331

1 INTRODUCTION

1.1 Problemdescription

Duringthelastcentury,theshareofmaritimetradein

thetotalvalueofworldtradeisconstantlyincreasing.

Nowadaysmorethan90%oftradeisestimatedtobe

transported by sea (EMSA 2015). This statistics 

portrays also that the world oil tanker account for

out30%ofglobalseabornetrade.Throughtheuse

of new technology of shipbuilding, modern

navigation and control systems, shipping in the

worldʹs become more secure. Despite this, the total

number of ships involved in accidents is still large.

Theaccidentsoftenresultfromcollision,losscontrol,

groundingandstructuraldamage,firesorexplosion.

Only from 2011 to 2014 ab

out 4620 cargo ships

involvedinaccidents,of which oiltankersrepresent

about 9% (415 units). In the last decade the total

volume of oil lost to the environment was

approximately 33 000 tonnes. As a result of tankers

incidents increases also the need to carry out the

cargo tra

nsfer between damaged ship and another

one in order to safe cargo (crude oil, petroleum

products, liquefied gas) and to mitigate emission to

theenvironment,calledinnavalterminologytheSTS

(Ship‐to‐ship) transfer operation. STS transfer

operation generally involve transshipment between

twoships,thelargecalledSBL(ShiptobeLightered)

and small one called SS (Service  Ship) positioned

alongside each other, either  while stationary or

underway in order to commence cargo tra

nsfer

(OCIMF/ICS 2005, OCIMF 2009). Usually this

operationiscarriedoutforhuge oil  tankersinopen

sea, when ship does not berth in port or jetty,

especially due to draught restrictions or the port

berthing charges. The moti

vation for performing

these operations is a lack of deep water ports and

economic aspects. These types of marine operations

are expected to increase significantly in frequency,

and expand into new geographical areas in the

comingyears.

Before mooring and cargo tra

nsfer start, the

ServiceShiphastoapproachtheShiptobeLightered,

whichmovesonaconstantheadingwithslowspeed

or drifts about zero. For this purpose basically a

collisionavoidancemanoeuvre hasto be carried out

Approach Manoeu

re During Emergency Ship-to-Ship

Transfer Operation with Oil Spill

A.Witkowska&R.Śmierzchalski

GdańskUniversityofTechnology,Gdańsk,Poland

P.Wilczyński

GdyniaMaritimeUniversity,Gdynia,Poland

ABSTRACT:OneofthemajoractivitiesduringShiptoShip(STS)transfer operationatseaistosafeapproach

theShiptobeLightered(SBL)whichmovesonaconstantheadingwithslowspeedordrifting.Inthepaper

describedthemanoeuvringproblemforapproachingduringemergencySTStra

nsferoperationwithoilspill.

The approach manoeuvre is considered as a sequence of navigation manoeuvres in specific navigational

environmentwithenvironmentalandoperationalconstraintsaswellasshipdynamicperformance.Additional

constraintsresultsfromSTStransferoperationguideandnavigationpractise. 

http://www.transnav.eu

the International Journal

on Marine Navigation

and Safety of Sea Transportation

Volume 11

Number 2

June 2017

DOI:10.12716/1001.11.02.17

332

in order to obtain the required safety distance

betweentwoshipsandtotakesidebysideposition.

The manoeuvring operations are individually

differentdependingon variation intheenvironment

condition, manoeuvring performance of the

individual ship (Pedersen et al. 2008, Husjord

&Pedersen 2009 , Husjord 2016). During emergency

STS

 transfer operation can appear additional

important aspects like ship and cargo condition

(transhipmentfromundamagedside),timelimits(to

ensure fast transhipment) as well as water area

constraints(closetoportarea),avoidancemovingoil

spillorotherrescueunits.

Our objective is to define Approach Manoeuvre

during emergency STS

transfer operation as a

problem of safe trajectory planning for approaching

taking into account weather condition (wind

direction),trafficdensityandstopandspeedcontrol

performance of the vessels involved. Trajectory of

approaching determined on available information

allowstotake propermanoeuvring decisionby ship

operatorusingruddersandpropellersand

tomitigate

oilspilltotheenvironment.

1.2 TheprincipleofastandardApproachManoeuvre

duringSTStransferoperation

The STS transfer operation requires proper

coordination, equipment in according to STS

operation plan and administration approval. The

purpose of the STS transfer operation plan is to

provide a step by

stepdescription of STS procedure

according to guidelines or recommendations from

Iranian Classification Society (ICS), Oil Companies

International Marine Forum (OCIMF) and the

InternationalMaritimeOrganization(IMO).Thisplan

shoulddealwiththefollowingstagesofoperation:

 Pre‐ApproachPlanning;

 ApproachManoeuvre;

 Mooring;

 CargoTransfer;

 Unmooring;



 DepartureManoeuvre.

