- Email: [email protected]
Sensing solutions for assessing and monitoring offshore structures
18 Sensing solutions for assessing and monitoring offshore structures M. H. KIM and J. M. LEE , Pusan National University, South Korea
DOI: 10.1533/9781782422433.2.550 Abstract: Structural health monitoring techniques for ship and offshore structures including recent literature survey in the related fields are summarized in this chapter. Typical hull monitoring systems (HMS) for ship structures are introduced with common sensor locations, as well as measurement parameters to be monitored. Long-based strain gauges (LBSG), accelerometers, and other types of sensors such as motion sensors and pressure sensors, which are employed for HMS, are introduced. Particular requirements for HMS in the context of the regulations and the codes by principal organization sectors are discussed. Moreover, emerging new technologies applicable for ship and offshore structures are introduced. The basic principles and the application of fiber optic sensors (FOS), acoustic emission (AE) sensors and crack detection (CD) sensors are reviewed in the later part of this chapter. Finally, new requirements and trends in structural health monitoring in marine industries are introduced. In particular, the importance of structural health monitoring technology applicable in a cryogenic environment is presented. Key words: ships, offshore structures, structural health monitoring, fiber optic sensors (FOS), long-based strain gauges (LBSG), fatigue strength, hull stress monitoring system (HSMS).
18.1 18.1.1
Introduction History of offshore structures
The rapid increase of oil and gas prices has resulted in the installation of a significant number of offshore structures for energy exploration in the ocean. The exploration depth is continuously increasing, from continental shelf to deep sea beyond 1000 m. Offshore structures include fixed jackets and concrete platforms, semi-submersibles, tension leg platforms (TLPs), spars, jack-ups, and floating production storage and offloading (FPSO). Common offshore structures are illustrated in Fig. 18.1. Fixed offshore structures are by far the most common kind of these structures.1 550 © 2014 Elsevier Ltd
Sensing solutions for monitoring offshore structures
Fixed platform
Complaint tower
TLP
FPSO
551
SPAR
18.1 Typical offshore structures (left to right): Fixed platform, complaint tower, TLP, FPSO and SPAR.
Steel jacket-type platforms have been widely developed for various purposes, including offshore drilling, processing, and support of offshore operations. It has been reported that about 5600 offshore structures have been installed and operated by oil and gas companies in the United States. Offshore structures in the Gulf of Mexico are getting old as well, and the average age is known to be 35 years. It has been reported that the age distribution of offshore structures installed in the UK continental shelf (UKCS) and the Norwegian continental shelf (NCS) shows that a relatively large number of installations have crossed of 20 years.2,3 Offshore structures used in petroleum activity are normally designed for various phases of their life. These phases include construction, float out from construction site to transport barge, transport to field, lifting from barge to site, operation on site and, finally, removal. The design codes normally include three limit states. These limit states are ultimate limit state (ULS), fatigue limit state (FLS), and accidental limit state (ALS).4 Structural integrity assessment of existing structures is performed to extend the service life of the facility, as new methods of production and new discoveries may result in a request for life extension.5,6,7 The most generally accepted standard for offshore structures is ISO 19900 ‘Petroleum and natural gas industries – Offshore Structures – Part 1: General Requirements’ ISO (2002). This standard provides general design rules and general rules for assessment of existing structures. The Norwegian regulations (PSA 2004) refer to NORSOK N-001 (NORSOK 2004) for structural design, which again refer to ISO 19900 (ISO 2002) for assessing existing structures. Other standards, such as API RP2A-WSD (API 2000) and ISO/DIS 13822 (ISO 2000), are also available for detailed procedures for integrity assessment of the existing structures.2,8 Although the removal of offshore structures is mainly an economic decision, the old structures should be properly maintained or removed to prevent any catastrophy. The Deepwater Horizon oil spill, or BP oil spill, is the largest accidental marine oil spill in the history of the petroleum industry, as shown
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Sensor Technologies for Civil Infrastructures
18.2 Deepwater horizon offshore drilling unit on fire in 2010. (Source: Courtesy of the United States Coast Guard.)
in Fig. 18.2. The spill caused extensive damage to marine wildlife habitats and the local community. This kind of catastrophic failure in offshore structures can be effectively avoided by proper regulation, inspection, and maintenance, as well as structural health monitoring.
18.1.2
Aims and scope
Structural health monitoring of the structures is essential for ensuring their safe operation. In this regard, the safety issues, including inspection, maintenance, and repair processes for offshore structures, have become of crucial importance in the offshore community. The design of a structural health monitoring system essentially requires consideration of the trade-off between initial costs and long-term operating costs. In particular, the difficult issues encountered in designing structural health monitoring systems for ships and offshore structures are: • •
Operation in hostile environment of salt water, storm waves, and cargoes. Having a huge area of steel surface, and difficulties in access.
Sensing solutions for monitoring offshore structures •
•
•
553
Use of instrumentation being more restricted. Instrumentation transducers and connecting wires have very limited durability in a hostile environment. Instrumentation and monitoring system are required to determine loadings, response, and performance characteristics of critical structural elements while the ship is in service. Uncertainties in loadings (environmental, operating) significantly influencing the monitoring system for validating structural response and performance analysis method.
The classes of damage considered for ships and offshore structures include ship collision, slamming induced by storm, ice impact, dropped objects, fatigue fracture, fire and blast, corrosion/material degradation to name a few. Unforeseen damage to platforms may occur due to accidents, storm damage, and various geological problems. Salvino et al. defined the main goals of structural health monitoring systems as:9 • Assessment of structural degradation and gradually degrading conditions • Verification of design assumptions associated with design loadings and structural responses • Assessment of potential failures due to errors in the design, fabrication and operation • Assessment of the operational utilization of the structure. The benefits of structural health monitoring (SHM) include reduced inspection costs, minimized preventive maintenance, increased asset availability, and extension of the remaining useful life of structures. In particular, future benefits of using SHM for offshore structures can be summarized as: • • • •
Design validation through sea trials Through-life load and usage monitoring Damage detection and diagnostic systems Prognosis, mainly for fatigue life predictions.
