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Assessment ofRiser System SelectionFloating Production, Storage and Offloading (FPSO) system has been used widely in the offshore oil andgas industries worldwide. In recent years, FPSO concept has been deployed in deep and ultra-deepwaterfield development including West of Africa, Offshore Brazil, Far East and Gulf of Mexico. Compared toother types of deepwater floaters (e.g. Spar, TLP, and Semisubmersible) the severe motion characteristicsof an FPSO make the riser concept selection and design much more challenging. Other constraintsimposed by field reservoir properties (e.g. high temperature\high pressure), disconnection requirementsfor early production FPSO system and significant increase in water depth even exclude some riserconcepts. This paper presents detailed assessment of potential deepwater riser concepts applicable for anFPSO. Among the deepwater concepts, steel catenary risers (SCR), flexible risers, free standing hybridrisers and lazy wave SCR have been successfully deployed to FPSO application. In addition, there arealso other viable riser concepts for FPSO application including Tension Leg Riser (TLR), HybridCatenary Riser (HCR), and more to come. The evaluation of deepwater riser concepts for FPSOapplication is conducted in terms of technical feasibility, design constraints, track records, fabrication,offshore construction and cost with focus on the existing and field proven riser concepts. Technicalchallenges and technology gaps are highlighted. The technical feasibility of some of the riser concepts forcertain FPSO application has been demonstrated through analysis and calculation. Novel ideas have beengenerated for particular requirements. Particular attention is paid to the discussion with the applicationto deepwater development in Asia. This paper shows the steel riser and hybrid riser are the most viableones for deep and ultra-deepwater FPSO. Flexible riser is also the preferred solution for FPSO.

TechnologyTechnology

I n recent years, exploration and production activi-ties have increased dramatically in deepwater nearlydoubling the water depth compared to the activitiesa couple of years ago. The offshore industry is

building systems today for even deeper waters, progres-sively using new technology and significantly extendingexisting technologies. The targeted water depths for oiland gas developments in areas such as Gulf of Mexico(GoM), West of Africa (WoA), Brazil, Asia, and NorthSea are increasing every year.

In view of the floater concepts that have beensuccessfully deployed for deepwater development,FPSO has been adopted nearly worldwide in particularOffshore Brazil, WoA, and Asia as compared with TLP,Spar and Semisubmersible, as shown in Figure 1 below.

For relative shallow water (e.g. water depth less than500 meters), FPSO is probably one of the most widelyused floater concepts.

Regardless of a floater concept for a deepwater fielddevelopment, there is always a need of riser systemconnecting the floater at surface and subsea facility onthe seabed. Riser system is one of the more complexaspects of deepwater developments. It became evidentthat riser system plays bigger and bigger role as part ofthe offshore infrastructures. There are different ways togroup deepwater risers. The most widely used and fieldproven deepwater riser concepts include SCRs, hybridrisers with a single line or bundled multi-lines, flexiblerisers with different configurations, and top tensionrisers (TTR) as shown in Figure 1.

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Comparing the motion characteristics of FPSO withother type deepwater floaters (e.g. Spar, TLP, andSemisubmersible) its severe motions in particular forheave and pitch makes the riser concept selection anddesign much more challenging. Other constraints im-posed by field reservoir properties (e.g. hightemperature\high pressure), disconnection requirementsfor early production FPSO system, and significant in-crease in water depth even exclude some riser concepts.

This paper presents detailed assessment of potentialdeepwater riser concepts applicable for an FPSO.Among the deepwater concepts, steel catenary risers(SCR), flexible risers, hybrid risers, and lazy wave SCRhave been successfully deployed to FPSO applica-tion. In addition, there are also other viable riserconcepts for FPSO application including Tension LegRiser (TLR), Hybrid Catenary Riser (HCR), and more tocome. The assessment of deepwater riser concepts forFPSO application is conducted in terms of technicalfeasibility, design constraints, track records, fabrica-tion, offshore construction, and cost with focus on theexisting and field proven riser concepts. Technicalchallenges and technology gaps are highlighted. Thetechnical feasibility of some of the riser concepts forcertain FPSO application has been demonstratedthrough analysis and calculation. Novel ideas havebeen generated for particular requirements. Sufficientanalysis is performed to demonstrate the governingdesign issues. Particular attention is paid to the de-tailed discussion with the application to offshoreAsia. This paper shows the steel catenary riser, flex-

ible riser and hybrid risers are the most viable ones fordeep and ultra-deepwater FPSO.

