Risers – Part II

Risers for FPSOsAssessment of

Riser System SelectionRiser System Selection

Riser Installation

Riser Types• Fixed Static Risers • Free Standing Dynamic Risers• Hybrid Risers – Combination of both• According to material type – Rigid pipe , Flexible pipe • Rigid pipe Risers - Only those clamped or fixed on Fixed

Platforms, J-tube riser, SCRs, TTRsPlatforms, J-tube riser, SCRs, TTRs• Flexible pipe – Simple Catenary riser, Lazy wave riser,

Pliant wave riser, Steep wave reiser etc• Rigid + Flexible - Hybrid risers

• Others- Hybrid Tower Risers, Single Line Offset Risers (SLOR), i.e. Single Hybrid Risers, GroupedSLOR

Risers for FPSO

• Among the deepwater concepts, steel catenary risers (SCR), flexible risers, free standing hybrid risers and lazy wave SCR have been successfully deployed to FPSO application.

• In addition, there are also other viable riser concepts for FPSO.

• The steel riser and hybrid riser are the most viable ones for deep and ultra-deepwater FPSO.

• Flexible riser is also the preferred solution for FPSO.

Risers for FPSO• FPSO has been adopted nearly worldwide in particular Offshore Brazil,

WoA, and Asia as compared with TLP, Spar and Semisubmersible, as shown in Figure

• Regardless of a floater concept for a deepwater field development, there is always a need of riser system connecting the floater at surface and subsea facility on the seabed.

• Riser system is one of the more complex aspects of deepwater • Riser system is one of the more complex aspects of deepwater developments.

• It became evident that riser system plays bigger and bigger role as part of the offshore infrastructures.

• There are different ways to group deepwater risers.

• The most widely used and field proven deepwater riser concepts include SCRs, hybrid risers with a single line or bundled multi-lines, flexible risers with different configurations, and top tension risers (TTR) as shown in Figure.

Risers for FPSO

• Comparing the motion characteristics of FPSO with other type deepwater floaters (e.g. Spar, TLP, and Semisubmersible) its severe motions in particular for heave and pitch makes the riser concept selection and design much more challenging. concept selection and design much more challenging.

• Other constraints imposed by field reservoir properties (e.g. high temperature\high pressure), disconnection requirements system, and significant increase in water depth even exclude some riser concepts.

FPSOs Response

• Compared to other types of deepwater floaters such as TLP, SPAR, and semisubmersible which have been in service worldwide FPSO is in general less riser-friendly floater.

• A typical FPSO’s pitch, roll and heave motion natural periods are closer to wave peak periods of storm condition, e.g. hurricane or typhoon.

• This generates sever FPSO motion response characteristics, as illustrated in Figure .

Risers for FPSOs• In some cases, the risers are connected to the turret of which

the vertical motion in particular vertical acceleration is amplified due to FPSO pitch motion.

• The relative larger vertical motion at riser top termination location makes the riser engineering more challenging.

Risers for FPSOs• There are some ways to mitigate FPSO motion including bilge

keels, dynamic positioning (DP) system

• The vortex shedding and viscous damping from the bilge keels of FPSO can significantly reduce its roll motion.

• The combined environmental forces cause the FPSO hull to form an oblique angle to the wave.form an oblique angle to the wave.

• It will significantly increase the wave frequency horizontal and vertical motions, which is the main driver behind the excessive compression and fatigue damage in particular to SCRs.

• The FPSO with DP can align the vessel with wave direction to reduce the wave frequency motion.

Risers for FPSOs

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

• This is why hybrid riser is also a kind of popular concept for FPSO application, e.g. in WoA (West of Africa) and GoM.

Potential Riser Concepts for FPSOApplication

Factors Affecting FPSO Riser Selectionfor FPSOs

Different riser concepts have been implemented or are being studied for FPSO application. The following major factors need to be taken into account for FPSO riser selection:•Water depth• Riser dimensions• FPSO motion• Metocean data• Metocean data• Riser termination location and method• Field layout and footprints• Thermal performance requirement• Installability• Schedule• Cost

Risers for FPSOs

Risers for FPSOs

• Basically, three riser concepts have been actually implemented in FPSO based field development, namely hybrid riser, flexible riser and steel catenary riser (SCR).

• In addition, there are also other types of riser • In addition, there are also other types of riser concepts are in the development that could be applied to FPSO.

• It includes tension leg riser (TLR) and hybrid catenary riser (HCR).

• Selectively SCR and hybrid riser concepts for FPSO application are illustrated in the Figures

Cascade & Chinook FPSO – Free Standing Hybrid Riser

SCRs

• SCRs were initially used as export lines on fixed platforms.

• SCRs clearly have similarities with free-hanging flexible risers, being horizontal at the lower end and generally within about 20 degrees of the vertical at the top end. within about 20 degrees of the vertical at the top end.

• In this arrangement, the riser forms an extension of the flowline that is hung from the platform in a simple catenary.

• Relative rotational movement between riser and platform can use a flexjoint, stress joint, and pull tube to offset the movement.

