59
1 INTRODUCTION
It is now more and more mobile water transport
objects (MWTO) with dynamic positioning systems
(DPS)onboard.WidevarietyofDPSapplicationsin
maritimeindustryrepresentsafetyofnavigationasa
complex problem. High precision navigation
processes interfere with environmental factors (EF):
wind,current,waves(Fig.1).
Figure1.Environmentalfactorsdisturbances.
Wind, waves and current produce nonlinear
external disturbances, which try to move MWTO
outside of the locally confined area permitted
boundaries.Environmentalforcesrepresentpotential
hazard to safety of navigation and technological
workperformance.
During dynamic positioning (DP), MWTO
experiences motion in 6 degrees of freedom (DOF).
The motion in the
horizontal plane referred to as
surge(longitudinalmotion,usuallysuperimposedon
the steady propulsive motion) and sway (sideways
safety of navigation motion). Heading or yaw
(rotation about the vertical axis z). The remaining
three DOFs are roll(rotation aboutthe longitudinal
axis x), pitch (rotation about the transverse axis y),
and heave (vertical motion). DPS implies
stabilizationofthesurge,swayandyawmodes.
DPS may be defined as a system that
automatically controls MWTO to maintain position
and heading exclusively by means of active thrust.
DPSmaysetatargetposition,calledastation,which
can be fixed or a
movable reference. DPS allows
MWTOs to safely maneuver within the confines of
portsandharbors.InmodernMWTO,theDPsystem
mayformpartofaMWTOintegratedcontrolsystem,
Safety o
f
Navigation During Dynamic Positioning on
Mobile Water Transport Objects
R.Gabruk&M.Tsymbal
OdesaNationalMaritimeAcademy,Odesa,Ukraine
ABSTRACT: This paper presentsan innovation methodology for quantity assessment thesafety of dynamic
positioninginlocallyconfinedareaoftechnologicalworkperformanceundernonlineardynamicdisturbances
of environmental factors.The methodology is based on new integratedpara digm of prediction coordinated
components interactions, which form
dynamic positioning system. Proposed methodology implementation
effectisbasedontheanalysisofcomplexmodelsthatformtheknowledgebase.Thecomprehensivereserveof
controllablethruster’sreactionswasadoptedasquantitativecharacteristicofdynamicpositioningsafety.
http://www.transnav.eu
the International Journal
on Marine Navigation
and Safety of Sea Transportation
Volume 10
Number 1
March 2016
DOI:10.12716/1001.10.01.06
60
communicatingbymeansofaredundantLocalArea
Network.
DPSalsoprovidesamanualjoystick controlthat
may be used for joystick control alone or in
conjunction with a position measuring system for
combinedmanualorautocontrol.Withoutaposition
measuring system, DPS can provide automatic
stabilizationandcontrol
oftheMWTOheadingusing
the gyrocompass as the heading reference. In
addition to the standard operational modes and
functions,varioustailoredfunctionsareavailableto
optimize MWTO operation for a wide range of
technological works (offshore loading, trenching,
dredging,drilling,pipeandcablelaying,etc.).
MWTO attitude is a
necessary feedback into the
DP system. The value of roll and pitch must be
allowedforinthecomputationsforpositionobtained
fromhydroacousticpositionreferencesystemsand,
insomecases,tautwiresystems.Boththesesystems
generate positional data derived from angular
measurements.Inordertoprovideageodetic
datum,
the vessel is provided with one or more Vertical
ReferenceSensors,orelectronicinclinometers.
InordertoprovideaDPfunction,thevesselmust
be equipped with an adequate spread of propellers
andthrusters.ForDPpurposes,aminimumofthree
thrusters are required, and most vessels are fitted
withmorethanthisminimum.Atypicalvesselmay
be fitted with twin propellers and rudders, a stern
tunnelthrusterandtwo(three)bowtunnelthrusters.
Now more and more vessels are equipped with
azimuththrusters.Azimuththrusters arereliabletool
withhighperformanceforDP.Alsocommonlyused
thrusters are
retractable thrusters. A semi‐
submersible drilling rig may be fitted with six or
eight azimuth thrusters. Many variations are
possible.Insomevessels,theruddersareDPactive,
toprovidetransverseforcesaft,whileinothervessels
theruddersarenotcontrolledbytheDPsystem.
