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Deep Water Hybrid Riser Systems Ange Luppi, Gilles Cousin (SEAL Engineering, Technip Group) Robby O'Sullivan (Technip Oceania Pty Ltd)

Copyright 2014, Offshore Technology Conference

This paper was prepared for presentation at the Offshore Technology Conference Asia held in Kuala Lumpur, Malaysia, 25–28 March 2014. This paper was selected for presentation by an OTC program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewed by the Offshore Technology Conference and are subject to correction by the author(s). The material does not necessarily reflect any position of the Offshore Technology Conference, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Offshore Technology Conference is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of OTC copyright.

Abstract

The worldwide development of deep water oil & gas fields has motivated the need to develop ‘Hybrid Riser’ concepts in complement to the “conventional” top tensioned risers, flexible or steel catenary risers.

The hybrid riser systems are well adapted technical solution (& cost effective alternatives) for offshore ultra-deep water oil & gas fields, due mainly to the decoupling effect between the floating production unit (wave responses) and the risers.

Hybrid risers allow for reduction of the (1) riser loads transmitted onto the floating production unit, (2) minimising riser fatigue issues and (3) installation planning risk decoupling as the hybrid risers can be pre-installed prior to the floating production unit site arrival date.

This paper firstly introduce the most “popular” deep water riser systems currently installed, including steel catenary risers, flexible risers and “first generation” hybrid riser systems.

The latest evolutions of hybrid riser systems are then further described: • The Free Standing Flexible Riser (FSFR), which is based on the use of (1) flexible risers from seabed to surface

level and of (2) a Flat Buoy, which purpose is to support and to tension the flexible riser pipe. The FSFR is a concept developed in order to optimise both the riser installation and in-situ behaviour;

• The Multi-Lines Free Standing Riser system, which is to be fully assembled offshore; • The Deep Steep Riser (DSR) system, which is an innovative concept developed for ultra-deep waters (e.g.

3,000m – 4,000m target) and consisting in a single leg tensioned riser, either ‘full flexible’ or ‘hybrid’ riser. These new hybrid riser alternatives would bring more cost effective & technical solutions allow for economic

development of both marginal and higher productivity fields and provide access to deeper field developments. These riser solutions have also been designed to integrate flexibility to local field conditions (e.g. metocean environmental conditions, supply chain capabilities) and would thus be well adapted to the emerging deep water Asia Pacific regions.

Introduction

Considerable growth in the number of deep and ultra-deep water oil & gas fields has inspired the development of hybrid riser concepts to complement the existing portfolio of technical riser solutions.

There are many technical, commercial, geographical and local content parameters which contribute to the selection process of a preferred riser configuration for any given development. One key feature of hybrid riser systems is the decoupling between the floating production unit (FPU) and the riser itself, both from a physical point of view and from a schedule (planning) point of view. Hybrid risers allow for reduction of the loads transmitted onto the FPU and reduced fatigue cycling at the steel riser components compared to steel catenary riser. In addition, a hybrid riser system can be pre-installed prior to the arrival of the FPU on site. This schedule decoupling or ability to take the riser installation off the project critical path can be a significant benefit to most field developments.

This paper briefly describes the main deep water riser systems in service today, including Steel Catenary Risers (both single catenary and lazy wave variations), Flexible Risers, Top Tensioned Risers and “1st generation” Hybrid Risers (including single leg and riser tower variations). Some of the primary features and advantages of those systems are presented.

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The paper then describes the latest evolution of hybrid riser systems. These new systems are being developed to address various technical, commercial and logistics challenges associated with current riser systems, but mainly to bring more cost effective solutions and to further enable the deeper field developments, targeting the 4000m water depth.

Existing Deep Water Riser Systems

Top Tensioned Risers Top tensioned risers are vertical steel risers which are used mainly for “dry tree” applications, i.e. where the wellhead tree

is on the topsides of the floating unit. This type of riser is suitable only for floater applications with very limited lateral excursions such as TLP’s and Spars. It cannot be used on ship shape floaters and generally is not suitable for semi submersibles. Top tensioned risers are supported at its upper part by hydraulic heave compensator system or via individual buoyancy tanks which are uncoupled from the floater vertical motions. This is necessary as the vertical risers must always remain in tension throughout their length.