Gendalo FPSO – Steel Catenary Riser

Musts and Preferred for Riser development

• In general, the following are regarded as “musts” for a specific field development:

• •Water depth • • Pressure and Temperature• • Conveyed fluids• • Size of each riser• • Number of risers• • Number of risers• • OHTC requirement (Overall Heat Transfer Coefficient)• • Riser top payload impact on hull termination location• • Constructability• The following are regarded as “preferred”:• • Functionality• • Operability• • Safety• • Schedule• • Cost including CAPEX and OPEX

Risers for FPSOs

• Tower / Hybrid Riser

• Basically, this riser adopts some principles of flowline bundle technology and it is a kind of flowline bundle technology and it is a kind of hybrid riser system consisting of rigid and flexible risers

Tower / Hybrid Riser Concept

• The main section of the hybrid riser consists of a central structural tubular, around which syntactic foam buoyancy modules are attached.

• Peripheral production and export lines run through the buoyancy modules and are free to slide axially in order to accommodate thermal and pressure induced expansion.

• The central structural member is connected to the riser base by way of a hydraulic connector and tapered stress joint.

• The peripheral lines are attached to the rigid flowline jumper on the • The peripheral lines are attached to the rigid flowline jumper on the base, which provides connection to the flowlines.

• On top of the riser, the peripheral lines are terminated in goosenecks. • Flexible jumpers are used to connect the goosenecks and the surface

FPSO, providing the flow path to the host platform whilst accommodating decoupling between the vessel motion and the platform.

Tower Design

• The capacity or cross section of the tower riser is predefined during the engineering phase.

• Any future expansion requirement must be built into the riser tower design. into the riser tower design.

• Extra expandability cannot be accommodated once the design is completed.

• Key tower riser design parameters are buoyancy capacity and soil foundation capacity to carry the up-thrust tension loading.

Risers for FPSOs

Flexible Riser

• Riser for catenary moored FPSO are generally designed using flexible pipe such examples include most of the FPSOs in offshore Brazil, some in WoA (e.g. Dalia, Agbami).

• Flexible riser is more or less the only concept for • Flexible riser is more or less the only concept for shallow water FPSOs since the flexible riser arrangement can be more compliant.

• This is normally achieved by lazy wave configuration, steep wave configurations, or compliant wave configuration.

Fabrication and Installation of Flexible Risers for FPSOs

• Fabrication• • Deepwater flexible risers are proprietary design from three

suppliers (Technip, Wellstream, NKT) only.• • Intermediate connection might be needed for the longer lines due

to manufacturing reel limitation or design considerations.• Installation• Installation• • Relatively simple installation from hydraulically powered reel.• • May require subsea tie-in / pull-in with flowline.• • For ultra deepwater, availability of installation vessel to handle the

tensions and collapse capacity is questionable, in particular if it is coupled with sweet and sour service requirements.

Risers for FPSOs

SCR Risers for FPSOs

• Steel Catenary Riser • SCR 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 mainly used for WoA and Campos Basin, e.g. Erha,

Bonga, AKPO.• As the result of FPSO motion, the SCR is susceptible to compression in the sag

bend area.bend area.• As a minimum, the following factors need to be evaluated when considering SCR

for an FPSO:• • SCR dimension• • Geotechnical data• • FPSO motion• • Internal turret location and configuration• • Metocean data• • SCR cross section design

Risers for FPSOs

• Lazy Wave SCR• Lazy wave SCR is basically a modified configuration of a free

hanging SCR by introducing buoyancy modulus for a certain length of the suspended SCR.

• The introduced buoyancy will absorb most of the dynamic motion imposed by FPSO motion so that the SCR touch motion imposed by FPSO motion so that the SCR touch down point (TDP) could land on the seabed “softly” hence reduce the frequent contact with the seabed significantly.

• This will improve the SCR fatigue life at touch down area and make the concept technically feasible.

• BC 10 field in Campos Basin is the first lazy wave SCR for FPSO in the world.

Pros and Cons of Lazy wave

Risers Installation

SUMMARY OF MAIN RISER TYPES• Risers for platforms may be broadly grouped into the

following categories:• riser for steel jacket platform;• riser for gravity base structure (concrete);• J-tube riser -this category is further discussed in (8);• caisson riser system; consists of a caisson which forms • caisson riser system; consists of a caisson which forms

a structural encasement and a number of riser pipes which are installed in the caisson.

• Caissons are protective devices to eliminate environmental loading on the riser pipes. Caisson riser systems may reduce the number of riser supports which otherwise would be required for conventional risers;

• Flexible riser for a floating facility

General• Risers are usually pre-installed with the jacket structure.

• Otherwise they can: be retrofitted onto existing platforms.

• This may be by the conventional method of lift, set and subsea tiein.

• Alternatively, one of the following methods may be used • Alternatively, one of the following methods may be used without the need for subsea tie-ins:

• stalk-on method;

• bending shoe riser method;

• barefoot riser;

• J-tube installation

Conventional method • Retrofitted risers are fabricated in sections, lifted from a barge and lowered into

suitable riser supports which may also be retrofitted onto the jacket.

