management of hybrid riser

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Management of Hybrid Riser Towers IntegrityJean-Luc Legras, Jean-Franois Saint-Marcoux (ASME), Subsea 7

Copyright 2011, Offshore Technology Conference

This paper was prepared for presentation at the Offshore Technology Conference held in Houston, Texas, USA, 25 May 2011.

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 beenreviewed 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, itsofficers, 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 toreproduce 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.

AbstractField developments target deeper and deeper waters, for which Hybrid Riser Towers (HRTs) have become one of the

solutions investigated almost systematically at bid stage. This is due, particular, to the capability of HRTs to accommodate

the requirement for large diameter risers, reduced load on FPSO, demanding flow assurance requirements, and robust layoutfor later developments phases.

Requirements of Integrity Management (IM) are nowadays at the heart of all offshore development; they are included in the

criteria for selection of a riser system. Application of IM principles to a HRT project are summarized with the perspective of

the Contractor. The first IM activity is a comprehensive risk assessment which requires the review of all components of the

subsea system from wellhead to FPSO; all parts of the preferred architecture of a Bundle HRT (BHRT) are addressed herewith emphasis on IM issues and the main failure modes identified.

Finally a typical monitoring system of a BHRT is described as well as inspection; indeed it is from their results that the IM

plan can be implemented during the operating phase of the project.

Introduction

HRTs include BHRTs and Single Hybrid Risers (SHRs) which have in common to be anchored in the seabed, include rigid

pipe over the most part of the water depth, be tensioned by a buoyancy tank at the top, and linked to a Floating ProductionUnit (FPU) by a flexible. BHRTs that are mainly considered here are more complex because of the plurality of pipes of the

column arrangement and of the specific installation method. Recent applications of BHRTs are offshore West Africa with

now more than ten years of operation on the Girassol field, but designs at FEED stage have been made for ultra deep water

fields of the Gulf of Mexico and Petrobras Pre-salt offshore Brazil.

A practical example application of an IM plan has been proposed and implemented by Total for the Girassol BHRTs(Chapin, 2005). Since then the industry has set in place a Joint Industry Program (SCRIM JIP, 2007) for the integrity

management of Steel Catenary Risers, and the program of this JIP has been extended to cover HRTs. The subject is alsoaddressed in the DNV RP F206. The main IM activities are summarized below.

From the Girassol (Rouillon, 2002) and Greater Plutonio (Sworn, 2005) experience, Subsea 7 (Alliot, Legras, 2006) has

analyzed the lessons learned from these bundle HRT based projects. This has lead to an architecture which will be detailed

hereafter and screened for the failure modes.

Dynamic risers require attention because of the dynamic nature of their loading, whether this is due to their environment or tothe fluid they convey. Companies have long recognized the importance of monitoring these and results are essential input for

the fitness statements to be carried out during operation of the riser system.


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Summary of Integrity Management of a BHT

Design phase

The first activity is a comprehensive risk assessment by means of a Failure Modes, Effects and Criticality Analysis

(FMECA). This used for the following tasks of the design phase:

- Select among design options (use the one involving less overall risk during installation and operation, but also theone which facilitates inspection and monitoring when relevant). Some examples are given in the following section.

- Define function requirements of the monitoring system.- Define criticality rating for the Quality Assurance for the procurement and assembly of component (level of

inspection, factory acceptance tests, etc).

- Preparation of procedures for operation of the field (contractor is generally not involved here and therefore processintegrity is not addressed below)

- Definition of the riser inspection and maintenance strategy.- Preparation of the detailed IM plan, in particular, methods to process, condense and interpret measurements from the

riser monitoring system and operating records, development of performance indicators, and periodical assessment of

fitness-for purpose.

Fabrication, installati on and pre-commissioni ng

The main tasks related to IM are:- Collect data-books of fabrication records, results of non-destructive inspection and as-built data.- Analyse impact of changes and non-conformity.- Estimate the fatigue damage experienced during installation and determine the damage left of the in-place life.

Commissioning, operat ion and abandonment

IM activities consist in the implementation of the IM plan

Contractors play a major role in IM activities during the two first phases, but they are rarely involved for the last one;indeed they could bring significant input, because of their good knowledge of the riser system behaviour, both for the

definition of the IM plan and the processing and interpretation of measured data.