Flexible Risers Risers are ‘dynamic’ systems which have to sustain surface floater wave induced motions and direct environmental

(waves, currents) loadings in addition to functional loads (pressure, temperature, corrosive fluids, etc.). Unbounded flexible pipes, the original enabling technology for floating production system applications, are the main riser

system suitable for dynamic applications in shallow water conditions. In most cases, the riser flexibility or ‘compliance’ requirements (due to floater large offsets) are obtained through the addition of buoyancy modules to form riser ‘loops’ which uncouple the riser bottom section (e.g. seabed touchdown zone) from the floating unit motions. This resulted in the development of comprehensive flexible riser configurations to suit different field conditions, such as “Lazy Wave”, “Steep Wave”, “Lazy-S”, “Steep-S”, “Pliant Wave”, etc.

Figure 1: Flexible Pipe with Multi-Layer Structures and Riser Configurations As water depths increase (typically 300m – 1000m range) the simple free-hanging catenary flexible risers became

feasible, as the water depth and corresponding riser catenary length provide sufficient flexibility for riser system compliance. Deep water free-hanging flexible risers were pioneered in Brazil and in the North Sea (e.g. Norway). Following many

years and many installations of single catenary flexible risers in deep waters, Brazil continues to push the limits of deep water flexible riser technology with installations in up to 2,500m water depth. For this water depth the use of buoyancy modules is mandatory to obtain a flexible riser “wave” configuration. Recently flexible riser technology have been qualified for 3,000m water depth, e.g. thanks to the use of composite materials (e.g. carbon fibre) and innovative solutions to resist corrosive fluids, potentially combined with high temperature (e.g. 150°C) and high pressure (e.g. 15kpsi) fluid.

In addition to Brazil and the North Sea, catenary flexible risers have been used on a number of West African offshore fields, in the Gulf of Mexico deepwater developments and more recently in Asia Pacific regions.

The flexible risers are technically suited to a wide range of water depth / floater type combinations. However in shallow waters and in ‘cyclonic’ environmental conditions, a high level of pipe flexibility and system compliance using mid-water supports (e.g. buoyancy modules, arches) is needed to accommodate large vessel offsets / motions. The flexible riser configurations are the most appropriated solution for dynamic applications in shallow water and ‘cyclonic’ conditions.

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Steel Catenary Risers As floating production systems moved into deeper water (>1000m) in the 1990’s, steel catenary risers (SCRs) also

became feasible as the longer length of pipe curve provides adequate compliance relative to the magnitude of vessel motions. The first application of steel risers in catenary configuration (SCRs) has been performed in 1993 on Shell’s Auger TLP in

the Gulf of Mexico for oil and gas exports. The SCRs were installed at a water depth of 870m using a flexible joint connector to the TLP pontoon. Since then similar SCRs have been installed on other TLPs such as Mars, Brutus, Ursa, and Ram-Powell. In 1997, the first SCR to a semi-submersible (Petrobras P18) was installed in the Marlin Field, Campos Basin offshore Brazil. In 2004, the Bonga SCRs were the first to be installed on an FPSO vessel (which has greater excursion than SPAR or TLP) offshore Nigeria.

Numerous SCRs have been installed, mainly in the Gulf of Mexico and some in West of Africa and offshore Brazil. The deepest installed SCRs reach a maximum water depth of around 2,470m and are installed on the Perdido Spar.

The Steel Lazy-Wave Riser (SLWR) is a steel riser fitted with buoyancy modules at its lower section to form the “wave” configuration, which provides additional compliance to the riser system. SLWR allows for (some) reduction of the fatigue cycling at the riser touch-down zone and reduction of the riser payload at the FPU balcony. SLWRs are currently being considered for deployment in ultra-deep waters (e.g. 2500m-2900m) in the Gulf of Mexico.