A special point that needs to be noted is that a structural health monitoring system for ships and offshore structures in marine operation always needs to consider the regulations and codes of principal organization sectors involved in the development such as: • •
Regulatory agencies Classification societies
554 • •
Sensor Technologies for Civil Infrastructures
Manufacturers, designers, builders and repairers Owners and operators.
18.1.3
Current SHM techniques for ships and offshore structures
The basic rule requirements of a hull stress monitoring system (HSMS) are in accordance with International Maritime Organization (IMO) recommendations for ‘fitting of HSMS (MSC/Circ.646 6th June 1994, Maritime Safety Committee)’. The major classification societies, such as LR (Lloyd’s Register), Det Norske Veritas (DNV), and American Bureau of Shipping (ABS), and the International Association of Classification Societies (IACS) not only provide similar monitoring guidance for minimum parameters to be monitored but also recognize the fitting of enhanced and more comprehensive monitoring systems through various notations. The notations and requirements of classification societies concerning the provision of the typical monitoring for bulk carrier and tankers are summarized in Table 18.1.
18.2
Hull response monitoring systems
The current state-of-the-art structural health monitoring system for ships is Hull Response Monitoring Systems (HRMS).10 HRMS is a system that measures and displays key ship motions and hull structural responses.11 Realtime ship motions and stresses monitored by HRMS provide the onset and severity of hull structural responses to the sea and ice to the operators. IMO, IACS, and other individual Classification Societies (LR; DNV; and ABS) have recommended the use of HRMS. HRMS consists of an affordable number of strain, temperature, and acceleration sensors placed in predetermined locations within ships’ structures. Strain sensors are typically placed in the vicinity of possible crack locations, Table 18.1 Notations and requirement of the classification societies Class
Notation
Gauges
LR DNV
SEA (HSS-4) HMON-1
ABS
HM2 + R
4 LBSG, 1 bow vertical Accelerometer 4 LBSG, 1 bow vertical Accelerometer, 1 midship accelerometer 4 LBSG, 1 bow vertical Accelerometer
Sensing solutions for monitoring offshore structures Processor/ display/recorder
555
Uninterruptible power supply Aft long-base strain gauge
Signal conditioning
Zener barriers
Midship port long-base Forward long-base strain gauge strain gauge
Midship starboard long-base strain gauge
Accelerometer forward
Pressure transducer forward
18.3 Typical hull monitoring system for a bulk carrier.
including various weld joints, as shown in Fig. 18.3. Additional sensors including global positioning system (GPS), hull hydrostatic pressure (external and in-tank), weather and motion prediction, and linkage to other ship instruments are provided. A hull monitoring system enables the operator of the vessel to monitor all relevant responses (motions, accelerations, loads, bending moments, stresses, etc.) and provides rational guidance on preventive measures in heavy weather conditions. HSMS is a system that provides real-time information, such as motions and global stress experienced by the ship, to the crew of the ship while navigating as well as during loading and unloading operations. While HRMS applications have matured as an industry, future applications on ice-class ships need more attention. Also, further research effort is required to relate the current HRMS system to at-sea operational guidance as well as route and schedule planning.
18.3
Fatigue monitoring sensors
Fatigue analysis consists of the characterization of short- and long-term cyclic conditions (loading and unloading of cargoes, hydrostatic pressure, hydrodynamic loadings, and machinery and equipment vibrations), the determination of the cyclic forces and strains in structural elements, and the determination of potential degradation in strength and stiffness degradation in structural elements. Fatigue damage to structural components arises because of the cyclic nature of wave or wind loadings. The quality of fatigue strength assessment depends on the accuracy of stress concentration evaluations, ductile and fatigue resistance materials, determination of cyclic loading history, robust (damage tolerant) design, and construction/quality assurance and control.
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18.4 Fatigue crack monitoring sensor.
Crack growth due to fatigue and stress corrosion is normally a slow degradation process up to a point, beyond which failure may be sudden and catastrophic. Detection as early as possible during this initial period of crack growth is essential if the consequences of unexpected failure are to be avoided. The fatigue gauge is commonly employed for the crack-growth measurement proportional to the cumulative fatigue damage for welded joints. These sensors are made of thin metal pieces and can be placed in front of stress-concentrated areas within structures for detecting fatigue cracks. These sensors are cheap, small, and made of thin metal pieces as shown in Fig. 18.4. When a fatigue crack propagates through the surface of the sensor, due to repeated fatigue loadings, the sensor can be used for measuring fatigue-crack length. Further development is required in offshore industry for easier installation of the sensors to the structure and robust sensors to offshore environmental circumstances.
18.4
Air gap sensing system
Air gap is defined as the positive difference between the highest crest elevation for the design return period and the underside of the lowest deck level.12 ISO 19902 standard on fixed offshore installations describes the necessity for measuring the air gap as:
Sensing solutions for monitoring offshore structures
557
Where air gap measurement devices are correctly set up, calibrated and maintained, continuous records of wave heights and tide can provide very useful information on environmental conditions. Where this can be combined with directionality data and ideally some method of estimating actions, the data can be used in analyses and assessment of defects and of remaining life, possibly reducing conservatism.
Simple methods, such as a tape measure from the cellar deck, have also been widely used, which have recently been replaced by radar measurement. Measured values are typically recorded and averaged over a given time period, for example, every three years. While periodic survey of air gap is sufficient most of the time, real-time continuous monitoring of the air gap is important in seismically active regions or in harsh local conditions.
18.5
Corrosion monitoring system
Ballast tanks and crude cargo tanks are continuously exposed to corrosive environment. Typically, ballast condition is worst in terms of corrosion damage, particularly in empty or partially filled conditions. Cathodic protection or coatings are commonly adopted for protecting structures from corrosive environment. Problems are due to improperly designed, applied, and maintained corrosion systems, and incompatibilities between structural and corrosion protection systems, such as flexible bulkhead covered with stiff coatings and corrosion cells set up between the parent materials and the weld heat affected zone (HAZ) region resulting in grooving corrosion.
