IBP2739_10 FIELD TEST OF A FLEXIBLE PIPE INTEGRITY MONITORING SYSTEM Jean-Philippe Roques 1, Benoit Balagué 1, Dominique Dion 2, Gregoire Audouin 2

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1 IBP2739_10 FIELD TEST OF A FLEXIBLE PIPE INTEGRITY MONITORING SYSTEM Jean-Philippe Roques 1, Benoit Balagué 1, Dominique Dion 2, Gregoire Audouin 2 Copyright 2010, Brazilian Petroleum, Gas and Biofuels Institute - IBP This Technical Paper was prepared for presentation at the Rio Oil & Gas Expo and Conference 2010, held between September, 13-16, 2010, in Rio de Janeiro. This Technical Paper was selected for presentation by the Technical Committee of the event according to the information contained in the abstract submitted by the author(s). The contents of the Technical Paper, as presented, were not reviewed by IBP. The organizers are not supposed to translate or correct the submitted papers. The material as it is presented does not necessarily represent Brazilian Petroleum, Gas and Biofuels Institute opinion, nor that of its Members or Representatives. Authors consent to the publication of this Technical Paper in the Rio Oil & Gas Expo and Conference 2010 Proceedings. Abstract Flexible pipes are used in all offshore fields to transport high-pressure and high-temperature fluids while flexing with variable subsea currents and wave action. The monitoring of flexible pipe integrity is a main concern, growing more significant as they increase in number and age. In a flexible pipe, gas from the transported fluids slowly diffuses into the pipe s annulus, a space located between two concentric plastic layers where the armors that support the whole structure are also found. The annulus gas is vented at the surface and flared to prevent a pressure buildup that could burst the pipe s outer sheath. The main concern for flexible pipe integrity is the presence of water in the annulus, either due to condensation or damage to the outer sheath. Seawater, or condensation water in the presence of diffused CO 2 or H 2 S, can lead to excessive corrosion of the armor layers and can significantly reduce the service life of the line (MCS International, October 2002). It is also possible for oxygen and moisture from the air to enter the annulus if the vent ports are not equipped with check valves. Annulus Outer Sheath 1 Engineer, TOTAL, S.A. 2 Engineer, Schlumberger Internal Carcass Pressure Layer Pressure Sheath Figure 1: Flexible pipe structure. Tensile Wire Currently, the only way to control the water tightness of the pipe s outer sheath and the quantity of condensed water in the annulus is to perform vacuum or pressure testing of the annulus (Bondevik, 2004). This operation is infrequent and costly, and does not allow for continuous monitoring of the pipe. Sometimes, it is complemented by measuring the rate at which gas exits the vent ports. However, this measurement is sensitive to temperature variations in

2 the annulus. A change in the rate of gas vent could be due to temperature effects, a change in gas diffusion rates, or water entry. To address these issues, TOTAL and Schlumberger have developed the subc-racs riser annulus condition surveillance system for monitoring flexible pipe integrity, which eliminates the need for vacuum tests. The first step was to define, together with the operators, the system design requirements. The highest priority was to provide a real-time alarm in case of rupture of the pipe s outer sheath. The system had to be adaptable to any existing flexible pipe equipped with vent ports and simple to install, operate, and maintain. It had to measure the free volume of the annulus, removing the need for regular annulus testing. The gas that permeates the riser pressure sheath is depressurized while measuring its pressure, temperature, and flow rate. Then, as in a well production test, the pressure drawdown and buildup curves are analyzed to give detailed information about gas diffusion rate, annulus gas-filled volume, and connectivity of annulus and vent ports. A detailed model of the riser annulus, its environment, and of the subc-racs instrument was used to understand the pressure transients during the operation of the system. At regular intervals depending on the gas diffusion and riser parameters, the gas pressure in the annulus is released. Prior to venting the gas, the pressure variations depend on annulus volume, gas diffusion rate, and temperature variations. During the gas vent, the rate of pressure decrease is governed by annulus free volume and the connectivity of the annulus from the vent ports to a few meters farther inside the pipe. After the solenoid valve is closed, the pressure buildup gives an indication of the free volume and of its connectivity farther away from the vent ports. A computer acquires pressure and flow rate from the flexible pipe measurement units. The pressure curve is correlated with riser fluid temperature to provide a correction for the effect of temperature variations in the annulus. Based on the model, algorithms were developed to calculate separately the annulus free volume, the gas diffusion rate, and the permeability to gas of the annulus and of the pipe s vent ports. Then, alarms are derived from the pressure curve and from deviations in the gas flow rate or volume measurements. The system was validated in the flow laboratory and on a riser sample before its installation offshore West Africa. Since the start of the field test in December 2009, results obtained have been within our expectations. We were able to demonstrate the robustness of the system and the reliability of the software. The free volume measurements are consistent with measurements obtained from previous annulus tests, and an alarm system has been implemented to allow operators to monitor the integrity of the risers on a daily basis. It is expected that the introduction of this system will improve safety and the planning of riser maintenance, and it should also provide reliable information on armor condition over the life of the flexible pipe. In this paper we briefly cover the operating principle of the system and focus on the operational experience attained during the field test in West Africa. Evidence is given on the value of the monitoring solution and its high sensitivity to production-related events that affect the flexible annulus environment. 1. Introduction Flexible risers are often selected as an alternative to rigid pipeline, especially in dynamic applications. However, the main issue with a flexible riser is the monitoring and follow up of the integrity of the line. One of the main failure modes for flexible risers is the breakage of the armor wires due to corrosion. This corrosion could be due to condensed water in the annulus, diffusion through the pressure sheath, or a rupture in the outer sheath allowing seawater in. At present, assessing the water tightness of the outer sheath and the quantity of condensed water in the annulus can only be achieved by performing pressure and vacuum tests. These test campaigns are costly operations that provide only intermittent information about the status of the annulus, and they do not allow for close follow-up of the riser. TOTAL and Schlumberger have developed new automated technology: the subc-racs system for continuous assessment of a flexible pipe s annulus condition and direct detection of damage to the outer sheath - without performing vacuum or pressure testing of the annulus. This system is able to monitor the free volume of the annulus, and other parameters related to the annulus environment, by controlling the venting of diffused gases and analysing the pressure transients when gas is vented. A full description of the system is detailed in OTC paper (Roques et al., 2010). The system has been installed in four risers in TOTAL s Moho field, Congo, since December All risers, about 1,000-m long, are connected to the Alima floating production unit (FPU) and are running to the seabed at 700-m depth with a lazy wave configuration. Risers that have been tested include two 8-in insulated production risers, a 6-in gas export riser, and a 15-in oil export riser. 2