Each stage consists of different procedures to

followandcheck‐liststocomplete.Astandardwayto

carryoutanSTS transfer operationiswhenthe SBL

maintainaconstantheadingatminimumcontrollable

speed(5knotsorless)ordriftwithwindandcurrents

but SS

approach the first one and berths normally

withitsportsidetothestarboardsideoftheconstant

heading ship (Fig.1). The standard Approach

Manoeuvre is divided into two phases. The initial

phase is basically a collision avoidance manoeuvre

fromcurrentpositionp

otofinalpositionpkinorderto

obtain the required safety distance between Service

ShipandShiptobeLightered.Thesafetydistanceis

called the Distance at Closest Point of Approach

(DCPA) and it is appropriate to the conditions.

DuringthisphaseSSmustapproachthefirstoneona

parallel course and

adjust its velocity to equal SBL.

The second phase which is operation of a ships

alongside takes place after the required safety

distance has been verified. On closer approach, the

manoeuvringshipshouldthenpositionitselfrelative

totheconstantheadingship.Contactismadebythe

manoeuvring ship, reducing the

 distance until the

fenderstouch.Subsequentlybothshipsareonparallel

courses with similar velocity and their manifold in

linetominimizeforceofberthingsimultaneouslyon

allfenders.

In the open waters the standard Approach

Manoeuvre begins at distance of 0.5 Nm from the

destinationpointandfinish at

DCPAapproximately

50‐100m off. The mooring lines startabout 20‐30m

away from each ship. Normally the manoeuver will

bemadewiththewindandseaahead,howeverlocal

conditions and knowledge may indicate an

alternative side. Usually transhipment is completed

after10‐24hdependingoncargo

qualityandweather

condition.

Throughout any berthing operation the visibility

shouldbegoodenoughforsafemanoeuvring,taking

into account safe navigation and collision avoidance

requirements. This standard Approach Manoeuvre

(Fig.1) is of assistance when ships are under power,

considering normal STS transfer operation. The

proceduresmayvaryfromthis

guidanceaccordingto

circumstances (emergency with oil spill, inshore

operation, limited geographical scope of operation),

dynamical and kinematical ship properties, weather

conditionandtrafficdensity.Ineachuniquesituation

Approach Manoeuvre almost base on knowledge,

experience and assessment of navigational situation

fromnavigators.



Figure1.TheprincipleofastandardApproachManoeuvre

in STS lightering operations (OCIMF/ICS 2005, OCIMF

2009)

The most common incident to occur during STS

operationsisacollisionbetweenthetwoshipswhile

manoeuvring alongside each other or sailing

(Ventikos & Stavrou 2013). Collision between two

ships typically occur for reasons which include:

incorrect approach angle between the manoeuvring

ships;approachingatexcessivespeed; failure ofone



orbothshipstoappreciatemeteorologicalconditions.

To mitigate the risk of incidents, guidelines will be

needed for the navigator of Service Ship, which

include information about reference trajectory for

approaching in meaning  of reference way points p

i:

position(x

i,yi )/or(headingi)andvelocityvitotake

333

a proper steering decision by ship operator at each

stageofshipmanoeuvring.

2 OPERATIONASPECTSDURINGEMERGENCY

STSAPPROACHMANOEUVRING

2.1 Accidentscenariowithoiltanker&STSoperationto

mitigateoilspill.

The following example accident scenario with oil

tanker is considered in this paper for trajectory

planning.

Product tankeraftercollision with general

cargo ship lost its ability to manoeuvre and start

drifting NE due to NW’ly wind. Immediate actions

were carried out to reduce oil  spill overboard,

arrangedtransfercargofromdamagedtanktoother

compatible tanks and increased heel to port using

ballasttanksto

keepallcracksonbulkheadabovethe

sea water level. Prepare floating cotton barrier to

reduceoilspotinthevicinityofship,startedoilspill

pumpandcollectedoilywatertoslopstanks.Atthe

same time all parties (administration, owner, 

charterer, insurer, SAR) were informed accordingly.

Small tanker with

 very good manoeuvring

characteristic was  designated to emergency STS

operation.

For considered accident with oil tanker two

variants of Approach Manoeuvres are proposed

based on good navigation practice and STS transfer

operation guide (OCIMF/ICS 2005, OCIMF 2009,

Wilczynski 2014). They depend on different

circumstances like wind direction, oil spill area

and

shipactuatorequipment(manoeuvringperformance).