18.6
Acoustic emissions monitoring sensors
Acoustic emission (AE) techniques can be used for monitoring corrosion in onshore tanks, but it is known that these would be much more difficult to apply on an FPSO, because the emission levels are too low and are easily masked by other noises on the installation. AE monitoring uses the transient acoustic stress-wave for detecting damage mechanisms such as cracks by characterizing the sound patterns due to structural anomalies induced locally in structures. The system has been used in areas with high risk of fatigue cracking and with difficult access for inspection.12 AE provides real-time information on fatigue crack initiation at the early stages of propagation and growth, and can be used with strain gauges to correlate the structural stress levels. AE systems have been applied extensively
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Sensor Technologies for Civil Infrastructures
18.5 AE sensor for use on underwater marine structure.
for the structural monitoring of critical structural members over recent decades in the offshore industry.13 AE monitoring can overcome various difficulties with crack detection (CD) in service, and is potentially a very promising method of inspection. It is sensitive to the propagating cracks, that is, the structurally significant defects, and provides information on growth rate under service loading conditions, guiding inspection and repair work for maximum cost effective maintenance. Cost effective structural monitoring based on an acoustic method is summarized in References [14,15]. This monitoring technology can provide improved assurance of overall integrity, justifying the additional work and cost involved. By continuously monitoring a vessel or offshore structure over a period of time, usually several weeks or months depending on the minimum acceptable defect size, enhanced assurance of structural integrity can be obtained. The design of an underwater sensor is shown in Fig. 18.5, attached to a marine structure (ballast tank in vessel or jack-up platform) where it was being used to evaluate the most effective sensor for monitoring.
18.7
Vibration-based damage assessment approaches
Vibration-based damage monitoring methods have been extensively used for structural monitoring of offshore platforms. The vibration-based damage monitoring system has been in operation since the late 1970s on a number of offshore jackets. Vibration parameters are monitored by on-line damage assessment techniques. They comprise techniques that are based on examination of changes
Sensing solutions for monitoring offshore structures
559
in natural frequencies, mode shapes, or mode shape curvatures (frequency domain methods). Doebling et al.16 and Sohn et al.17 published a state-of-theart review on vibration-based damage identification methods. Typically, accelerometers are placed on the topside of an installation with additional subsea accelerometers. The vibration response of the jacket to wave loading is continuously monitored. Any major structural damage to the platform is reflected by a change in structural response.18 The method is known to be sensitive enough to detect a frequency change of 0.5%. However, the method cannot detect minor damage, such as small defects or local fatigue cracks. Figure 18.6 illustrates an example of damage detection based on change in natural frequencies. By comparing the natural frequency data between the damaged and the undamaged platforms, the occurrence of damage in the platform can be easily determined.19 Loland and Dodds (1976) discuss practical experiences learned by monitoring three North Sea platforms over 6–9 months.20 Discussions of platform geometry, instrumentation, environmental conditions during measurements, and system cost are presented. In the paper, the following five requirements for the vibration-based damage monitoring systems are identified. 0.395 Data collected from undamaged platform
East/west frequency (Hz)
0.39
0.385
0.38
0.375
0.37 Data from damaged platform
0.365
0.36 0.355
0.36
0.365
0.37
0.375
0.38
0.385
0.39
0.395
North/south frequency (Hz)
18.6 An example of damage detection based on the change in natural frequencies.6
560 • • • • •
Sensor Technologies for Civil Infrastructures
Ambient (sea and wind) excitation must be used to extract the resonant frequencies. Vibration spectra must remain stable over long periods of time. Instruments must withstand environmental challenges. Mode shapes must be identifiable from above-water measurements The system must offer financial advantages over the use of divers.
18.8
Fiber optic sensors (FOS)
FOS have recently been employed in place of conventional strain gauges to determine the stress or loading regime in part of a structure. The strain measuring techniques have been widely applied across many industries, including the offshore industry. A typical Bragg grating type fiber optic sensor is shown in Fig. 18.7. FOSs exhibit advantages such as flexibility, embeddability, multiplexity, and electromagnetic immunity (EMI) immunity compared to traditional sensors. However, fiber optic strain sensors are limited in application for stress variation only, not absolute stress levels. Cabling, especially for conventional strain gauges in a humid environment, is another important issue to be considered. Among various fiber optic sensor methods, fiber Bragg grating (FBG) sensors are the most promising candidate for structural health monitoring application for offshore platforms.21 FBG sensors have been employed on offshore platforms for monitoring ship collisions and ocean wave loads.22 It is reported that the FBG sensors installed at the bottom of the central pillar of the jacket performed well and did not show any significant reduction of sensing performance, as shown in Fig. 18.8. However, conventional strain gauges failed to operate, due to the detrimental corrosion of sea water.22
18.7 Typical FOS-based on Bragg gratings (FBGs).
Sensing solutions for monitoring offshore structures
561
3# 4#
Boat collision position
2#
1#
18.8 Platform, model and location of FOS.8
Slit
Grating
18.9 Installation of FBG sensors in LNG insulation system.9
Another example of fiber optic sensor application in ships and offshore structures is in the structural health monitoring of the failure modes of insulation panels of liquefied natural gas (LNG) carriers. Insulation panels of LNG cargo tanks may include mechanical failures such as cracks, as well as delamination within the layers due to impact sloshing loads and fatigue loadings, and these failures cause a significant decrease of structural safety. Figure 18.9 shows a fiber optic sensor embedded within the insulation system of an LNG vessel. A structural health monitoring system has been reported that employs FOS for monitoring the various failures that can occur in LNG insulation panels.23
18.9
Riser and anchor chain monitoring
The riser and anchor chain monitoring system consists of an array of sonar positioned beneath the platform, which emits signals in and around the horizontal plant.12 Plate XVII in the color section between pages 374
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and 375 shows a typical riser and anchor chain monitoring system. The purpose of this technique is to monitor the position of risers and anchor chains, and alert the operator to any displacements outside of predefined limits. In addition, failure analysis can be performed using the resulting computerized data. The measured data can be used to determine the precise location of riser and anchor chains and to continuously monitor their position.
