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Copyright 2007, Offshore Technology Conference

This paper was prepared for presentation at the 2007 Offshore Technology Conference held inHouston, Texas, U.S.A., 30 April3 May 2007.

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AbstractDuring the last years the number of deepwater oil fielddevelopments has increased significantly, resulting in a strongdemand for pipe lay vessels. The continuously increasing day-rates of lay vessels is making the combined tow method for

deepwater pipelines and riser more competitive. Also arequirement for more sophisticated materials and strictqualification requirements to welded joints, due to sour serviceand fatigue performance, make the tow method a morepreferred solution since the pipeline is fabricated and weldedonshore. More than 50 bundles have been installed in the

North Sea towed out from the Wick fabrications site located inthe northeast of Scotland.

This paper describes a method of towing deep waterpipelines and risers, fulfilling both strength and fatiguerequirements. The concept incorporates presently availableequipment and technology. The proposed method isdemonstrated through two case studies presenting theinstallation of a gas export pipeline at a water depth of 800m

and a riser/pipeline string at a water depth of 300m.

IntroductionPipelines may be installed by the towing techniques wherelong sections of the line are made up onshore and towed withtug boats to the field. The design procedures for towed or

pulled lines are very dependent on the type of tow methodchosen. It is also important to control the submerged weight ofa towed line to minimize towing forces and at the same timehave sufficient weight for stability on the seabed in crosscurrents.

The combined tow concept reported in this paper

incorporate the following tow methods for deep waterpipelines and risers:

aPresently at J P Kenny, Norway

Figure 1 Controlled depth tow method

- The off-bottom tow method (with buoyancy tanksand chains) from the fabrication site to approximately

5km offshore.- The CDTM (Controlled Depth Tow Method, Figure

1) from 5km offshore to a temporary location wherethe buoyancy tanks are removed.

- The catenary tow method for the deep water tow tothe installation site (without buoyancy tanks and

chains).

For the off-bottom- and controlled depth tow method,buoyancy steel tanks are mounted at selected intervals. Chainsare also mounted at frequent intervals along the pipeline toovercome the excess buoyancy and keep the system stable.

During the catenary tow method the buoyancy modules andchains are removed and the submerged weight of the system

increases.

Tow MethodsIn order to use the tow methods, the pipeline is normallyconstructed at an onshore site with access to the sea. Once thepipeline sections are welded together to a determined length

and hydro tested, the pipe is de-watered and launched into thewater by a tow vessel attached to the lead end as seen inFigure 2. Onsite winches are attached to the trail end to ensureback tension and control the launch speed.

During this operation the varying curvature of the pipelinestring due to the tidal current is continuously monitored and

corrected by the lead tug. When the whole length of thepipeline is launched, submerged weight checks are carried outto ensure that the pipeline is suitable for towing. Chains areadded or removed to achieve the desired submerged weightbefore the tow vessels begin to tow the pipeline along thepredetermined tow route.

OTC 18797

Combined Tow Method for Deepwater Pipeline and Riser InstallationAlf Roger Hellest, Daniel Karunakaran, and Trond Grytten

a, Subsea 7, and Ove Tobias Gudmestad, Statoil and U. of

Stavanger


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2 OTC 18797

Figure 2 Tow out from shore

For pipelines that are to be towed into deepwater,pressurized nitrogen can be introduced into the buoyancytanks to prevent collapse or buckling of the tanks under highexternal hydrostatic pressure.

Pipeline installation by towing can be divided into threemain methods:

- Off-bottom tow- Mid-depth tow - CDTM- Catenary tow

The choice of method is dependent on the followingfactors:

- The submerged weight of the pipeline- Length of the pipeline- The seabed environment and presence of existing

pipelines along the selected tow route

The combined tow concept reported in this paper look intoa combination of the 3 methods presented above i.e. Off-bottom tow method from the shore to a suitable depth, CDTM

method from this point to entering deeper waters and Catenarytow for the deepwater tow to the installation site. In the off-bottom and controlled depth tow phases, the pipeline issupported by external buoys and chains which are removed byan ROV before the catenary tow phase. A short introduction tothe different tow methods used in the combined tow concept is

presented herein.

