OTC 17696

Inspection and monitoring of Girassol hybrid riser towers G. Chapin, Total E&P Angola

Copyright 2005, Offshore Technology Conference This paper was prepared for presentation at the 2005 Offshore Technology Conference held in Houston, TX, U.S.A., 2–5 May 2005.

This paper was selected for presentation by an OTC Program Committee following review of information contained in a proposal submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the Offshore Technology Conference and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the Offshore Technology Conference, its officers, or members. Papers presented at OTC are subject to publication review by Sponsor Society Committees of the Offshore Technology Conference. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of the Offshore Technology Conference is prohibited. Permission to reproduce in print is restricted to a proposal of not more than 300 words; illustrations may not be copied. The proposal must contain conspicuous acknowledgment of where and by whom the paper was presented. Write Librarian, OTC, P.O. Box 833836, Richardson, TX 75083-3836, U.S.A., fax 01-972-952-9435. Abstract More than three years after installation of the Girassol Hybrid Riser Towers, this paper aims at presenting the choices made in terms of inspection and monitoring on such equipment and their inspection results.

The access for inspection is very restricted and the riser tower design is based on a zero inspection and maintenance concept: FMEA studies conducted during the design phase have led to incorporate changes in the final design and the project has worked to eliminate the areas of concern.

However, external cathodic protection measurements, general and close visual inspection by ROV are regularly performed.

These inspection activities are implemented within the framework of a general sub-sea inspection strategy defined through a Risk-Based Inspection approach.

The results of inspection are compiled in a dedicated Computerized Inspection Management System (CIMS).

So far, the riser towers have not shown any significant defect or anomaly.

The temperature of the production spools at the base of the towers is monitored with sensors. The data recovered from these sensors, together with data collected from topside instruments, allows monitoring the thermal performance of the towers. This is critical in ensuring that the risk of hydrate formation in the system is minimized.

Description of the system: a world premiere

Each of the three Girassol towers connects the sub-sea system to the topside FPSO carrying production, water or gas

injection, gas-lift and service fluids. It is principally made of six main parts:

• buoyancy tank • taper joint / transition pieces • current / foam section • umbilical arches • bottom of tower • suction anchor

The sub-sea riser system also includes for each individual vertical riser tower lines:

• spool pieces from bottom tower to the horizontal bundles

• flexible jumpers from the buoyancy tank top to the FPSO

Figure 1 presents the general view of the three towers where all the mentioned elements appear.

Fig. 1: Overall tower view

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Each riser tower comprises the following pipes (see figure 2): • 1 carbon steel central core pipe ensuring a

mechanical link between the riser base and the buoyancy tank

• 6x8” carbon steel pipes dedicated either to oil production, gas or water injection

• 4x3” carbon steel gas-lift lines

• 2x2” stainless steel methanol service lines

• Insulation material

Fig. 2: Riser cross-section

This set-up was the first development in such depth. Many new technologies were used with significant technical challenges. At the time of the design, there was no existing monitoring philosophy of such deepwater system. Thus the monitoring strategy was only defined during the development process.

As for the inspection strategy, it was defined at the very beginning of the field operation.

Integrity and inspection strategy The methods and organization implemented with respect to Girassol Hybrid Riser Towers are part of a general sub-sea equipment inspection plan. The aim of this plan is to ensure the structural and functional integrity of the installation to contribute to the prevention of any incident or accident that could have an impact on staff, environment and production availability throughout the Field life. More specifically, the Field Operation team had to achieve the following tasks:

• Optimize inspection criteria and planning to minimize operational costs without compromising the level of safety.

• Take into account the specific operational constraints of the Girassol field.

• Implement relevant procedures, inspection databases and datasheets to maximize understanding and reduce sub-sea inspection team training.

• Implement a CIMS to ensure that all sub-sea items assessment and event history are safely and perennially stored over field lifetime.

