441
1 INTRODUCTION
During last years the demand for natural resources
rapidly increased. In addition to land‐based sources
the submarine deposits are exploited intensively. In
parallelwiththeextractionofseabasinsresourcesthe
renewableenergysourcesarebuiltandexploitedon
the sea. These facts necessitate the transfer of
hydrocarbons and energy ashore. Under the bottom
ofthesea,numerousnetworksofpipelinesandcables
have been laid. Technological development allows
layingpipelinesandcablesontheevergreaterdepths
and the int
ensification of exploration and extraction
materialsfromunderbottomdepositsresultedina
significant increase in the amount of undersea
i
nfrastructure. China, for example, have a 3000 km
undersea pipelines and in the next decade are
planning to triple the length of this infrastructure.
Linkedtothisfactitrisestheproblemofthesafetyof
such structures and the safety of the marine
environment at risk of failure or damage. Statistical
data indicat
e that a significant threat to underwater
pipelines is the ships traffic especiallywith it the
riskofdamagebytheanchorsordirectlythehullhit.
Other dominant factors of damage underwater
infrastructure arefalling objects from oil rigs and
shipsduringcargoandsparepa
rtstransshipmentand
pipelines corrosion. The statistics of damage to
offshoresectorpipelinesaregatheredinthePARLOC
2001database.Dataarecollectedinyears1960‐2003
andconcernpipelinesintheNorthSea.Thedatabase
includes 1,567pipelines witha total length of 24837
km.
2 LITERATUREREVIEW
Risk of the offshore i
nfrastructure damageis very
important issue for companies operating the oil and
gas fields as well for the classification societies and
safety institutions which create rules and
recommendations for them. They can be found for
example in Det Norske Veritas or HSE
recommendations[1],[2],[3],[5].Moreover,thereare
scient
ific researches concerning safety and risk
assessmentinareaoftheriskforunderwaterpipeline
systems. Authors continue research contained in
Criteria of Accidental Damage by Ships Anchors of
Subsea Gas Pipeline in the Gdańsk Bay Area
K.Marcjan,R.Dzikowski&M.Bilewski
M
aritimeUniversityofSzczecin,Szczecin,Poland
ABSTRACT:Thispaperdiscussesissuesoftheaccidentalanchordamagetooffshoresubseapipelineasoneof
themostsignificantthreattounderwaterinfrastructure.Thedensityofvesseltrafficoverthepipelinebetween
platform Baltic Beta and Wladyslawowo power plant has been analyzed. In order to determine the most
common damages associated with heavy ship t
raffic, the authors used the risk model for underwater
infrastructure.Forthispurposeshipsanchorequipmenthasbeencategorizedandastheresultsthecriteriaof
damagetothepipelinehavebeendiscussed.
http://www.transnav.eu
the International Journal
on Marine Navigation
and Safety of Sea Transportation
Volume 11
Number 3
September 2017
DOI:10.12716/1001.11.03.07
442
papers [6], [7] for systems situated in the Polish
economiczone.
Figure1.Underwater pipeline damage statistic. Data from
PARLOC2001database
DatabasePARLOC2001groupsthemostcommon
causes of damage to the: damage by anchors of
vessels passing above the pipeline, hit by the shipʹs
hull,corrosion,technicaldefects,defectsinmaterials,
natural hazards, structural defects, technical
maintenance, human error, operational problems,
others.Fig.1.
DNVGLclassificationsocietyobservedincreasing
amount of the lost and dragged anchors. In Fig. 2
thereisthestatisticoflostanchors.
Figure2.Anchorlostper100shipyear.DatafromDNVGL
Scenarios of the subsea pipelines system risk
assessment should take into consideration the
following factors: vessels passing the pipeline,
including oil rig support ships (supply vessels,
floating cranes, ships, surveillance and diving
vessels), merchant ships and ferries, fishing vessels.
