security monitoring of the mooring and
boatsonthemooringsite,
andgenerally,useitexceptionally,whenthereare
nootheroptions.
For fore and aft mooring the distance between
buoyscanbeapproximately3lengthsoftheboat,as
seen per the central line of the boat, while the
transversedirection(clearwidthforoneboat)canbe
twotothree
breadths(Figure7),butnotlessthanthe
breadth of the largest vessel increased by 1 to 2 m
[Technicalstandards,2002].
Figure7. Area for fore and aft mooring [ROM,
2007]
From observing boats at anchorages, as well as
fromsimulations,boatsatsinglepointmooringtend
toalignsimilarlywithinamicrolocationtheyarein.
Undertheinfluenceofcurrent,wind,andwaves,they
roughly align in the same
direction, with extremely
rare instances of exceeding +/‐90° from the resulting
direction of the current and/or wind. Crossing this
boundaryonlyoccursinsituationsofsuddenchanges
in wind and/or current, accompanied by different
types of connections and/or non‐typical shape and
sizeoftheship.
In nautical simulations (Transas
NTPro 5000), a
seriesofsimulationswereconductedtodeterminethe
behaviourofvariousvessels(lengthofabout10to20
m) in different currents and winds. The simulations
were carried out by setting the appropriate initial
value of current and wind, and then moderately
changing the value of the
wind direction or current
(Figure8).Inallsimulations,thevesselsweredirected
in the same way, or all vessels were within courses
within 180°. Only in extreme, almost unrealistic,
situations of wind and/or current changes, it was
possibleforsomevesselstotemporarilydirectinthe
oppositedirectionofthe
others.
Table1.Swingradiusfordifferentwaterdepthsandvesselslengths
___________________________________________________________________________________________________
Chart Scope ELT EHT Anchorline SwingofMooring Swingradius(m)
depth(m) (m) length anchorline line fordifferentvesselslength(m)
(m)(m) (m) (m) 10 12,5 15 17,5 20
___________________________________________________________________________________________________
2.5 1.75 2 4 7 6.7319.7 22.2 24.7 27.2 29.7
5 1.46 4.5 6.5 9.5 8.4321.4 23.9 26.4 28.9 31.4
7.5 1.33 7 9 12 9.7322.7 25.2 27.7 30.2 32.7
10 1.26 9.5 11.5 14.5 11.0 324.0 26.5 29.0 31.5 34.0
12.5 1.21 12
14 17 12.0 325.0 27.5 30.0 32.5 35.0
15 1.18 14.5 16.5 19.5 13.0 326.0 28.5 31.0 33.5 36.0
17,5 1.16 17 19 22 14.0 327.0 29.5 32.0 34.5 37.0
20 1.14 19.5 21.5 24.5 14.8 327.8 30.3 32.8 35.3 37.8
22,5 1.13 22 24 27
15.7 328.7 31.2 33.7 36.2 38.7
25 1.11 24.5 26.5 29.5 16.4 329.4 31.9 34.4 36.9 39.4
27,5 1.10 27 29 32 17.2 330.2 32.7 35.2 37.7 40.2
30 1.10 29.5 31.5 34.5 17.9 330.9 33.4 35.9 38.4 40.9
___________________________________________________________________________________________________
152
Figure8. Circling of ships moored to a buoy (wind 15 kn
NE,current2kn:000,090,180,270)
5 PROBABILITYOFCONTACTINSWING
MOORING
Ifamooringareaisdesignedsothattheswingcircles
ofthelargestvesselsdonotoverlap,evenduringthe
lowestwater, then theprobability of contact goes to
zero,orexistsonlyinsituationsofhigherforcesthat
cannot be influenced and cannot
be predicted. The
probabilityofcontactoftwovesselswhengoinginto
partialoverlapofswingcircleswillbeanalysedinthe
followup.
Whentheswingcirclesoverlap,thesurfaceofthe
overlap does not increase linearly with the
approachingofthebuoys(centresofthecirclesofthe
swingradii). Iffor twoswing circles of20 mradius
thereisanoverlapontheconnectionofthecentresat
6m(whichis30%ofthetotalpossibleoverlapof20
m, i.e., to the buoy) the overlap of the surfaces is
about6.8%.Fortwoswingcircles
whereoneistwice
aslargeastheotherforthesameoverlap(6mor30%)
the overlap of the surfaces is about 7.9% (100%
overlapofsurfaceswouldbewhenthesmallercircle
of the swing radius touches the centre of the larger
circle of the swing radius).