Given the above target, implementing the inspection strategy consisted of defining a geographic and technical breakdown of the equipment, performing a risk assessment combined with a RBI approach as well as creating and feeding a CIMS to generate inspection plan and programs. This applies to the whole sub-sea equipment and will lead to a specific strategy for the riser towers.

Equipment breakdown

The sub-sea inspection plan is based on an accurate description of the systems to be operated.

The technical hierarchy is defined in accordance to inspection activities and takes into account the actual marking of the sub-sea structure (for the towers: North, South or Center) so as to identify clearly every item. Note that the inspection techniques are not different from an object to another but the generic breakdown underlines the three main categories of equipments:

• FPSO & Buoy • UFL • SPS

These categories are made of various groups such as Riser Towers (but also production sealine bundles, injection lines etc…), characterizing the areas on which the inspection activities will be carried out.

These groups are broken down into object types, considered as the lowest level on which inspection reports is required. In this general classification, riser towers belong to the UFL category (2) and are the second group (2) of this category. This leads to the following hierarchy on each riser tower with generic basis “22X”:

• Type 221 = Goosenecks (GN) • Type 222 = Buoyancy tanks (BT) • Type 223 = Taper joint (TP) • Type 224 = Foam sections (FS) • Type 225 = Umbilical arches (UA) • Type 226 = Bottom of tower (BOT) • Type 227 = RT Anchor base (RTA)

On each of these sub-assemblies, relevant inspection areas (e.g., ROV panels, Cathodic Protection, Painting / Marine Growth) can be defined and will be further used to generate inspection worksheets and reports.

This technical hierarchy is seconded by a geographic breakdown. It will lead to assign a unique tag number to the equipment, which allows extracting the relevant objects from the database when an inspection is planned. In the case of Girassol Hybrid Riser Towers, this will only refer to their cardinal position (North, South or Center). For instance, the foam section of the Riser Tower South will be tagged RTS-FS.

OTC 17696 3

Risk Assessment and RBI Approach

The RBI approach is well developed in the topside industry. Inspection techniques are well known and most of the items are accessible (externally and internally) for inspection using various numbers of non-destructive technic (X-rays or US for instance) providing accurate data on the current status of the equipment. This consequently drives to a fine adjustment of the related inspection frequency.

The approach for sub-sea items is quite different due to the lack of past experience for innovative equipment to anticipate the inspection planning. Besides, the access for inspection is often very restricted, or even impossible, and the inspection techniques are limited to external Cathodic Protection measurements, General and Close Visual inspection by ROV.

Baseline risk evaluation

Risk assessment related to Girassol sub-sea asset has been first performed by the Project contractor to match the overall FRAME model. Based on a classic model definition (criticality = probability x consequence), this baseline risk evaluation has been performed using results from HAZID, QHR and QRA for assemblies an sub-assemblies and FMEA for components.

Using the classic 5x5 matrix classification (Consequences vs. Likelihood), a 3x3 matrix was defined to accentuate the logarithmic scale on the Consequences axis. Figure 3 illustrates the method and shows the resulting baseline risk or criticality categories.

Negligible Minor Significant Major Catastrophic0.05 0.1 0.2 0.4 0.8

Probable 0.9 0.05 0.09 0.18 0.36 0.72

Possible 0.7 0.04 0.07 0.14 0.28 0.56

Low probability 0.5 0.03 0.05 0.1 0.2 0.4

Remote 0.3 0.02 0.03 0.06 0.12 0.24

Very Remote 0.1 0.01 0.01 0.02 0.04 0.08

Matrix reduction

0.25 0.32 1.92

0.08 0.1 0.6

0.07 0.08 0.48

HIGHMEDIUM

LOW

Baseline risk value = 2Baseline risk value = 1

≥ 0.6between 0.08 and 0.60

≤ 0.08

Baseline risk value = 3Risk assessment results

C o n s e q u e n c e s

L i k

e l

i h o

o d

3

2

F3

1

F1 F2

2 3 3

2 3 3

1 2 3

Baseline Risk /Criticality

Re-evaluation factor

1

2

3

Inspection category 1 @ Baseline + 5 years

Inspection category 2 @ Baseline + 2 years

Inspection category 3 @ Baseline + 1 year

Fig. 3: Reduction to a 3x3 Risk Matrix

Inspection category evaluation

The above classification takes into account neither the production availability nor the contingency intervention means and intervals to repair. A complementary analysis was then performed to re-evaluate the categories according to these factors. This way, the items can be classified into inspection categories giving information about the attention to be paid to the equipment both in terms of inspection and follow-up activities.