Figure 2: Typical Steel Lazy Wave Riser Configuration (lower line) vs. Flexible in Lazy Wave (upper line)

All types of steel risers are prone to Vortex Induced Vibrations (VIV) fatigue issues, particularly in deep waters where long lengths of risers are exposed to current (multi-directional) loadings. Consequently, steel risers are generally fitted with anti-VIV devices, including VIV strakes or fairings.

Hybrid Risers The term “Hybrid Riser” generally refers to a system which incorporates both steel pipe and flexible pipe technologies.

The steel pipe can be a vertical leg or an SCR connected to a subsurface buoy support and the flexible pipe is used to complete the fluid flow path from the subsurface buoy to the FPU. The configuration is designed such that most of the dynamic motions are absorbed by the flexible pipe, hence the vertical pipe can be considered to be “quasi-static” along its entire length. Hybrid riser arrangements have been conceived to meet the challenge of deep waters and minimise the drawbacks inherent to the steel riser systems, e.g. coupling effect with FPU wave motions and subsequent fatigue issues.

The hybrid riser systems provide reduced payload on the FPU and flexibility of field layout by permitting the routing of flowline to be independent of the approach lay azimuth angle of the riser top hang-off porch. They also bring schedule flexibility by permitting the installation of maximum riser components prior the FPU arrival on site. In addition, the hybrid risers provide the required flexibility to accommodate a disconnectable FPU, which is one of the preferred solutions for fields in harsh environments (i.e. prone to hurricane and typhoon conditions).

As a consequence this type of riser is often preferred in deep water West of Africa (FPU planning risks) and was used for the first disconnectable FPSO in the Gulf of Mexico. In a similar manner, it would be suitable for ‘future’ deepwater regions of Asia Pacific such as South China Sea, offshore Western Australia or Indonesia in harsh environmental offshore conditions.

The most commonly used hybrid riser systems to date are the Free Standing Hybrid Riser (FSHR) in ‘single’ leg configuration or several lines bundled in a Hybrid Riser Tower (HRT).

Hybrid Riser Towers consist in steel bundled risers in straight ‘tensioned’ configuration by means of a large subsurface buoyancy tank (positionned at some 50-200m water depth level) and is connected to the FPU by flexible jumpers.

The HRT applications have been pioneered by the Green Canyon 29 & Grand Banks 388 installations in the Gulf of Mexico and the Total Girasol (1350m water depth) installation in offshore Angola. The riser tower solution is mainly used in Angola (Girasol, Rosa, CLOV, and BP Greater Plutonio) where there is a dedicated onshore fabrication yard.

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Riser towers are mostly suitable for towed installation method, whereby the bundle is assembled on land, towed to site horizontally and upended to its final configuration. The submerged weight of the riser tower is minimised by buoyancy foams which are distributed along the length of the tower and can be either integral to the bundle cross section or clamped around its circumference. The riser tower components being assembled onshore (at a location suitable for towing) provide a high local content profile.

A derivative of the HRT solution is the single leg FSHR, which is based on the same concept of a steel riser supported by

a subsurface buoyancy tank and connected to the FPU by flexible jumper. This configuration includes one line per riser rather than a bundle of lines. Several FSHRs have been installed, mainly offshore West of Africa (e.g. Angola, Nigeria), in Brazil and in the Gulf of Mexico. This includes for example the 18”oil export riser on the P-52 FPU for the Roncador field in Brazil (1800m water depth) and the deepest FSHRs installed for the Cascade & Chinook fields in the Gulf of Mexico at 2,500m water depth.

Figure 3: FSHR System Overview – Upper & Lower Assemblies (typical) The payload capacity of the FPU is not affected by the water depths when using hybrid risers. The increasing length of

the vertical steel riser section is compensated by increasing the size of the buoyancy tank. The independent/uncoupled buoyancy tank also means that the hybrid riser can be used with a floater that has relatively large dynamic motion responses and excursions, which is a key advantage in deep water developments.

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Emerging Variations of Hybrid Risers A number of alternative hybrid riser variations have been developed to address specific optimisations and improvements

to the traditional hybrid riser configurations. These novel configurations and the specific design drivers which led to their development are described in the following sections.