Vital to the safety
of any DP operation is the
continuity of the power supply. The power plant
mustalwaysbeconsideredasanintegralpartofthe
DPsystem.Anyinterruptioninthesupplyofpower
canhaveheavyeffectson thepositioningcapability
of the MWTO. All MWTO with DP capability are
particularly
vulnerable to blackout or part‐blackout
situations.
DP‐capable MWTO must have a combination of
power, maneuverability, navigational ability and
computer control in order to provide reliable
positioningability.Thisformsanintegratedsystem,
whichconsistsofdifferentelements.
Nonlinear forces of external disturbances pla y a
major role. This forces
(Rd) are generated by wind,
currentandwavesandtryingtomoveMWTOfrom
holding position. DPS compensates these
disturbances by means of controls that produce the
necessary control forces R
c. Forces of controlled
reactions should be equal or greater than forces of
environmentaldisturbances.
n3
Xcp X j
p1 j1
n3
Ycp Y j
p1 j1
n3
XOYcp XOY j
p1 j1
RR,
RR,
RR
MM.
M,
,
M
d
cd
d
cd
d
whereR
c‐forceofcontrolledreaction;Mc‐moment
of controlled reaction; R
d‐nonlinear force of EF
disturbances(wind,currentandwavesforces);M
d–
nonlinearmomentofEFdisturbance;R
Xc,RYc,RXd,RYd
‐ projections of forces on the corresponding axis of
MWTO coordinate system; p‐number of MWTO
thrusters;j–numberofEF.
A comprehensive reserve of controlled thrusters
reactions∆R
c (∆Rc = Rc max – Rc) considered to be a
quantitativecharacteristicofDPsafety.Thecomplex
dynamic system of threshold type MWTO – EF
functioningnormallywhentheprocess ofnonlinear
disturbance and typical parameters of the system
don`textendbeyondallestablishedlimits.
Presently safety of DP operations assessed by
dynamicpositioningoperator(DPO)
onthe basisof
Company Safety Management System (SMS)
requirements, Failure Modes & Effects Analyses
(FMEA)andusingcapabilityplotdiagrams.
Capability plot diagrams commonly used for
environmentalforcesaffectassessment(Fig.2).
Figure2.TypicalDPcapabilityplotdiagramshape.
CapabilityplotdiagramsprovideDPOmaximum
safeacceptablewindspeedwithconsideredrotatable
current in MWTO coordinate system. Almost all
diagrams for self‐propelled MWTO have large
uninformative protrusions in sectors around 0 and
180degrees(mostlymorethan80knots).Capability
plot diagrams do not provide information about
active thrust
forces distribution under real
environmental forces disturbances. All this lacks
donotallowtoformaclearpictureforDPO.
61
Redundancy, according to the International
Maritime Organization (IMO), is the ability of
component or system to maintain or restore its
function, when a single failure has occurred. The
essence of redundancy is to enable the MWTO to
safelyterminateaDPoperationafterlosingacritical
componentorsystem.This
conceptis referredtoas
“Single Point Failure” mode. Most MWTO have
propulsion/thruster configurations beyond the
minimum, as these provide control options such as
minimumpowerconsumption,finepositioncontrol,
and barred zones for azimuth thrusters to protect
equipment. It also allows redundancy, which is the
abilityof avessel
tomaintain positionand heading
despite losing a component within its DP system.
Redundancy gives a MWTO added time to safely
shut down an operation should an failure occur in
theDPsystem.PotentialhazardassociatedwithaDP
operation is key in determining the level of
redundancy. On this basis
DP systems are divided
intothreeclasses.
The following methodology is proposed for
guaranteeing safety of navigation during DP
operationsandDPprocessoptimization.Italsocould
beapplicableforverificationofthedecisionvalidity
atrealrisksofcommercialMWTOexploitation.Itis
applicabletoMWTOofallDP
classes.
2 DPPROCESSDECOMPOSITION
DPsystem isa complex combinationof subsystems
interacting to automatically maintain a MWTO
position and heading with active thrust. Thrusters
arepositionedtoprovideoptimumDPcapabilityand
movement, with minimum interference with other
thrusters and sensors, using minimum fuel
consumption. It is also important
that thrusters are
positioned tocontrol the MWTO with minimal fuel
consumptionandtoreducethewearonthethruster.
