21
1 DYNAMICALLYPOSITIONEDUNITSAND
THEIRAPPLICATIONS
Fastgrowthofdemandforfossilfuelsandinabilityto
meet it only from land‐based reservoirs created a
need to start exploiting offshore oilfields. To do so,
however,specialpositioningtechnologywasneeded,
allowing drilling ships to maintain position in most
hydro‐ and meteorological condit
ions. That is how
dynamic positioning systems were invented. Det
Norske Veritas (DNV, now DNV‐GL after merging
with Germanischer Lloyd) provides following
definition of dynamically positioned (DP) vessel
(DNV,2013):
‘A vessel which automatically maintains its
position and heading (...) exlusively by means of
thrusterforce’
DPvesselsareused by offshoreindust
ry,mainly
for:
maintainingpositionofdrillingshipsifanchoring
isfoundtobeimpractical;
keeping pipe‐laying vessels exactly over the
predeterminedtrackofthepipeline;
maintaining safe distance between two units
during cargo or crew transshipment from one to
another;
otheroperations,wherestablepositionorheading
iscrucial.
To ensure safety of operations, redundancy is
introduced. For each pa
rt of equipment, a backup
devices must be ensured to guarantee proper
functioningofthesystemevenifsomedevicesfailto
operate.DPsystemsaredividedintoclassesfromDP‐
1(no redundancy, failure ofsingle device can cause
the whole system to collapse) to DP‐3 (syst
em
continuesto workevenif oneengine roomis under
fire or flooded). Decision on which class of the
equipmentshallthevesselpossesstoperformhertask
is made by operator according to required level of
reliabilit
y. It must be underlined, however, that DP
systems are recommended to support offshore
operation only when no failure is detected. When it
occurs, the operation should be suspended – DP
system’s (no matter its class) objective is to ensure
safewithdrawalfromtheoperation(IMO,1994).
SLAM – Based Approach to D
y
namic Ship Positioning
K.A.Wróbel
GdyniaMaritimeUniversity,Gdynia,Poland
ABSTRACT:Dynamicallypositionedvessels,usedbyoffshoreindust
ry,usenotonlysatellitenavigationbut
alsodifferentpositioningsystems,oftenreferredtoas‘reference’systems.Mostofthemusemultipletechnical
deviceslocatedoutsidethevesselwhichcreatessomeproblemswiththeiraccessibilityandperforma nce.Inthis
paper, a basic concept of reference system independent from any external device is presented, ba
sing on
hydroacoustics and Simultaneous Localization and Mapping (SLAM) method. Theoretical analysis of its
operabilityisalsoperformed.
http://www.transnav.eu
the International Journal
on Marine Navigation
and Safety of Sea Transportation
Volume 8
Number 1
March 2014
DOI:10.12716/1001.08.01.02
22
Vessel’s position and heading is constantly
monitored and controlled by dynamic positioning
system, consisting of sensors, displays, operator
panels, thrusters etc. A very important part of DP
systemisalsoapositioningreferencesystem.
2 REFERENCESYSTEMS
Precision and accuracy of systems widely used by
merchant marine is insufficient for
dynamic
positioning purposes. Therefore, some special
reference systems must be introduced and the most
important and frequently used of them will now be
brieflydescribed:
Satellite navigation – global navigation satellite
systemsarebasedonreceivingdatafromsatellite
unitsandprocessingitbythereceiverinorderto
compute global‐referenced position and other
parameters, like heading for instance. Errors
produced by ionospheric disturbances,
inaccuracies in satellites’ constellation
measurements etc., can be reduced by using
differentialcorrections (calculatedbyshore‐based
stations and transmitted either by them or by
commercial satellites). Costs of obtaining highly‐
precise corrections prove to
be too high for
shipping but negligible in offshore industry.
Global coverage is the biggest advantage of such
systems;
Hydroacoustic–thosesystems’workingprinciple
is based on using sound wave emitters and
receivers, located both on the seabed (referred to
as ‘transponders’) and ship’s hull (‘transducers’).
Transponders serve
as objects, distances or
bearings towards which can be determined. The
biggest advantage of using hydroacoustic
navigationsystemsisthatprecisionofnavigation
is higher than in dead reckoning and can be as
high as 5 millimeters (Rowiński, 2008). On the
otherhand,thebiggestlimitationofsuchsystems
is
that transponders must be precisely placed on
the bottom of the sea before even operations can
startandthatmakesthemsuitableonlyfor long‐
term operations performed in a relatively small
area.Moreover,thetranspondersshallberaisedto
the sea surface periodically in order to clean it
from
algae and other marine organisms
(Rutkowski,2013);
Taut wire – in those systems, a clumpweight is
loweredto theseabed. Bymeasuring theamount
ofwirepaidoutandtheanglebetweenwireand
vertical, position can be calculated in relation to
position where the weight was initially lowered.
