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The use of imagery in monitoring existing sub surface structures
- 1. ! APPLICATIONS & RESEARCH IN HYDROGRAPHIC SURVEYING! ! The use of imagery in monitoring existing sub surface structures! ! ! Friday 3rd April 2015! ! ! ! ! ! ! ! ! 0806514! !! ! ! INTRODUCTION! ! Monitoring of subsea structures is essential in the offshore energy industry to ensure structural integrity of assets within an unpredictable erosive environment. Applications include assessment of drilling platform supports, offshore wind-farms, waterfront facilities, and examining the health of subsea pipelines to certify that they are safe and fit-for- purpose. Subsea monitoring can be performed in a variety of ways including close- inspection using divers, remotely operated vehicles (ROVs) or autonomous underwater vehicles (AUVs) to obtain structural integrity information. Longer-term continuous monitoring can be provided by underwater camera deployment. For example with Kongsberg’s harsh environment CCTV systems (Kongsberg, 2015). However, project requirement, expense and water conditions may dictate the use of a particular method. Lack of accurate analysis can lead to structural failure that can have serious implications to life, the environment, and the economy.! ! This paper will present challenges of acquiring subsea imagery while outlining advances and applications in subsea imaging for sub-surface structural monitoring. This will focus mainly on underwater photography and video but will also look briefly at imaging generated by sonar and underwater laser scanning.! ! Common challenges of the underwater environment for divers, ROVs and AUVs include unpredictable currents, turbid water, unclear water, interruption from marine-life, difficulty of positioning within proximity of the target and low light at depth.! ! ! OPTICAL SYSTEMS! ! Photographic and video subsea optical systems typically consist of an underwater camera and an illumination system. Underwater cameras are housed within water-tight enclosures with a depth-rated lens (Bonin et al. 2011).! ! Recent advances in optics have seen; higher quality cameras become more affordable, processing software become faster and less expensive, the development of digital holographic cameras used to compose 3D video, the development of digital signal processors capable of executing algorithms for real-time applications, increased accuracy of oceanic parameter simulations and models, the development of image processing algorithms capable of processing multi-source data and matching feature points from each source, and digital data compression that reduces storage requirements and improves data transfer rates (Caimi et al. 2008).! ! 0806514|1 8A&R - Hydrographic Survey
- 2. According to Bonin et al. (2011) the biggest challenge of acquiring subsea imagery is the refraction, scattering and absorption of light in water (Bonin et al. 2011). Firstly, refraction of light between camera and target is an issue as light must bend through water to glass and then glass to air due to the camera housing. This distorts the apparent size, shape and position of an object and must be accounted for to obtain accurate imagery (Bonin et al. 2011). Secondly, scattering of light impacts the range at which an object can be observed underwater, the larger the distance between the camera and the object the more the light is attenuated (Bonin et al. 2011). Figure 1.1 below highlights the effect that scattering has on a beam of light in water.! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !! Lens defects and misalignments can also affect image quality but can be minimised through instrument calibration.! ! Backscatter and forward scatter occurs from the presence of particles or organisms within the water column between the light source and the target object (Bonin et al. 2011). Examples of light refraction, backscatter and forward scatter are shown in figure 1.2 below.! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! 0806514|2 8A&R - Hydrographic Survey Image Source: Bonin, Burguera & Oliver (2011) Figure 1.2: Example of backscatter, forward scatter and light refraction underwater Image Source: Stephan, T. (2011) Figure 1.1: Red light scattered within water