ThemainelementofMWTOsystemisaHullthat
has mass, hydrodynamic and aerodynamic
characteristics (Fig. 3). It contains all other
subsystems.TheEFinfluenceontheHullsubsystem.
Energysubsystemactsasaresourceofsafetyfor
the MWTO. Input and output data from different
sensors, position meas uring systems and MWTO
subsystemsformdataflows.
Information subsystem distributes data flows
circulation during the interaction of DPS functional
elements. As well, it provides user interface and
operationstationinputs.
Computation
subsystemonthebasisofprocessed
data, which Information subsystem provides,
computes signals that should compensate forces of
externalinfluences.Mathematicmodelisdesignedto
computethedifferencebetweenthesetpointvalues
and offset values of heading and position.
Mathematic model issues forces to counteract
offsetting forces to return
MWTO to the set point
heading and position. This special calculating
modulecontinuallycalculatesMWTOresponsetoEF
nonlineardisturbances.Forcesdistribution algorithm
calculates force for each thruster. Computation
subsystem should be considered as an appropriate
open system that connects data from the various
sources and produces control signals for the
Controlledthrusterssubsystem.
The number of control computers will depend
upon the level of redundancy available, which in
turnwillrelatetotheDPequipmentclass.Insimple
terms,singlecomputersystemsarefittedinMWTO
withoutacriticaldependenceuponpositionkeeping.
This provide no redundancy. Higher levels of
reliability are required for MWTO undertaking
technological work involving higher risk. Dual or
triple control computer systems can provide extra
redundancy.
Figure3.MWTO‐EFsystemscomplexinteraction.
Controlled thrusters subsystem produces
appropriate control force vectors, which are
necessary for the MWTO reaction to EF nonlinear
disturbances. At the next decomposition level, each
of these subsystems could be represented as
compositionofinteractedsubsystems.
62
EF system EF consists of Water and Air
subsystems. On the fluidity property basis, it is
possible to describe these subsystems as va rious
models of fluid. Models of fluid motion reflect
followingsubsystems:Wind,WavesandCurrent.
Mathematical models of interacting subsystems
representedbyequationsofarigidbodymotion
ina
fluid, equations of hydrodynamics and
aerodynamics, equations of electric drives
electrodynamics, equations of thruster`s mechanics,
equations, that describe processes in DP control
systems.
3 METHODOLOGY
The proposed methodology is based on a new
integratedparadigm of coordinated elementactions
to support decision‐making process regarding DP
safetyin
locallyconfinedareaoftechnological work
(Fig.4).
Figure4.Informationandlogicalmodelofmethodology.
Data, that describe MWTO physical
characteristics, enter at the beginning. These data
couldbeenteredmanuallyorcouldbeselectedfrom
the library of existing objects. Mathematical
description of complex MWTO system is based on
thesedata.
Asanexampleproposedmethodologyappliedfor
typical AHTSS vessel UT 733‐2 project
(Fig.5). The
MWTOequippedwithtwo bow thrusters(800 BHP
each) and two azimuth stern thrusters (3600 BHP
each).
Figure5.AHTSSUT733‐2.
Step1.MWTOcharacteristicsdetermination.The
determinationof hullcharacteristicrequire building
an adequate hull model. At present, there are a lot
surface‐modeling programs, which allow to
determine MWTO hull principal dimensions,
hydrodynamic and aerodynamic characteristics,
added masses and moments of inertia. Following
modelswerebuiltusingFREE!shipsoftware
When
MWTO model is built, before proceeding
with calculation of characteristics, appropriate
surfaces should be checked for any errors and
disjointsegments.
Gaussian curvature is most common method,
whichcouldbeusedtocheckthemodel(Fig.6).The
model is shaded in colors, based on the discrete
Gaussian curvature in each
point. Most hulls are
curved in two directions, called the principal
curvatures. Gaussian curvature is the product of
these two principal curvatures. There are 3
possibilities.
Figure6.MWTOmodelGaussiancurvature.