Off-bottom tow methodTo control the pipelines submerged weight, buoyancy

modules are mounted at selected intervals. Further chains aremounted at frequent intervals along the pipeline to overcomethe excess buoyancy and keep the system stable on the seabedin cross currents. During the off-bottom tow, the submergedweight of the pipeline together with the buoyancy modules

and chains are equal to the weight of chain links resting on theseabed. By controlling these weights the pipeline can belocated above the seabed at a predetermined height during thetow, see Figure 3. The advantages of this method is thatexisting pipelines can be crossed by placing concrete mats orsandbags over these lines allowing the hanging chains to be

dragged over the mats and not damage the coating. Off-bottom

tow is only feasible up to a certain depth as the buoyancybecomes more expensive as the water depth increase. Despiteof the longer exposure time (due to lower towing speed)compared to CDTM the total accumulated fatigue damage issmaller since the pipeline is further away from the surfacewave action. Off-bottom tow is used at locations where the

bottom conditions on the tow route are known and smaller

towing loads and fatigue damage are required.

Figure 3 Off-bottom tow method

Controlled depth tow method (CDTM)In controlled depth tow method, see Figure 4, the pipeline

is kept between the lead and trailing tug. The total submergedweight of the pipeline, floats and chains is negative. Duringtow, the drag on the chain creates a 'lift force' shown in Figure5, which reduces the systems submerged weight. An

increasing flow velocity is consistent with an increasing anglebetween the drag chains and the vertical. The submergedweight is hence also determined by the relative water velocitypast the chain.

Figure 4 Controlled depth tow method

Figure 5 Forces on the tow chain during tow

At the design tow speed, the pipeline will lift off from theseabed and adopt the desired mid-depth CDTM configuration.The lift is dependent on:


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- Speed- Type of chain- Number of links

By controlling the tow speed and tow wire length, the

pipeline configuration is maintained within acceptable limits,as defined by static and dynamic tow analyses. These essential

parameters are continuously monitored during the tow, andadjusted if necessary. Figure 6 illustrate how the systemssubmerged weight and tow speed affect the lift force. If thepipeline/floats system total buoyancy decrease, less chain is

needed to achieve the design submerged weight (heavybundle) and a higher tow speed is needed to obtain anadequate lift.

Figure 6 Lift force as function of tow speed

The advantages of the CDTM is the tow speed which ishigher than for off-bottom tow, and the absence of contactwith the sea bottom which allows passing severe slopes or

rocky bottom conditions. The maximum tow speed conductedfrom bundle projects today is close up to 3.5 m/s. Normal towspeed is in the range of 2.0 2.5 m/s.

Catenary towIn the catenary tow, the pipeline is in a catenary

configuration between the tugs, see Figure 7. The requiredbollard pull of the two tugs increases as the water depth

decreases. A catenary tow is not possible in shallow depthsince the required horizontal bollard pull forces to keep thepipeline sag-bend of the seabed are too high for conventionaltugs. The installation on site is simply done by paying out onthe tug winch wires while controlling the touch down routingwith the vessel position.

Figure 7 Catenary tow m ethod

Combined tow method for deep water installation

A main challenge with towing pipelines in deep water hasbeen the buoyancy needed to support the pipeline. The

required wall thickness in deep waters to avoid collapse makes

the steel buoyancy tanks too heavy and also the use ofsyntactic buoyancy at these depths is expensive. Furthercontrolled removal of buoyancy in deepwater may be a timeconsuming and expensive task because the buoys may need tobe recovered individually.