So, if we take that the
smalleroverlapsofswingcirclesareuptoabout10%,
thiscorresponds toapproaching the buoys byabout
30‐40%. Even if we go to the maximum overlap
(radiusoftheswingcircleequalsthedistancebetween
thebuoys)itisstilllessthan50%
ofpossibleoverlaps
ofsurfaces.Theprobabilitythattwovesselswillbein
theareaofoverlapcanbeestimatedinthefollowing
way.AssumingtheprobabilitythatthevesselAisin
theareaofoverlapP(A),orforvesselBtheprobability
P(B).TheprobabilityP(A),orP(B),
inprinciplecanbe
representedby the ratio ofthe overlap areaand the
total area of the swing circle. The probability of
findingoneandtheothershipintheoverlaparea(Po)
is:
Po=P(A)∙P(B)
Forexample,theprobabilityoffindingtwovessels
inanoverlappingareaof30%,
fortwoswingcirclesof
the same radius (C1=C2) is about 4.6∙10
‐3
, and for
C1=2∙C2itisabout6.2∙10
‐3
.
Figure 9 shows how Po changes for different
percentages of change of the overlapdistance (100%
whentheswingcircleofoneboattouchestheadjacent
buoy). It is noticeable how the probabilities change
minimallyforoverlappingdistanceupto20~30%.
Figure9.Changeintheprobabilityoffindingintheoverlap
area
Calculated probabilities apply to the random
movement of vessels around the mooring/anchoring
point. However, in practice, vessels follow thesame
patternofmovementforthesameexternalforces,and
deviations from this are very rare. Accordingly, the
real probabilities will be significantly less. If one
wants to calculate the actual probability
of contact,
theninadditiontotheoverlappingareasoftheswing
radii,thefollowingshouldbetakenintoaccount:
theprobabilityofcontactexistsonlyforanatypical
vessels,whichdoesnotcircleasthemajority,
the largest vessels will not always be present, so
the probability of contact also depends on the
percentageofthemooringsoccupiedbythelargest
vessels,
swing radii are defined for extreme water levels,
andtheseextremeshavetheirownfrequenciesthat
canalsobetakenintoaccount,
mooringsystemfailure,
shipaccidents,etc.
Also, the occurrence of a contact does not mean
that an accident with harmful consequences will
occur. Since we are talking about boats and small
vessels with minimal movement speeds, normal
fendersaresufficientforshockabsorption.Thisisalso
confirmed by statistics which show that in the
east
Adriaticcoastareatherearealmostnocollisionswith
consequential damages caused by insufficient buoy
distances. Having said all this, it is quite
understandable that in practice there are examples
wherebuoysareplacedatsmallerdistancesfromeach
other than expected, taking into account the
calculationsofthe
swingradiusof the largest vessel
and for the most unfavourable situations for which
thisisformallydeclared.
6
THESIZEOFAMOORINGFIELDANDSOME
SPECIFICRECOMMENDATIONS
In selecting mooring areas as concession fields, the
primarygoaloftheconcessionaireiscertainlytoplace
asmanybuoysaspossibleinaslittleareaaspossible.
153
Eachvesselatthemooringoccupiesacorresponding
area,whichinpracticeisusuallydescribedbyacircle
(formooringatasinglebuoy)orarectangle(foreand
aft mooring). In fact, the total area of the mooring
field is the pure sum of the areas allocated to each
individual vessel. In the case of single swing
moorings, comparedto fore and aft mooring, things
are more complicated, mainly because the area
allocatedtoeachvesselisdefinedbyacircleandthese
circlesendupbeingplacedinthegeometricshapesof
squares,rectangles,trapezoids,andthelike.
Thus,the
goalistoplaceasmanycirclesaspossiblewithinthe
defined(regular)geometricfigures,i.e.,toarrangethe
circlessideby sidesothattheydescribeassmallan
area as possible, assuming that the swing circles do
not overlap and thatʺnʺ different types of ships of
classʺiʺ can be accommodated in an anchorage
occupyingacertainareaʺAʺ.Inotherwords,thegoal
istohaveaslittleunusedareaaspossiblewithinthe
mooringarea.
1
n
i
i
i
A
NS min
A‐totalsurfaceofamooringarea
S(i)‐surfaceofindividualanchorageforclassʺiʺships
N
i‐numberofclassʺiʺships(fields)
Inthesimplestcase,theswingcirclesarearranged
in a series of squares (or trapezoids) to define the
outerboundaryoftheberthingarea(Figure10).