Negative Gaussian curvature. These areas are
shadedblueandhavetheshapeofasaddle,sincethe
curvature in one direction is positive while the
curvatureintheothermustbenegative.
ZeroGaussiancurvature.Atleastoneofthetwo
principal curvatures is zero, sothe surface is either
flat
orcurvedinonlyonedirection.Inbothcasesthe
63
surface is developable (This is in fact a very
important property of developable surfaces). These
areasareshadedgreen.
Positive Gaussian curvature. The curvature in
bothdirectionscanbepositiveornegative, butmust
have the same sign. These areas are convex or
concaveandshadedred.
Zebrashadingis
anotheroptionmethodtocheck
the model (Fig.6). Regions with a constant light‐
reflectionintensityareshadedinbands.
Figure6.MWTOmodelzebrashading.
Thisissimilartothewaythehumaneyedetects
unfair spots on a surface since the shininess and
shadowsvaryinthoseareas.Iftheedgesofthezebra
stripes are curved smoothly then the surface is
smooth in these areas. At knuckle lines they vary
abruptly.
Wind forcesare
calculated by using well‐known
formula, where major role aerodynamic coefficients
play:
2
2
2
,
2
,
2
,
2
a
AX x Sх wr
a
AY y Sy wr
a
AMZ mz Sy Sy wr
p
RCFV
p
RCFV
p
M
CFLV
where R
AX‐longitudinal force; RAY‐lateral force;
M
AMZ‐yawing moment; Cx‐longitudinal force
coefficient;C
y‐lateralforcecoefficient;Cmz‐yawing
momentcoefficient;p
a‐densityofair;FSx‐transverse
projectedwindarea;F
Sy‐lateralprojectedwindarea;
L
Sy‐distancetocentroidoflateralprojectedarea;Vwr
‐relativewindspeedbetweenwaterlevelandtopof
superstructure.
DeterminationofaerodynamiccoefficientsC
x,Cy,
C
mz is carried out for MWTO hull (excluding
underwater part) and superstructures using
superposition method. As MWTO superstructure
consists of many adjacent elements, which can
overlap each other, care should be taken during
calculation of appropriate wind exposed surfaces
areas(Fig.7).
Figure7.MWTOelementsexposed towindinfluence.
Toincreaseaccuracyofthemodelitissuggested
to include in calculation bulwarks, masts, massive
deckequipment(towwinch,windlass,etc.).
Added masses and moments of inertia can be
understoodaspressure‐inducedforcesandmoments
duetoaforcedharmonicmotionoftheMWTOhull
proportionaltoitsacceleration.
The concept of fluid kinetic energy is used to
derivetheaddedmassterms.Moreover, anymotion
oftheMWTOwillinduceamotionintheotherwise
stationaryfluid.InordertoallowtheMWTOtopass
throughthefluid,itmustmoveasideandthenclose
behindtheMWTO.
For completely submerged MWTO it will be
assumed that the added mass coefficients are
constantand thusindependentof the wavecircular
frequency. In generally it represents by matrix of
addedmassesandmomentsofinertia.
11 12 13 14 15 16
21 22 23 24 25 26
31 32 33 34 35 36
41 42 43 44 45 46
51 52 53 54 55 56
61 62 63 64 65 66
Matrix consists of 36 elements, which represent
fulladdedmassesandmomentsofinertia.
For the MWTO with partly submerged hull
definitionisundertheassumptionofanidealfluid,
no incident waves, no sea currents, and zero
frequency.
For most MWTO (like considered AHTSS) it is
common to decouple
the surge mode from the
steering dynamics due to xz‐plane symmetry. As
well,theheave,pitch, androllmodes are neglected
undertheassumptionthatthesemotionvariablesare
smallanddonothavegreatinfluenceontheMWTO
during DP operations in locally confined area of
technological work. In
this case, it is possible to
represent simplified matrix of added masses and
momentsofinertia.
64
11 13 15
22 24 26
31 33 35
42 44 46
51 53 55
62 64 66
000
000
000
000
000
000
Bothmatricesaresymmetricalrelativetothemain
diagonal. Further simplifications could be done on
the assumption that during DP only MWTO
movement in horizontal plane is controlled by DP
system.Itmeansthataddedmassesonxandyaxis
andmomentofinertiaonzaxisplaysignificant
role
(numbersinthematrix11,22and66respectively).