This re-evaluation has been made using factors defined as follows:

• Factor F1 = Item not directly impacting production availability - Interval to Maintenance /repair considered as short.

• Factor F2 = Important impact on production availability - No additional work force mobilization - No extra detailed engineering (such as metrology performance) required within the interval to maintenance /repair

• Factor F3 = Serious impact on the production and/or interval to repair critical with contingency plan to be developed and work force to be mobilized.

For sub-sea items, and in particular for riser towers, it was found convenient to directly associate risk-category values to pre-set inspection periods as shown in Figure 4.

Fig. 4: Final criticality matrix A re-evaluation was finally issued, taking into account Total company rules when their requirements were more stringent. For instance, items within the FPSO 500-m zone require to be inspected every two years, even if initially classified in category 1 (see Figure 5).

Inspection results / status evaluation

Based on the FRAME model and company rule, the defects are categorized with a grade that drives the decision for action. Three levels are considered when equipment status evaluation is performed:

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Gr• Level 1: equipment fit for service. No or minor damage are reported

oup Code Description Baseline Risk

Factor Category Final Classification

RT 3 3GN Goose necks 1 F1 1 2BT Buoyancy tank 1 F2 2 2TJ Taper joint 1 F2 2 2FS Foam sections 2 F1 2 2UA Umbilical Arches 1 F1 1 2

BOT Bottom of riser structure

1 F1 1 2

RTA RT Anchor base 1 F3 3 3

• Level 2: equipment still fit or partially fit for service. Close follow-up, maintenance or repair works are required before next programmed inspection.

• Level 3: equipment unfit for service. Repair or replacement is required as soon as possible.

These levels are associated to a specific color code (1=green, 2=orange, 3=red) that will be added in the CIMS. The latter will then provide a basis for decision making in terms of intervention.

The CIMS will also generate alarms in order to ensure that the 5-year inspection program is properly covered for each item.

Sub-sea inspection operational management process: CIMS

All database including objects identification, construction / installation and exploitation data are first compiled in Excel spreadsheets. All related electronic document files (drawing, sketches, datasheets and pictures) are named according to a specific numbering system and stored on a server in Luanda following clear rules of access paths.

The main features of the CIMS are: • Management of inspection / intervention events and

their periodicity, including the generation of events schedules.

• Generation of inspection plans.

• Archiving and management of inspection / intervention reports and related documents, including filled inspection reports, pictures and scanned documents, videos, sketches, spreadsheets and other documents in various text formats.

• Generation of event reports.

• Management of Non Conformity Reports.

An example of equipment inspection worksheet is given in Figure A-1.

Application on Girassol Riser Towers

The inspection of the Hybrid Riser Towers is derived from the approach described previously.

Applied to Girassol riser towers, the classification obtained is described in Figure 5.

Note: baseline risks for buoyancy tank and taper joint were reduced thanks to a double barrier installed on each tank compartment (two different types of valves) and “reinforced” welding on taper joints. Factor was also reduced due to new fabrication design.

Fig. 5: Classification of riser tower equipment

Inspection schedule has been built accordingly and so far, the ROV visual inspections have taken place showing no major degradation for any of the components.

However, a few findings might be reported:

• Gooseneck P60 on RTS: the insulation grease bag was found less full than the others. No leak was detected but the status is closely followed (at every inspection).

• Buoyancy tanks on RTN and RTS: some anodes for cathodic protection have been found damaged or missing due to a degradation of fixation. Equipment potential is closely followed before taking any action.

