65

1 DECISIONMAKINGPROCESSAND

CONVENTIONALEVALUATIONMETHODOF

COLLISIONRISK

The subject of Decision Making for collision

avoidance is strict requirements in terms of risk

mitigation. There are three steps in the process of

decision making. The first step is information

collection,wherevariousinformationlikeasAISdata,

radardata,chartdataandma

nyothervoyagerelated

data used for ship operation are collecting. The

second step is information processing, where all

collected information would be processed and

analyzedbytheteamondutyatbridgetorecognize

about the own ship current situation, for instance,

encountering collision risk and the performa

nce of

own ship in a current navigation environment. The

third step is the decision of own ship action, where

captain and/or navigator select own ship action that

satisfies safety, legality and efficiency. According to

thenearmisscausesurveyeddoneinthepast

[1]

,43%

of cause by the navigator themselves, 17% for

teamwork and 23% for supporter, so most of the

causeofnearmissexistsinhumans.So,thereisneed

to review the function of decision making process.

Improvementfortheinformationcollectionhasbeen

made,suchasloadingAIS.Andwith ap

pearanceof

GPS,itbecameeasytoknowthetruemotionoftarget.

But nothing has been improved for the information

processing and overall improvement of decision

making has not progressed. Among the information

processing,itisnecessaryto improve theevaluation

method of collision risk. The conventional collision

risk evaluation method use the Distance of Closest

PointofApproa

ch(DCPA)andTimetoCPA(TCPA)

of target which are related to relative motion (see

Figure1).

Evaluation Method of Collision Risk by Using True

Motion

H.Imazu

TokyoUniversityofMarineScience&Technology,Tokyo,Japan

ABSTRACT:ItisnecessarytodevelopausefulapplicationtousebigdatalikeasAISforsafetyandefficiency

ofshipoperation.AISisveryusefulsystemtocollecttargetsinformation,butthisinformationisnoteffective

useyet.Theevaluationmethodofcollisionriskisoneofthecausedisturb.Usuallythecollisionriskofshipis

evaluated by the va

lue of the Closest Point of Approach (CPA) which is related to a relative motion.So, it

becomesdifficulttofindoutasafetypassinacongestedwater.Here,LineofPredictedCollision(LOPC)and

ObstacleZonebyTarget(OZT)forevaluationofcollisionriskareintroduced,theseva

luesarerelatedtoatrue

motionanditbecamevisibleofdangerousplace,soitwillmakeeasytofindoutasafetypassinacongested

water.

http://www.transnav.eu

the International Journal

on Marine Navigation

and Safety of Sea Transportation

Volume 11

Number 1

March 2017

DOI:10.12716/1001.11.01.06

66



Figure1.Collisionriskevaluationmethodbyusingrelative

motion

This method has several problems for collision

avoidanceasfollows;

1 It does not consider the ability for collision

avoidanceofownship.Thespeedofownshipis

deeplyrelatedtotheabilitytoavoidcollision.

2 It must set own ship action for evaluation in

advance, even before decision

 making. So, this

method is a trial and error method to find out

safetyaction.

3 Thebaseofthismethodevaluatesstraitlinepass,

but in congested area it is necessary to evaluate

notonlystraitlinepassbutalsobrokenlinepass.

4 It is necessary to predict

and respond quickly to

thetarget’sactionchangeinacongestedarea.But

therearemanydifficultiesforpredictinganeffect

oftarget’sactionchangeonthecollisionriskand

planningpass.

5 It is not easy to compare the collision avoiding

action with the chart data. It is necessary to



convertnumericaldatatopositiondata.

Therearesomanyproblemstousethisevaluation

methodforcollisionavoidanceinacongestedarea,so

that the collision risk alarm equipped in ARPA has

notbeenusedsomuch,especiallyinacongestedarea,

duetothesereasons.Foractiveuse

ofbigdatasuchas

AISfor collision avoidance,we needto improve the

evaluationmethodofcollisionrisk.

2 LINEOFPREDICTEDCOLLISION

Therearetwowaystoexpresstheprocessofcollision

betweentwodots.Thefirstway,itistheclosestpoint

ofapproach(CPA)asshownin

Figure1,thisvalueis

related to the relative motion. The second way, two

dots arrive at the same place at the same time from

theirownposition.Here,thesameplaceisnamedthe

Point of Predicted Collision (PPC), this is related to

the true motion. Since, there are

many problems in

usingtherelativemotion,soIwilladoptPPC.



Figure2.PointofPredictedCollision

Figure2showswherethePPC(P)looksfromship

A and ship B. Length between AP and BP is

proportiontotheirspeedasfollows,

tVtVVVBPAP

BABA

:::



 (1)









max

min

 (2)

TwoPPCaredeterminedeverytimeelapsed(t)as

showninthefigure.

Figure 3 shows the Line of Predicted Collision

(LOPC)

[2]

thatisformedbyconnectingPPCtogether.