Addedmassescouldbedefinednumericallybya
method of successiveapproximations at the
decision of integrated equations Fredholm of 2‐nd
sortconcerning density of hydrodynamic features
(sources‐effluents) distributing continuously on
surface MWTO hull withtheaccountand
without
takingintoaccounteffectofabottomand
a trim. Thus we believe, that a fluid ideal,
incompressible, and movement potential. MWTO
hull isconsideredasthesolid limited to the
closedsurfaceandmovinginaboundlessfluid.
Step 2. MWTO dynamic equations
transformation. It is possible to represent
MWTO
movementmathematicaldynamicsmodelinCauchy
formbyonenonlineardifferentialgeneralequation:
(,,),
dv
f
dct
dt
where ʋ ‐ MWTO state vector; d‐vector of
disturbances; c‐vector of controlled reactions; f‐
non‐linearfunction;t
0≤t≤ti‐simulationstepinterval.
TheMWTOstatevectorgeneralformincludes12
variables:
[, ,,,,,,,, , , ].
i ig ig ig ix iy iz i i i ix iy iz
vxyzVVV
When analyzing MWTO motion it is convenient
to define the following well‐known coordinate
systems.
TheNorth‐East‐Down(NED)coordinatesystemis
defined as relative to the Earthʹs reference ellipsoid
(World Geodetic System 1984). It is defined as the
tangent plane on the surface of the Earth moving
withtheMWTO,butwithaxespointingindifferent
directionsincomparisonwiththebody‐fixedaxesof
the MWTO. For this system, the x‐axis points
towards true North, the y‐axis points towards East
while the z‐axis points downwards normally to the
Earthʹs surface. During MWTO DP
operations in a
local confined area, it is possible to consider that
longitude and latitude are constant. Therefore, it is
convenient to use for navigation an Earth‐fixed
tangentplaneonthesurface.Thisisusuallyreferred
toasflatEarthnavigationanditwillforsimplicitybe
denotedas
theNEDframe.ForflatEarthnavigation,
itispossibletoassumethattheNEDframeisinertial
suchthatNewtonʹslawsstillapply.
MWTO coordinate system (Fig. 1) is a moving
coordinateframe,whichisfixedtotheMWTO.The
MWTO orientation uniquely determined by the
appropriate angles between
axis of coordinate
systems.
In MWTO complex mass center flat
movementconsideration, some state variables were
excluded, because they are not controlled by DPS.
These state variables are: complex mass center
movement along the vertical axis of the MWTO
coordinatesystem,projectionofthelinearspeedona
verticalaxis
ofthe NEDcoordinate system,angular
movements around the longitudinal and transverse
axes of the MWTO coordinate system, angular
speeds aroundthe longitudinal and transverseaxes
of the NED coordinate system. Thus, MWTO state
vectorincludessixvariables:
[, ,,,, ].
iigigixiyiiz
vxyVV
DPS controls the movement of MWTO in the
planeofthehorizonanddoesn`tcontrolthevertical
movement, rolling and pitching. This significantly
simplifies the model without harming desired
scientific results. A Vector of controlled reactions
represents the projection of forces and moments
generated by controlled thrusters. A vector of
externaldisturbancesrepresentstheprojectionofEF
forcesandmoments.
Step 3. Controlled thrusters mathematical
description. During mathematical description of
controlled thrusters subsystem following should be
the considered: thrusters and hull interaction,
forbidden zones formation for each thruster,
influenceofMWTOdynamiconthethrusteffect,all
possible DP thrusters allocation
modes. Forbidden
zones and allocation modes are depend of MWTO
type, position measuring equipment, nature of
technologicalwork.
Open water propeller characteristics could be
describedbyfollowingformula:
22
(),
pnn VnPOBTVVMWTO
RTnTVnTV
whereR
p‐propellerforce;Tnn,TVn,TVV‐coefficients
of dynamic influence; n‐rotational speed of
propeller;V
MWTO‐MWTOspeed.