18.10 Conclusion and future trends In this article, various techniques for monitoring the structural integrity of ship and offshore structures are discussed. Typical HMSs are introduced with common sensor locations as well as measurement parameters to be monitored. The long based strain gauge (LBSG) is typically installed at the midship and quarterdeck of vessels for monitoring hull girder strength. Ship motion and slamming pressures are measured by accelerometers and pressure sensors located at the bow of the vessel. Different requirements for the hull monitoring system are summarized for each classification society. Then a review of the introduction of emerging new technologies that are applicable for ship and offshore structures follows. The basic principles and application of FOS, AE sensors, fatigue CD sensors, vibration-based damage monitoring, and riser and anchor chain monitoring systems are reviewed. Finally, new requirements and future trends in structural health monitoring in marine industries are introduced. In particular, the importance of structural health monitoring technology that is applicable in cryogenic environment is indicated. Another important issue in structural monitoring is subsea system integrity monitoring. The integrity and safety assessment, as well as remaining life assessment, of deep sea pipelines and the related subsea systems for oil and gas exploration are of critical importance and need special attention. In conclusion, the structural health monitoring technique is an integral system for ensuring the safety of ships and offshore structures, as well as for protecting environment.
18.11 References 1.
2. 3.
Haritos, H (2007), ‘Introduction to the analysis and design of offshore structures– an overview’, Electronic Journal of Structural Engineering, Special Issue: Loading on Structures. pp. 55–65. Ersdal, G (2005), ‘Assessment of existing offshore structures for life extension’, Doctoral thesis, University of Stavanger, Stavanger, Norway. Minerals Management Service (2004), History of the Offshore Oil and Gas Industry in Southern Louisiana, Outer Continental Shelf Study – MMS 2004– 050, Minerals Management Service, Louisiana, USA.
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4. Det Norske Veritas (2007), Rules for Classification of Offshore Drilling and Support Units, Special Provisions for Ageing Mobile Offshore and Self-Elevating Structures, OSS-101, Det Norske Veritas, Høvik, Norway. 5. NORSOK (1997), Condition Monitoring of Load Bearing Structures, NORSOK N-005, NORSOK, Oslo, Norway. 6. International Standards for Organization (2004), Performance Parameters for Condition Monitoring of Structures, ISO 16587:2004, International Standards for Organization, Geneva, Switzerland. 7. American Petroleum Institute (2007), Structural Integrity Management of Fixed Offshore Structures, ANSI/API recommended practice 2SIM (Draft), American Petroleum Institute, Washington, DC, USA. 8. American Petroleum Institute (2000), Offshore Structures Design Practice, API RP 2A, 21st edition. American Petroleum Institute, Washington, DC, USA. 9. Salvino, LW and Collette, MD (2009), ‘Monitoring marine structures’, in Boller, C., Fou-Kuo Chang and Fujino, Y. (eds), Encyclopedia of Structural Health Monitoring, John Wiley and Sons, Ltd. 10. Kim, M and Kim, D (2009), ‘Ship and offshore structures’, in Boller, C., FouKuo Chang and Fujino, Y. (eds), Encyclopedia of Structural Health Monitoring, John Wiley and Sons, Ltd. 11. Ship Structure Committee (1997), State of the Art in Hull Response Monitoring System, SSC-401, Ship Structure Committee. 12. Health and Safety Executive (2009), Structural Integrity Monitoring – Review and Appraisal of Current Technology for Offshore Applications, RR685, Health and Safety Executive, Norwich, UK. 13. Gorman, M (2009), ‘Acoustic emission’, in Boller, C., Fou-Kuo Chang and Fujino, Y. (eds), Encyclopedia of Structural Health Monitoring, John Wiley and Sons, Ltd. 14. Mecon Limited (2005), Cost Effective Structural Monitoring – An Acoustic Method, Phase II, RR325, Health and Safety Executive, Norwich, UK. 15. Mecon Limited (2005), Cost Effective Structural Monitoring – An Acoustic Method, Phase IIb, RR326, Health and Safety Executive, Norwich, UK. 16. Doebling, SW, Farrar, CR, Prime MB and Shevitz, DW (1996), Damage Identification and Health Monitoring of Structural and Mechanical Systems from Changes in Their Vibration Characteristics: A Literature Review, LA-13070-MS, Los Alamos National Laboratory, New Mexico, USA. 17. Sohn, H, Farrar, CR, Hemez, FM, Shunk, DD, Stinemates, DW, Nadler, BR and Czarnecki, JJ (2004), A Review of Structural Health Monitoring Literature 1996 – 2001, LA-13976-MS, Los Alamos National Laboratory, New Mexico, USA. 18. Mojtahedi, A,Yaghin, MA, Hassanzadeh, Y, Ettefagh, MM, Aminfar, MH and Aghdam, AB (2011), ‘Developing a robust SHM method for offshore jacket platform using model updating and fuzzy logic system’, Applied Ocean Research, 33, pp. 398–411. 19. Farrar, CR and Doebling, SW (1997), ‘An overview of modal-based damage identification methods’, Proceedings of the International Conference on Damage Assessment of Structures, International Conference on Damage Assessment of Structures, Sheffield, UK.
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20. Loland, O and Dodds, JC (1976), ‘Experience in developing and operating integrity monitoring system in North Sea’, Proceedings of the 8th Annual Offshore Technology Conference, Offshore Technology Conference, Houston, Texas, USA, pp. 313–319. 21. Peters, K (2009), ‘Fiber Bragg grating sensors’, in Boller, C., Fou-Kuo Chang and Fujino, Y. (eds), Encyclopedia of Structural Health Monitoring, John Wiley and Sons, Ltd. 22. Ren, L, Li, HN, Zhou, J, Li, DS and Sun, L (2006), ‘Health monitoring system for offshore platform with fiber Bragg grating sensors’, Optical Engineering, 45, 8, 084401-084401-9. 23. Kim, MH, Kim, DH, Kang, SW and Lee, JM (2006), ‘An interlaminar strain measurement for insulation panels of LNG carriers’, Strain – An International Journal for Experimental Mechanics, 42, 2, pp. 97–106.
