IBP1572_09 REMOTE EROSION AND CORROSION MONITORING OF SUBSEA PIPELINES USING ACOUSTIC TELEMETRY AND WET-MATE CONNECTOR TECHNOLOGY Howard Painter 1, Stewart Barlow 2, Daniel Clarke 3, Dale Green 4 Copyright 2009, Brazilian Petroleum, Gas and Biofuels Institute - IBP This Technical Paper was prepared for presentation at the Rio Pipeline Conference and Exposition 2009, held between September, 22-24, 2009, 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 s opinion, or that of its Members or Representatives. Authors consent to the publication of this Technical Paper in the Rio Pipeline Conference Proceedings. Abstract This paper will present a novel approach for monitoring erosion and corrosion using proven subsea technologies: intrusive erosion and corrosion monitoring, acoustic telemetry and wet-mateable connector technology. Intrusive metal loss based monitoring systems on subsea pipelines are increasingly being used because of their ability to directly measure erosion and corrosion. These systems are integrated with the subsea production control system or located close to the platform and hard-wired. However, locations remote from a subsea control system or platform requires a dedicated communication system and long lengths of cable that can be cost prohibitive to procure and install. The system presented consists of an intrusive erosion or corrosion monitor with pressure and temperature transmitters, a retrievable electronics module with an acoustic modem, a data storage module, and a replaceable power module. Time-stamped erosion and corrosion data can be transmitted via an acoustic link to a surface platform, a vessel of opportunity or to a relaying modem. Acoustic signals can be transmitted up to 6 km from the monitoring location. The power module along with data module and acoustic modem are mounted on the erosion and corrosion module using wet-mateable connectors, allowing retrieval by remotely operated vehicles. The collected data can be used to assess the cumulative erosion and corrosion as well as use the real-time metal loss rate data to correlate with operational parameters. Benefits include optimization of corrosion inhibitor dosage rates, mitigation of damage caused by solids production, and increased flow assurance. 1. Introduction Intrusive high-resolution metal loss probes are capable of providing real-time erosion and corrosion data. This allows operators to optimize both the production flow vs. sand production and the chemical inhibitor (CI) dosage rates. Integration of temperature and pressure transducers into the intrusive probe provides additional data for flow assurance managements. For susbsea applications, the monitoring of intrusive probes has been either through a subsea control module at the sea floor or via cable directly wired to the platform. Figure 1 shows a system concept that allows remote corrosion monitoring from as far as 6 kilometers from a platform, surface vessel, or subsea control module through the use of acoustic telemetry. The system consists of an electronics module with an acoustic modem, a data module, and a power module. The modules are mounted on the intrusive erosion/corrosion monitor with remotely operated vehicle (ROV) retrievable wet-mateable electrical connectors. The electronics module houses the circuitry that compares the metal loss of the probe with a pressure and temperature compensated reference element to produce unbiased and calibrated real-time metal loss data. 1 Principal Engineer - Teledyne ODI 2 Technical Director, Europe Teledyne ODI 3 Technical Manager R&D Teledyne Cormon 4 Chief Scientist Teledyne Benthos
This data is then periodically transmitted using an acoustic modem that is also housed in the electronics module. A separate retrievable data module logs the metal loss data and can be retrieved by ROV. The power module houses batteries capable of powering the system for up to 5 years depending on the data transmission frequency. Figure 1. Remote Corrosion, Temperature and Pressure Monitoring System with Acoustic Telemetry and Retrievable Modules 2. High Resolution Metal Loss Technology Recent developments in metal loss technology have increased resolution by more than two orders of magnitude over conventional electrical resistance (ER) ratio-metric methods. Figure 2 shows the single-piece construction that provides both the sensing and reference elements. Fabricating both the sensing and reference electrode from the same material assures uniform material properties. The elements have a spiral profile to increase the resistance path in a compact form. Erosion or corrosion of the sensing element changes the cross section and, in turn, the resistance of the element. Figure 3 shows the elements integrated into a probe with a mounting flange and electronics housing. The electronic housing processes the resistance data with a noise rejection algorithm and outputs metal loss data. The electronics also calculates temperature from the resistance of the reference electrode and outputs pressure if a pressure transducer is installed in the probe. 2
Figure 2. One-Piece Sensing Element and Reference Element with a Spiral Profile to Increase the Resistance in a Compact Form Figure 3. Sensing Element Integrated into a Flange Mounted Probe Figure 4 shows the schematic of the end of the probe, where the reference electrode is pressure compensated with the production fluid, assuring no difference in bulk resistance between the sensing element and reference element due to differential strain. The thermal conduction through the core of the probe provides temperature compensation between the two elements, assuring no difference in bulk resistance of the elements due to differential in temperature. Thus the only difference in resistance between the sensing element and reference element is due to the metal loss on the 3