FPSO Motion ResponseCompared to other types of deepwater floaters such as

TLP, SPAR, and semisubmersible which have been inservice worldwide FPSO is in general less riser-friendlyfloater. A typical FPSO’s pitch, roll and heave motionnatural periods are closer to wave peak periods of stormcondition, e.g. hurricane or typhoon. This generatessever FPSO motion response characteristics, as illus-trated in Figure 2.

In some cases, the risers are connected to theturret of which the vertical motion in particularvertical acceleration is amplified due to FPSO pitchmotion. The relative larger vertical motion at risertop termination location makes the riser engineer-ing more challenging.

There are some ways to mitigate FPSO motionincluding bilge keels, dynamic positioning (DP) sys-tem [Ref. 4]. The vortex shedding and viscous damp-ing from the bilge keels of FPSO can significantlyreduce its roll motion. The combined environmentalforces cause the FPSO hull to form an oblique angleto the wave. It will significantly increase the wavefrequency horizontal and vertical motions, which isthe main driver behind the excessive compressionand fatigue damage in particular to SCRs. The FPSOwith DP can align the vessel with wave direction toreduce the wave frequency motion.

In addition to mitigate the FPSO motion response,another efficient way is to develop riser concept that can

Figure 1. Illustration of Deepwater Floater and Riser Concepts

Figure 2. Natural Periods of Floaters vs. Wave Period

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decouple FPSO motion and riser response. This is whyhybrid riser is also a kind of popular concept for FPSOapplication, e.g. in WoA and GoM.

Potential Riser Concepts for FPSOApplication

Riser concept selection and system design is one of themost important tasks accomplished during a FPSOproject. The number one challenge of selecting anddesigning a riser for FPSO is the sever FPSO motionresponse that impose significant dynamic response ofthe riser causing motion fatigue damage. Among 6 DOFmotion of an FPSO, pitch, roll and heave are the mostcritical to the riser response.

For environmental conditions in GoM, FPSO willfreely weathervane or rotate around turret. Risers willalso be terminated at the turret. Deepwater riser imposerelative high top tension loads on the turret bearing,which need to be taken into account during the bearingcapacity design. Alternatively, different riser conceptcan be explored to minimize the top tension loadsimposed on the turret bearing.

Factors Affecting FPSO Riser SelectionDifferent riser concepts have been implemented or

are being studied for FPSO application. The followingmajor factors need to be taken into account for FPSOriser selection:

• Water depth• Riser dimensions• FPSO motion• Metocean data• Riser termination location and method• Field layout and footprints• Thermal performance requirement• Installability• Schedule• Cost

Turret Configuration vs. Riser ResponseUnless in a very benign environment (WoA, offshore

Indonesia) or an environment with high directionality(Campos Basin), an FPSO has to be turret moored toallow weathervaning. If the turret is positioned far fromthe mid-ship section, pitch angle response of the FPSOis of outmost importance. The closer to the bow the

turret is located, the higher the first order excitationexperienced by the riser will be. Therefore, bringing theturret closer to mid ship section is a desired option.When the turret is located at midship section or near it,bow and/or stern thrusters are usually installed in orderto keep the FPSO’s heading conveniently oriented. Onthe other hand, it has been recently shown that direc-tional stability with respect to current and waves can beassured by means of special stabilizers of the rudder-type, for a wide range of turret installation positions.

Obviously, in the case of a FPSO turret configura-tion, the riser positioning problem is actually re-stricted to the problem of determining the optimumturret position along the ship’s centerline. This is not,however, a simple task.

The best turret position depends upon a balancebetween the ship heading stability with respect to cur-rent loading and the wave induced vertical motions onthe turret point.

Feasible Riser ConceptsWith respect to deepwater application, FPSOs have

been widely used in offshore Brazil and West of Africa.For relatively shallow water applications, FPSOs aremostly used in the North Sea, in particular NorwegianSector, and offshore China.

In view of riser concepts have been applied to FPSO,brief summary is given in Table 1.