Free-Standing Flexible Riser The Free Standing Flexible Riser (FSFR) is a concept developed in order to optimise the riser installation tension and in-

situ dynamic behaviours. The basic concept is similar to a traditional FSHR, with the following main features: • The vertical leg of the riser system from seabed to the subsurface buoy is a flexible pipe, instead of a rigid steel

pipe. This is a fundamental requirement of this concept as the low bending radius of the flexible pipe is required for the ‘double-catenary’ installation method. On the other hand, as the pipe is always vertical and in tension along its entire length during operation, it eliminates any touchdown point curvature cycling of the pipe and associated armour wire buckling design limits encountered in very deep water, for large diameter flexible pipes.

• The traditional “slender” buoyancy tank is replaced by a “flat buoy” where the aspect ratio between the height and diameter is decreased to approach that of a typical oil offloading buoy (typically height / diameter ratio of approximately 0.5).

• The Flat Buoy features good hydrodynamic behaviour, i.e. involving less excursions and minimised Vortex Induced Motions and Rotations (VIM / VIR) with comparison to ‘typical’ FSHR slender buoy.

Some of the primary drivers for the FSFR concept development are as follows:

• The flat buoy can be fabricated onshore, lifted or launched into the sea at a quayside, and towed to site at sea surface where it is ballasted to its final sub-surface location. This eliminates the requirement for an offshore heavy lift capability, thereby significantly optimising the cost profile of the overall solution;

• Prior to ballasting down of the “Flat Buoy”, a winch pre-installed on the flat buoy allows pull in and support the weight of the vertical flexible pipe. Hence the flat buoy would support half of the flexible pipe full installation loads, thereby reducing the installation vessel tension capacity by 50%. As a consequence, in most cases, a lower capacity (& lower cost) flexible pipe installation / construction vessel can be used for the installation of the FSFR system.

Figure 4: FSFR System Overview

Figure 5: Flat Buoy (flexible lines not installed)

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The Flat Buoy is made of several compartments, allowing to control the net uplift by ballasting / deballasting steps and provide safety compartment(s) in case of accidental flooding during the operational phase.

Two I-tubes are implemented in the Flat Buoy to allow the passage of the vertical flexible riser and flexible jumper, which are hung and connected (by means of a rigid gooseneck) at the top level of the buoy.

Comprehensive campaign of wind tunnel and wave basin tests complement by CFD calculations have been performed to validate the hydrodynamic stability of the system under “in-place” current conditions. Particular attention has been paid to the fluid-structure interaction phenomena, such as Vortex Induced Motion (VIM) and Vortex Induced Rotation (VIR), which are typically observed for slender cylindrical structures. Wind tunnel and basin test campaigns have been performed to “benchmark” the numerical CFD full scale models and confirming the improved stability of the ‘Flat-Buoy’ riser system.

A fully detailed installation procedure has been developed for the FSFR system, which has been subjected to full

constructability review and installation ‘hazid’ processes with offshore installation engineers and management teams. The main steps of the FSFR installation procedure are as follows:

1. Flat Buoy is deployed in water at quay (e.g. slipway, crane lift) and towed to site; 2. Pulling winches are deployed at the Flat Buoy; 3. First end deployment of the vertical flexible riser to the flat buoy and pull-in through the I-tube using a

messenger line and the pull-in winch cable;

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Figure 6: FSFR Outline Installation Procedure – Steps 2 and 3

4. Riser hang-off connection at the flat buoy hang off clamp; 5. First end deployment of the flexible jumper to the flat buoy and pull-in through the 2nd I-tube using a messenger

wire and the pull-in winch cable; 6. Jumper hang-off at the flat buoy hang-off clamp and spool-piece connection between the vertical flexible riser

and the flexible jumper at the flat buoy deck level;

Figure 7: FSFR Outline Installation Procedure – Step 6

7. Demobilisation of the pulling winches from the flat buoy; 8. Deployment of the remainder of the vertical flexible riser from the installation vessel and transfer of its entire

weight to the flat buoy; 9. Vertical riser pull down to its final subsea (pre-installed) foundation, through a combination of flat buoy

nitrogen fillings and pull down wire tensioning from the support vessel;