Byassumingemergencysituations,thereshouldbe
consideredsituationsofemergencyanchors
dropping
or dragging (important data aretype and weight of
anchors) and bottom trawling (network type, trawl
thrust)aswellasthecharacteristicsofthepipeline:
thetype(steel,flexible),thedepthofthebasin,deep
depressions in the ground, pipeline diameter, wall
thicknessandpipelagging.Modelofrisk
assesement
is presented in Fig. 3. To analyse the damage
importantistocheckifthedredgedanchorsareable
tocatchthepipelinesystem,whatisanalysiedinthis
paper.
Figure3.Modeloftheriskanalysisforoffshorepipeline
3 OFFSHOREACTIVITIESINTHEPOLAND’S
ECONOMICZONE
The exploitation of underwater resources by Polish
companiestakesplaceinPolandʹseconomiczone.In
the complex process of subsea exploitation, the
extraction of oil and/or gas is oneof the last stages.
Startingfromthedevelopmentofageologicalmodel
of
productionsitesmakinguseofgeophysicalsurvey,
the operations include the assembly and fixing of
drilling and production platforms and underwater
systems of pipelines and networks, seaborne
transport of hydrocarbons to land,movement of
drilling rigs to new locations and periodical
reconstruction of existing wells.Given below are
upstream activities
related to the oil and gas
production and operation and maintenance of
existingwellslocatedinthePolisheconomiczone:
1 explorationisperformedby:
seismic reflection survey vessels Polar Duke
andSt.Barbarathatcarryout3Dseismicsurvey
withinthelicensedareaofexplorationinfields
B21and
B16(Fig.4),anddrillingofexploration
holes.
drilling holes for geophysical survey and
measurements,executedbyPetrobaltic,Lotos
Petrobalticrigs.
2 productionperformedbyvarioustypesofrigs:
jackup–stationaryproductionrigs,
jackup–mobiledrillingrigs,
jacket–stationaryunmannedproductionrigs.
3 exploration
andexploitationwellsareestablished
inlicensedareas;exploitationwellsareusedfor
oil and gas extraction as well as injection of
depositwaterandseawaterfilteredtooptimizethe
production.
4 transfer of gas via an underwater pipeline to
Wladyslawowo.
Anchors/trawl/hull
empiricimpactmodel
Pipelinesystem
description
AISTraffic
information
VMS‐fishing
vesselstraffic
Information
Accidentsdatabase
Tolerablerisk
citeria
Shippingdata
analysis
(FEM)FiniteElementMethod
pipelinedamageevaluation
Pipelineis
threatened?
ON
Impactstudy
consequencemodel
443
5 transshipment from a single buoy mooring
(SBM)situatedneartheBalticBetarigandcarriage
ofoilbythemtIKARUSIIItoGdańsk.
6 transshipment from a single buoy mooring
(SBM) situated nearthe LotosPetrobaltic rig and
carriageofoilbythemtApatythtoGda
ńsk.
7 continuoussuppliestotherigsbyoffshorevessels
and supervision provided by standby vessels; at
present,thevesselsemployedforthepurposeare
thetugsAgath,BazaltandKambrandsupportships
AphroditeIandSeaForce.
8 jackuprigstowagetonewdrillinglocations.
9 submarine
work: divingand maintenance,use of
remotelyoperatedvehicles(ROV).
The system of gas transfer from Baltic Beta to
Wladyslawowo consists of the following items: The
gascompressionstationonanoilrigBalticBeta.
1 submarinetransmission pipeline with a lengthof
82kmandadiameterof115mm.
Thepipelinehas
been constructed according to the technology of
PrecisionTube Technology of Houston company.
Steelpipesareinsulatedwithpoliethylan
2 the station separation,gas storage and
preparation of fuel for the power plant with a
capacityof120000Nm3/day
3 Wladyslawowo power plant with
2 gas turbines
and two heat recovery boilers of total electric
powerapprox.12MWeandheatapprox.18MWt,
3peakboilersforgasandoilwithatotalcapacity
of15MW.
Underwater pipeline and the underwater
infrastructure in Baltic Beta safety zone is shown in
the(Fig.4).Pipeline
characteristic:
Material of pipe: Steel X65C, pipe tension
parameters: Re= 455 Mpa, Rm = 540 Mpa.
Protection: 3 layers: Valsparepoxide material,
Dupont epoxide polymer, polyethylene material.