TheexamplesareshowninFigures2‐3.



Figure2. The example trajectory for approaching in STS

transferoperationswith2controlmodes(I,II)‐variantA



Figure3. The example trajectory for approaching in STS

transferoperationswith3controlmodes(I,II,III)‐variantB

Both manoeuvres are from leeward and to the

starboard side of the constant heading ship. They

differonlyinpossibilitytorunDynamicPositioning

control mode (variant B) or without possibility of

runningit(variantA).

2.2 ControlmodesduringemergencySTSprocedure

The possibility of using different control modes

during

 STS procedure depends ( among other) on

ship current velocity and actuator equipment. The

Service Ships dedicated for conducting STS transfer

operation are mostly equipped with aft main

propellerusuallyinconjunctionwithrudderandbow

tunnelthruster.Mainpropellerproducethenecessary

surge force needed for transi t and rudder

produce

yawmomentwhichcanbeusedforsteeringcontrol.

Thetunnelthrusterproducesaswayforceandisonly

effectiveatlowspeed.Thissetofthreeactuatorscan

realizeTrajectoryTrackingoperationofApproaching

and then Berthing. Azimuth thrusters can produce

two force components surge and sway in

 the

horizontal plane. They are attractive in Dynamic

Positioningsincetheycanproduceforcesindifferent

directions(Fossen,T.2011).

To control the movement of the Service Ship

during STS Approach Manoeuvre a  few general

control modes are possible, in order to achieve the

final Distance from SBL, parallel course

and equal

speed(Fig.2).Theyconsistof:

1 Trajectory Tracking  (moderate or high‐speed

manoeuvring)

2 StoppingManoeuvre(stopship)

3 DynamicPositioning(low‐speedmanoeuvring)

The control modes mention above are classified

accordingtocontrolobjectivesduringSTSApproach

byusingdifferenttypesofavailableactuatorsinclude

propulsionsystem,

thrustersandrudders.

Trajectory Tracking (Tomera 2016): The first

control objective is to minimize a tracking error

betweenadesiredtrajectorygivenbyadesiredtime‐

varying position and velocity reference signals.

334

DuringSTSoperationtrackingcontrolcanbeusedfor

course‐changing manoeuvres and speed‐changing

control separately or simultaneously. The task is

mostly realized at the first phase of Approach

Manoeuvre to assume moderate or high ship speed

(more than 2m/s). At this speed rudder for course

control and main

 propeller for speed control (ROT

andruderanglecontrol)areonlyeffective.

Stopping Manoeuvre (ABS, 2006): The second

controlobjectiveisrelatedwithareductionofcurrent

shipvelocityalongtrajectorysegmentsbetweenway

points. The stopping ability of the vessel is judged

using emergency stop manoeuvre or normal stop

menouvre. The emergency stopping test must be

performed starting from the test speed. After the

steadystateisachieved,the“fullastern”commandis

given from the engine control mode on the bridge.

Thetestisconsideredtobecompletedwhentheship

speedisaboutzero.

Duringstop manoeuvrethe

operator shouldstop

itsengineandonlycoursekeepingbyusingrudders

amidships. The test is considered to be completed

whentheshipspeedisdeadonthewater.

Dynamic Positioning (Fossen, 2011, Witkowska

2013). The third control objective consist in

manoeuvringtheshipatlowspeed(lessthan

2m/s).

The only course and position control are associated

withthismode.Atthesespeedshipsteeringiscarried

outbyusingmostlyazimuththrusters,bowthrusters,

sternthrusters,waterjets.Theefficiencyofruddersat

low velocity significantly decreases. Dynamic

Positioning mode can be activated at last phase of



approaching(afterspeedreduction)torealizevarious

kindofshipmovementlikelongitudinal,transverse,

rotationarounditsaxisorsidemanoeuvreatcertain

angle.

After Approach Manoeuvre by using I, II or/and

III control modes, the ships should manoeuvres

alongside at the required safety distance (DCPA).

That means both SS

and SBL keep their constant

heading

SS SBL



  and constant speed

SS SBL





or drifting  about 0. In this condition the Berthing

operation by using tunnel thruster and Mooring

procedurebyusinglinescanstart.

2.3 Stoppingandspeedcontrolcharacteristics

Comprehensive details of the ship stopping and

speed control characteristics are included in the

manoeuvring booklet. This booklet is required to be

on board and available for navigators. Most of the

manoeuvring information in the booklet can be

estimated but some should be obtained from trials.