• Umbilical arch on RTC: scratches were found on the structure, no impact but to be followed.

• Bottom of Tower on RTC: there was a direct interference between one injection line spool and the riser tower counterweight half pipe. Two ROV survey have been performed to evaluate the impact. On 14/07/02, with gas line under service

On 29/11/02 without gas injection.

A protection sleeve was then installed (Contractor remedial work) around the spool to avoid direct contact between the spool and the ½ pipe of the deflector in order to eliminate friction and abrasion (Figure 6).

Spool

Sleeve

Deflector

Contact Point

½ Tube

Fig. 6: Bottom tower / spool configuration

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In addition, specific ROV-based inspections have also been performed to check thermal leaks, though it is not part of the actual inspection plan. These inspections have shown the evidence of thermal leaks due to seawater convection movements inside the insulation covers at tower bottom (see also temperature monitoring section).

Monitoring of the riser towers

As the Girassol development was a first in this domain, the monitoring philosophy of such a system was not well developed.

For the riser towers, monitoring was implemented in order to: • Confirm adequacy of engineering tools to predict

system thermal behavior (model calibration)

• Provide additional data to assist in production management (flow assurance issues such as hydrate prevention)

• Have means to check the integrity of the structural components

• Confirm adequacy of engineering tools in predicting system dynamic behavior.

For that purpose the monitoring on Girassol riser towers was constituted of:

• Stress monitoring (load cells) on taper joints

• Motion Recording Units (MRU) on the foam section (at -200 m & -895 m)

• Temperature monitoring probes on the bottom riser production spools (4 per riser tower)

The two first monitoring means were used during installation and for a one-year-monitoring period whereas the second one is still in place and followed regularly.

Riser tower integrity monitoring The equipment failure mode assessment demonstrated the

following catastrophic failures might occur to the structure of the riser tower:

• Water ingress in the buoyancy tank (loss of buoyancy)

• Failure of the core pipe

• Failure of the riser tower anchor (latch unit or suction anchor)

Monitoring devices were installed on the riser towers during towing out and installation on site (Figure A-2). The monitoring of the central riser tower was left in place to continue data acquisition during one year.

Stress monitoring on taper joints

During engineering, the transition zone between the riser bundle and the buoyancy tank (called taper joint, see Figure 7) was identified as a highly stressed area subject to fatigue, especially during installation.

Taper joint

Riser tower buoyancy tank

Riser tower bundle

Fig. 7: Stress load cells localization

In order to control this phenomenon, a stress monitoring container was installed. This container is linked to four load cells (Figure 8) attached to the riser taper joint to measure stresses and access tension and bending moment in this area.

These measurements were made during towing and also recorded after installation.

During towing operation (600 km tow), the aim was to ensure that installation parameters remained within allowable limits (this could impact on fatigue life). The measured stress modulus range was found between 328 MPa and 410 Mpa.

Fig. 8: Stress load cells

The “in-place” configuration monitoring was used by UFL contractor to verify that the actual behavior was in line with the engineering predicted behavior and to follow the integrity of the riser tower during one year. Under operating conditions, the stress modulus was found at a value of around 246 MPa.

6 OTC 17696

Dynamic behavior monitoring

The aim of this type of monitoring was to give a feedback about the Riser Tower effective dynamic behavior, which could be compared to the dynamic model predictions. This could validate the expected lifetime of the tower.

In addition, the evaluation of potential water ingress inside the buoyancy tanks was expected to be obtained by a periodic monitoring of the riser tower head position and motions. A significant loss of buoyancy would indeed result in an alteration of the excursion compared to other towers.

Two MRU were fixed on the foam section at -200 m and -895 m on one of the three towers (RTC). These are ROV removable units that can detect displacement magnitude up to 1.5 m and angular motions up to 0.5°. MRU data acquisition containers were conceived as autonomous for a mean duration of six months, allowing a 20-minute recording every four hours.