DOI: 10.1533/9781782422433.2.550 Abstract: Structural health monitoring techniques for ship and offshore structures including recent literature survey in the related fields are summarized in this chapter. Typical hull monitoring systems (HMS) for ship structures are introduced with common sensor locations, as well as measurement parameters to be monitored. Long-based strain gauges (LBSG), accelerometers, and other types of sensors such as motion sensors and pressure sensors, which are employed for HMS, are introduced. Particular requirements for HMS in the context of the regulations and the codes by principal organization sectors are discussed. Moreover, emerging new technologies applicable for ship and offshore structures are introduced. The basic principles and the application of fiber optic sensors (FOS), acoustic emission (AE) sensors and crack detection (CD) sensors are reviewed in the later part of this chapter. Finally, new requirements and trends in structural health monitoring in marine industries are introduced. In particular, the importance of structural health monitoring technology applicable in a cryogenic environment is presented. Key words: ships, offshore structures, structural health monitoring, fiber optic sensors (FOS), long-based strain gauges (LBSG), fatigue strength, hull stress monitoring system (HSMS).
18.1 18.1.1
Introduction History of offshore structures
The rapid increase of oil and gas prices has resulted in the installation of a significant number of offshore structures for energy exploration in the ocean. The exploration depth is continuously increasing, from continental shelf to deep sea beyond 1000 m. Offshore structures include fixed jackets and concrete platforms, semi-submersibles, tension leg platforms (TLPs), spars, jack-ups, and floating production storage and offloading (FPSO). Common offshore structures are illustrated in Fig. 18.1. Fixed offshore structures are by far the most common kind of these structures.1 550 © 2014 Elsevier Ltd
Sensing solutions for monitoring offshore structures
Fixed platform
Complaint tower
TLP
FPSO
551
SPAR
18.1 Typical offshore structures (left to right): Fixed platform, complaint tower, TLP, FPSO and SPAR.
Steel jacket-type platforms have been widely developed for various purposes, including offshore drilling, processing, and support of offshore operations. It has been reported that about 5600 offshore structures have been installed and operated by oil and gas companies in the United States. Offshore structures in the Gulf of Mexico are getting old as well, and the average age is known to be 35 years. It has been reported that the age distribution of offshore structures installed in the UK continental shelf (UKCS) and the Norwegian continental shelf (NCS) shows that a relatively large number of installations have crossed of 20 years.2,3 Offshore structures used in petroleum activity are normally designed for various phases of their life. These phases include construction, float out from construction site to transport barge, transport to field, lifting from barge to site, operation on site and, finally, removal. The design codes normally include three limit states. These limit states are ultimate limit state (ULS), fatigue limit state (FLS), and accidental limit state (ALS).4 Structural integrity assessment of existing structures is performed to extend the service life of the facility, as new methods of production and new discoveries may result in a request for life extension.5,6,7 The most generally accepted standard for offshore structures is ISO 19900 ‘Petroleum and natural gas industries – Offshore Structures – Part 1: General Requirements’ ISO (2002). This standard provides general design rules and general rules for assessment of existing structures. The Norwegian regulations (PSA 2004) refer to NORSOK N-001 (NORSOK 2004) for structural design, which again refer to ISO 19900 (ISO 2002) for assessing existing structures. Other standards, such as API RP2A-WSD (API 2000) and ISO/DIS 13822 (ISO 2000), are also available for detailed procedures for integrity assessment of the existing structures.2,8 Although the removal of offshore structures is mainly an economic decision, the old structures should be properly maintained or removed to prevent any catastrophy. The Deepwater Horizon oil spill, or BP oil spill, is the largest accidental marine oil spill in the history of the petroleum industry, as shown
552
Sensor Technologies for Civil Infrastructures
18.2 Deepwater horizon offshore drilling unit on fire in 2010. (Source: Courtesy of the United States Coast Guard.)
in Fig. 18.2. The spill caused extensive damage to marine wildlife habitats and the local community. This kind of catastrophic failure in offshore structures can be effectively avoided by proper regulation, inspection, and maintenance, as well as structural health monitoring.
18.1.2
Aims and scope
Structural health monitoring of the structures is essential for ensuring their safe operation. In this regard, the safety issues, including inspection, maintenance, and repair processes for offshore structures, have become of crucial importance in the offshore community. The design of a structural health monitoring system essentially requires consideration of the trade-off between initial costs and long-term operating costs. In particular, the difficult issues encountered in designing structural health monitoring systems for ships and offshore structures are: • •
Operation in hostile environment of salt water, storm waves, and cargoes. Having a huge area of steel surface, and difficulties in access.
Sensing solutions for monitoring offshore structures •
•
•
553
Use of instrumentation being more restricted. Instrumentation transducers and connecting wires have very limited durability in a hostile environment. Instrumentation and monitoring system are required to determine loadings, response, and performance characteristics of critical structural elements while the ship is in service. Uncertainties in loadings (environmental, operating) significantly influencing the monitoring system for validating structural response and performance analysis method.
The classes of damage considered for ships and offshore structures include ship collision, slamming induced by storm, ice impact, dropped objects, fatigue fracture, fire and blast, corrosion/material degradation to name a few. Unforeseen damage to platforms may occur due to accidents, storm damage, and various geological problems. Salvino et al. defined the main goals of structural health monitoring systems as:9 • Assessment of structural degradation and gradually degrading conditions • Verification of design assumptions associated with design loadings and structural responses • Assessment of potential failures due to errors in the design, fabrication and operation • Assessment of the operational utilization of the structure. The benefits of structural health monitoring (SHM) include reduced inspection costs, minimized preventive maintenance, increased asset availability, and extension of the remaining useful life of structures. In particular, future benefits of using SHM for offshore structures can be summarized as: • • • •
Design validation through sea trials Through-life load and usage monitoring Damage detection and diagnostic systems Prognosis, mainly for fatigue life predictions.