Preferred architecture

With due respect to the requirements of specific projects, there is a necessity to standardize the design of BHRTs. Thisnecessity was clearly expressed by the clients, by the contractors project management team, and by the engineering team, in

order to cut short unnecessary over design, costs, and schedule delays.

For future projects, a comprehensive review of design options was conducted to select the preferred architecture. This

design has been proposed for ongoing deepwater and ultra deepwater projects in Brazil, Gulf of Mexico (GOM), and West

Africa.

Various design alternatives were considered for each function of the tower. This exercise was conducted with allstakeholders of the contractor project team: engineering, procurement, fabrication, installation and project management.

The preferred architecture for a BHRT consists of a bundle of several rigid pipes, anchored to the seabed and tensioned bymeans of a buoyancy tank. It is connected to the FPU by means of flexible jumpers and to flowlines and pipelines by means of

spools (rigid jumpers). A general view of the architecture is shown below (Fig. 1).

The components are briefly described hereafter from the bottom to the top (Legras, 2008).


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Fig.1 Illustration of the BHRT preferred architecture (patent pending)

those used for the mooring system of the FPUs. Additional ballast weight is

add he pile.

ow friction bushings and washers. The connection system between column and

fou s a Rotolatch component.

Foundation

It is preferably a suction pile similar to

ed in receptacles fitted around t

Articulation

The design that has been used in the past is a flexible joint including several layers of elastomer with steel reinforcements.An alternate design is a universal joint with l

ndation include

Riser Bundle

The core pipe is the main structural member of the BHRT linking the articulation at the bottom and the Upper Riser

Tower Assembly (URTA). It can be used for fluid transfer per API RP 2RD and may be internally lined. It is of largerdiameter than the other risers because it has to sustain compression during installation (upending) and large tension after the

buoyancy tank has been installed and deballasted.Rigid risers are attached to the URTA at the top and are free to expand at the bottom (see fig. 2).The risers are evenly

distributed around the column so that, on the cross section, they are practically tangential to the inside of the same circle. This

minimizes the risk of galloping under the effect of strong currents (see Fig.2).Because the risers are not enclosed they can be visually inspected by ROV. Risers are guided at regular interval along the

tower by guiding frames attached to the core pipe which ensure the relative axial displacement between riser and core pipe

with low friction.The BHRT is fabricated onshore and towed to site by towing, buoyancy modules are necessary to make it almost

neutrally buoyant. Buoyancy modules are half-shells attached to the core pipe.

Buoyancy modules are made of Glass Syntactic Epoxy Foam (GSEF). GSEF consists of glass micro-spheres of typically

30 microns and larger spheres (a few mm in diameters) wound with glass or carbon fibers, all in a matrix of epoxy.

Mechanical properties of Epoxy deteriorate by hydrolysis in presence of water at high temperature (above 65C to 70C) and


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signed to

allo water circulation. Natural convection of seawater is e foam from high temperature.

Fig. 2 Bundle configuration

pper Riser Termination Assembly (URTA)

tural link between the core pipe and the buoyancy tank by means of an articulated joint and a tubular

me

pers, through the goosenecks,ers,

he URTA must be buoyant during tow to site and upending of the BHRT. Foam blocks provide the necessary buoyancy.

uration is selected to avoid using a highly stressed

tap

ed. A double barrier of valves is used to prevent loss of buoyancy by ingress of water

in t e buoyancy tank compartments.

or the flexibles which can be of conventional design: the maximum depth is not largeeno

the upper layers of the ocean.

Ne rtheless the condition of the jumper can be checked by monitoring the annulus pressure.

, or others.In a

U includes a support above water level (half rings on the flexible

term

he upper end of the bend stiffener is fitted with a connector that is latched to the lower end of the J-tube.

high pressure. It is therefore important to maintain the foam blocks at low temperature and avoid direct contact with hot lines

or insulated lines. In the BHRT preferred architecture, the gaps between the buoyancy foam blocks and risers are de

w very effective to protect th

U

The main functions of the URTA are to:

- provide a rigid structural link between the risers and the core pipe,

- provide a struc

mber or a chain,

- allow fluid transfer between the riser bundle assembly and the flexible jum- support the potentially diverless tools for installing the flexible jump

- in some cases access for coil-tubing intervention inside rigid risers.T

Buoyancy Tank

The buoyancy tank is connected to the URTA by a tether. This config

er joint, even though this was successfully made on previous projects.The buoyancy tank is a steel cylinder including a number of compartments to minimise the consequences of an accidental

flooding. The compartments are pressurised with nitrogen during installation and therefore designed to sustain minimal

differential pressure. The tank includes a ballast system with ROV operated valves at top and bottom of each compartment.