Each MRU (see Figure 9) includes: • 3 fiber-optic gyrometers to measure the angles

• 1 inertial measurement unit with 3 accelerometers

• 1 real-time computer to calculate velocities and displacements

During towing, the MRU were used to record the tower movements at the most expected, critical locations from a fatigue point of view.

Fig. 9: MRU container preparation and installation

On Girassol, integrity monitoring of the riser tower was principally used during towing and installation.

Additional integrity monitoring: operational feedback

During the project and after the first oil in December 2001, some additional integrity monitoring means were identified though not implemented directly on the riser towers. However, it is of interest to mention them briefly:

• Buoyancy tank monitoring: leaks or water ingress has been identified as a potential risk. A receptacle for transponder was provided on the top of the buoyancy tanks in order to record the excursion of the latter and analyze the data combined to oceanographic conditions and FPSO excursion at the same time. This monitoring has not been implemented yet but

this will be done in the near future. Note that a double water tightness barrier on each of the buoyancy tanks compartments was put in place. Testing of this barrier was judged unappropriate because it might be an additional source of leaks. It was then assessed that the integrity of the buoyancy tank would be ensured over field lifetime.

• Foam behavior monitoring: the behavior of the foam overtime and in actual operation conditions is not known. In order to evaluate their potential degradation, it was suggested to place ten foam samples along the tower at two different locations (-800 m & -1350 m) and retrieve a sample of each every year on a 5-year program. This was abandoned as no simple way was found to put these samples in actual operating conditions.

• Erosion monitoring: there is no monitoring directly on the riser tower mostly because the implementation of such a system (probes, acoustic or cable transmitters, receptor) was not simple. However, goosenecks (at the top of the buoyancy tank) where fluid velocities are the highest are made of Super Duplex. Erosion is monitored at wellhead and riser arrival on FPSO.

Temperature monitoring Girassol specific context

On Girassol Field, one of the major challenges in terms of Flow Assurance is hydrate prevention. Indeed, due to seabed low temperature conditions (≈ 4°C), shutdown and restart conditions expose the sub-sea system to the risk of hydrate formation. During the project phase, the hydrate formation temperature was calculated given the Girassol fluid characteristics and the most restrictive pressure conditions in the sub-sea lines. This value equals 18°C and a margin was taken to consider 20°C as a limit. This temperature was used as a reference in the design specifications of the equipment, especially for the qualification of thermal insulation material.

The field preservation strategy imposed specifications for the sub-sea lines and in particular for the riser towers with a minimum 16-hour cool-down time (down to 20°C from initial 51°C temperature) in liquid filled conditions and 4 hours for the risers in gas conditions. Contractor has even guaranteed an 8-hour cool-down time performance in gas conditions for risers. The 16-hour also applies to production lines.

CFD models were used to validate the design from a theoretical point of view. Those models were fed by the geometric and thermal characteristics of the lines as well as surrounding environment conditions. One of the key parameters used is the overall heat exchange coefficient of the equipment (U-values in W/m2.K), which accounts for the heat flux between the production fluid inside the lines and the sea through pipe and insulation material.

OTC 17696 7

Calculations were also performed in flowing, steady-state conditions so that another critical target is achieved: getting a fluid at riser with a minimal temperature of 40°C to avoid wax formation.

According to design, calculated values for the different section of the riser towers are given in Table 1.

Equipment designation U value (W/m2.K)

Flexible jumper 4.5

Buoyancy tank 6.3

Taper joints 5.4

Riser bundle 1.4 to 1.6 *

Bottom tower spool 2.9

Table 1: Design U-values

*depending on configuration (# of lines producing in tower)

In order to monitor the thermal insulation integrity of the risers, which is a key issue for the hydrate and wax prevention strategy, riser towers have been equipped with temperature sensors on the tower bottom production spools. This allows to assess riser tower thermal insulation performance both in flowing and cool-down conditions, at least qualitatively, and to detect anomalies in case of significant differences.

Technical description of the system

Temperature gauges are located on the spools close to the bottom of the tower (see Figure 10), which is considered as a potential “cold spot” of the riser due to the riser-spool connector proximity. This connector is insulated by an insulation cover (“dog-house”) designed on purpose and clamped on the connector after tower installation / production lines hook-up.