3 2. Field Test System Description: Hardware and Software The subc-racs system implementation is described in Figure 2. System hardware consists in one riser unit for each riser that requires monitoring, a smart junction box, and a dedicated PC running supervisory control and data acquisition (SCADA) software. Figure 2: subc-racs system implementation on FPU Alima. Each riser unit holds instruments to measure pressure, temperature, and flow rate, as well as an interface to the gas sampling equipment, and solenoid and gas relief valves. The riser units and the smart junction box interfacing them to the SCADA computer are all located on the FPU deck. This equipment is designed for explosive atmosphere Zone I use and for deployment in marine environments. For the Moho field test, the subc-racs computer was installed in the main control room. Afterwards it will be interfaced to the FPU s integrated control and safety system (ICSS). RTAC real-time acquisition and control software has various screens that allow the offshore operator to monitor the status of the risers. An overview screen (Figure 3) displays the principal measurements and calculations for up to 16 risers, highlighting the measurements that change significantly. For each riser, a single screen (Figure 4) displays the measurements, the calculated results, their trend versus time, and a summary of alarms and events. 3

4 Figure 3: Overview screen, to monitor up to 16 risers. Figure 4: Riser screen. The SCADA software also has other menus and functions for log-in control and the ability to plot data or results, display alarms and events, or export data to be included in a report. 4

5 3. Data quality control A main goal of the field test was to assess the quality of the annulus free volume and gas migration rate computations. The system is intended to work consistently while unattended, so the computations must be robust, with as few false alarms as possible. Calculations are made every few days, and, if the riser bore and annulus environments do not change, the results should be very repeatable. However, if the value of annulus free volume or gas migration rate changes significantly from one gas vent to the next, the subc-racs system must be able to discriminate if this is due to a riser integrity issue, an equipment failure, or a production-related event. During the field test period, from December 2009 to June 2010, some production-related events affected the riser and annulus environment. For example, from 15 to 25 June, several changes in production rate in the Bilondo L riser impacted the annulus temperature and the gas diffusion rate. The corresponding annulus pressure changes, highlighted in Figure 5, caused a wrong annulus free volume figure to be calculated following the gas vent of June 22. In contrast, the quality of the annulus free volume calculated from the June 28 gas vent data is good. Flow rate variations June 22 gas vent June 28 gas vent Figure 5: Annulus pressure of the Bilondo L riser, June 15 to July 5. To avoid false alarms, a software feature currently under development uses the subc-racs and/or process data to automatically recognize production stops and flag the next calculations of volume and gas migration rate as being of low quality. This disables the alarms until the following gas vent. 5