Accuracy
decreaseswithdepthofwateras ocean
currentmaycurvethewire.Horizontaltautwires
canalsobeapplied,measuringpositioninrelation
tofixedstructure;
Microwave – those systems’ working principle is
basedondeterminingvessel’spositioninreference
to fixed structure by using radar. In various
systems,
thiscan beachieved inmany ways, like
for example by detecting the difference from
parallel thatthetwo antennas areone to another
and using the signal to drive two coupled servo
motors to move the antenna on the station,
bringing it parallel to the counter station’s one
(Artemis) or
determining distance between
interrogatingantennaandtra nsponder attachedto
the fixed object together with angle between line
normal to the interrogator and transducer’s
direction e.g. by measuring the phase shift
(RADius,RadaScan);
Laser – infrared laser beams can be used to
determine the distance between two objects by
measuring
time in which light is produced by
vessel’s rotating antenna, reflected and received.
Special reflectors must be placed on the latter
objecttomakethispossible.Mutualorientationof
those two is determined by sensing the angle
betweenlaserandship’shull’splaneofsymmetry
together with her heading: Fanbeam, CyScan
(Rutkowski,2013).
Other reference systems’ working principles are
basedonusinginertialnavigationtogetherwithother
navigational devices. To ensure proper redundancy
andreliabilityofpositionfixing,atleastthree(voting
2 of 3) reference systems of different working
principlesshouldbeusedinatime.
Oneof the greatest
disadvantagesof above‐listed
reference systems (except for taut wire) is that they
requirespecial off‐vessel devicesor infrastructure to
operate. This creates a risk of failures the vessel’s
crewcouldnotcopewithorneedofdistributinge.g.
hydroacousticnavigationsystems’transpondersprior
to operation which can be found
economically
ineffective for short‐time activities. The solution of
this problem is finding a fixed objects, which
navigational parameters can be measured at open
seas.Sailorsin18
th
oreven15
th
centurywouldclaim
celestial bodies to meet the requirements, but the
precision and accuracy of celestial navigation is
insufficient for today’s applications. An only fixed
objecttodetermineafixtoappearstobeaseabed.
Vessel’s velocity in relation to seabed can be
measuredbyDopplerVelocityLogs(DVL).
However,
their precision and accuracy is highly dependent on
sound’s speedin water, which inturn changes with
depth(temperature andsalinity to be more precise).
Errors created by this effect disqualify DVLs from
being used in highly‐precise dynamic positioning.
There is, however, a theoretical possibility of
determining vessel’s motion
components by using
other hydroacoustic devices, initially designed for
different purposes, like for instance imaging sonar.
Here, a pulse of sound is directed downwards,
reflected bythe seabed and receivedbythe rotating
transducer (Figures 1 and 2). The signal is then
divided into a number of beams by highly
sophisticated algorithms and converted into water
depth values, creating 3D image of the seabed
(MarineElectronics,2014).Toachievehighresolution,
ahydroacousticwaveshouldbeofahighfrequency
(200 through 400 kHz) and low beamwidth.
Unfortunately, such a high frequency causes sea
watertodispersesoundwaveprettyquickly
andfor
that reason, transceivers must be placed relatively
close to the seabed (up to 500 meters in best case).
Thismethodissuitableespeciallyforlowspeedover
groundorwhenthevesselistomaintainherposition
which circumstances are common in offshore
industry.
23
Figure1: Imaging sonar working principle (Marine
Electronics,2014)
Figure 2: Imaging sonar 3D survey results (Marine
Electronics,2014)
3 SLAMDESCRIPTION
SimultaneousLocalizationandMapping(SLAM)isa
set of methods enabling navigation of autonomous
robotsinanunknownenvironment.Here,arobot(or
anyothervehicle,adynamicallypositionedvesselfor
instance)itselfusesonboardsensorstocreateamap
ofareainwhichitoperates andnavigates
usingthis
map.Toachievethat,probabilisticmethodsareused
to determine the spatial relationships between
characteristicpointsof–inunderwatercase–seabed
topography. Those points are then referred to as
‘landmarks’.Aprioriknowledgeregardingtheareaof
operationisnotrequired.