Fig. 10: Sensors location

Temperature data are not directly transmitted to the FPSO but they can be read out either at the replacement of the batteries or any time by connecting an ROV “override cable” to allow real time monitoring. Data are retrieved on a laptop and available for analysis.

The probes are installed on the pipe wall and are topped by the insulation coating that covers the spool (Figure 11). Two probes are installed on each spool: only one is used at a time and in case of failure of one probe, the second one can be taken as a spare. A junction box is clamped on the line to allow data transmission to a recording container attached at the bottom of the riser tower. The connection between a junction box and the container is made by a ROV bus bar.

Fig. 11: Probe installation

Each container (one per tower) includes five layers of seven batteries to replace every 6-8 months, the Data Acquisition and Recording System (DARS) as well as the override cable connection for online monitoring.

Since Girassol has started producing, temperature data has been retrieved periodically during battery change outs. This data is then used for cool-down thermal performance assessment (and to a little extent for flowing conditions follow-up). The “live” monitoring was also performed during the start-up of each production loop in order to feed the overall loop thermal signature test. This test provided a baseline for the thermal performance of all equipment: manifolds, sealine bundles, risers and flexible jumpers.

Steady state thermal assessment

Riser foot temperature data cannot be displayed and stored in the Girassol Control System. The only way to get live data is to connect a ROV override cable on the container: the data is then retrieved directly on a laptop at the surface. This implies

Temperature probe

Junction boxes

insulation foam

Vertical dog-house

Temperature sensors

riser spool

8 OTC 17696

ROV operations, so this functionality of the temperature monitoring system can only be used when a significant gain is expected.

At the very beginning of the field life, knowing what were the actual thermal performances of the sub-sea system was a good enough reason. Thus, thermal tests were conducted for each production loop in order to calculate an approximate U-value for sealine bundles and riser tower. Such a calculation required in particular to have the temperature at the bottom of the riser tower. In order to do simple calculation, it was decided to get the data directly from the system using the ROV and synchronize them with other temperature data along the lines.

envref

envprobeprobebulk T

Tθθ

θθθθ

−∆−

−∗∆+=

7568.00865.1 −∗= probebulk θθ

Figure 12 is an illustration of the test conducted for a given production loop. The different points of measurement were used to estimate the U-value of each line section (bundles, risers).

⎥⎦

⎤⎢⎣

⎡−−

∗∗=∗∗∗envend

envstartp TT

TTLnCmLIDU 'π

9175.00938.1 −∗= probebulk θθ

( )envactual

envextrapenvenvactualextrap θθ

θθαθθθαθ

−

−=+−∗= 0

0

with ,

Fig. 12: Loop configuration for U-value estimation

Once temperature is known at both ends of a given line, the calculation is made by using Equation 1, derived from the Fourier’s Law (Jq = -k dT/dx) for heat conduction:

Eq. 1: calculation of U-value

Where: • m’ : circulation mass flow rate (kg.s-1),

• Cp : fluid specific heat (J.kg-1.K-1),

• ID : internal diameter of the line (m),

• L: length of the line (m).

The test was conducted by circulating oil in the loop from the topside at a controlled flow rate so that all data were known to calculate the U-value.

Because of the non intrusivity of the temperature probes, fluid temperature has to be extrapolated from the wall temperature. Lab testing has shown that in steady state conditions, a bulk fluid temperature is 51°C (θref) induces a temperature

difference value of 1.5°C (∆T) when system is surrounded by water at 4°C (θenv). From this, bulk temperature was extrapolated in steady state following the formula given in Equation 2.

Eq. 2: Extrapolation formula in steady state

This extrapolated fluid temperature is then used in the U-value calculation.

Cool-down thermal performance assessment

Every six to eight months, data are retrieved from the containers and a post-analysis of the riser bottom thermal performance is done. During the available time period, exploitable cool-down cases are identified and the thermal inertia around the tower bottom is compared with the expected one (16 hours from 51°C to 4°C). It is an intricate process divided into several steps.