- 3. Thirdly, the natural absorption of light in water reduces light intensity which can lead to the capture of blurry imagery.! ! In essence, sub-surface structural monitoring requires image capture systems that overcome these challenges and produce clearer, high definition imagery with increased contrast and field of view to improve ease of anomaly identification. Advances in the field are now seeing the emergence of systems that can also quantify deformation by comparing time-series data. A system development trend is emerging that focusses on improved accuracy and range with real-time data collection and analysis.! ! Optics are commonly fitted onto an ROV or AUV combined with an illumination system that is fitted as far apart from the camera as possible to increase the image contrast (Bonin et al. 2011). Varying forms of illumination systems are available including; Halogen bulbs, High Intensity Discharge (HID), Hydrargyrum Medium-Arc Iodide (HMI) lamps, High Intensity Fluorescent (HIF) lamps, Light Emitting Diodes (LEDs), and Infrared or Laser (Bonin et al. 2011).! ! Optics are a navigational requirement for ROVs but they are less commonly mounted onto AUVs as they are programmed to navigate a user-defined route. However, they are now being included for inspection purposes. For example, subsea pipeline and rig jacket inspections etc. Companies such as Kongsberg (2015) are also providing deployable underwater CCTV solutions that offer continuous subsea monitoring. Such systems are attached to blow out preventers (BOPs), anchors, risers and well heads on rigs and drill ships to monitor these subsea components and the operations they carry out (Kongsberg, 2015). Kongsberg (2015) note that the monitoring system attached to the riser can run down the pipe with a trailing umbilical to monitor BOP landings and inspect seals but also inspect the riser pipe and joints for structural health assessment (Kongsberg, 2015). The anchor camera system can be used to identify the anchor orientation and eliminates the need to ballast which saves rig time (Kongsberg, 2015).! ! The ability to hover an underwater vehicle is a problem that has recently been addressed to allow more stable positioning that aids image capture, for example the hover capabilities delivered by Subsea 7’s autonomous inspection vehicle (AIV) (Subsea 7, 2012). Recent technological advances have also seen shallow waters become more accessible to ROVs, allowing the capture of near-shore imagery. For example, the development of the CR200 underwater walking robot. Jun et al. (2011) explain that the hydrodynamic body shape and pressure sensitive feet allow the robot to align itself with the tidal force for maximum stability (Jun et al. 2011). This reduces bubbles in the water, allowing the capture of clearer imagery. This may be useful for assessing the structural integrity of waterfront facilities such as piers. It may also be effective for dam inspections, eliminating the need for diver inspections that require a halt in operations for health and safety purposes. The CR200 toolset includes optical cameras for clear water applications and a high resolution scanning sonar that can be utilised in unclear or turbid waters. The CR200 is shown overleaf in figure 1.3. As annotated, the front-facing camera possesses zoom, tilt and pan functionality that are also commonly available with other imaging systems.! ! ! ! ! ! ! 0806514|3 8A&R - Hydrographic Survey
- 4. ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! One of the advantages of scanning sonar is that it has a full 360o view so offers situational awareness. This helps in ROV navigation but can also highlight potential hazards that surround the structure and may damage it in the future. This allows the hazard to be identified and arrangements made for monitoring or removal.! ! ! SONAR! ! The use of sonar in monitoring sub-surface structures is prevalent as sonar can propagate through water much more efficiently than light. Side-scan, single beam and multi-beam sonar are frequently used in offshore industries for structural inspections as data can be obtained in real-time from a system attached to a vessel or underwater vehicle. Significant strides in system development include tailoring vessel transducer installations and integrating the system with positioning and inertial measurement systems to obtain accurate georeferenced imagery. Recent advances have seen high definition sonar carried on underwater vehicles for closer inspection, observable in figure 1.3 above, that are often more effective than optics in unclear water conditions. MiniROVs are also being marketed that use sonar and