6 4. Interpretation and alarms The system continuously monitors various parameters indicative of the riser annulus condition, as explained below and depicted in Table 1: - The annulus free volume is the most important parameter to monitor since a reduction in its value is an indication that liquid has entered the annulus. The curve of free volume versus time gives clear information about water condensation or about the filling up of the annulus due to a leak in the pipe s outer sheath. A significant decrease in annulus free volume without an increase in the gas migration rate shows the formation of a plug, such as an accumulation of condensation water in a sag bend area. The combination of a decrease in annulus-free volume and an increase in gas migration rate shows water entry. - The quantity of gas vented gives an average rate of its migration toward the riser s vent ports. If the free volume and annulus temperature are constant from one vent to the next, then all the gas vented comes from diffusion through the pressure vault. In case of a leak in the outer sheath, the gas migration rate increases due to gas pushed out of the annulus as water comes in. Over time, a gradual change in the gas diffusion rate - but not in riser fluid pressure or temperature - indicates aging of the pressure vault polymer. Together with gas analysis from samples, this enables the flexible pipe manufacturer to recalibrate the gas diffusion model. - The pressure transient analysis allows us to monitor the gas mobility separately at the vent ports and along the length of the riser. After venting some gas from the annulus, pressure returns slowly to equilibrium, and like in well testing interpretation, the time constant of the pressure stabilization is representative of gas mobility. Too low gas mobility deep inside the annulus can indicate the condensation of water vapor. Likewise, an accumulation of corrosion by-products or water at the vent ports will reduce gas mobility close to the sensors, which is easy to detect. Table 1. Interpretation of subc-racs system measurements Annulus Condition Annulus Free Volume Gas Migration Rate Outer sheath rupture Slow leak into the annulus Water condensation Formation of a water plug (e.g. in the sag bend) Change in gas diffusion rate Alarms are triggered if a breach in the outer sheath is detected, if the gas diffusion rate changes abruptly, or if the system either leaks or fails to vent. 5. Annulus monitoring process with subc-racs system As shown during the field test, it is possible to retrofit the subc-racs system on an existing floating production facility, and, from the time of installation, begin to acquire and interpret new data about the condition of the annulus. With a new project, the interpretation of subc-racs system data can be enhanced if the riser annulus condition has been established when the riser is fresh from the factory and verified as early as possible after installation, before gas diffusion starts. At the time of the factory acceptance test (FAT), the annulus volume, vent port performance, and gas mobility are usually at their highest. The data recorded during FAT is the standard against which all future data will be compared. 6

7 When there is no gas diffusion, the subc-racs system can be set up to operate on demand. This is done either by injecting some nitrogen into the annulus, or by extracting with a vacuum pump some of the gas present in the annulus, changing the annulus pressure by only 100 mbar or less. Operating procedures have been established to make measurements with the subc-racs system either during the FAT vacuum test at the flexible pipe s manufacturing plant, or offshore by injecting a small quantity of nitrogen into the annulus. Riser manufacturer data FAT Vacuum test with subc-racs Model subc-racs operation. Select operating parameters Commissioning; Low pressure nitrogen test with subc-racs Commissioning report Update model and operating parameters Every 6 months subc-racs Operation New gas analysis Interpretation Biannual report Real-time display: Data, status, trends and alarms Figure 6: Process for riser annulus monitoring. The complete process for annulus monitoring with the subc-racs system is depicted in Fig. 6: - From the riser s manufacturer data, establish a model of gas movement in the annulus due to venting. This model is used to estimate the time needed to achieve pressure stabilization during FAT, and to set the subc-racs system parameters for this test. - Perform a subc-racs system test as part of the riser FAT. This gives the initial values of annulus volume, vent port performance, and gas mobility. - Commission the subc-racs system as early as practical after riser installation. The commissioning includes a test with nitrogen injection to verify the riser condition after installation, before gas diffusion has started. Update the model and operating parameters. - As gas diffuses into the annulus, pressure increases and the gas is vented automatically every few days. At any time, offshore operators can see the subc-racs system in operation and follow its results on the dedicated computer display. In case of an alarm, the detailed raw data present on the computer hard drive can be analyzed to pinpoint the cause of the alarm; and in case of an outer sheath rupture, the data can illuminate the location of the damage. 7

8 - Every 6 months a simple report should be prepared to present the trends of annulus volume, gas diffusion rate, gas composition, and vent port performance versus time. At that time the model and operating parameters are reviewed and updated if necessary. 6. Data management For the field test s duration, subc-racs system data and results were stored on a dedicated PC. The next development for the Moho field will be to incorporate the data into the overall monitoring system already in place for the riser follow up. The present monitoring system is collecting wave measurements, FPU motion, and riser operating data, i.e., pressure, temperature, flow rate, fluid composition, and chemical injection rate. All data is transferred daily from the FPU to a special Web site where it is stored and analysed by a dedicated engineering company in charge of riser surveillance. The engineering company is responsible for collecting and storing the data and providing yearly assessments of the status of the line, based on the operating data collected. Data from the subc-racs system will be implemented within this monitoring system and will be transferred from the subc-racs computer to the ICSS and then forwarded onshore. 7. Conclusion The field test system has now been operating since December 2009 without issue. TOTAL E.P. Congo is now preparing for installation of the subc-racs system on the entire riser configuration operating in the Moho field. Generally, the subc-racs system is becoming the standard equipment to be installed and used on all new TOTAL field developments where flexible risers are deployed. 8. References BUNDEVIK, J.V., HAAKONEN, R., LUNDE, S. Annulus testing for condition assessment and monitoring of flexible pipes. OMAE , June MCS International. UKOOA Guidance Note on Monitoring Methods and Integrity Assurance for Unbonded Flexible Pipe, October ROQUES, J-P., BALAGUE, B., DION, D., AUDOUIN, G. Flexible pipe integrity monitoring: A new system to assess the flexible pipe annulus condition. OTC20973, May