In normal operation all valves are clos

h

F lexible Jumpers

There are no severe requirements fugh to cause concern for collapse

Overall Heat Transfer Coefficient (OHTC) is not stringent because sea water is warmer in

ve

I nterf ace with the FPU

The BHRT design would accommodate any of the following FPU types: TLP, Spar, semi-submersible, FPSOsreas with extremely severe sea state conditions, the flexible jumpers can be attached to a disconnectable turret.

The jumper guiding/attachment system on the FP

ination head) and guides along the hull (J-tube).

T


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s very much congested, much improvement in interference issues is brought by an arrangement

wh e umbilicals are supported by arches attached on the bundle at some elevation above the bottom of riser (Girassol

illon 2002).

Fa

tter controlled processes for

we on lines is also made easier. This is particularly

imp eral techniques can be used at a reasonable cost:

l 625,

welding of Corrosion Resistant Alloy (CRA) clad or lined tubulars.

Heat treatment also may be c

Fig. 3 BHRT fabrication in sections

Ins

lines is increased allowing the HRT to reach a stable subsurface configuration that isma

l is always present.

clump weight and connected to its foundation. The following steps take

platank to the URTA,

installation of the flexible jumpers (this last step can be delayed if the BHRT is installed before mooring of the FPU).

The installation is reversible which allows recovering the tower at any step in the procedure and after service.

Umbilical arch

When the field layout i

er

BHRTs) (Rou

brication

There are different possible methods for the onshore fabrication, depending on the characteristics of the available site.

Ideally it may be entirely assembled on the yard and launched in the open sea in a single operation. Alternatively the BHRTcan be fabricated in sections in a yard (see Fig. 3). Yard fabrication allows more advanced and be

lding and field joint coating than offshore operation. Lining of water Injecti

ortant for fatigue sensitive welds, where sev

- welding with material with high fatigue properties, such as Incone

- improving root and cap as appropriate,-

onsidered.

tallation

The completed HRT is towed to the field using two configurations. At the start it is towed at surface (see Fig. 4), then

when the water depth is sufficient, and if required to minimize fatigue during transportation, it is towed sub-surface. The

length of the lead and rear chain towintained for the rest of the route. Tows over hundreds of miles can be envisaged even in areas such as offshore West

Africa where swel

Upon arrival on site the BHRT is upended using a

ce afterwards:

- connection of the buoyancy- deballasting of the tank compartments and flooding of the risers,

- hook-up of the spools,

-


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Fig. 4 Surface tow of an HRT

Review of failure modes

The typical failure modes associated with risers are listed in the SCRIM 2007 JIP:

- Temperature- Pressure

- Product fluid composition

- Service loads

- Fatigue- Corrosion

- Installation

- Fabrication- Accidental Damage

These failure modes have been gathered in three categories that are listed hereafter:

- Produced Fluid related: internal corrosion (fluid composition), pressure, and temperature,

- Loads: pre-service (fabrication and installation), and service loads, fatigue

- Environmental impact: external corrosion, interference with other facilities, Simultaneous Operations (SIMOPS)

Most failure modes are common to all types of risers. In this paper only the ones specific to the BHRT are developed.

Produced Fluids related

Internal corrosion/erosion

Internal corrosion may affect flexible jumpers, the riser bundle, the URTA and the core pipe if used for fluid transfer

The material selection for an HRT follows the same rules as for any other riser system, with regards to:- H2S and CO2 partial pressure, (NACE MR-0175, NORSOK M 506) at the operating conditions during the life cycle

- Erosional velocity (ISO 13703/API RP 14E), and erosion

Erosion is sensitive to the radius of curvature, which is a particular concern for goosenecks. For that reason, goosenecksare usually made from CRA clad pipe and sometimes retrievable.

PressureMinimum, maximum, and pressure fluctuations must be considered for the design of each line.