However, a new buoyancy tank design proposed by Subsea7 together with making use of pressurized nitrogen to reduce

the required wt can solve the weight and handling challenges.The nitrogen filled steel buoyancy tank design hastorispherical ends and consists of two compartments, seeFigure 8. One compartment (with a concave internal end) will

be flooded to reduce the buoyancy during buoy removal. Theother compartment has internal pressure that will be relievedthrough proper valve system as the buoys return to the surface,see also (Cruz 2005).

Figure 8 Buoyancy tank design

By having all the buoyancy modules only slightly buoyant,they can be connected together by a line and de-attached fromthe pipeline and recovered to the surface as a continuous stringshown in Figure 9. Thereafter they can be surface towed to

shore by tugs and demobilized. Before changing to a catenarytow or installing a riser section where the chains also have tobe removed the chains can be connected together with thebuoyancy tanks as one assembly and recovered together with

the floats.

Figure 9 Buoyancy tanks recovery

Riser installations at location

The riser/pipeline string is gradually lowered to sea bed inthe parking area subsequent to arrival on location, where itsettles in an equilibrium position above sea bed with the lowerportion of chain links resting on sea bed. From the parkingarea the riser/pipeline string will be towed in off bottom mode

until the towhead reaches the determined position. Thereafterthe riser/pipeline string will be pulled in to the platform with awire that goes through a sheave connected to an anchorlocated at the platform leg and back to the lead tug.

Riser pull-in to production facilityPrior to start of the riser hang-off pull-in, the ballast chain

and buoyancy tanks will be removed from the riser section

Lift

Heavy

Bundle

Light

Bundle

SubmergedWeight

Speed


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after the pipeline/riser string is positioned on the sea bed, seefigure 10.

Figure 10 Positioning of a riser prior to pull-in to a productionfacility

The riser will then be pulled in to the structure using a pullin winch. During the whole operation the trial tug will adjustthe hold back tension and vessel position at the trail tow headto control the riser catenary curvature and sidewaysmovement. ROVs will monitor the touch down point and thelead tow head. When the riser end is at the platform deck itwill be permanently fixed at hang off flanges and the pull inwire will be disconnected.

DeflectionsIf any horizontal deflection due to routing is required it can

be controlled by connecting clump weights along theriser/pipeline string while the trial tug is setting of sideways.The deflection tension force required will depend on thecurrents deflecting the pipeline/riser string, the seabed friction

and at what degree of deflection that is needed. After thesubsea end is located in the target box the remaining buoyancy

tanks will be removed from the pipeline section ensuring on

bottom stability.

Design checksThe pipeline/riser integrity during tow out and installation

should be checked against DNV Submarine Pipeline Systems,OS-F101, DNV (2005). For pipeline design issues, see alsoBai (2001).

Case studiesTwo case studies are presented, these are:

Deepwater pipeline installation, 800 m water depth

Large diameter riser and pipeline installation in mediumwater depth, 300 m.

Riser/Pipeline data for case studiesThe pipeline properties used for the case studies are shown

in Table 1 below:

Table 1 Riser/pipeline data

Pipeline Riser/pipeline string

Length [m] 3000 625/1375Wt [mm] 25 28.6OD [mm] 609.6 666.8Material API 5L X65 ASTM Grade 23/API 5L X65

CoatingThe riser section consists of different coating layers along

its length. The pipeline has a 3mm coating layer with density1550 kg/m3.

Buckle arrestors

Even if the pipeline is designed to resist collapse from

external pressure, welded external sleeve type buckle arrestorsare needed to limit the damage caused if a buckle is initiated.The buckles may be induced during installation when thepipeline is empty.

TowheadsThe lead towhead shown in Figure 11 is designed to have a

total submerged weight of 1 tonnes and enables connectionbetween the pipeline and the tow wires. The lead and trialtowheads body will have typically 4 and 2pipework/valvesattached, for flooding & venting purposes. Manifolds andpigging systems can be integrated in the pipeline midline orend sections (as towheads). In these cases, the towed pipeline

is termed as a towed production system.