Figure10.Swingcirclesinmooringfieldsandforeand aft
moorings
Ontheexampleoftwovessels,thismeansaʺloss
of spaceʺ of 21.46% (ratio of the area of the swing
circle and the square or trapezoid in which it is
arranged).Usingtheexampleofafieldwith4buoys
andadistancebetweenthebuoysof40m(for
vessels
ofabout10mlength),atotalareaofatleast6400m2
mustbeprovidedtoavoidoverlappingoftheswing
circles.Incaseofmaximumoverlapping,therequired
area can be reduced down to 4000 m2 (the distance
betweenthebuoyscorrespondstotheswingradiusof
thelargestvessel).Forforeandaftmooring,evenless
than1000m2canbesufficient(eachvesselisassigned
anareaofwidththreetimesthewidthofthevessel,
and the distance between the buoys, along the bow
andsternlines,isthreetimesthelengthofthevessel).
It is clear that the fore and aft moorings can
accommodate the largest numberof vessels perunit
area, however, as mentioned earlier, these moorings
require greater investment and more complicated
maintenance. In addition, mooring and unmooring
manoeuvresaremorecomplicated,andtheinfluence
of wind, waves, and currents at
berth is more
significant. Accordingly, swing moorings
predominateinpractice,andwiththemtheproblem
ofoverlappingswingcircles.
As for the percentage of occupation of the cove,
especiallysmallerones,examplesfrompracticeshow
thatitmostlygoesupto20%,rarelyover50%ofthe
total available area of
the cove [Pušić, Lušić, 2022].
Accordingly, but also taking into account
environmentalandsafetyfactors,itcanbeconfirmed
thatthetotalareaofanchorfieldsshouldnotexceed
50%ofthecove.Certainly,itshouldalwaysbetaken
into account that anchor (mooring) fields should
interfereas
littleaspossiblewithothertraffic,andnot
toconstrainaccesstothecoastfromtheseaandvice
versa,regardlessofusers.Theconductofshipsatsea
is governed by the International Regulations for the
Prevention of Collision at Sea, which apply to all
vessels.
The anchorage selection process
is always a
compromisebetweensafetyandecologyononehand
andeconomicfactorsontheother.Someofthemain
criteriacanbegroupedasfollows:
Safety of Navigation criteria (vicinity of traffic,
distance from the shore and other dangers;
underwatercablesandpipelines;bottomtypeand
typeofshoreline;availablespace;protectionfrom
the wind, currents, waves; vessel type and size,
etc.);
Economiccriteria(sizeof field, numberof buoys,
vicinityofpopulardestinations,ports,roads;etc.)
Environmental criteria (type of anchor, distance
from the open sea, shape of a cove, ship waste
collectionservice,localheritagevalues,etc).
Thegeneralcriteriaandgeneralrecommendations
are well known and have already been analysed in
section 2 of the paper. However, some specific
recommendations, which can be drawn from
the
general ones, good practice, and especially from the
examplesofmooringfieldsinSplitDalmatiaCounty
[Račić et all, 2019], [Action Plan, 2013], previous
research [Pušić, Lušić, 2022] and manuals [Pianq,
2014], [ROM, 2007], [Quartermaster, 2013] can be
summarizedasfollows:
3.
Shapeandsizeofthefield
The shape of mooring field should follow the
coastline as much as possible and strive for
simpler geometric shapes like circle, square,
rectangle,trapeze;
Mooringfieldboundariesshouldreflectknown
vessellengthsandestimatedswingradius;
For small and closed coves, the mooring area
shouldnottakeupmorethan50%ofthecove;
Minimal width of the field should not be less
thandoubleswingradius of thelargestvessel
andnotlessthan40m,exceptionallyforextra
smallboatsofupto20m.
The field should be divided into groups
accordingtothelengthofvessels,e.g.,LOAto
10m,from10to15m,from15to20m,etc.
4.
Safetydistances
154
The distance between buoys in single buoy
mooring should be determined so that the
swing circles do not overlap. The calculation
should be based on the length of the largest
vessel for which the mooring is intended,
taking into account extreme water levels.