Protection from excessive pressure can be provided by HIPPS. In general, this system should be located at the manifoldside, in order to protect both the risers and the flowline against excessive shut-in pressure. For High Pressure fields, the weak

point could be the flexible jumpers. If necessary to protect the flexible jumper against the Wellhead Shut-in pressure

(WHSIP), a HIPPS could be located at the URTA.The effect of pressure is therefore limited to the minimum that would affect any other riser type.

Temperature

Maximum and minimum temperature conditions affect the selection of material. The minimum temperature can be

onerous on gas services in start-up and depressurizing conditions.


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In the riser system the monitoring of the temperature is essential to verify the integrity of the thermal performance of the

system. Steady state conditions can be monitored with the temperature probes located at the manifolds and the FPU. However

this gives no information on how thermal transient performances are achieved. This can be provided by:

- a temperature probe located at the riser base on the production line (Taxy and Lebreton, 2004)- continuous monitoring of the temperature using fiber optics with, for example Bragg grating.

Monitoring the Girassol BHRT with a temperature probe was accurate enough to detect a thermal leak at a doghouse.

Degradation of composite material at high pressure and elevated temperature can affect Glass Syntactic Epoxy Foam

(GSEF) as well as Glass Syntactic Polyurethane (GSPU). It must be noted that in the proposed design the buoyancy foam iskept at low temperature thus inherently mitigating the issue.

By design the temperature effects are limited to the minimum that would affect any riser type.

Loads

Fabrication

Loads during fabrication are generally not onerous and therefore not further detailed.

Installation

Towing contributes to fatigue damage. This is particularly significant in West Africa where long swell will persist for the

duration of the tow. It is therefore necessary to estimate the fatigue damage during tow either from measurement of themotion of the BHRT or from calculations based on characteristics of the environmental conditions.

In-service condition-general

The fatigue damage in-service is low and therefore does not require direct stress monitoring.On the other hand the effectiveness of the upthrust provided by the buoyancy tank is a key element of the integrity. This

can be monitored by:

- The position of the HRT to be compared to a design envelope.

- The tension in the core pipe.

In-service condition-effect of current

By design the top of the Hybrid Riser Tower is located below the active water layer of the ocean. That water depth is

about 70m In West Africa and deeper, about 200m, in other areas of the world such as the Gulf of Mexico and Brazil. Thecurrent velocity there is typically half of the surface current. Nevertheless the effect of current must be reviewed for the

following issues:

- Vortex Induced Motion of the Buoyancy Tank- Galloping of the BHRT column

- Vortex Induced vibrations of the of the BHRT column

- Vortex Induced vibrations of the risers within the bundle

Vortex Induced Motion of the Buoyancy Tank

Extensive review and tests were performed at the Girassol project and the decision was made not to install strakes(Rouillon, 2002). The current observations confirm that this was not necessary.

Galloping of the BHRTFor a circular cross-section, outside of the range of VIV, the average lift on the bundle cross section is equal to zero by

symmetry. This is not the case for a typical BHRT cross-section and the average lift load varies with the current incidence,

which may induce galloping motion in horizontal displacement and/or rotation about the longitudinal axis.

In order to analyze the potential for galloping of a non-circular cross-section the normal procedure consists of performingmodel tests to determine the drag and lift coefficients at various incidence of the flow (Fig 5). The criteria for stability of the

column are given by Blevins, 1997. CFD calculations are more and more accurate and then now can assist to assess the riskof galloping.

To avoid galloping the cross-section should be as circular as possible.


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Fig. 5 Model test of an HRT cross-section

Vortex Induced Vibration of the BHRT columnAs any other riser the BHRT can be subject to VIV. However, as explained above the BHRT is located in an area where

the current velocities are lower than at the surface. Furthermore, the BHRT cross section may be assimilated to a very rough

cylinder and then subject to much smaller VIV excitation than a smooth pipe. In addition it should be observed that one of

the leading VIV software, SHEAR 7 is specifically designed for vertical risers, and therefore directly applicable to BHRTs.

Vortex Induced Vibrations of Individual Risers

In the preferred architecture, already implement in a recent West Africa project, the individual risers are free to expand intheir guides.

It is therefore necessary to let them slide and provide a gap for doing so.

The risers must be refrained from slamming into their guides by appropriately designed guiding devices.