Riser Top Left

Production Bundle

Production Bundle

Riser Tower

Left

Riser Tower Right

Riser Bottom Left

Riser Bottom Right

Riser Top Right

First, as for steady state, the temperature measured does not exactly correspond to the actual fluid temperature. It is different from the steady state case due to particular convection motions occurring in the fluid during cool-down. Engineering studies based on CFD models were realized and they gave a simple transfer function between the probe and the bulk fluid temperatures (réf. [1]). For Girassol, transfer functions, depending on the nature of the fluid, are (Equations 3&4):

Eq. 3: Transfer function for dead oil

Eq. 4: Transfer function for live crude Then, to be able to compare different cool-down cases and to compare them to design previsions, an extrapolation to a common initial cool-down temperature (51°C, θ0

extrap) is done. This extrapolation is based on the hypothesis that the behaviour of the heat transfer coefficient characterising the surroundings of the sensors (typically the insulation cover + riser spool) does not depend on this initial temperature. Thus, the difference between fluid temperature and the environment temperature follows an exponential law with the same exponent in both cool-down cases. The extrapolation formula is then (Equation 5):

Eq. 5: Extrapolation to 51°C initial temperature

Once all cool-down cases are extrapolated to a unique initial temperature, it is possible to follow the evolution of the tower

T1

T2

T3

T5

T4

Flow Circulated Q

Manifold

OTC 17696 9

bottom thermal inertia either by comparing the cool-down time or by doing a U-value calculation. In order to use the maximum number of cool-down cases, it is assumed that the cooling is mainly due to radial exchanges between the hot fluid inside the 8” line and the environment. That is true when temperature difference is sufficient and no specific operation is performed (initial preservation, fluid displacement, line depressurisation for instance).

The temperature theoretically follows the Newton’s Law of cooling, which leads to Equation 6:

pmCtAU

envenv et∗∗

−

×−+= )()( 0 θθθθ

Eq. 7: Cool-down equation

Where: • t time (s)

• U thermal conductivity (W.m-2.K-1)

• Cp fluid specific heat (J.kg-1.K-1), here 1900 J/kg.K

• A area of exchange (m2)

• m mass of fluid (kg)

Considering the geometry of the pipe it leads to Equation 8:

pC

Ut

envenv et Φ−

×−+= ρθθθθ4

0 )()( Eq. 8 : Pipe equation

Where: • ρ fluid density (kg/m3), 875 kg/m3 (845 for diesel

oil)

• Φ diameter (m), 0.203 m

From this formula, it is possible to get the U-value for t>0. It is given by the formula:

)(4

0 tULntC

env

envp =⎟⎟⎠

⎞⎜⎜⎝

⎛−−

×Φ

θθθθρ

Eq. 9: U calculation

Figure A-3 shows a typical example of this calculation.

When considering all the risers, this leads to an overall cartography of the available cool-down time of the Girassol Riser Towers given in Figure A-4.

Conclusions After 3 years of operation, inspection and monitoring of the Girassol Riser Tower has shown no major issues.

The management of the inspection plan through the CIMS quickens the inspection campaigns preparation and the reporting, giving more time for the actual analysis and potential troubleshooting.

Structural monitoring of the towers was mostly used during towing and installation whereas temperature monitoring has

been used since the production start-up in December 2001. Analysis of temperature data has thus permitted to launch a campaign of visual inspection of the vertical connector dog-houses at the towers bottom. Thermal leaks due to water convection inside the cover were detected after start-up, leading the manufacturer to modify the design to limit the heat loss around the connectors. The degradation of the insulation coating can be assessed overtime giving more time to anticipate interventions.

The main conclusion is that requirements for monitoring have to be defined early in the project so that they can be included in the sub-sea packages and duly qualified, tested and calibrated. The interest of the monitoring has to be clearly identified so that a proper action plan is set and the data are not only collected but treated and analyzed.