Figure 11 Typical Towhead Design

Buoyancy modulesThe steel buoyancy tanks have torispherical ends and

consist of two compartments, see Figures 8 and 12. Onecompartment (with the concave internal end) will be flooded

to reduce the buoyancy during buoy removal. The othercompartment has internal pressure that will be relieved

through proper valve system as the buoys return to the surface.The buoyancy tanks are mounted at predetermined lengths toachieve the design submerged weight to the system. Eachbuoyancy tank applies approx. 3 tonnes buoyancy to thesystem.

Figure 12 Typical Buo yancy Tank


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Ballast ChainsThe present pipeline chain configuration is composed of a

long chain and two short chains for any 36m long section. Thepipeline becomes negatively buoyant at zero tow speed. In thiscase, some chain links will be suspended between the pipelineand the sea bed while other links will be resting on the sea

bed, see figure 13. The submerged weight of the chain links

resting on the sea bed will be equal to the total submergedweight of the system. Table 2 summarizes the submergedweights.

Table 2 Submerged weight of pipeline section and chains

Pipeline Riser/pipeline

Resulting buoyancy incl.floaters [N/m]

-98 -83/-155

Submerged chain weight alongsection [N/m]

153 154/226

Resulting submerged weight[N/m]

55 71

Figure 13 Chain configuration

Design and Operational criteriaThe following design criteria are governing for the towoperations

- The dynamic pipeline Von Mises Stress andLongitudinal Stress shall be within 90 % of the yieldstress

- The maximum allowable fatigue damage duringinstallation shall be limited to 0.1

A local buckling check based on the Load controlledcondition according to DNV-OS-F101 (DNV, 2005) is alsoperformed.

Weather restricted operationsUncertainties in weather forecasts are included by applying

an operational limiting sea state less than the design limitingsea state. The operational versus design criteria ratio is set

according to DNV Rules for planning and execution of marineoperations, DNV (1996). The relevant reduction factor (alpha

factor) is 0.63 for HS,D greater than 4m and an operationalperiod less than 72hrs. Therefore each phase of the towoperation is planned as a single marine operation within areference period less than 72 hours, referring to the design

criterion from DNV (1996). The design parameters for the towroutes are presented in Table 3 and Table 4 below.

Table 3 Design parameters for the deepwater tow operation

Deepwater Pipeline tow Off-bottom Controlled-depth

Catenary

Tow velocity [m/s] 0.7 2.5 2.5Planned operation [hrs] 4 59 34

Table 4 Design parameters for the medium tow operation

Medium water depthriser/pipeline tow

Off-bottom Controlled-depth

Tow velocity [m/s] 0.7 2.5Planned operation [hrs] 4 55

Several wave headings have been considered for each set

of HS,Dand TZ. A summary of all the analyzed sea states areincluded in Table 5.

Table 5 Applicable sea states for design criteria simulations

Wave heading[deg]

Tz[s] Hs,D[m] Hs,OP[m]

0 [6, 7, 813]45 [6, 7, 813]90 [6, 7, 813] 5.5 3.5

135 [6, 7, 813]180 [6, 7, 813]

Figure 14 present the proposed tow routes used for the twocase studies. The tow route for the deepwater pipeline involvesa combination of all three tow methods.

Figure 14 Proposed tow routes for the combined tow operations

Analysis methodsRIFLEX for Windows v.3.4.5 (Marintek, Sintef 2005 a and

b) has been used for the pipeline tow simulations at 800 mwater depth and Visual OrcaFlex v.8.6.d has been used asdesign tool for the riser and pipeline tow and installationsimulations at 300 m water depth.