Average distance may be expected between 3
and
4 lengths of the vessel, for minimal tide
range;
The calculated swing radius should be
increasedby10%of the vessellength,butnot
less than 1 m (smaller boats), (according to
Pianq:forstandardanchoring:minimum20m,
forfishingandpleasurecraftitmaybereduced
to5m);
For multi‐buoy mooring (forward and aft)
minimal (transversal) distance between buoys
should notbe lessthan2 to3 breadths of the
vessel;
Thedistancebetweenmooringfieldboundaries
andthecoast(danger)shouldnotbelessthan
the length of the largest vessel, and not less
than20m;
Incaseofthetransittrafficvicinity,thedistance
fromtheshoreline(safeisobath),shouldnotbe
lessthandoublelengthofthelargestvesseland
notlessthan40m;
Inthenauticalmooringareatowardstheshore
and 150 m from the nautical mooring field
towardstheopen sea there shouldnot beany
other artificial installations nor objects,
includingmoorings or maritime trafficcontrol
measures;
The distance from the beach should be more
than100m;
Theareashouldbedeepenough,makingsure
that during the lowest tide the depth should
neverbelessthan1mmorethantheexpected
vesseldraft.
5.
Typeofanchor
Strive to modern anchoring techniques, like
helical anchors, to avoid destruction of reefs,
seagrassmeadowsandthebenthicecosystems
ingeneral;
For standard anchoring the scope can reach 5
and more, for concrete anchors it should be
between2.5 and1, andfor helicaland similar
embeddedanchorsbetween1.5and1.
The risk assessment for mooring/departing and
stayingatthebuoy/anchoringisusuallybasedonthe
periodofuseofmooring,
andtheworst‐casescenario
shouldbeconsideredforthisperiod.Theinstallation
ofayear‐roundmooringisalwaysmoredemanding.
Forseasonal(tourist)use,usuallyduringthesummer
months, more favourable hydro meteorological
factors prevail, but theworst case shouldalways be
assumed.
7
CONCLUSION
In determining size, shape, and outer borders of a
mooring area,it is essential totake into account the
accommodationcapacity,andthatisprimarynumber
andsizeofvessels.Ifthesecapacitiesarenotknown,
theycanalwaysbeestimatedfrompractice,statistics,
or expectations regarding the location of
the
anchorage. Nautical anchorages are primarily
intendedforsmallervessels,andpracticeshowsthat
these vessels are usually up to 20 m in length.
Accordingly, the distance between buoys (their
anchors) is usually chosen conforming to vessels
whose length isbetween 10 to20 meters. Therefore,
whenplanningthefield,
itisalwaysrecommendedto
draw a series of swing circles based on known or
estimatedvesselsizesanddefinethefieldboundaries
as tangents to the drawn circles. Smaller vessels
shouldbeclosertoshore,largervesselsclosertothe
open sea. As for the shape of the fields, simple
geometric shapes (square, rectangle, trapezoid, ...)
should be aimed for, which should also follow the
coastlineasmuchaspossible..
Thedistancebetweenbuoys,i.e.,theiranchorpins,
should allow the vessel to turn free of all hazards.
This is a basic recommendation, the application of
whichisunproblematicin
practiceforlargervessels.
However, for smaller vessels,especially tourist ones
and boats in general, there are deviations, or more
precisely,situationswheretheoreticallyswingcircles
canoverlap.Thisisthecasewhensimplercalculation
models are used, such as determining the distance
betweenbuoysbasedonincreasingthelengthof
the
largestvesselby acertain percentage,usually2 to 3
times the length of the largest vessel. It should be
emphasizedthatthekeyelementinriskassessmentis
the accurate calculation of the swing radius and its
non‐intersectionwiththeswingradiiofothervessels
or other
hazards. Overlapping of the swing circles
should be avoided,althoughatsmall percentagesof
overlap, up to 20‐30% of the swing radius of the
largestvesselsforwhichthefieldisdesigned(andfor
theworstscenario),theprobabilityofcontactisvery
small. Even if contact does occur, the
probability of
consequential damage is minimal due to the low
approachspeeds.
Ifthereisnotenoughmooringspace,itisgenerally
recommendedthatothermooringtechniquesbeused,
such as Fore and Aft mooring. Of course, always
make sure that the location of the mooring field
provides sufficient protection
from external factors
andminimisestheimpactontheenvironment.Inthat
respect avoid ecologically sensitive areas and use
modernmooringsolutionsthatarebasedondrilling,
i.e., firmly anchored in the seabed. In addition,
mooring areas should be located at a safe distance
from facilities, installations, beaches, hazards,
commonroutes
ofothervessels,andshouldbeofan
appropriatesizesoasnottoimpedetransitoraccess
tothecoastingeneral.
FUNDING
This research received no external funding, but the
equipment used is obtained by the project “Functional
integration of the University of Split, PMF / PFST / KTF
through the development of scientific and research
infrastructure in the three faculty (3F) building” grant
number:KK.01.1.1.02.0018.
155
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