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Flat Buoy Installation Vessel

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Figure 8: FSFR “Double-Catenary” Installation Procedure – Steps 8 and 9

10. Base jumper installation; 11. Flexible jumper cross-haul to FPU after its arrival on site.

A cost comparison study has been performed with the purpose of comparing the FSFR concept to other hybrid riser

systems (e.g. FSHR). The analyses have been performed for both a single riser development and for a multiple riser development. The following main conclusions can be drawn from this analysis:

• Single Riser Comparison o The FSFR is approximately 30% cheaper than the FSHR when considering a single riser installation. o While the procurement cost of the flexible riser is higher than the equivalent steel riser (FSHR), this

cost differential is fully compensated by less onerous marine spread (e.g. lay tension) that is required for the FSFR installation.

• Multiple Risers Comparison o When considering an equivalent case of five riser systems in equally remote location where the FSHR

system must consider inter-continental mobilisation of a heavy lift vessel and high top tension pipe lay vessel, the cost differential between the FSHR and FSFR becomes marginal. This is explained by the fact that the intercontinental mobilisations are amortised over a greater number of riser installations, thereby reducing the relative “unit” cost of a riser system. This cost comparison exercise should be considered on a case by case basis to ensure that the optimum solution is selected for any specific development scenario.

The FSFR riser system is hence a competitive solution that can be applied in nearly all deep to very deep water areas due

to its versatility and intrinsic stability of the ‘Flat Buoy’. It also brings specific enhancement to development cases either requiring low number of risers to be installed (e.g. limited field expansion) or in regions where appropriate specialised marine spread (heavy lift) is not available.

In conclusion, the FSFR system complements the current flexible riser technologies, its main benefits are as follows: • It provides access to the generic advantages of hybrid riser systems, in particular:

o Minimisation of total loads applied on the FPU; o Decoupling between riser installation and FPU arrival (planning risk).

• It optimise the inherent benefits of flexible pipe design: o Dynamic loads and motions are mostly exerted on the flexible jumper, which needs (only) to be a

“shallow” water design;

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o Deepwater (vertical) flexible riser is maintained under tension and does not experience high curvature or compression loads.

• It eliminates the requirement for offshore heavy lifting and optimises the marine spread requirements for installation.

Multi-Lines Free Standing Riser In many deep water developments the riser function requirements per reservoir can be typically:

• Production line combined service line to form an “hybrid loop” for round-trip pigging; • Water injection line to maintain reservoir pressure and production; • Gas-lift at the production riser base for produced fluid artificial lift.

The Multi-Lines Free Standing Riser system is currently developed to provide all above functional requirements, as a

combined assembly in a single free standing riser system which is to be fully assembled offshore. Typically, this concept could be considered as an alternative to a Riser Tower system or to an array of FSHRs.

Figure 9: Multi-Lines Free Standing Riser (3D Artistic View)

The Multi-Lines Free Standing Riser system is an innovative concept, gathering several rigid risers to be fully assembled

offshore and hung onto a single top riser assembly / buoyancy tank system. The Multi-Lines Free Standing Riser features the following main components:

• A single buoyancy tank, which provides the required net uplift and restoring moment versus the environmental and functional loads;

• A stem pipe-in-pipe (PIP) riser fitted with top and lower riser assemblies, acting as the tendon (load transition) between the riser foundation and the near surface buoyancy tank. The annulus of the pipe-in-pipe is used to inject the gas lift at the riser base;

• A Top Riser Assembly (TRA) allowing connections of several flexible jumpers in double-catenary shape, e.g. production & gas-lift, service line and water injection;

• A Lower Riser Assembly (LRA), located at the bottom part of the PIP and performing the connection with the riser foundation;

• Two lateral risers (e.g. water injection & service) installed in between the top and lower riser assemblies and maintained “in-place” by spacers distributed along the central PIP riser. The lateral risers are hung at the TRA and are free to slide within the spacers and the LRA to allow for thermal expansions;

• A foundation system (e.g. suction caisson with counterweights) which sustains the riser lower tension loads; • Three Riser Base Jumpers (RBJ, i.e. production, service & water injection), either made of flexible or rigid

pipes, connect the LRA to the flowline end terminations (FLET) and are designed to absorb flowline expansions and LRA angular deflections.