A special point that needs to be noted is that a structural health monitoring system for ships and offshore structures in marine operation always needs to consider the regulations and codes of principal organization sectors involved in the development such as: • •
Regulatory agencies Classification societies
554 • •
Sensor Technologies for Civil Infrastructures
Manufacturers, designers, builders and repairers Owners and operators.
18.1.3
Current SHM techniques for ships and offshore structures
The basic rule requirements of a hull stress monitoring system (HSMS) are in accordance with International Maritime Organization (IMO) recommendations for ‘fitting of HSMS (MSC/Circ.646 6th June 1994, Maritime Safety Committee)’. The major classification societies, such as LR (Lloyd’s Register), Det Norske Veritas (DNV), and American Bureau of Shipping (ABS), and the International Association of Classification Societies (IACS) not only provide similar monitoring guidance for minimum parameters to be monitored but also recognize the fitting of enhanced and more comprehensive monitoring systems through various notations. The notations and requirements of classification societies concerning the provision of the typical monitoring for bulk carrier and tankers are summarized in Table 18.1.
18.2
Hull response monitoring systems
The current state-of-the-art structural health monitoring system for ships is Hull Response Monitoring Systems (HRMS).10 HRMS is a system that measures and displays key ship motions and hull structural responses.11 Realtime ship motions and stresses monitored by HRMS provide the onset and severity of hull structural responses to the sea and ice to the operators. IMO, IACS, and other individual Classification Societies (LR; DNV; and ABS) have recommended the use of HRMS. HRMS consists of an affordable number of strain, temperature, and acceleration sensors placed in predetermined locations within ships’ structures. Strain sensors are typically placed in the vicinity of possible crack locations, Table 18.1 Notations and requirement of the classification societies Class
Notation
Gauges
LR DNV
SEA (HSS-4) HMON-1
ABS
HM2 + R
4 LBSG, 1 bow vertical Accelerometer 4 LBSG, 1 bow vertical Accelerometer, 1 midship accelerometer 4 LBSG, 1 bow vertical Accelerometer
Sensing solutions for monitoring offshore structures Processor/ display/recorder
555
Uninterruptible power supply Aft long-base strain gauge
Signal conditioning
Zener barriers
Midship port long-base Forward long-base strain gauge strain gauge
Midship starboard long-base strain gauge
Accelerometer forward
Pressure transducer forward
18.3 Typical hull monitoring system for a bulk carrier.
including various weld joints, as shown in Fig. 18.3. Additional sensors including global positioning system (GPS), hull hydrostatic pressure (external and in-tank), weather and motion prediction, and linkage to other ship instruments are provided. A hull monitoring system enables the operator of the vessel to monitor all relevant responses (motions, accelerations, loads, bending moments, stresses, etc.) and provides rational guidance on preventive measures in heavy weather conditions. HSMS is a system that provides real-time information, such as motions and global stress experienced by the ship, to the crew of the ship while navigating as well as during loading and unloading operations. While HRMS applications have matured as an industry, future applications on ice-class ships need more attention. Also, further research effort is required to relate the current HRMS system to at-sea operational guidance as well as route and schedule planning.
18.3
Fatigue monitoring sensors
Fatigue analysis consists of the characterization of short- and long-term cyclic conditions (loading and unloading of cargoes, hydrostatic pressure, hydrodynamic loadings, and machinery and equipment vibrations), the determination of the cyclic forces and strains in structural elements, and the determination of potential degradation in strength and stiffness degradation in structural elements. Fatigue damage to structural components arises because of the cyclic nature of wave or wind loadings. The quality of fatigue strength assessment depends on the accuracy of stress concentration evaluations, ductile and fatigue resistance materials, determination of cyclic loading history, robust (damage tolerant) design, and construction/quality assurance and control.
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18.4 Fatigue crack monitoring sensor.
Crack growth due to fatigue and stress corrosion is normally a slow degradation process up to a point, beyond which failure may be sudden and catastrophic. Detection as early as possible during this initial period of crack growth is essential if the consequences of unexpected failure are to be avoided. The fatigue gauge is commonly employed for the crack-growth measurement proportional to the cumulative fatigue damage for welded joints. These sensors are made of thin metal pieces and can be placed in front of stress-concentrated areas within structures for detecting fatigue cracks. These sensors are cheap, small, and made of thin metal pieces as shown in Fig. 18.4. When a fatigue crack propagates through the surface of the sensor, due to repeated fatigue loadings, the sensor can be used for measuring fatigue-crack length. Further development is required in offshore industry for easier installation of the sensors to the structure and robust sensors to offshore environmental circumstances.
18.4
Air gap sensing system
Air gap is defined as the positive difference between the highest crest elevation for the design return period and the underside of the lowest deck level.12 ISO 19902 standard on fixed offshore installations describes the necessity for measuring the air gap as:
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Where air gap measurement devices are correctly set up, calibrated and maintained, continuous records of wave heights and tide can provide very useful information on environmental conditions. Where this can be combined with directionality data and ideally some method of estimating actions, the data can be used in analyses and assessment of defects and of remaining life, possibly reducing conservatism.
Simple methods, such as a tape measure from the cellar deck, have also been widely used, which have recently been replaced by radar measurement. Measured values are typically recorded and averaged over a given time period, for example, every three years. While periodic survey of air gap is sufficient most of the time, real-time continuous monitoring of the air gap is important in seismically active regions or in harsh local conditions.
18.5
Corrosion monitoring system
Ballast tanks and crude cargo tanks are continuously exposed to corrosive environment. Typically, ballast condition is worst in terms of corrosion damage, particularly in empty or partially filled conditions. Cathodic protection or coatings are commonly adopted for protecting structures from corrosive environment. Problems are due to improperly designed, applied, and maintained corrosion systems, and incompatibilities between structural and corrosion protection systems, such as flexible bulkhead covered with stiff coatings and corrosion cells set up between the parent materials and the weld heat affected zone (HAZ) region resulting in grooving corrosion.