It is clearly shown that more versatile riser conceptshave been applied to FPSO, which facilitate morewidespread use of FPSO, offering improved productionand export capacity for larger developments, and reduc-ing cost. Basically, three riser concepts have beenactually implemented in FPSO based field develop-ment, namely hybrid riser, flexible riser and steel cat-

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enary riser (SCR). In addition, there are also other typesof riser concepts are in the development that could beapplied to FPSO. It includes tension leg riser (TLR) andhybrid catenary riser (HCR). Selectively SCR and hybridriser concepts for FPSO application are illustrated in theFigures 3 and 4.

In addition to the above riser concepts that have beenimplemented in actual field application, other conceptssuitable for FPSO application are also available includ-ing hybrid catenary riser (HCR).

Technical Assessment of FPSO RisersAssessment Criteria

One of the most important issues associated with thedefinition of the deepwater riser concept is the cleardefinition of the criteria of selection. It is necessary to

define what are “musts” and “preferred”. Quite oftenriser assessment criteria definitions are not well thoughtout in particular at early development phase. A “must”is more or less a threshold that must be mandatorilyachieved. An example of this would be the OHTCrequirement for the riser system. It is a “must” to achievethe minimum thermal performance requirement. It is a“preferred” to lower the OHTC, but not mandatory.

In general, the following are regarded as “musts” for aspecific field development:

• Water depth• Pressure and Temperature• Conveyed fluids• Size of each riser• Number of risers• OHTC requirement• Riser top payload impact on hull termination

location• ConstructabilityThe following are regarded as “preferred”:• Functionality• Operability• Safety• Schedule• Cost including CAPEX and OPEX

Assessment of FPSO RisersTower / Hybrid Riser

Basically, this riser adopts some principles of flowlinebundle technology and it is a kind of hybrid riser systemconsisting of rigid and flexible risers.

ConceptThe main section of the hybrid riser consists of a

central structural tubular, around which syntactic foambuoyancy modules are attached. Peripheral produc-tion and export lines run through the buoyancy mod-ules and are free to slide axially in order to accommodatethermal and pressure induced expansion. The centralstructural member is connected to the riser base byway of a hydraulic connector and tapered stress joint.The peripheral lines are attached to the rigid flowlinejumper on the base, which provides connection to theflowlines. On top of the riser, the peripheral lines areterminated in goosenecks. Flexible jumpers are used toconnect the goosenecks and the surface FPSO, provid-

Figure 3. Cascade & Chinook FPSO – Free Standing Hybrid Riser

Figure 4. Gendalo FPSO – Steel Catenary Riser

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ing the flow path to the host platform whilst accommo-dating decoupling between the vessel motion and theplatform.

Functionality• The capacity or cross section of the tower riser is

predefined during the engineering phase. Any fu-ture expansion requirement must be built into theriser tower design. Extra expandability cannot beaccommodated once the design is completed.

• Key tower riser design parameters are buoyancycapacity and soil foundation capacity to carry theup-thrust tension loading.

Fabrication• Fabrication and testing of tower riser is performed

onshore at lower average unit cost compared withregular offshore construction.

• Some risk and security issues associated with theonshore fabrication sites in areas of political insta-bility, e.g. WoA.

Installation• Field layout and construction sequence affect the

riser tower location relative to FPSO.• Tower riser requires diverless subsea tie-in between

riser base and flowline.• Flexible construction sequence.• Low cost marine spreads (tugs) can be used for

towing.• High risk operation during towing and upending. A

single tower riser scenario offers no contingency.

Operation• Monitoring system needs to be in place for the sake

of assistance, assurance and verification.• Potential for heat up and return lines via dedicated

lines.• Pigging challenge for the product risers.• Compact field layout with FPSO.• Low load transfer to the FPSO through the riser

porch.

Maintenance• Very difficult to inspect individual riser externally.• Very difficult to maintain or repair a single indi-

vidual riser within the tower. This can be addressedby inclusion of spares.

Safety and Risk• Gas line and production line bundled together

would increase operating risk.• Riser base connection using flex joint, roto-latch or

other type of connector should be designed to bewell within the capability of the component. Sec-ondary restraint could be considered as redundancy.

• The whole system may be closely tied to a few singlecomponent with no redundancy.

• No future expandability once the tower riser isfabricated.

To summarize the tower riser concept, Table 2 lists thepros and cons.

Flexible RiserRiser for catenary moored FPSO are generally de-

signed using flexible pipe such examples include mostof the FPSOs in offshore Brazil, some in WoA (e.g. Dalia,Agbami). Flexible riser is more or less the only conceptfor shallow water FPSOs since the flexible riser arrange-ment can be more compliant. This is normally achievedby lazy wave configuration, steep wave configurations,or compliant wave configuration.