Internal diameter 114,3 mm, external diameter 101,6
mm.FirsttransmissionofnaturalgaswasinAugust
2002andfirstheatingseasonusing
naturalgasstarted
inWladyslawowo–Autumn2003r.
Figure4. Underwater pipeline from Baltic Beta oil rig to
Wladyslawowo power plantand scheme of the
underwater infrastructure. Source: http://www.lotos.pl/,
ENCchart
4 SHIPSPASSINGDISTRIBUTIONOVERTHE
UNDERWATERPIPELINE
TheAISdata ofvessel trafficoverthe pipelinehave
been decoded and analyzed. Character of encoding
AIS information’s is based on 6‐bit the ASCII (the
totalnumberofcharactersinthe6‐bit codeis 64) as
opposed to theASCII
7 bit. An exampleof decoded
AISmassageshasbeenpresentedinthetableFig.5.
Figure5.DecodedAISdataintableform.
In order to determine the frequency and to
indicate the places where the ships pass over the
submarine pipeline the AIS data from two summer
months (June and July of 2011) were used. In those
month’stherewashighestvesseltrafficdensity.Inthe
Fig. 6and 7 below the ships passing
distribution is
presenteddependingonthelatitudeoftheexactplace
where ships were crossing over the pipeline. The
pipelineisdividedinto13 sections (every3minutes
of latitude) orapproximately every3 nautical miles.
IndesignatedsectionintheperiodfromJunetoJuly
2011thepipelinewas
passedby2,334shipsnearly39
shipsaday.
Figure6. Ships passing distribution over the underwater
pipelinewithbathymetrycurve
Figure7. Ships passing distribution over the underwater
pipeline
Tolerable risk criteria for pipeline damage by
dredgedanchor
444
Various parameters have an hooking subsea
pipelinebydredgedanchor.Oneofthemisgeometric
parameteroftheanchorversuspipelinediameter.
Themost importantislength betweenanchor shank
and top of the anchor fluke. A principle sketch is
showninFig8.
Figure8.Anchorhookinggeometry
TheminimumflukelengthLcanbecalculatedas
follow:
L=

α
21 α
max
Dsin
cos
–ΔL (1)
where:
D
max–maximumpipelinediameter,
α–anglebetweenshankandfluke,
ΔL–correctionforwidthoftheshank.
For diameter of the analysedpipeline 115 mm
the minimum length of the fluke is 158 mm.
ComparingwiththecatalogueofanchorsFig.9every
available anchor is able to
hook this pipeline. Fluke
lengthisEvalueinFig.9.
Figure9.Exampleoftheanchordimensioncatalogue.
Secondconditionisdependonverticaldistanceof
theanchor chain can reach during dragging.
Anchor chain is never vertical due to interaction
betweenchainandwater.ThatisillustratedintheFig
10.
Figure10.Forcedistributionindredgedanchorchain
Equationofdredgedanchorforcescanbewritten
inaccordanceNewton’sLawasfollow:
,0FmalT
 (2)


,
nt
ma l T Tt f n f t mgk
x

(3)
where:
m‐massofthechainperunitlength,
g‐accelerationofgravity,
,alT
‐accelerationofthechain,
l–coordinatealongthechain,
n
f
–normaldragforceperunitlength,
t
f
–tangentialdragforceperunitlength,
n
–normalunitvector,
t
–tangentialunitvector,
Fluke
Dmax
L
Shank
445
k
–unitvectorinthedirectionofgravity.
Therearetwocomponentsofdragforce:
n
f
=
2
2
Dn w
D
Cv
cos
2
α (4)
t
f
=
2
2
Dt w
D
Cv
sin
2
α (5)
where:
D
n
C ‐normaldragcoefficient,
D
t
C ‐tangentialdragcoefficient(DNV301),
w
–seawaterdensity.
D‐anchorchaindiameter.