They contain (among other relevant data)

characteristics of main engine, stopping test results

(emergency and normal) and deceleration

performance. The characteristics of main engine

contain possible engine

 order (Full Sea Ahead, Full

Ahead,HalfAhead,SlowAhead,DeadSlowAhead,

Dead Slow Astern, Slow Astern, Half Astern,  Full

Astern), propeller revolution, speed, power, pitch

ratio.

Stopping ability is measured by the track reach,

head reach, side reach, time required to speed

reductionandfinalcourse(Fig.4).



Figure4.Definitionusedinstoppingtest.

It covers the following modes of stopping

manoeuvers: from Full Sea Ahead to Full Astern;

fromFullAheadtoFullAstern;fromHalfAheadto

Full Astern; from Slow Ahead to Full Astern; from

Full Sea Ahead to stop engine; from Full Ahead to

stop engine; from Half Ahead to stop

 engine; from

SlowAheadtostopengine.

Deceleration performance concern track reach,

headreachandtimerequired.Itcoversthefollowing

modes:fromFullSeaSpeedtoFullAhead;fromFull

Ahead to Half Ahead; from Half Ahead to Slow

Ahead;fromSlowAheadtoDeadSlowAhead.When

the

vessel travels along a straight line with the

original course (autopilot is on) the track reach and

time reach values are taken as the longest travelling

distance and the maximum time to decelerate ship

velocity.

3 TRAJECTORYPLANNINGFORAPPROACHING

The Service Ship trajectory

 for approaching is

defined as a set of turning points





, ,...,

pp pP on ship route from current

position (initial point)

 to the destination (final

point)

. The way points









, , , 0,1, ,

iiii

pxyv i k

 of desired trajectory

haveposition

y determinedtoavoidobstacleson

considered area with respect to a top ship speed





, 0,1, ,

vi k on each way points. The way

pointsdivide trajectoryintoa set





,,,

Sss s

of trajectory segments with a lengths





,,,

Ddd d. The

s  compose of the path

position sequences between way points on straight

line. The way points components are respectively

reference ship position and speed on each turning

point.

PlanningofthesafetrajectoryduringSTSassumed

that each of trajectory segment





, 1, ,

si k ,

between way





, 0, ,

ik points does not cross

in the area of the environment with the static and

dynamicobstacles.Thechoiceoftopspeedelements

v 





0,1, ,ik at each way points of desired

trajectory depend on set



,





0, , , , ,

FA HA SA DSA

Vvvvv  where the following

engineordersareconsidered:FullAhead(

v ),Half

Ahead (

v ), Slow Ahead (

v ), Dead Slow Ahead

(

v ).

The designed trajectory satisfies  deceleration

conditioniftheshipisableoneachtrajectorysegment

335



 todecelerateshipvelocityfrom.Itmeansthatfor

a given starting reference speed

v  at

 it is

possibletoapproachbyshiptheendingone

1ii







at



 with segment length



. The feasibility of

trajectoryischeckedbasedonstopandspeedcontrol

constraints collected in manoeuvring booklet. When

thevesseltravelsinastraightlinealongtheoriginal

course the segment length value can’t be less than

track reach needed for speed deceleration or stop

ship:

d track reach



  (1)

where

track reach



 isthetravellingdistanceneed

todecelerateshipvelocityfrom

v  to



.

The planning of the last way points





, 2, 1,

ik k k  , 2k   depend additionally

on ship manoeuvrability constraints during STS,

results by using variants A or B of Approach

Manoeuvre.

3.1 Waypointsplanning‐variantA

Theinitialwaypoint

 consistofacurrentposition



y andvelocity

v  ofServiceShipwhenitstart

Approach Manoeuvre (Fig.  5). The destination point



, ,

kkkk

xyv  hasaparallelposition



sSBL

ll  in

asafetydistance(DCPA)frompositionofShiptobe

Lightered and the some velocity

vv , to allow

starting manoeuvring alongside. When emergency

STStrajectoryisplanningtheSBLmaintainitscurrent

position



y  constant and speed about zero,

0v  . In this case the initial p0 and destination pk

pointsareapproximatelyconstantandchosenbythe

operator or calculated by the simple geometric

relationship:



| , ,

kSSSSSBL

ll l   (2)



0, | | ,

vDCPApp

 (3)

where







| , , | , ,

kkk

xy p x y

l straightlinecoversSSdiametricalline,

SBL

l  straightlinecoversSBLdiametricalline.