The experience on Girassol has also contributed to improve the design for the Rosa tie-back development (Figure A-5). The top arrangement has been modified as the buoyancy tank is linked to the tower with a chain and the flexible jumpers are attached below. In addition, all lines inside the riser tower are individually isolated and the surrounding foam has only a function of buoyancy, contrary to Girassol where it also provides thermal insulation with potential convection phenomena occurring. It must be noted that thermal performance requirements are much more stringent on Rosa due to longer sealines (up to 19 km), which explains also why the design required improvements.

From Girassol to Rosa, the challenge of monitoring the integrity of the equipment will remain: methods and tools are in place, the ability to exploit them will need to be maintained.

Acknowledgements The author would like to thank Sociedade Nacional de Combustíveis de Angola (SONANGOL), Esso Exploration Angola (Block 17) Ltd., BP Exploration (Angola) Ltd., Statoil Angola Block 17 A.S., Norsk Hydro and Total for permission to present the information contained in this paper.

Nomenclature CIMS: Computerized Inspection Management System

CFD: Computation Fluid Dynamic

DARS: Data Acquisition and Recording System

FMEA: Failure Mode & Effect Analysis

FPSO: Floating Production Storage & Offloading

FRAME: Functional Reliability Availability Maintainability Evaluation

HAZID: HAZard IDentification

MRU: Motion Recording Unit

NCR: Non Conformity Report

10 OTC 17696

QHR: Qualitative Hazard Review

QRA: Qualitative Risk Assessment

RBI: Risk-based Inspection

RTC: Riser Tower Center

RTN: Riser Tower North

RTS: Riser Tower South

SPS: Subsea Production System

UFL: Umbilical & FLowlines

US: Ultra-Sound

ROV: Remotely Operated Vehicle

References

[1] Stéphane Taxy & co-writers: “Use of Computational Fluid Dynamics to investigate the Impact of Cold Spots on Subsea Insulation Performance”; OTC 16502, Houston, May 2004.

OTC 17696 11

Fig. A-1: Typical Inspection Worksheet

12 OTC 17696

Fig. A-2: Overview of Riser Towers monitoring equipment

Evolution of heat transfer coefficient

2.63

1.59

2.12 2.22

1.77

0.0

0.5

1.0

1.5

2.0

2.5

3.0

déc-

01ja

nv-0

2fé

vr-0

2m

ars-

02av

r-02

mai

-02

juin

-02

juil-

02ao

ût-0

2se

pt-0

2oc

t-02

nov-

02dé

c-02

janv

-03

févr

-03

mar

s-03

avr-

03m

ai-0

3ju

in-0

3ju

il-03

(W/m

2.K

)

Riser foot temperature 08-02-2003 to 17-10-2003

0102030

40506070

8/2/03 30/3/03 19/5/03 8/7/03 27/8/03 16/10/03

date

T (°

C)

Extrapolation to common Tinit

0

10

20

30

40

50

60

00:00 02:00 04:00 06:00 08:00 10:00 12:00 14:00 16:00 18:00 20:00 22:00 00:00

Duration (h)

Text

rap

(°C

)

juil-02 sept-02 nov-02 avr-03 juin-03 1st f luid

Cool down comparison

0

10

20

30

40

50

60

00:00 02:00 04:00 06:00 08:00 10:00 12:00 14:00 16:00 18:00 20:00 22:00 00:00

Duration (h)

T (°

C)

juil-02 sept -02 nov-02 avr-03 juin-03 1st f luid

C over chang e

not comparable(dif ferent f luid)

Fig. A-3: Typical temperature study case

OTC 17696 13

North Riser TowerP40R : 18h00P40L : 15h30P50R : 15h00P50L : 15h30

South Riser TowerP10R : 9h00P10L : 13h00

Center Riser TowerP20R : N/AP20L : 14h00P30R : 15h30P30L : 15h00

Fig. A-4: Cartography of Girassol riser towers thermal inertia

Fig. A-5: Rosa Tower Design