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Figure 10: Top Riser Assembly (3D View)

Different options are considered and developed for the Lower Riser Assembly: either flexible or rigid base jumper.

Figure 11: Lower Riser Assembly with Riser Base Flex tails to PLET.

Different installation procedures have been defined for the Multi-Lines FSR, depending on the available vessels and

related lifting capacities. The following outline installation procedure has been defined for high crane capacity installation vessel:

• Pre-installation of gravity based foundation (e.g. suction pile combined with additional counterweights) to react to the riser tension load (note that 1-2 months soil curing is not required for gravity-based foundation);

• Step 1: LRA and production PIP riser are assembled to form a vertical string; • Step 2: TRA is connected to the last upper joint of the PIP riser on board the installation vessel; • Step 3: The complete PIP riser system (+LRA & TRA) is transferred to a dedicated overboard hang-off platform; • Step 4: Water injection riser is deployed to form a vertical string suspended from the installation vessel; • Step 5: The Water injection riser is over boarded and connected to the TRA dedicated hang-off porch;

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Figure 12: Outline Installation Procedure – Steps 3 to 5

• Step 6: Using counterweights, the lateral riser is pulled into the spacer guides, under ROV monitoring and the

guides are then closed; • Step 7: Service riser is deployed and hang-off in identical manner to the water injection riser; • Step 8: Buoyancy tank is lifted from the vessel deck, up-ended and connected to the TRA by mean of the flexible

joint (e.g. rotolatch); • Step 9: The completed riser system with its buoyancy tank are lifted off the hang-off platform, over boarded and

lowered below the sea surface,

Figure 13: Outline Installation Procedure – Steps 7 to 9

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• Step 10: Once the buoyancy tank is properly (sea water) flooded, the crane will lower the LRA into the foundation rotolatch receptacle under ROV monitoring. The crane (with heave compensation stretcher) will then pay-in to engage the rotolatch mechanism and ROV to activate the rotolatch receptacle safety pins;

• Step 11: The crane will maintains a positive uplift tension (using stretcher or cranemaster device) during the buoyancy tank de-ballasting, i.e. nitrogen injection (e.g. umbilical) into each buoyancy tank compartments;

• Step 12: The riser base jumpers are installed to make the connection to the flowline end terminations; • Step 13: The flexible jumpers are installed with first end connection to the TRA and second end cross-over to the

FPU.

Figure 14: Outline Installation Procedure – Steps 12 to 13

A cost study has been performed to compare the Multi-Lines FSR system with existing hybrid riser solutions:

• Hybrid Riser Tower; • Free Standing (Single Leg) Hybrid Riser array.

The cost comparison is based on a 1,700m water depth field development featuring 3 production areas, i.e. 3 insulated

pipe-in-pipe production lines (gas-lift is required at riser base), 3 non-insulated service lines and 3 water injection lines. For this specific case study, the “as-installed” cost of a Multi-Line Free Standing Riser is approximately 17% lower than

the equivalent development considering only a Free Standing Hybrid Riser array (9 off FSHRs). It is approximately 12% lower than the equivalent development considering only Hybrid Riser Towers (3 off).

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Deep Steep Riser The next challenge for the offshore Oil & Gas industries (e.g. within next 5-10 years) would be the 4000m ultra deep

water frontier developments, e.g. in Brazil, Gulf of Mexico, West of Africa. For this water depth target, a novel ‘flexible’ riser system is being considered for further technical feasibility studies and cost analyses.

This innovative concept consists in a single leg tensioned riser, which can be either ‘full flexible’ or ‘hybrid’ riser solutions. The riser system is hence basically composed of a flexible jumper at its upper section to decouple from the FPU motions. The lower riser vertical section can be either a flexible or a rigid steel riser to reach the seabed foundation.