The previous way point



 has position

determinedonstraightline

l  parallelto

SBL

l .



| ,

kSSSSSBL

lll





  . (4)

The reference speed



 is modelled as

minimum controllable speed

v  (Dead Slow

Ahead)forsafetymanoeuvringincloseproximity.

kDSA



  (5)

with satisfying feasibility condition of trajectory

segment

s  :



1 2

, ,

| |

kk kk

kk k k

xy xy

d p p track reach







  (6)

where

track reach  is the travelling distance need

todecelerateshipvelocityfrom

v  toabout0.

The way point



 is determined on the arc

 between the end points A  and

 satisfying



. The arc is a part of a circle





, ,

Op AO 

witharadius

0.5AO   nauticalmilesofcellsand

central angle

0, 30



. We also assume that

reference velocity

2kDSA



  is predetermine as

minimumcontrollable.



| ,

kABss

LAl







 (7)

2kDSA





 (8)

where





, , , 0,30 , 0.5NM

AB k

LOpAO AO





3.2 Waypointsplanning‐variantsB

ThedifferencebetweenvariantsA(Fig.5)andB(Fig. 

6) lie only in modelling  two way points





,2,1

pi k k



. Because of using Dynamic

Positioning control mode, the maximum reference

velocity



 of approaching doesn’t exceed 1 knots

in the safety  area  of 0.3 NM [17] from destination

point. The DP system at low speed allow to realize

variouskindofshipmovement,sothepoint



 is

determined on the larger arc

L  with a central

angle

0,90





.

1 ,

vknot





 (9)



kCD







, .



 (10)

where





, , , 0,90 ,

CD k

LOpCO



 





0.5NM 0.3NMCO

.

The way point



 is determined on the arc

LAB between the end points

A  and

 satisfying



. The arc is a part of a circle



, ,,

Op AO 

with a radius

0.5 AO  nautical miles of cells and

centralangle

0,90



.Weassumethatreference

velocity



 is predetermine as minimum

controllable (Dead Slow Ahead) for safety

manoeuvring:

2kDSA





 , (11)







LAB,



 (12)

where

336



, , , 0,90 , 0.5NM.

AB k

LOpAO AO







Figure5.Modelingwaypoints‐variantA 



Figure6.Modelingwaypoints‐variantB 

To satisfy deceleration condition on trajectory

segment



 a  distance



 can’t be less than a

track reach calculated on stop and low speed

characteristics:



12 1 2

, ,

| |

kk kk

kk k

xy xy

dp p



 

 

track reach



  (13)

where

track reach



 isthetravellingdistanceneed

toreduction



 to



 1knot.

Additional constraints on way points depend on

winddirection(sideofmanoeuvreifitpossiblefrom

leeward) and emergency condition (side of

manoeuvreapproachfromundamagedside)

4 CONCLUSIONS

The paper formulate the problem of Approach

ManoeuvreduringemergencySTStransferoperation

withoilspillasaproblemof

trajectoryplanning for

approaching. The trajectory is considered as a

sequence of way points with reference position and

velocity and straight line segments between them.

Thewaypointplanningprocessresultsfromtransfer

operationguide,shipoperationconstraintsandwith

respect to additional constraints depended on ship

speedandstopping

meneuverperformance.

The presented in above way trajectory planning

issue can be consider as an example of classical

avoiding collisions at sea. It can be reduced as a

multi‐criteria, nonlinear optimization problem with

navigational time, safety and economy criteria with

navigationalconstraints:

 stationaryobstacles (land,islands, shallowwater,

restricted

area)

 dynamical obstacles (SBL ship, other ships,

modeling of the prediction of the oil  spill area

(Łazugaetal.2012)

 modelingofshipsandobstaclesbydomains

 ships and obstacles domains position, course,

speed.

 weathercondition(winddirection)

 stoppingandspeedcontrolcharacteristics

Several

solutionscanbeusedtosolvetheproblem

of trajectory planning for approaching defined as

optimization task. One of them contain Genetic or

Evolutionary Algorithm (Kuczkowski &

Smierzchalski 2014), Particle Swarm Algorithm

(Lazarowska 2015), Simulated Annealing (SA). The

results of using Evolutionary Algorithm with taking

into account speed deceleration and stopping



characteristics for trajectory planning during

emergency STS transfer operation is the aim of the

followingstageofresearch.

ACKNOWLEDGEMENT

Thepaperpresentstheresultsdevelopedinthescope

oftheHAZARDprojecttitled“MitigatingtheEffects

of Emergencies in Baltic Sea Region Ports” that has

receivedfundingfromtheInterreg

BalticSeaRegion

Programme 2014‐2020 under grant agreement No

#R023.https://blogit.utu.fi/hazard/

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