18.6
Acoustic emissions monitoring sensors
Acoustic emission (AE) techniques can be used for monitoring corrosion in onshore tanks, but it is known that these would be much more difficult to apply on an FPSO, because the emission levels are too low and are easily masked by other noises on the installation. AE monitoring uses the transient acoustic stress-wave for detecting damage mechanisms such as cracks by characterizing the sound patterns due to structural anomalies induced locally in structures. The system has been used in areas with high risk of fatigue cracking and with difficult access for inspection.12 AE provides real-time information on fatigue crack initiation at the early stages of propagation and growth, and can be used with strain gauges to correlate the structural stress levels. AE systems have been applied extensively
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18.5 AE sensor for use on underwater marine structure.
for the structural monitoring of critical structural members over recent decades in the offshore industry.13 AE monitoring can overcome various difficulties with crack detection (CD) in service, and is potentially a very promising method of inspection. It is sensitive to the propagating cracks, that is, the structurally significant defects, and provides information on growth rate under service loading conditions, guiding inspection and repair work for maximum cost effective maintenance. Cost effective structural monitoring based on an acoustic method is summarized in References [14,15]. This monitoring technology can provide improved assurance of overall integrity, justifying the additional work and cost involved. By continuously monitoring a vessel or offshore structure over a period of time, usually several weeks or months depending on the minimum acceptable defect size, enhanced assurance of structural integrity can be obtained. The design of an underwater sensor is shown in Fig. 18.5, attached to a marine structure (ballast tank in vessel or jack-up platform) where it was being used to evaluate the most effective sensor for monitoring.
18.7
Vibration-based damage assessment approaches
Vibration-based damage monitoring methods have been extensively used for structural monitoring of offshore platforms. The vibration-based damage monitoring system has been in operation since the late 1970s on a number of offshore jackets. Vibration parameters are monitored by on-line damage assessment techniques. They comprise techniques that are based on examination of changes
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in natural frequencies, mode shapes, or mode shape curvatures (frequency domain methods). Doebling et al.16 and Sohn et al.17 published a state-of-theart review on vibration-based damage identification methods. Typically, accelerometers are placed on the topside of an installation with additional subsea accelerometers. The vibration response of the jacket to wave loading is continuously monitored. Any major structural damage to the platform is reflected by a change in structural response.18 The method is known to be sensitive enough to detect a frequency change of 0.5%. However, the method cannot detect minor damage, such as small defects or local fatigue cracks. Figure 18.6 illustrates an example of damage detection based on change in natural frequencies. By comparing the natural frequency data between the damaged and the undamaged platforms, the occurrence of damage in the platform can be easily determined.19 Loland and Dodds (1976) discuss practical experiences learned by monitoring three North Sea platforms over 6–9 months.20 Discussions of platform geometry, instrumentation, environmental conditions during measurements, and system cost are presented. In the paper, the following five requirements for the vibration-based damage monitoring systems are identified. 0.395 Data collected from undamaged platform
East/west frequency (Hz)
0.39
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0.37 Data from damaged platform
0.365
0.36 0.355
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18.6 An example of damage detection based on the change in natural frequencies.6
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Ambient (sea and wind) excitation must be used to extract the resonant frequencies. Vibration spectra must remain stable over long periods of time. Instruments must withstand environmental challenges. Mode shapes must be identifiable from above-water measurements The system must offer financial advantages over the use of divers.
18.8
Fiber optic sensors (FOS)
FOS have recently been employed in place of conventional strain gauges to determine the stress or loading regime in part of a structure. The strain measuring techniques have been widely applied across many industries, including the offshore industry. A typical Bragg grating type fiber optic sensor is shown in Fig. 18.7. FOSs exhibit advantages such as flexibility, embeddability, multiplexity, and electromagnetic immunity (EMI) immunity compared to traditional sensors. However, fiber optic strain sensors are limited in application for stress variation only, not absolute stress levels. Cabling, especially for conventional strain gauges in a humid environment, is another important issue to be considered. Among various fiber optic sensor methods, fiber Bragg grating (FBG) sensors are the most promising candidate for structural health monitoring application for offshore platforms.21 FBG sensors have been employed on offshore platforms for monitoring ship collisions and ocean wave loads.22 It is reported that the FBG sensors installed at the bottom of the central pillar of the jacket performed well and did not show any significant reduction of sensing performance, as shown in Fig. 18.8. However, conventional strain gauges failed to operate, due to the detrimental corrosion of sea water.22
18.7 Typical FOS-based on Bragg gratings (FBGs).
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3# 4#
Boat collision position
2#
1#
18.8 Platform, model and location of FOS.8
Slit
Grating
18.9 Installation of FBG sensors in LNG insulation system.9
Another example of fiber optic sensor application in ships and offshore structures is in the structural health monitoring of the failure modes of insulation panels of liquefied natural gas (LNG) carriers. Insulation panels of LNG cargo tanks may include mechanical failures such as cracks, as well as delamination within the layers due to impact sloshing loads and fatigue loadings, and these failures cause a significant decrease of structural safety. Figure 18.9 shows a fiber optic sensor embedded within the insulation system of an LNG vessel. A structural health monitoring system has been reported that employs FOS for monitoring the various failures that can occur in LNG insulation panels.23
18.9
Riser and anchor chain monitoring
The riser and anchor chain monitoring system consists of an array of sonar positioned beneath the platform, which emits signals in and around the horizontal plant.12 Plate XVII in the color section between pages 374
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and 375 shows a typical riser and anchor chain monitoring system. The purpose of this technique is to monitor the position of risers and anchor chains, and alert the operator to any displacements outside of predefined limits. In addition, failure analysis can be performed using the resulting computerized data. The measured data can be used to determine the precise location of riser and anchor chains and to continuously monitor their position.