TheLOPCchangetheshapeaccordingtotheratioof

twoship’sspeed(N),

N

 (3)

Iftwoship’sspeedareequal(N=1)thenLOPCis

theperpendicularbisector,andotherwiseitiscircler

shapesurroundingaroundlowspeedship.



Figure3.LineofPredicted Collision

The PPC of the ship is an intersection of course

lineof theshipand LOPC.Leftside ofthe Figure4

shows PPC ofship A (P

A) in eachcourse ofship A,

andrightsideshowsPPCofshipB(P

B)ineachcourse

ofshipBrespectively.Iftheshipchangescourse,then

PPCmoveonLOPCaccording tothenewcourseline.

So, we can predict very easily which direction PPC

willtake.IfP

AandPBaresameposition,itmeansthat

twoship’sdestinationissame,thentheywillcollide

at the PPC, and if their position are difference, two

dotswillnotcollide(DCPA>0).

67



Figure4.PPCandLOPC,thearrowrepresentsmovementof

PPCwhentheshiphassteeredtostarboard.

Figure5showstheshapeofLOPC(N=2).HereTis

the contact pointof tangency drawn from ship B to

the LOPC, where the angle of ∠TAB is 90°, and

tangentangleofLOPC(∠ABT)is



.



Figure5.ShapeofLOPC(N=2)

The elements of LOPC can be obtained by the

followingequations

OT N AO







 (4)

So,theshapeofLOPCchangewithspeedratio(N)

andlengthofbaseline(d).TheradiusofLOPC(OT)

tendtoshortenwhenthespeedratioNdepartsfrom

1.0asshownintheFigure3.Ifhighspeedshiptryto

seelowspeedshipfrom

thebowatananglegreater

thanβ,thenthecourse line ofhighspeedshipdoes

notcrosstheLOPC,sonocollisionoccursregardless

ofthecourseoflowspeedship.Likethis,highspeed

ship has capability to avoid collision by its own

action.Severalvaluesofβare

showninTable1.

Table1.Sampleofangleβ



3 OBSTACLEZONEBYTARGET

However,shipisnotadotithassizeandvolume.So,

we must take into consideration the safety passing

distance(SD)betweentwoships.ThecourseofshipA

A) to keep the SD between the ship B can be

obtained by using next method which all navigator

knowswell(seeFigure6).



Figure6. Obtaining method of Collision Course, where

collisioncourseisC

A1〜CA2

1 First,drawacircleofradiusSDcenteredonship

A,asindicate“CircleofSD”inthefigure.

2 DrawtangentlinestothiscirclefromshipB,and

contactpointsaresetasT1andT2.

3 DrawthemotionvectorofshipBbehindshipB,as



indicate“OB”inthefigure.

4 Drawa circleof radius ofship Aspeed centered

onpointO,asindicate“circleofshipAspeed”in

thefigure.

5 D1 and D2 are intersections of tangent lines and

circle of ship A speed. The direction of line

connectingO

andD1(CA1)andOandD2(CA2) 

becomecollisioncoursestobeobtained.

ThecoursebetweenC

A1andCA2wouldbethatthe

DCPA is less than SD, so that this course is called

collision courses. But, in case of a congested area,

collisioncoursespereachtarget willbeoverlapand

noteasytofindoutsafepassageroutebyusingthese

collisioncourses.So,Iaddthe

followingaction,

68



Figure7.ObtainingtheObstacleZonebyTarget

1 Shiftthecollisioncourses(CA1andCA2)toshipA

asshownintheFigure7.

2 Andfindthezonewhichpartofthecourselineof

shipBcutoutbythesecollisioncourses.Thiszone

is named the Obstacle Zone by Target

(OZT)[3][4][5].

SinceshipAwill

collidewithshipBwhenpassing

through this zone, which means that the collision

coursecanbeconvertedtotheinformationofcollision

place.



(a)OZTbyshipA(b)OZTbyshipB

Figure8. The Course Line of each ship and OZT, where

N=1.5

Figure8showstheOZTincaseofN=1.5.Leftside

of the figure shows the OZT in each course of low

speed ship A (OZT

A), since ship A is inside of the

LOPC, every course line of ship A cross the LOPC

andgenerateOZT

Aattheintersectionasshowninthe

figure. Right side figure shows the OZT in each

course of high speed ship B (OZT

B), since ship B is

outsideof theLOPC, basically twoOZT(OZT

B1and

OZT

B2)aregeneratedwhenthecourselineofshipB

crosstheLOPC,andwhenthecourselineiscloseto

the contact point, one long OZT

B is generated. OZT

hasfollowingadvantages,

 It can be visualized the collision risk as danger

place.

 OZTisontheLOPCasshownintheFigure8.So,

positionofOZTwillmoveontheLOPCaccording

with target action change. It becomes easy to

predict the new

position of OZT with target’s

actionchangebyusingLOPC.