A vehicle is moving through
an unknown
environment, taking observations of an unknown
landmarks by using sensors (imaging sonar,
multibeam echosounder etc., Figure 3). At a time k,
followingparametersaredefined:
x
k – the state vector, describing the location and
orientationofthevehicle;
u
k – the control vector, applied at a time k‐1 to
drivethevehicletoastatex
k;
m
i – a vector, describing the location of the i
th
landmark whose true location is assumed time
invariant;
z
ik– anobservation taken fromthe vehicle of the
locationofthei
th
landmarkintimek.
Figure3:SLAMmethodprinciple(Durrant‐Whyte&Bailey,
2006)
Inadditiontothat,thehistoryofvehiclelocations
(x
0:k)andcontrolinputs(u0:k),setofalllandmarks(m)
andsetofalllandmarkobservations(z
0:k)arecreated.
In a probabilistic approach to SLAM problem, a
probabilitydistribution:
P(x
k,m|z0:k,u0:k,x0:k ) (1)
iscalculatedforalltimesk.Bythat,relativelocations
of landmarks m
0:k are determined. The accuracy of
landmarksobservationsincreasestogetherwiththeir
number, as the observations can be considered as
nearly‐independent. Even if landmark m
i observed
from position x
k is not visible from position xk+1, its
locationiscorrectedintimek+1,becauseitsrelationto
other objects remains known. Some errors are
introduced by sensors imperfections and inaccuracy
of x
kvector determination (Durrant‐Whyte&Bailey,
2006).Afteramapoflandmarksiscreated,aposition
of thevehicle itself can be determined in relation to
them–afixisobtainedbymethodsofold‐fashioned
pilotnavigation.Ergo,positionandorientationofthe
vehicle is determined in relation
to a grid of
landmarks. Furthermore, exact global referenced
positions(latitudeandlongitude)oflandmarksneed
nottobeknownasavesselcannavigateandoperate
inlocalsetofcoordinatesfromthetimek.Inthiscase,
however, coordinates of all infrastructure elements
mustbeknowninrelation
tolandmarks’gridbesides
or rather than being known in global set of
coordinates.
ConsiderDPvesselequippedwithimagingsonar,
proceeding on sea surface over the seabed. Its
orientationisheadingmeasuredbygyrocompassand
position–latitudeandlongitudemeasuredbyGNSS
(x
k). At time k, relative positions of landmarks are
measuredbysoundingandthenprocessedintoamap
of spatial relationships between landmarks
themselves.Relativepositioninthiscasemeansthree
distances from transducer: to port/starboard side,
24
ahead/astern (by measuring respective beam angles)
and vertical (by measuring time of sound wave
travelling in water). As the transducer is fixed to
ship’s hull, those distances and angles change only
when ship’s position or heading change (or, if the
transducer is rotating, its angle of rotation must be
taken
into account). In other words, by measuring
changeofthoseanglesanddistancesinkandk+p,the
changeofship’sorientationandpositioninrelationto
landmarks’ grid can be determined. GNSS fixing or
other reference systems can be used in order to
compareresultsorasabackup.
4 ANALYSISOFINITIALCONDITIONSOF
APPLYINGSLAMASAREFERENCESYSTEM
Ability to determine vessel’s position in relation to
fixedobjectswithincreasingprecision(inoppositeto
decreasing precision as in dead reckoning) makes
SLAM one of solutions to problem of positioning
vessels without relying on external sources of
information. Due to requirement of ensuring
redundancyinDPoperations,SLAMcanonlybeone
ofatleastthreepositioningsystems.Eitherway,there
aresomeconditionstomakethismethodapplicable:
Sensorsaccuracyandprecision–sensorsusedby
thesystemmustbeofhighestqualityavailableon
the
market to ensure satisfying precision and
accuracy of landmarks’ relative position
measurements. The most important technical
parameters of the sensors would be their pulse
length(andbythat–altituderesolution),number
of beams and maximum range. Resolution of
today’shydroacousticsensorscanbeashighas1.5
centimeters which
enables good positioning
precision.Specificrequirementsregardingsensors’
performanceandprecisioninbathymetricsurveys
can be found in(IHO, 2008). Themore beams of
smallerbeamwidthareused,thehigherhorizontal
resolutioncanbeachieved;
Depthofwater–asitwasmentionedbefore,using
highfrequencysoundwavecauses
problemswith
wavedispersionbyseawater.Thelongerdistance
the sound travels in water, the less accurate the
soundings become. Modern imaging sonars have
maximumrangeofsome400metersandthiscan
be the most serious limitation of described
system’s operational performance. However, the
devices are still being
developed and this
disadvantage’s significance can be reduced in
future, for example by progress in field of
parametric hydroacoutics. Loss of precision
characteristicsinwatersdeeperthanpreferableas