Tow models- The pipeline and towheads are modeled by FEM

principles using discrete beam elements.- The static tow configurations are established.- The tug wave response is modeled by RAO data for

relevant tugs.- The buoyancy modules are modeled as clump weight

types- The ballast chains are modeled as drag chains.- Wave modeling is defined by using irregular waves

according to the JONSWAP energy spectrum.


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Tow riggingAn OrcaFlex winch model is used for simulating the

connection between the tug and the tow wire at eachpipeline/riser end. The winches are directed to pull in or payout tow wire if the tension is outside a specified interval. Thiswinch mode will also be used during the tow operation which

prevent that peak loads and slack wires appear. The tow wires

are modelled as line items. Drag and inertia forces areaccordingly included to obtain the correct catenary behaviourduring tow.

Buoyancy tanks and ballast chains

The buoyancy tanks are modelled as clump types attachedon selected intervals to keep the resultant submerged weight

constant during tow.Ballast chains are modelled by the drag chain facility with

properties to achieve the design submerged weight. The chaindrag and lift coefficients vary with the incidence angle of therelative flow. Figure 15 shows the pipeline section with thechain and buoyancy attached modeled in Orcaflex.

Figure 15 Pipeline section w ith the chain and buoyancy attached.

Environmental conditionsWave conditions are defined by regular waves for the

strength design check and sensitivity analysis during tow.Irregular waves according to the JONSWAP energy spectrumare used during the fatigue analyses. The main parameters for

defining the spectrum are the significant wave height, HS,

spectrum peak period, TPand the peakedness factor .

Hydrodynamic coefficientsA drag coefficient CD = 1.0 is used for the pipeline and

buckle arrestors in the analysis. Added mass Cahas also beenset to 1 for the pipeline, towhead, buckle arrestors and tow

wires. For the riser/pipeline string analyses a drag coefficientCD = 1.2 is used to include the strakes and coating propertieson the riser section.

Sensitivity analysisSensitivity analyses are performed in order to set the worst

wave heading and peak period for each tow phase. The

limiting sea states presented in this paper are purely derivedfrom analyses and operational aspects (i.e. deck handling) arenot considered.

Fatigue analysisThe stress time series are calculated based on the stored

force time series (axial and bending stresses) and the pipelineparameters using specified SN-curves and rain-flow cyclecounting. The S-N curves are found in DNV-RP-C203, DNV

(2005). The fatigue damage for the deepwater pipeline iscalculated for a specified number of points on the tubecircumference (here: 16 points). The irregular sea state issimulated for 60 minutes to obtain a stable statistical basis for

the fatigue damage calculations.

The fatigue calculations for the riser/pipeline string towoperation are based upon Rain flow analysis and 8 theta values(directions) around the riser/pipeline circumference. Theirregular sea state has been simulated for 45 minutes to obtaina proper standard deviation for the riser/pipeline stress. Thedamage is furthermore scaled according to the specified

exposure time.

The fatigue sea state has been defined by HS,OP and thecombination of TZ and wave heading that implies greatestriser/pipeline string loading obtained from the sensitivityanalyses.

Static ResultsTow analysis

A summary of the results for the static tow configurationsis included in Table 6 below. The values obtained are used asinput in the dynamic sensitivity analysis.

Table 6 Results from static tow analysis

Deepwater Pipeline Medium water depthRiser/pipeline

Tow method Off -bottom

CDTM Catenary Off-bottom

CDTM

Tow velocity [m/s] /[knots]

0.7/1.4 2.5/4.9 2.5/4.9 0.7/1.4 2.5/4.9

Length of leadingtow wire [m]

80 120 94 410 340

Length of trailingtow wire [m]

80 120 94 400 235

Maximumhorizontal tow wiretension [kN]

192 364 752 212 895

Maximum VonMises stress [MPa]

35 37 105 38 49

Riser pull in analysisFrom the riser/pipeline pull-in analysis, three worst case

conditions were assessed as shown in Figure 16:

- Maximum Von Mises stress in riser during towheadlift off

- Maximum declination angle for pull-in wire at hangoff

- Maximum pull-in wire tension at the hang off justbefore final pull-in

1

2

3Figure 16 Riser/pipeline pull-in analyses, worst case conditions


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OTC 18797 7

The maximum values are presented in Table 7 below.