 OZTwillbedisplayed on thecourselineof each

targetindividually,soitiseasytoidentifywhich

ship’sOZTis,anditmakeeasytofindoutsafety

placetopassinacongestedarea.

 OZTshows

location,soitcanbesuperimposedon

thechartanditisusefulforpreventinggrounding

atsametime.

So, the perspective should be changed to true

motion from relative motion, then the collision risk

willchangefromthenumericalformlikeTCPAand

DCPAtovisualizedinformation,soit

canbeeasyto

useeveninacongestedarea.

The collision course C

A is obtained by the

followingequation.



sin sin( )

AZ Z B

CA NA C





   (5)

where A

Z is the azimuth angle of ship B, CB is the

courseofshipBandαis ∠ABTintheFigure6.

4 LOPCANDOZT

Figure 9shows how LOPC and OZT change with a

speedratioN.Here,thespeedratioNis1.5,1.25and

1 respectively. The OZT by ship A (OZT

A) and the

OZT by ship B (OZT

B) occurs at the intersection of

LOPCandcourselineofshiprespectively.Thelength

of OZT

B becomes longest in a point of tangency of

LOPC,seeOZT

B1inthefigure.



Figure9.ChangeofSpeedandOZT

Figure10showstheOZTAandtheOZTBincaseof

bothship’sdestinationisapointoftangency.

69



Figure10. Difference of OZTA and OZTB at the point of

tangencyofLOPC

ThereisabigdifferencebetweenOZTAandOZTB.

For ship A to avoid passing through OZT

B, it is

necessary to take large angle course change, for

instancewherestarboardsidefromC

AtoCA.S,orport

sidecasefromC

AtoCA.P.Ontheotherhand,ifshipB

avoid passing through OZT

A, it is sufficient to take

smallanglecoursechange,wherestarboardsidefrom

B to CB.S or port side from CB to CB.P respectively.

This difference leads to gap in the start time of the

avoidanceactionbyeachship.

5 DISTRIBUTIONOFOZTINACONGESTED

AREA

Figure 11 is an example of OZT distribution in a

congestedarea,itshowsOZTbythetargetship.On

thescreen,thereare21

targetshipsatsametime,itis

quite congested but OZT in the angle range of 10

points(112.5°)oneachsidefromthebowofshipA

areonly10places,Z1toZ10asshowninthefigure,

these OZT will obstruct the passage of ship A. The



targetshipformingtheOZTcanbeeasilyidentified,

so you can identify the target ships that need

attention. From the OZT distribution shown in the

figure,itiseasytofindasafetyroutewhichisstraight

line or broken line avoiding through these OZT.

Thinking about behavioral conditions

like as safety,

legality and efficiency, we will select route of own

ship,amongthosesafetyroutescandidate.Thereare

fourOZT(Z5,Z6,Z7andZ9)nearthecourselineof

shipA.IfshipApassesthroughthesefourOZT,itis

difficulttogetenoughspacebetweeneach

OZT,and

since the ship A will becomes an overtaking ship

against the target forming Z5, the ship A will be

necessary to take avoiding action against this target

beforereachingZ5.



Figure11.SampleofOZTdistribution

Figure12showsasimilarrouteadoptedbyshipA.

First,shipAchangeditscoursetoport20degree to

avoid congestion, and go straight about 3nm (10

minutes). After passing Z6, ship A put the course

back and took starboard 20 degree. This route and

OZTcan be superimposed

on the chartas shownin

the figure. So, at the same time it can be used to

preventthegrounding.



Figure12.Safetyrootplanning

6 CONCLUSIONS

After this study, it appears that the evaluation of

collisionriskusingtruemotionhasmanyadvantages

asfollows;

1 Inthismethod,theabilityforcollision avoidance

ofownshiphasbeenconsidered.Thereisastrong

relationbetweentheabilityofcollisionavoidance

70

and LOPC. The ability for collision avoidance is

highwhentheshiplocatesoutsideLOPC.

2 It is easy to find out safety route and action for

collision avoidance by using the distribution of

OZTaroundownship.

3 The influence by target’s action change can be

regarded as sifting

 position of OZT. And new

position of OZTby target’s action change can be

easilypredictedbyusingLOPC.

4 It is easy to compare the planning pass obtained

bydecisionmakingwithchartdata.

And,LOPCcanberegardedasamodelinthecase

where SD=0 in OZT.

Therefore, all the two ships

encounterconditioncanbeexplainedbyusingLOPC.

So, it become possible to evaluate collision risk

quicklyperthetarget’sdynamicinformation.

Since this theme is fundamental to ship safety

operation,itcanbeutilizedallmattersrelatedtoship

safetyoperation,likeasshiptraffic

flowanalysisand

designofshiptrafficflowetc.

The standard of collision avoidance action by

usingLOPCandOZTwillbemadeinfuturework.

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