per sonar’s technical specification can be
determinedbyinspectionandmaybeasubjectof
furtherstudy;
Fixrate–assuming
soundspeedinseawatertobe
approximately1500mpsanddepthofwaterof400
meters,soundwavewouldrequiremorethan0.5
seconds to cover the distance from transducer to
landmark and back. Next pulse shall not be
transmitted sooner to avoid identification and
interferenceissues.Thus,positioncan
beobtained
not more often than this (plus the time the
transducer might require to rotate in angle
enabling sufficient number of landmarks to be
located), decreasing proportionally as depth of
waterinareaofoperationincreases;
Topographyoftheseabed–SLAMrequiressome
distinguishable points (‘landmarks’) to determine
vessel’s position in relation to them – the very
operational principle of the system is based on
isolating specific points from the background
(seabed) and identifying them using e.g. their
shapes. To do so, the system’s processing unit
must find points of depth significantly different
than those in vicinity and
then identify them as
eitheri
th
ori+1
th
landmark.Thiswouldrequirethe
seabedinoperationalareatobepleatedenoughto
isolateandthenproperlyidentifylandmarks.The
moreobjectscanbeisolatedfromseabed’simage,
thebetter precisioncanbe achieved.It shouldbe
noted that underwater infrastructure of oilfield,
such as pipelines or manifolds, can
serve as
landmarks. To improve probability of finding
properobjects,theswathareacanbeincreasedby
usingsensorsofgreateroradjustableswathwidth
(upto200deg.);
System’s design – reference system based on
SLAM method can be divided into hardware
(sensormeas uring relativepositionoflandmarks)
and
software–aprocessingunit. Thiswouldnot
only convert data regarding soundings into
vessel’sposition,speedandheading,butalsotake
otherfactorsintoaccountinprocessofdatafusion
includingangleofheelandtrim.Anotherfunction
ofprocessingunitcouldbeadjustingthesensor’s
operatingparameterslike
pulselengthdepending
onthedepthofwater.
5 SLAM‐DPADVANTAGESAND
DISADVANTAGES
Hereby presented system using SLAM methods
would have some advantages and disadvantages,
listedinTable1.
Table1. Advantages and disadvantages of SLAM
applicationindynamicpositioning
_______________________________________________
AdvantagesDisadvantages
_______________________________________________
autonomous operations (not maximum depth of water limit;
dependant on external inaccurate when flat seabed;
devices); possibility of interferences from
ship’s mobility not limited by hydroacoustic navigation
anchors etc.; systems;
constant or even increasing system’s precision and
precision; accuracy can be found
enables data regarding both poorer than those of satellite
position and heading; systems, making it
position relative to point of unsuitable for DP;
interest (manifold etc.) – ability of proper system
no need to know its maintenance reduced by
global-referenced position. sensor’s location under the
waterline.
_______________________________________________
6 SUMMARY
Preliminary concept of positioning reference system
basedondepthsoundingandSLAMpost‐processing
hasbeenpresented.Thedescribedsystemeliminates
somelimitationsoftoday’sreferencesystemssuchas
25
dependenceonexternaldatasources.However,ithas
itsowndisadvantagesaslistedinthepaper.Further
researchisrequiredtodeterminewhetherthedesired
precisioncanbeachievedusingthismethodandifits
performance can meet the requirements of dynamic
positioningprocess.
REFERENCES
DetNorskeVeritas.2013.RulesforClassificationofShips‐
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Durrant‐Whyte, H. & Bailey, T. 2006. Simultaneous
LocalisationandMapping(SLAM):PartITheEssential
Algorithms.Robotics&AutomationMagazine,IEEE13(2).
99‐110.
Grelowska,G.& Kozaczka,S.&Szymczak,W.2009.
Some
methods of the sea bottom recognition. Hydroacoustics
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International Hydrographic Organization. 2008. IHO
Standards for Hydrographic Surveys – Special
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th
Edition. Monaco: International
HydrographicBureau.
International Maritime Organization. Maritime Safety
Committee.1994. GuidelinesforVessels with Dynamic
PositioningSystems.MSC/Circ.645.
Marine Electronics Ltd. http://www.marine‐
electronics.co.uk/retrieved28‐01‐2014.
Rowiński, L. 2008. Pojazdy głębinowe – budowa i
wyposażenie.Gdańsk:WydawnictwoWiB.
Rutkowski, G. 2013. Eksploatacja statków
dynamicznie
pozycjonowanych.Gdynia:Trademar.
ACKNOWLEDGEMENTS
Author would like to express his gratitude to Prof.
Pawelski, J. of Gdynia Maritime University for his
supportandassistanceinpreparingtheherebypaper.