Dynamic coefficients T
nn, TVn, TVVare
representedbyformula:
4
3
3
(3) ,
(2) ,
(1) ,
nn tp w р
Vn tp w р
VV tp w р
TK pD
TK pD
TKpD
whereK
tp – specific polynomial, Ktp = (‐0,1060;‐
0,3246; 0,4594), for particular case; p
w‐density of
water;D
p–diameterofpropeller.
65
Mostly MWTO with azimuth thrusters are
equippedwithaccelerationnozzles.Nozzlesprevent
propellersfromdamageandincreasetheirefficiency.
Where acceleration nozzles are applicable, open
water propeller force should be multiplied by
respectiveaccelerationcoefficient.
Power resources, which are represented by
Energysubsystem,shouldbesimulatedaccordingto
MWTO classification
society approved Power
ManagementPlan.
Step 4. MWTO motion control mathematical
description. During this step, hierarchical
performance of Computation and Information
subsystems are described. Computation subsystem
mathematical description involves development of
station keeping and low speed maneuvering
algorithms on MWTO mathematical model basis.
Informationsubsystemprovidesdata fromdifferent
sources
for these algorithms. This data include
MWTO position, kinematics and dynamics,
disturbance and reaction forces in different
coordinate systems, power reserve, thruster
allocation and set point, etc. Forces distribution
algorithm computes necessary forces for each
availablethrustertoperformDPoperations.
Stage 2. EF system dynamics mathematical
description is carried out
on the basis of
meteorological data and DP locally confined area
analysis. At this stage, depending on the used
method and accuracy requirements, the following
shouldbedetermined:EFcharacteristics,characterof
hydrological features (Froude number, tide heights,
etc.). The EF conditions could be also defined as a
one minute
mean maximum wind velocity, a most‐
probable significant wave height, and a most‐
probablewavemodalperiod.
Thelibraryofwaterareasiscreatedonthisstage.
SystematicEFwaterareadataincludethefollowing:
generalweatherconditionsinlocallyconfinedareaof
DPoperations,characteristicsofwindspeedandits
direction, characteristics of sea state (wave heights,
swelldirection),characteristicsofcurrentspeedand
itsdirection(peculiaritiesofchanges).
Step1.Nonlinearwindmodel.Windisdefinedas
the movement of air relative to the surface of the
Earth. Mathematical models of wind forces and
momentsimproveperformance androbustness
ofthe
system in extreme conditions.The nature of the
nonlinear component depends on the water area of
MWTO operation. The appropriate spectral
characteristicsmayalsobeused.
Step 2. Nonlinear wave model. The process of
wave generation due to wind starts with small
wavelets appearing on the water area surface.
This
increasesthe dragforce, whichinturn allowsshort
wavesto grow.Short wavescontinue to grow until
they finally break and their energy is dissipated. A
developingsea,orstorm,startswithhighfrequencies
creating a spectrum with a peak at a relative high
frequency.Astorm,which
haslastedforalongtime,
creates a fully developed sea. After the wind has
stopped, a low frequency decaying sea or swell is
formed. These long waves form a wave spectrum
with a low peak frequency. If the swell from one
storminteractswiththewavesfromanotherstorm,a
wave spectrum with two peak frequencies may be
observed. In addition, tidal waves will generate a
peakat alowfrequency. Hence,the resultingwave
spectrummightbequitecomplicatedinwaterareas,
wheretheweatherchangesrapidly.
Step 3. Nonrotational current model. Current is
definedashorizontalmotionof
watersystemswitha
constantaveragespeed.Verticalmovementofwater
particlesfromonelayertoanotherisnotconsidered.
Stage 3. MWTO‐EF interaction. Step 1. MWTO‐
wind interaction. Assumption of wind flow
homogeneityandquasi‐stationarypropertiesplay s a
major role in the mathematical description of the
MWTOreaction
towinddisturbance.Calculationof
wind forces and moments acting on MWTO results
fromrelativewindspeedandangleisdone.
Step 2. MWTO‐wave interaction. The interaction
of MWTO hull with waves is a complex physical
process.ThemathematicaldescriptionoftheMWTO
reaction on the wave disturbance requires
consideration on regular and irregular waves.In
the first case, the model will have deterministic
nature,andinthesecond‐astochasticnature.Wave‐
inducedforcesandmomentonMWTOarecalculated
using force transfer function. Wave height is a
governing factor of this function. Research of the
stochastic model is
more complicated and time
consuming,butresultsmoreaccuratelydescribethe
reactiontothewavedisturbance.