allow significant ROV rotation. For example, the range offered by Seabotix. Seabotix note that although 2D information is being obtained by the miniROV system, free rotation allows the pilot to generate a 3D perception of the object (Seabotix, 2015). Seabotix also explain that the improved image quality produced by their Explorer 3000 sonar system is due to increasing the physical transducer count by 30% and using a new lens prescription in order to produce a higher resolution from each transducer (Seabotix, 2015). Technological advances are enabling higher quality real-time image capture from mobile equipment that is progressing towards video-like performance from sonar.! ! ! 0806514|4 8A&R - Hydrographic Survey Image Source: spectrum.ieee.org Figure 1.3: The CR200 ‘Crabster’ Walking Robot
- 5. LASER SCANNING! ! The offshore oil and gas industry employs laser scanning for much of its dimensional control but is now adopting it for close-range underwater inspection. The rate of data collection and high precision of the method is often unrivalled. However, it can suffer from refraction, scattering and absorption underwater. This can result in the capture of noisy imagery, so is often carried out using ROVs and AUVs at close range. In 2011, Lockheed Martin trialled a 3D sonar solution on it’s Marlin AUV and are now developing a similar 3D laser scanning solution with underwater laser developers 3D at Depth (Reeves et al. 2014). Close-in inspection with High Definition Scanning (HDS) will enable 3D modelling that can be analysed for change detection and quantify such changes. Despite promising results of underwater laser scanning O’Byrne et al. (2014) note that concerns still exist over the portability, power consumption and data-transmission deficiencies (O’Byrne et al. 2014). However, it appears that it will only be a matter of time before these issues are resolved and the technology is more widely adopted. Figure 1.4 below shows the level of detail that is currently achievable from an underwater laser scan. If imagery is taken simultaneously then a colour map can be applied to the scan to display a truer appearance of the object that may aid the identification of anomalies.! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! CONCLUSION! ! In summary, the acquisition of imagery for structural integrity monitoring is much more challenging than simply mounting a camera within a water-tight housing and placing it at depth. An appropriate illumination system is often needed and the effects of light refraction, scattering, and absorption in water must be accounted for to produce useful clear imagery that can be used to inspect a sub-surface structure effectively.! ! Sub surface imaging systems have applications offshore in the renewable energy and oil and gas industries including; wind farm foundation inspection to ensure wind turbine stability, attachment to BOPs to ensure structural integrity, riser mounting for pipe inspection and BOP landings, anchor monitoring and wellhead attachment. Imaging systems are most common on ROVs and AUVs for pipeline, rig and sea-bed production structure inspections.! ! 0806514|5 8A&R - Hydrographic Survey Image Source: http://www.2grobotics.com/ Figure 1.4: Well Inspection - Underwater Laser Scan Point Cloud
- 6. Advances in sub-surface imaging have seen higher quality camera equipment become faster, more compact, efficient and affordable. Processing software has become cheaper and faster with increased functionality. Image processing algorithms are becoming more sophisticated. Camera developments now allow 3D video to be obtained and signal processor developments enable real-time applications. Also, more efficient digital data compression methods have reduced storage requirements and increased data transfer rates, resulting in reduced processing times.! ! These trends to improve sub-surface imaging look set to continue in the future to enhance image quality, range and cost effectiveness. It also appears that other image capture techniques such as sonar and laser scanning will become more widely adopted, with technological advances allowing them to rival or even replace optical image capture in some areas.! ! 0806514|6 8A&R - Hydrographic Survey