18.10 Conclusion and future trends In this article, various techniques for monitoring the structural integrity of ship and offshore structures are discussed. Typical HMSs are introduced with common sensor locations as well as measurement parameters to be monitored. The long based strain gauge (LBSG) is typically installed at the midship and quarterdeck of vessels for monitoring hull girder strength. Ship motion and slamming pressures are measured by accelerometers and pressure sensors located at the bow of the vessel. Different requirements for the hull monitoring system are summarized for each classification society. Then a review of the introduction of emerging new technologies that are applicable for ship and offshore structures follows. The basic principles and application of FOS, AE sensors, fatigue CD sensors, vibration-based damage monitoring, and riser and anchor chain monitoring systems are reviewed. Finally, new requirements and future trends in structural health monitoring in marine industries are introduced. In particular, the importance of structural health monitoring technology that is applicable in cryogenic environment is indicated. Another important issue in structural monitoring is subsea system integrity monitoring. The integrity and safety assessment, as well as remaining life assessment, of deep sea pipelines and the related subsea systems for oil and gas exploration are of critical importance and need special attention. In conclusion, the structural health monitoring technique is an integral system for ensuring the safety of ships and offshore structures, as well as for protecting environment.
18.11 References 1.
2. 3.
Haritos, H (2007), ‘Introduction to the analysis and design of offshore structures– an overview’, Electronic Journal of Structural Engineering, Special Issue: Loading on Structures. pp. 55–65. Ersdal, G (2005), ‘Assessment of existing offshore structures for life extension’, Doctoral thesis, University of Stavanger, Stavanger, Norway. Minerals Management Service (2004), History of the Offshore Oil and Gas Industry in Southern Louisiana, Outer Continental Shelf Study – MMS 2004– 050, Minerals Management Service, Louisiana, USA.
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4. Det Norske Veritas (2007), Rules for Classification of Offshore Drilling and Support Units, Special Provisions for Ageing Mobile Offshore and Self-Elevating Structures, OSS-101, Det Norske Veritas, Høvik, Norway. 5. NORSOK (1997), Condition Monitoring of Load Bearing Structures, NORSOK N-005, NORSOK, Oslo, Norway. 6. International Standards for Organization (2004), Performance Parameters for Condition Monitoring of Structures, ISO 16587:2004, International Standards for Organization, Geneva, Switzerland. 7. American Petroleum Institute (2007), Structural Integrity Management of Fixed Offshore Structures, ANSI/API recommended practice 2SIM (Draft), American Petroleum Institute, Washington, DC, USA. 8. American Petroleum Institute (2000), Offshore Structures Design Practice, API RP 2A, 21st edition. American Petroleum Institute, Washington, DC, USA. 9. Salvino, LW and Collette, MD (2009), ‘Monitoring marine structures’, in Boller, C., Fou-Kuo Chang and Fujino, Y. (eds), Encyclopedia of Structural Health Monitoring, John Wiley and Sons, Ltd. 10. Kim, M and Kim, D (2009), ‘Ship and offshore structures’, in Boller, C., FouKuo Chang and Fujino, Y. (eds), Encyclopedia of Structural Health Monitoring, John Wiley and Sons, Ltd. 11. Ship Structure Committee (1997), State of the Art in Hull Response Monitoring System, SSC-401, Ship Structure Committee. 12. Health and Safety Executive (2009), Structural Integrity Monitoring – Review and Appraisal of Current Technology for Offshore Applications, RR685, Health and Safety Executive, Norwich, UK. 13. Gorman, M (2009), ‘Acoustic emission’, in Boller, C., Fou-Kuo Chang and Fujino, Y. (eds), Encyclopedia of Structural Health Monitoring, John Wiley and Sons, Ltd. 14. Mecon Limited (2005), Cost Effective Structural Monitoring – An Acoustic Method, Phase II, RR325, Health and Safety Executive, Norwich, UK. 15. Mecon Limited (2005), Cost Effective Structural Monitoring – An Acoustic Method, Phase IIb, RR326, Health and Safety Executive, Norwich, UK. 16. Doebling, SW, Farrar, CR, Prime MB and Shevitz, DW (1996), Damage Identification and Health Monitoring of Structural and Mechanical Systems from Changes in Their Vibration Characteristics: A Literature Review, LA-13070-MS, Los Alamos National Laboratory, New Mexico, USA. 17. Sohn, H, Farrar, CR, Hemez, FM, Shunk, DD, Stinemates, DW, Nadler, BR and Czarnecki, JJ (2004), A Review of Structural Health Monitoring Literature 1996 – 2001, LA-13976-MS, Los Alamos National Laboratory, New Mexico, USA. 18. Mojtahedi, A,Yaghin, MA, Hassanzadeh, Y, Ettefagh, MM, Aminfar, MH and Aghdam, AB (2011), ‘Developing a robust SHM method for offshore jacket platform using model updating and fuzzy logic system’, Applied Ocean Research, 33, pp. 398–411. 19. Farrar, CR and Doebling, SW (1997), ‘An overview of modal-based damage identification methods’, Proceedings of the International Conference on Damage Assessment of Structures, International Conference on Damage Assessment of Structures, Sheffield, UK.
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20. Loland, O and Dodds, JC (1976), ‘Experience in developing and operating integrity monitoring system in North Sea’, Proceedings of the 8th Annual Offshore Technology Conference, Offshore Technology Conference, Houston, Texas, USA, pp. 313–319. 21. Peters, K (2009), ‘Fiber Bragg grating sensors’, in Boller, C., Fou-Kuo Chang and Fujino, Y. (eds), Encyclopedia of Structural Health Monitoring, John Wiley and Sons, Ltd. 22. Ren, L, Li, HN, Zhou, J, Li, DS and Sun, L (2006), ‘Health monitoring system for offshore platform with fiber Bragg grating sensors’, Optical Engineering, 45, 8, 084401-084401-9. 23. Kim, MH, Kim, DH, Kang, SW and Lee, JM (2006), ‘An interlaminar strain measurement for insulation panels of LNG carriers’, Strain – An International Journal for Experimental Mechanics, 42, 2, pp. 97–106.