Figure 15: Deep Steep Riser (3D Artistic View) As depicted in the above schematic, the Deep Steep Riser (DSR) system features:

• An upper flexible riser in “wave” configuration formed by means of distributed buoyancy modules at its “hog” section in a similar manner to the many ‘shallow water’ flexible riser wave systems;

• A lower ‘steep’ riser section using slender buoyancy tanks for in-place ‘up-righting’ moment, e.g. free-standing. This riser section can be either composed of flexible or rigid pipes. Varying riser cross-section designs are being considered to optimise the weight, tensile capacity and hydrostatic collapse capacity of relevant riser sections, thereby providing an overall optimal riser design and weight budget;

• Seabed foundations e.g. fitted with a rotolatch system in case of hybrid DSR to allow for some flexibility of the steep steel riser.

The DSR configuration will limit the riser loads at the FPU balcony or at the mooring turret in case of floater

weathervaning requirement. Two outline installation procedures have been defined, depending on the presence (or not) of the FPU during the riser installation phase:

• In the base case (FPU on location) the lower riser section is deployed (partially flooded buoyancy tanks) by means of the upper flexible riser section, by the method dubbed ‘flexible pipe follower’, the lay vessel tensioners are providing the required lay & lowering tension.

• In the alternative case the lower riser section is pre-installed (using conventional A&R winch method) and free-standing (by way of the slender buoys) waiting for the FPU arrival, thereafter the upper flexible riser is pull-down, connected to the vertical riser top subsea connector, and finally its 2nd end is cross haul to the FPU.

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The lay tension to be sustained by the installation vessel is limited, even in ultra-deep waters, thanks to the additional uplift provided by the slender buoyancy tanks and distributed buoyancy modules over the riser length.

The following figure depicts the maximum laying tension distribution in the case of a hybrid DSR in 4,000m water depth, This lay tension could be further reduced (e.g. some 250-300Te) if required by adding temporary buoyancy modules.

Figure 16: 11”ID Hybrid DSR Tension Distribution in 4,000m Water Depth (Typical)

The DSR configuration is expected to enable the future challenge of ultra-deepwater field development to some 4000m

water depth. It allows in particular extending the use of ‘flexible pipe’ technology in term of water depth as it is designed to ensure that the flexible risers remain under tension during both installation and operational phases, thus reducing the effect of external pressure and related collapse compression phenomena.

Full Flexible DSR systems featuring 6” internal diameter and carbon armour layers are currently under development for water depth applications in excess of 3,000m. Hybrid DSR systems are also under development to include larger internal diameter, e.g. 11”ID for gas production (& transportation) targeting the 4,000m water depth frontier.

Conclusions

This paper presents a number of deepwater hybrid riser concepts, some of which have already been successfully deployed, e.g. FSHRs in West of Africa, Brazil and GoM. Other hybrid riser systems are under development to address the future ultra-deep water needs. All riser concepts have been developed considering issues related to technical feasibility, local content and cost effectiveness to satisfy increasingly calls for field economics and reduced EPCI costs.

The emerging deep water developments in Asia Pacific would require innovative riser solutions to ensure that the economics of deepwater fields are not hampered by the requirement to procure expensive materials and mobilise expensive installation vessels for long periods from other parts of the world.

Acknowledgement

The authors wish to thank Technip ITC (Innovation & Technical Centre) for their continuous supports for the development of hybrid riser concepts.

Abbreviations A&R Abandon & Recovery CFD Computer Fluid Dynamic DSR Deep Steep Riser EPCI Engineering, Procurement, Construction & Installation FLET Flow Line End Termination FPSO Floating Production Storage and Offloading FPU Floating Production Unit FSFR Free Standing Flexible Riser FSHR Free Standing Hybrid Riser HRT Hybrid Riser Tower LRA Lower Riser Assembly PIP Pipe-in-pipe RBJ Riser Base Jumper ROV Remotely Operated Vehicle SCR Steel Catenary Riser SLWR Steel Lazy Wave Riser TLP Tensioned Leg Platform TRA Top Riser Assembly VIV Vortex Induced Vibrations