- 7. REFERENCES! ! Bonin, F., Burguera, A., Oliver, G. (2011) Imaging Systems For Advanced Underwater Vehicles. Journal of Maritime Research. [Online] Spanish Society of Maritime Research 8 (1) p. 65-86. Available from: http://www.jmr.unican.es/index.php/jmr/article/view/ 146/143 [Last Accessed: 20th March 2015].! ! Caimi, F.M., Kocak, D.M., Dalgleish, F., Watson, J. (2008) Underwater Imaging And Optics Advances. Oceans 2008. [Online] IEEE Xplore Digital Library. Available from: http:// www.ieeeoes.org/pubs/newsletters/oes/html/spring10/UnderwaterImaging.pdf [Last Accessed: 20th March 2015].! ! Jun, B.H., Shim, H., Kim, B., Park, J.Y., Baek, H., Lee, P.M., Kim, W.J., Park, Y.S. (2011) ! Preliminary design of the multi-legged underwater walking robot CR200. Oceans 2012. [Online] IEEE Xplore Digital Library. Available from: http://ieeexplore.ieee.org/xpl/ login.jsp?tp=&arnumber=6263600&tag=1&url=http%3A%2F%2Fieeexplore.ieee.org %2Fstamp%2Fstamp.jsp%3Ftp%3D%26arnumber%3D6263600%26tag%3D1 [Last Accessed: 20th March 2015].! ! Kongsberg. (2015) Harsh Environment CCTV Systems. [Online] Available from: http:// w w w . k m . k o n g s b e r g . c o m / k s / w e b / n o k b g 0 2 4 0 . n s f / A l l W e b / AD2EEEDE75BA7D6AC125776C003954D1?OpenDocument [Last Accessed: 30th March 2015].! ! Kongsberg. (2015) Offshore Drill Support CCTV Systems. [Online] Available from: http:// w w w . k m . k o n g s b e r g . c o m / k s / w e b / n o k b g 0 3 9 7 . n s f / A l l W e b / A 8 C 8 6 C F 0 1 4 B F 6 1 E D C 1 2 5 7 5 6 8 0 0 3 C 1 D A 0 / $ fi l e / Kongsberg_Offshore_Drill_Support_CCTV_Systems_A4.pdf?OpenElement [Last Accessed: 30th March 2015].! ! McLeod, D., Jacobsen, J., Hardy, M., Embry, C. (2013) Autonomous Inspection Using An Underwater 3D LiDAR. [Online] IEEE Xplore Digital Library. Available from: ftp:// dns.soest.hawaii.edu/bhowe/outgoing/IEEEOES_2013/papers/130503-107.pdf [Last Accessed: 20th March 2015].! ! O’Byrne, M., Pakrashi, V., Schoefs, F., Ghosh, B. (2014) A Comparison Of Image Based 3D Recovery Methods For Underwater Inspections. EWSHM - 7th European Workshop on Structural Health Monitoring. [Online] Available from: https://hal.inria.fr/hal-01020414/ document [Last Accessed: 20th March 2015].! ! Reeves, T., Mcleod, D., Embry, C., Nickerson, B. (2014) AUV-Based 3D Laser Inspection for Structural Integrity Management in Deepwater Fields. Offshore Technology Conference. [Online] Available from: https://www.onepetro.org/presentation/ OTC-25381-PT [Last Accessed: 18th March 2015].! ! Seabotix. (2015) ROV Based HD Sonar Enables Realtime 3D Perception. http:// www.seabotix.com/about_seabotix/news/news12/aris.htm?story_link=email_msg [Last Accessed: 18th March 2015].! ! Stephan, T. (2011) Principles Of Underwater Vision. Proceedings of the 2011 Joint Workshop of Fraunhofer IOSB and Institute for Anthropomatics, Vision and Fusion 0806514|7 8A&R - Hydrographic Survey
- 8. Laboratory. [Online] p. 63-80. Available from: https://books.google.co.uk/books? hl=en&lr=&id=KmGOCgBGVjMC&oi=fnd&pg=PA63&dq=offshore+underwater +imaging&ots=gvBuMRZQl2&sig=dNFMrQAaZ2sNxSBADV4x3Sq95v0#v=onepage& q&f=false [Last Accessed: 18th March 2015].! ! Subsea 7. (2012) Autonomous Inspection Vehicle (AIV). [Online] Available from: http:// www.subsea7.com/content/dam/subsea7/documents/technologyandassets/ LOF_AIV.pdf [Last Accessed: 18th March 2015].! ! ! ! FURTHER READING! ! ! ROV Tech Systems. (2015) Underwater and Nuclear Harsh Environment Remote Inspection and Intervention Systems. [Online] Available from: http:// www.rovtechsystems.co.uk/ [Last Accessed: 30th March 2015].! ! Smart Light Devices. (2015) Subsea Miniature Camera. [Online] Available from: http:// www.smartlightdevices.co.uk/files/9313/7207/0834/SLD_MC1.pdf [Last Accessed: 30th March 2015].! ! Subsea World News. (2015) Bowtech Supplies Miniature Cameras for Isotek. [Online] Available from:http://subseaworldnews.com/2014/12/09/bowtech-supplies-miniature- cameras-for-isotek/ [Last Accessed: 30th March 2015].! ! Subsea World News. (2015) Kongsberg Builds Subsea Camera for Pipeline Repair Monitoring. [Online] Available from: http://subseaworldnews.com/2015/03/05/ kongsberg-builds-subsea-camera-for-pipeline-repair-monitoring/ [Last Accessed: 30th March 2015].! ! Subsea World News. (2015) Kongsberg Underwater Cameras in High Demand. [Online] Available from: http://subseaworldnews.com/2015/01/21/kongsberg-underwater- cameras-in-high-demand/ [Last Accessed: 30th March 2015].! ! Teledyne Bowtech. (2015) Teledyne Bowtech Cameras. [Online] Available from: http:// www.bowtech.co.uk/products-bowtech.php?range=Cameras&pcid=1&category [Last Accessed: 30th March 2015]. 0806514|8 8A&R - Hydrographic Survey