surface of the sensing element that reduces the cross-sectional area. The resistance data is then digitally filtered to reject noise and converted to metal loss using calibration data specific to the resistance elements. The metal loss of 4 mils/year can be detected within 4 hours for a typical 8 mm thick subsea element. The metal loss data can be output using standard subsea communication methods, such as CANBUS RS485 or 4-20 ma. Figure 4. Schematic of Sensing Element and Electronics Module This technology has proven accurate and reliable over the past 10 years with numerous surface and subsea applications. Application for high-resolution metal loss instrumentation includes optimizing corrosion inhibitors, monitoring corrosion & erosion, and monitoring sand production. Corrosion inhibitors are widely used to protect carbon steel piping in hydrocarbon service and are a major expense. Optimizing corrosion inhibitor dosage under actual operating conditions can be achieved with real-time corrosion data. When reservoir formation sand or other solids are entrained in oil and gas streams, they can have damaging erosive effects. This resulting metal loss can be detected by an intrusive high resolution monitoring system in real time, which allows operators to mitigate the effect by controlling the production of solids. For subsea applications, a high-resolution sensor with extended probe life can be combined with pressure and temperature transmitters in a single instrument. This can provide operators with critical flow assurance data throughout the design life. 3. Acoustic Telemetry Acoustic modems provide the capabilities to transmit data up to a 6 kilometer distance. Recently, mode technology has expanded to incorporate release functionality, to provide an extensive signal process infrastructure, to support massive real-time data recording, and to provide navigation aids for UUVs and other platforms. The modems are very flexible and efficient in their use, power consumption, and packaging. Figure 5 shows the sizes of the current acoustic modem technologies and Table 1 gives the performance characteristics of the modems. 4
Figure 5. The compact modem and Telesonar (4 th Generation) modem compared with a 14 cm ruler Table 1. Performance Characteristic of Acoustic Modems 4 th Generation Modem Compact Moden TM Size 2 Boards 1 Board 5.7 x 2.5 x 2.8 1.6 x 6 x 1 Max Freq 70 khz 70 khz Bandwidth 5-20 khz 5-20 khz Low Power 12 mw (3 tone wakeup) 2-20 mw (Hibernate) Power-Active 500 mw 400 mw Input Voltage 10-60 vdc 6-12 vdc Digtal I/O 4 inputs 1 input 5 outputs 1 output Serial Ports/UART 2 UARTS (RS-422 or CMOS level) 1 UART (only CMOS Level Transmit Power 20 W 2 W One of the most demanding requirements that has been placed on the Telesonar modem comes from the National Oceanographic and Atmospheric Administration (NOAA). Their requirement was to provide a modem that could support the development of the national tsunami warning system. This application does not require transmission of large volumes of data nor does it require high data rates. It does, however, demand very long life with few batteries, and an absolutely guaranteed transfer of sensor data into the modem, ready for transmission. These two requirements resulted in the development of a very low power sleep mode, but one in which the modem would awaken immediately to either of two conditions: a) the arrival of an acoustic message, or b) the arrival of data over the RS232 port. In neither case can data be lost. Meeting this requirement resulted in a modem that sleeps at only 12 mw, whereas the normal fully awake (receive) mode consumes approximately 500 mw. This technology is ideal for the corrosion monitoring application where data will only need to be transmitted when high metal loss rates are detected or on a weekly basis to monitor long-term erosion trends. The Telesonar modems have found wide application, especially in support of the oil and gas industries. The following figure shows an artist s rendering of a system developed for providing stress monitoring along a long subsea pipe. Data at any of the nodes is accessed by a remote modem positioned some distance away and, from there, retransmitted to a surface unit. See Figure 6. 5
Figure 6. Artist s Rendering of an Array of Modems Deployed for Pipeline Stress Monitoring with Central Uplink Station It is not at all necessary to use the simple master-slave architecture of the system. Indeed, the Telesonar modems form the heart of the US Navy s Seaweb program, which has developed an extensive, and extensively tested and refined, set of protocols to facilitate true undersea networks. These types of networks support many conventional protocols, such as request to send, clear to send, various acknowledgements, and extensive traffic routing. Networks of up to 40 nodes have repeatedly been deployed. Below is a summary of the actual applications, communication distances and data rates. This proven technology provides reliable long-distance communication from the corrosion sensor to a platform, subsea control module, or vessel of opportunity. Multiple modems can be arrayed as described above, providing a network along the length of the pipeline. Characteristic of Installed Acoustic Communication System Buzzards Bay, Massachusetts: 10 m max water depth 5 to 7 km at 800 bps, 4 to 6 km at 1200 bps Hawaii, 200 ft water depth 2 km at 1200 bps (did not test limits) St. Margaret s Bay, Canada 10 km at 800 bps Gulf of Mexico 1700 m vertical at 10 kbps Communications at 800 bps with platforms moving in excess of 20 kts St. Andrews Bay, Florida, 10 m water depth reception of 3 simultaneous waveforms at 80 bps (asynchronous multi-access signaling for underwater GPS) 4. Wet-Mate Connector Technology Wet-mate connector technology has a proven track record in subsea control systems and wellhead monitoring systems. The acoustic modem, electronics module, and battery can be integrated with wet-mate connector technology to allow replacement subsea via a remotely operated vehicle (ROV). Figure 7 shows an acoustic modem integrated with a wet-mate ROV-operated connector. Wet-mate connectivity will allow the battery to be replaced on 5-year intervals. The modem and electronic module can be replaced in the event of a failure. 6
Figure 7. An Acoustic Modem Integrated with an ROV-Operated Wet-Mate Electrical Connector 5. Conclusion By combining proven high resolution corrosion monitoring technology, acoustic telemetry, and wet-mate connector technology, a system can be constructed that can effectively monitor pipelines erosion and crorosion at remote locations. 6. References PAINTER, H.E, FLYNN, J. Current and Future Wet-Mate Connector Technology Developments for Scientific Seabed Observatory Applications, Ocean 2006. PAINTER, H.E, CLARK, G.R., WRIGHT, P, Subsea to Shore - The Challenges and Solutions for Subsea Connections. Rio Oil & Gas 2006, IBP1773_06. Eng., v. 14, n. 2, p. 137-158, 1996. 7