Step3.MWTO‐currentinteraction.The natureof
the MWTO reaction depends on the current speed
and direction. All hull protractions hydrodynamic
effectiscalculatedonthisstepaswell.Consideration
of current speed
changes, caused by hydrological
featuresofthewaterarea,improvestheaccuracyof
results.
Inordertoreceiveparametersofthestatevector
to evaluate MWTO safety of DP operations, which
undergononlinearEFdisturbances,itisnecessaryto
make description of the MWTO subsystems
interaction, which are coordinated to solve
the
followingtasks:
Decompositionofcomplexsystemsandtasksinto
moresimple(typicalorstandard).
Relationship determination between selected
componentsinthelogicalgorithmsform.
Distributed processing by Computation
subsystem of various primary data from
Information subsystem. During this process,
navigation regarding MWTO dynamic
positioning in locally
confined space are formed
andrepresentedtoDPO.
Distributed secondary processing of aggregation
orgroupdata, whichtogetherreflectMWTOstate
vector characteristics and EF disturbance main
vector characteristics (course over ground,
heading, speed, direction and force of
disturbances).
Determination of threat directions that form
extremesituationconcerningDP
safety.
Parameters identification (for components of
Controlled thrusters and Energy subsystems),
which are required for safe and effective DP
processrealizationinlocallyconfinedarea.
Realization of identified parameters by DP
processcontrollaws.
66
Stage4.Processingandanalysisofresults.Step1.
MWTO thrust reserve (load) plots and polar
diagrams development. EF disturbancelead to DPS
reaction, which through the forces distribution
algorithm specifies load set points to controlled
thrusters.Thecomprehensivereserveofcontrollable
thruster’s reactions represents a quantitative
characteristic of MWTO
DP safety.The curves of
thrust reserve received for specific weather
conditions, together with the assessment of the
MWTOstatevectorprovideanopportunitytoassess
the DP safety and execution of technological work
expediency.
However,thesecurvesdescribeonlyonepossible
MWTOheading.Thepictureofthrustreservewill
be
differentif MWTO headingwill change duringDP.
This happened due to changes in MWTO‐EF
interactionparameters.A similarsituationobserved
whenparametersofEFchanged.
For the complex situation assessment, it is
necessarytoknowhowthrustreservedistributeson
allpossibleMWTOheadings.For thispurpose, it
is
convenient to use polar diagrams that characterize
spatialdistributionsofthrustreserve.
Step 2. Polar diagrams analysis. Polar diagrams
represent thrust reserve (load) on MWTO possible
headings range (360 degrees) and expand proposed
methodology opportunities. Polar diagrams allow
DPO to ensure MWTO safety of navigation during
DP, establish safe
abortion route in case of any
emergency,optimizeDPprocessandreducecostsof
MWTO commercial exploitation. Identified
limitations,imposedbythecharacteroftechnological
workandEF, haveagreatimportanceandshouldbe
reflectedinthediagram.
Library formation of existing MWTO objects,
limitations dueto EF disturbance and
technological
worktakeplaceduringmethodologyperformance.
Decisionmaking bythe DPOis a crucialpart of
the methodology. Accordingly to the philosophy of
proposed methodology, the main control element,
whichensuresafetyof DPisDPO.To makeproper
decisionDPOshouldconsiderresultsofsolvingthe
followingtasks
duringlimitedtimehorizon:
AssuranceofDPprocesssafetyandoptimization
in locally confined area of technological work
execution.
Assessment of the current situation, which can
rapidly change because of external disturbances
or because of MWTO factors (failure of DPS
components,etc.).
Situational decision‐making, especially in
the
criticalandextremeconditionsofDPoperations.
Implementation of decisions taken according to
identified resourcesand limitations of active
control. Thatprovided adaptation of the MWTO
operativecontrolsynthesizedlawstothepractice
ofachievingtargetedresults.
4 IMPLEMENTATION
Implementation of proposed methodology was
conducted on board DP‐
1 vessel project UT 733‐2.