Table 7 Riser/pipeline pull-in analyses, worst case conditions

Condition Static Value Distance from hangoff to riser end [m]

1. Max Von Misesstress in riser [MPa]

357 265

2. Max wire declinationat hang off [deg]

32 186

3. Max wire tension athang off [kN]

1060 1

Sensitivi ty analysisDynamic simulations are performed for incoming waves onstarboard side only by assuming that the longitudinal vesselaxis has a symmetry plane. The vertical response at thestern/bow roller located on the longitudinal vessel axis, will

hence be identical for e.g. quartering head sea on starboardand port side. Each sea state simulation does not cover theentire planned operation period. However, the largest waverise/fall during the operation period has been extracted foreach set of HS,D and TZ. The overall maximum Von Mises

Stress and Longitudinal stress over the complete pipeline andriser/pipeline string is included in the figures below.

0

5

10

15

20

25

30

0 45 90 135 180

Lead tug

Trail tug

Wave heading

MaxLongitudinalstress[Mpa]

Figure 17 Max axial stresses at lead/trial tug of the gas pipelinefor 8 s peak period at different wave headings in off-bottom tow.

0,00

20,00

40,00

60,00

80,00

100,00

120,00

140,00

0 45 90 135 180

Lead Tug

Sagbend

Trail Tug

Wave he ading [deg]

MaxLongitudinalstress[Mpa]

Figure 18 Max axial stresses at lead/trial tug and in the sag bendof the gas pipeline for different wave headings in catenary tow.

0

10

20

30

40

50

60

70

80

6 7 8 9 10 11 12 13

Mean zero crossing period [s]

Max

VonMisesstress[MPa]

0 degrees

45 degrees

90 degrees

135 degrees

180 degrees

Figure 19 Max Von Mises stresses from the riser/pipeline CDTMtow

Figure 17 shows that the stresses in the pipeline are minorduring the off-bottom tow. The analysis for the pipeline

catenary tow shown in Figure 18 demonstrate that a peakperiod of 8 seconds and quartering seas (45 and 135) inducethe largest dynamic loading along the pipeline. The pipeline

section exposed to the largest stress is located approximately614m from leading tug end.

Figure 19 shows that a mean zero crossing period of 12seconds and a wave heading of 180 involve the largestdynamic loading along the riser/pipeline during the CDTM

tow. The riser/pipeline section exposed to largest Von MisesStress is located approximately 113m from trailing tug end.

The dynamic response analysis from all the different towmethods shows that the pipeline has sufficient strengthcapacity. The most critical section for buckling and Von Misesstress is at the sag bend in the catenary tow mode. It istherefore important to constantly monitor the distance between

the tugs during the tow.

Sensitivity analysis for the riser pull in is also performed.The worst wave headings and mean zero crossing periodswere found for each load condition, see Figure 16. The overallmaximum Von Mises stress (condition 1); wire declination(condition 2) and tension (condition 3) at riser hang off areincluded in Figures 20 to 22 below.

350

352

354

356

358

360

362

364

366

368

370

6 7 8 9 10 11 12 13


MaxVonMis

esstress(Cond.

1)[MPa]

0 degrees

45 degrees

90 degrees

135 degrees

180 degrees

Figure 20 Maximum Von Mises stress i n riser during tow head lif toff.


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30,0

30,5

31,0

31,5

32,0

32,5

33,0

6 7 8 9 10 11 12 13


Max

Declination(Cond.

2)[Deg]

0 degrees

45 degrees

90 degrees

135 degrees

180 degrees

Figure 21 Maximum declination angle for pull -in wire at hang off

1000

1020

1040

1060

1080

1100

1120

1140

1160

1180

1200

6 7 8 9 10 11 12 13


MaxEffec

tiveTension(Cond.