• The number of riser sections depends on the water depth and the length of the barge.

• The riser normally consists of an upper section behind the jacket bracing (to provide safety against boat impact) which is connected to lower sections positioned on the outside of the jacket. After installation, a subsea tie-in is made positioned on the outside of the jacket. After installation, a subsea tie-in is made to the pipeline.

• A form of retrofit riser clamp may be installed after a jacket has been in service for some time.

• In this case, provision shall be made for aligning the clamps/guides.

• This is achieved by connecting the riser clamp/guide which is also clamped to a structural jacket member or stub, depending on the size of riser. Retrofitting of these clamps/guides involves considerale diver time.

• Alternatively, a riser ladder, or more simply riser support stubs, may have been installed on the jacket in the fabrication yard for future retrofitting of risers.

• Retrofitting methods without the need for subsea tie-ins are described below.

Stalk-on riser method

• For shallow water, this is the most commonly used riser installation method. • After the pipeline has been laid with its end on the sea bottom and close to the

platform, the lay barge is moored in position. • The riser bend which will eventually connect the horizontal pipeline to the

platform deck is measured and the location at which the pipeline will be cut for connection to the bend is marked.

• The pipeline is then lifted from the seabed by applying tension to the pipe. • The pipeline is then lifted from the seabed by applying tension to the pipe. • In very shallow water with small diameter pipelines, this is not a problem;

however, larger lines in deeper water require a substantial length of pipe to be supported off the bottom to avoid overstressing the pipe.

• The pipe is then cut at the mark, the bend is welded onto the free end of the pipe and the pipe and bend are lowered down.

• This process of adding pipe is continued until the pipe reaches the bottom. • The riser is then secured to the platform using diver-operated clamps.• Expansion spods can be set simultaneously with this method.

Stalk-on riser method

• In deeper waters, the handling of the pipe and riser becomes increasingly difficult and even hazardous to both pipe, equipment and personnel.

• The main advantages of this technique are:• • weld connections are made above surface and can be fully inspected, ensuring weld• quality;• • diver activities are relatively simple, requiring only a normally-skilled team using

standard tools. • The expense and time delay involved in mobilising specialised contractor personnel • The expense and time delay involved in mobilising specialised contractor personnel

are avoided;• • there is no requirement for underwater welding.• The disadvantages of this technique are:• • the lifting, welding and lowering operation is vulnerable to environmental

conditions;• • careful planning and strict compliance with the predetennined lifting and lowering

procedures are vital to avoid overstressing the pipeline and riser;• • greater adjustability in the riser clamps is required because the riser cannot be

moved fore and aft once it is welded to the pipeline.

Installation

• dfsd

Risers for FPSOs

• dfsd

Bending shoe riser method

• This method, consists of installing a curvature limiting shoe on the platform during onshore fabrication.

• The pipeline is then laid to the structure and positioned under the bending shoe either by manoeuvring the barge or attaching cables to the line as it is laid and pulling it under the shoe.

• Once the line is in the correct orientation with respect to the centreline of the bending shoe, specially designed hydraulic clamps centreline of the bending shoe, specially designed hydraulic clamps on the platform capture and secure the riser.

• These clamps may be installed either during onshore fabrication or immediately before the riser is installed offshore.

• In very deep water or for pipe with low stiffness, it may be necessary to install auxiliary cables on the riser to assist with the installation.

• Other than inspection, this method of riser installation requires a minimal amount of underwater work.

Barefoot riser method

• The method is simple and should find many applications, especially for deepwater platfonns.

• The pipe weight and wall thickness are selected such that the pipe can be lifted vertically at the water surface without exceeding a specified, non-buckling, bend in the sag portion of the line.

• The method consists of approaching the platform with the pipe suspended vertically at the water surface. vertically at the water surface.

• The pipe is then positioned at a tangent to and in contact with the upper end of a series of pipeline clamps on the platform.

• The lifting load is decreased according to a prescribed schedule which forces the riser pipe into each successive clamp and puts the bottom span into compression.

• The riser is then clamped to the platform once the desired curvature in the sag-bend is achieved.

• The necessary hydraulic or electrically operated riser clamps can be installed offshore using a rail guidance system to land each clamp at a predetermined elevation.

• Diver time, other than for inspection, would be minimal for this method of riser installation.

Subsea tie-in methods

• SUBSEA TIE-IN METHODS• In most cases, tie-in of the pipeline to the offshore facility is

achieved by inserting an expansion spool piece.• The purpose of the spool piece is to absorb expansion loadings,

and accommodate the installation tolerance on the pipeline.and accommodate the installation tolerance on the pipeline.• The spool piece connections may be made up using one or a

combination of the following methods:• • mechanical connectors;• • flanged tie-in using RT J swivel ring flanges, miss alignment

flanges; or• • hyperbaric welding.

Risers for FPSOs

• Spoolpieces

• General Principle

• This method is probably the most popular method of tie-in for flowlines/pipelines. Diversmethod of tie-in for flowlines/pipelines. Divers

• measure and then assist the installation of a piece of pipe to fit in between the two ends of

• flowline to be tied together.