Functionality• Brazil has vast experience of flexible risers attached

to deepwater FPSOs.• Technip and Wellstream offer flexible pipe design

suitable for deepwater application with some limi-tations.

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• Significant track record with compliant systems forinterfacing with FPSOs in shallow water (< 500m).

• Flexible riser configuration can be optimized tominimize cost and interface loads, but provideflexibility to accommodate FPSO motion.

Fabrication• Deepwater flexible risers are proprietary design

from three suppliers (Technip, Wellstream, NKT)only.

• Intermediate connection might be needed for thelonger lines due to manufacturing reel limitation ordesign considerations.

Installation• Relatively simple installation from hydraulically

powered reel.• May require subsea tie-in / pull-in with flowline.• For ultra deepwater, availability of installation ves-

sel to handle the tensions and collapse capacity isquestionable, in particular if it is coupled with sweetand sour service requirements.

Operation• Flexible thermal insulation requirement for flexible

riser is a challenge since it can only provide limitedinsulation which may not be capable of meetingoperating criteria.

Maintenance• Pigging normally not needed as riser is a composite

structure.• External ROV inspection may be needed focusing

on critical areas such as TDP, connections, andbend stiffener.

Safety and Risk:• Gas egress through the membrane.• ESDV requirements and its location.• Technical risk since the Cascade will push the

existing flexible technology further in terms of waterdepth, temperature, pressure and size.

• Schedule risk due to qualification and fabricationrequirements.

• Commercial risk due to only one or two supplierscapable of this job.

Steel Catenary RiserSCR concept has been widely used in the recent

years in various kinds of floaters especially for TLP,Spar, and Semi. For FPSO application, it is mainlyused for WoA and Campos Basin, e.g. Erha, Bonga,AKPO. As the result of FPSO motion, the SCR issusceptible to compression in the sag bend area. Asa minimum, the following factors need to be evalu-ated when considering SCR for an FPSO:

• SCR dimension• Geotechnical data• FPSO motion• Internal turret location and configuration• Metocean data• SCR cross section designMore detailed discussion of deepwater SCR technol-

ogy can be found in Ref. 4.

Lazy Wave SCRLazy wave SCR is basically a modified configura-

tion of a free hanging SCR by introducing buoyancymodulus for a certain length of the suspended SCR.The introduced buoyancy will absorb most of thedynamic motion imposed by FPSO motion so that theSCR touch down point (TDP) could land on theseabed “softly” hence reduce the frequent contactwith the seabed significantly. This will improve theSCR fatigue life at touch down area and make theconcept technically feasible. BC 10 field in CamposBasin is the first lazy wave SCR for FPSO in the world.

ConclusionsThis paper presents the unique challenges for

deepwater riser engineering for an FPSO application.Feasible and field proven riser concepts have been

APR-JUN 2012 57

reviewed. Detailed assessment of each riser concept hasbeen conducted with pros and cons outlined. In addi-tion to SCR, flexible riser and hybrid riser concepts,other viable riser concept has been discussed briefly.

Based on the technical assessment and qualitativereview it can be concluded that SCR, hybrid riser andflexible riser are popular concepts for FPSO application.

Reference1. API RP 2RD: Design of Risers for Floating Produc-

tion Systems (FPSs) and Tension-Leg Platforms(TLPs), 1998.

2. DNV OS-F201: Dynamic Risers, Det Norske Veritas3. Zimmermann, C., Petruska, D., Duggal A. Effec-

tive riser solution for a deepwater FPSO, Pro-

ceedings of OMAE 2002 - 283764. Song, R., Stanton, P.: Advances in Deepwater Steel

Catenary Riser Technology, Part I – Design, Proceed-ings of OMAE 2007 - 29329

5. Song, R., and P. Streit,: Design of the World’s DeepestHybrid Riser System for the Cascade & Chinook

Development, Proceedings of OTC 2011, OTC Paper# 21338

This publication thanks Ruxin Song and KalyanUppu, Riser Engineering Department, Genesis Oiland Gas, Houston, USA, for providing this paper,which was presented by Kalyan Uppu at DeepwaterTechnology Asia 2012, which was held in Jakartaon the 19th and 20th March 2012.

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