3)[kN]

0 degrees

45 degrees

90 degrees

135 degrees

180 degrees

Figure 22 Maximum pull-in wire tension at the hang off just beforefinal pull-in

It is seen that the wave action has very little effect on theresponse of the system and therefore the pull-in operation canfind place in a significant sea state of 3.5m, independent of the

wave period and direction.To investigate how the riser pull in operation is influenced

by the current, a new set of analyses was performed for theworst wave heading and mean zero crossing period obtainedfrom the sensitivity analysis. Three different current directionswere analyzed, inline, outline and perpendicular to the risercatenary. The maximum riser loading from the analyses,including current effects corresponds to an utilization of 54%according to the design criteria.

FatigueThe maximum accumulated damage for the deep water

pipeline was found to be 0.028 for the present weatherwindows. The pipeline section exposed to the worst damage islocated 614 m from the leading tug end. Figure 23 presents the

total fatigue damage along the pipeline during installation atHsOP= 3.5 m.

Total fatigue damage Hs = 3.5 m

0,00E+00

5,00E-03

1,00E-02

1,50E-02

2,00E-02

2,50E-02

3,00E-02

0

258

456

660

857

1055

1259

1457

1655

1859

2057

2255

2459

2656

2854

Total fatigue

damage

Pipeline length [m]

Figure 23 Total fatigue damage for the deep water pipeline tow

For the medium water riser/pipeline tow, the maximumaccumulated damage due to fatigue is 0.0003 for the presentweather windows. The riser/pipeline section exposed to worstdamage is located 106m from the trailing tug end.

ConclusionIn this paper, two case studies have been investigated; the tow

of pipelines in deepwater using three combined tow methodsand towing of a riser/pipeline string including pull in to theproduction facility at the field. The governing criteria for theseinvestigations are the resistance against local buckling, towwire tension and fatigue damage during tow out/installation.

As seen from the analyses the stresses during the toware very low in an operational sea state of 3.5 m withworst wave heading and period and there is no risk ofbuckling the pipeline and riser/pipeline string.

The maximum accumulated damage due to fatigue is0.0003 for the riser/pipeline and 0.028 for the gaspipeline deepwater tow.

Use of the combined tow method is therefore shownto be a robust method of transporting deepwaterpipelines and risers from the fabrication site to thefield.

Focus should be made on the design and handling ofthe buoyancy modules. A system that can minimizethe offshore time for disconnection of the buoyancymodules and recover this system back onshore shouldbe looked further into.

AcknowledgementsThe authors would like to acknowledge Subsea 7 for

permission to publish the results presented in the paper. It is

emphasized that the conclusions put forth reflects the views ofthe authors alone, and not necessarily those of Subsea 7.

ReferencesBai, Y., 2001: Pipelines and risers, Volume 3, Elsevier, The

NetherlandsCruz, I., 2005: Towed Risers Installation Study, Towing

methods evaluation report, Subsea 7 BrazilDNV, 2000: Environmental conditions and environmental

loads, Classification notes No.30.5DNV-RP-C203, 2005: Fatigue Design of Offshore Steel

Structures, 2005DNV-RP-F105, 2002: Free Spanning Pipelines

DNV, 1996: Rules for Planning and Execution of MarineOperationsDNV-OS-F101, 2005: Submarine pipeline systemsHellest, A. R, 2005: Towing of deepwater pipelines and

risers MSc thesis, University of StavangerMARINTEK, SINTEF, 2005: RIFLEX- Flexible Riser

System Analysis Program Users Manual, V3.4,Trondheim

MARINTEK, SINTEF, 2005: RIFLEX- Flexible RiserSystem Analysis Program Theory Manual, V3.4

ORCINA, 2006 Virtual Orcaflex Manual

installation analysis

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