VesselperformedDPinlocallyconfinedareaduring
supplyoperations ofoffshorefacilityinPersianGulf
(Fig.8).
Figure8.Supplyoperationprocess.
Thefollowingweatherconditions wereobserved
in the locally confined area of technological work:
winddirection25°,speed11knots;currentdirection
50°, speed 0.9 knots; wave direction 300° degrees,
observedsignificantwaveheight1m.
Company SMS requirement: 80% maximum
thruster load. Following polar diagram of thruster
loadswascalculated
(Fig.9).Onthediagram,which
referredtoNEDcoordinatesystem,loadsofthebow
thrusters and azimuth stern thrusters were
representedasMWTODPSreactionsonconcreteEF
disturbances.Reference toNED coordinate system
allows DPO to plan easier MWTO movements and
abortion route. This diagram could be attached
to
navigationchartorputnearECDISscreenforquick
reference.
Loads greater than 80% form “EF limitation”
sectors. These limitations correspond to currently
observed weather conditions. If the EF values
change,thepolardiagramwillalsochange.
67
Figure9.Polardiagram.
MWTO’s cargo and cargo hoses connections are
locatedonthesternofthevessel (whichiscommon
to all AHTSS). It means that during supply
operations the vessel should be parallel or stern to
the offshore facility. This condition superimposes
with the “EF limitation” sectors and forms
“Technologicalworklimitation”sector.
The remaining sector forms “DP safe headings
sector”.Allheadingswithinthissectoraresafeand
allow performing supply operations. 300° heading
wasselectedasfinalpracticalheadingtoconductDP.
Using described above methodology, it is also
possibletopredictthemaximumweatherconditions
that the MWTO is able
safely to continue DP
operations (maintain position and heading within
specifiedlimits,takingintoaccountboththeaverage
environmentalloadandthevesseldynamics).
SafetyofDPprocessisguaranteeaccordingtothe
proposed methodology by using researched
analyticalregularitiesofinterdependenceof parallel
processesofdiscretenavigationandcontinuousDPS
control of MWTO state vector under nonlinear EF
disturbances.
5 CONCLUSIONS
The proposed ergatic interaction structuration
provides adaptation processes of high performance
and operational problems solving, which arise
duringDPoperationsinextremesituationswherethe
timefactoriscrucial.Describedabovemethodology
allows ensure safety and increase efficiency of
DP
operations and is applicable to all types of MWTO
andtheirpropulsionconfigurations.
The formed knowledge base allows DPO to
ensure MWTO DP safety in various conditions.
Introduced polar diagrams referred to NED
coordinate system, have an adaptive and dynamic
nature and favorably differ from known capability
plot diagrams, which
are static and referred to
MWTOcoordinatesystem.
REFERENCES
Bray,D.J.1999.Dynamic PositioningOperator Training. The
official guide to The Nautical Institute training standards.
2ndedition.London:TheNauticalInstitute.
Fossen, T. I. 2002. Marine Control Systems. Trondheim:
NorwegianUniversityofScienceandTechnology.
IMCA M 140. 2000. Specification for DP Capability Plots.
London:IMCA.
IMCA M 178.
2005. FMEA Management Guide.London:
IMCA.
IMCA M 117. 2006. The Training and experience of Key DP
Personnel.London:IMCA.
IMCA M 182. 2006. International Guidelines for the Safe
Operation of Dynamically Positioned Offshore Supply
Vessels.London:IMCA.
IMCAM103.2007.GuidelinesfortheDesignandOperationof
Dynamically
PositionedVessels.London:IMCA.
IMCAM190.2011.Guidance for Developing andConducting
Annual DP Trials Programmes for DP Vessels. London:
IMCA.
IMO MSC Circular 645. 1994. Guidelines for vessels with
dynamicpositioningsystems.London:IMO.
Morgan, Dr. M. J. 1978. Dynamic Positioning of Offshore
Vessels. The Petroleum Publishing Company. Tulsa,
Oklahoma.
Sizov,V.G.2003.Shiptheory.Odessa:Fenix.
UKOffshoreOperatorsAssociation/Chamberof Shipping.
2002.Safe Managementand Operation of Offshore Support
Vessel.London.