Environmental Impact

External corrosion

Because of its open design, there is a free circulation of cold seawater around the core pipe and guide frames.

Consequently the traditional sacrificial cathodic protection system can be used. Corrosion protection of the individual riserscan be handled as for any other riser system type.

Interference with other facilities

In the design of a deepwater system the interference between the mooring, the risers and the umbilicals usually drive the

layout of the field.It is also important to be able to plan for future development. The bundle HRT can provide a robust layout (Blevins et al.

2007), because the effect of risers interference, including the effect of wake shielding and wake instability is kept minimal.

Design of a monitoring system

Based on the above observations, the monitoring system for a bundle HRT should be capable of monitoring:- pre-service conditions,

- in-service conditions.

The pre-service condition monitoring system is essentially designed to monitor the condition during towing and upending.This system consists of depth sensors and possibly Motion Recording Units (MRU) located at the top, middle and bottom of

the bundle HRT. They are designed to be retrieved at the end of the installation operation, as the information they provide

would be of limited value in service.

The permanent in-service monitoring system must address:

- The integrity of the flow assurance systems: thermal insulation, riser base gaslift- The integrity of the applied top-tension

- The integrity of the flexible jumpers

This can be achieved with:


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aslift injection point)

ial load in the core pipe at URTA

lev

ts, or internal pressure, is not considered necessary, as the

determination of a flooded compartment can be carried out by ROV, after such event has been detected through monitoring

of e

of each individual riser. Careful ROV visual inspection can beery effective to detect any damage. Besides visual inspection, ROVs allow the monitoring of cathodic protection.

internally inspected using intelligent pigs.

hieved with the Preferred design. The project

incorporate these requirements around the standard design.

he BHRT is fitted with convenient monitoring which allows integrity assessments to be made during the operation phase of

he authors are indebted to Subsea 7 for supporting this paper. This paper reflects the opinion of its authors and does not

rsement by the company to which acknowledgements are made.

.for

7683, Houston, TX.

r

hore Technology Conference, paper 17397, Houston, TX0. Taxy, S., Lebreton, E., (2004), Use of CFD to investigate the impact of cold spot on subsea insulation performance Application to

deepwater fields, Offshore Technology Conference, paper 16502, Houston, TX.

- Monitoring of the temperature and pressure in production lines at riser base (if relevant, both downstream and upstream

of the g

- Monitoring of the position in space of the buoyancy tank and/or measurement of the axel.

- Check of the integrity of the Flexible jumpers from their annulus pressure at the FPU.

Continuous measurement of the content of tank compartmen

ither its displacement or a change in the core pipe axial load.

The preferred design allows for visual inspection by ROV

vRisers can be

ConclusionsThe design of the BHRT, selection of material, and fabrication are of primary importance to its integrity. A standard design

allows capturing the lessons learned of previous projects. This is what is acspecific requirements come in addition to suit the particular needs of the project, but it is essential, in order to maintain cost

and schedule, to

T

the BHRT life.

AcknowledgementsT

imply full endo

References1. DnV RP F-206 Riser Integrity Management

2. MCS SCRIM JIP (2008) Steel Catenary Riser Integrity Management Joint Industry Project3. Alliot, V., Legras, J-L., (2006), Lessons Learnt from the Evolution and Development of Multiple-Lines Hybrid Riser Towers

Deepwater Production Applications, Offshore Technology Conference, paper 1

4. Blevins, R., D., Saint-Marcoux, J-F., Hybrid riser Tower for Deepwater Drill Centers, OMAE, San Diego, Ca., June 2007(OMAE 2007-29289).

5. Blevins, R.,D., (1997) Flow Induced Vibrations, Kreiger, Malabar, FA.6. Chapin, G., (2005), Inspection and Monitoring of Girassol Hybrid Riser Towers, Offshore Technology Conference, paper 17696,

Houston, TX.7. Legras, J-L, Saint-Marcoux, J-F, (2008) Plug and Play Deepwater Minimum Production Riser System, Deep Offshore TechnologyConference, paper ID48, Houston, TX.8. Rouillon, J., (2002), Girassol The Umbilicals and flowlines presentation and Challenges, Offshore Technology Conference, pape

14171, Houston, TX.9. Sworn, A., (2005), Hybrid Riser Towers from an Operators Perspective, Offs1

management of hybrid riser

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