In consequence we can write two differential
equations:
22
sin cos 0
2
Dt w
dt D
Cv mg
dl
 (6)
22
cos sin 0
2
DN w
dD
TC v mg
dl

(7)
For the above differential equation initial
conditionsareasfollow:
T(0)=W
anchor,α(0)=0
Weightoftheanchorinthewaterisreduced:
Wanchor=manchor g(1‐
)
w
s
teel
(8)
Parameters ofships anchors equipment have
been found using Equipment Number (EN) in
classification societies regulations. Theclasses of the
shipshavebeencorrelatedwithshipslength.
Solvingtheequationswecandeterminedifchain
anchor systemare capable to reach the subsea pipe.
That can be done using
Runge‐Kutta method. C‐
sharpprogramminglanguagehasbeenused.
Figure11. Vertical distance of dragged anchor chain for 4
classesoftheshipspassingoverthesubseapipeline.
Figure12. Results of the vertical distances of the dredged
anchorchainshipclassIIforvariousspeeds.
For the calculation of the speed of vessels, with
whichtheanchor will bein contact withthe bottom
dependingonthelengthofthevessel, the following
classesofvessels:
Class I: vessels of 30m‐60m (Fig.11‐brown) for
thisclassofshipwastheweightoftheanchor
in
therangeof360kg‐900kg,chainlength123.8m‐
178.8m.Shipsofthisclassaredragginganchorata
depth of 80mwhile moving ata speed 5.8w≤V
<10.8in.
ClassII:shipswithalengthof60m‐80m(Fig.11‐
green)forthisclassofassumed
importanceanchor
in the range of 1020 kg‐1590 kg, chain length
179m‐206.3m. Ships of this class are dragging
anchoratadepthof80mwhilemovingataspeed
10.8w≤V<14.6in.
ClassIII:vesselsof80m‐93m,whileoiltankersup
to123m(Fig.11‐blue
color);adoptedforthisclass
of anchor weightin therange 1700kg‐3500 kg,
thechainlengthof212m‐250m.Shipsofthisclass
are dragging anchor at a depth of 80m while
movingataspeedof14.6≤V<20.8in.
IVclassvesselsover93m,while
thetankerswitha
lengthofmorethan123m.Forthoseshipsanchor
regardlessoftheirspeedwillbedraggedatdepths
greaterthan80m.
In Fig.11 80 m depth line is marked, which is
maximumdepthoverthepipeline.
Towed anchor arrangement of passing ship will
stabilizedatcertainwater
depth.Thedragforcesare
proportional to the velocities squared. This implies
thatthetowdepthislessforhighvelocities.Example
isshowninFig.12.
5 CONCLUSIONS
1 To determine risk of damage of the underwater
infrastructure it is necessary to check the ships
trafficoverpipelinesystems.
Oneofthemethodis
toanalyzeAISdata.
2 The classification rules are source to determine
anchorsystemsequipmentofthepassingships.
3 Assignthecorrectanchorequipmenttoeachship
isdonebysegregationtheshipsusingtheirlength
andcorrelatingwithequipmentnumber.
446
4 Besides length and weights of ships anchors
systemspeed is important value to determine
verticaldistanceofdredgedanchors.
5 Using the results of the research it is possible to
findwhichshipsarethreatforpipeline.
BIBLIOGRAPHY
[1]DNV,DNV‐OS‐301, OffshoreStandard, Position
Mooring,2010,
[2]DNV,DNV‐OS‐F101SubmarinePipelineSystems,2010,
[3]DNV‐RP‐F107, Risk Assessment Of Pipeline Protection,
October2010,
[4]Dzikowski R.,Ślączka W., Analysis of IWRAP mk2
applicationforoilandgasoperationsintheareaofthe
Baltic Sea in view of fishing vessel traffic, Scientific
JournalsofTheMaritimeUniversityofSzczecin,2014,
[5]HSE,Guidelines For Pipeline Operators On Pipeline
AnchorHazards,2009,
[6]Kyriakides S., Mechanics of Offshore Pipelines, The
University of Texas at Austin and Edmundo Corona
UniversityofNotreDame,2007,
[7]
MacDonal M., “PARLOC 2001” The update of loss of
containment data for offshore pipeline”, Prepared for
The Health and Safety Executive, The UK Offshore
Operators Association and The Institute of Petroleum,
12June2003