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They say there’s something about the mountain air in Colorado. Well, there must be something about the water, too. The state of Colorado has birthed a remarkable number of LiDAR companies and organizations focused on addressing challenging remote sensing problems. In the 1970’s, the National Oceanic and Atmospheric Administration’s Environmental Technology Laboratory initiated their legacy of developing and deploying research-caliber, directdetection and coherent-detection LiDAR systems for wind sensing and constituent mapping.
Then, in the mid-1980’s, Coherent Technologies, Inc. (now part of Lockheed Martin) appeared and has focused on heterodyne detection LiDAR products for both atmospheric sensing and hard target imaging. In parallel, companies such as Ophir Corporation, Ball Aerospace, and Aret Associates (Longmont office) have produced their share of LiDAR development systems and products. Shrinking the standoff distance, Boulder’s Zolo Technologies, Inc. and SPEC Inc. focus on short-range (quasi in situ) measurements to characterize constituents in industrial and atmospheric research applications. And this is not a complete listing, especially when factoring Colorado-based companies that develop and supply key enabling technologies and components for LiDAR sensors.
It should come as no surprise, then, that yet another Colorado-based LiDAR company is emerging. However, given that the state is located more than 500 miles from the nearest ocean, eyebrows may rise when it is revealed that this new addition to Colorado’s LiDAR ecosystem is developing LiDAR for underwater imaging applications.
Boulder’s 3D at Depth initiated its first significant development through a Research Partnership to Secure Energy for America (RPSEA) contract in January of 2011. Since then, the company has made incredibly fast progress toward the first product release of its precision imaging underwater LiDAR, which is planned for the first half of 2013. A nighttime picture of the LiDAR operating in a test pool is shown in Figure 1. 3D at Depth is focused on several offshore oil and gas applications. In these cases, their sensor and software provide the functional equivalent of the types of jobs traditional terrestrial LiDAR scanners are performing `topside’ on platforms.
Specific application areas include asset mapping and inspection in support of subsea construction. As well, associated as-built verification activities are required for operations, maintenance and sustainment. Such metrological measurements are becoming increasingly critical for subsea tieback applications that have become the predominant deep water production strategy in the 21st century. Subsea tiebacks connect new fields to existing production infrastructure and rely on accurate dimensional measurements to support the fabrication of components which "tie" assets together. 3D at Depth has developed specific operational workflows designed to greatly reduce the time to perform the required surveying in support of subsea tieback operations at depths of as much as a few thousand meters. Additional oil and gas applications will include pipeline inspection from a moving platform and life of field asset management and risk mitigation. Complementary freshwater applications are also envisioned to include bridge and dam construction and verification.
Because scanning underwater LiDAR can make these measurements more quickly and precisely than most other technologies, operational efficiency is improved, further contributing to better bottom-line performance. In addition, whether before or after a construction project is completed, the sensor can support damage inspection resulting from severe weather and other events.
3D at Depth’s patented technology is called Depth Perception and the first product from the company is called the DP1, shown in Figure 2. The DP1 will support underwater survey applications mounted on a tripod and tethered to a Remotely Operated Vehicle (ROV) for power and communications. This integration with the industry’s leading Schilling ROV is shown in the opening photo, where Carl Embry, Chief Technologist, is joined by Partners Brett Nickerson and Mark Hardy. "We feel our initial product design is a good balance between performance and affordability," Embry said. 3D at Depth’s demonstrator unit measures roughly 8 inches in diameter and 29 inches in length. The product release planned for the first half of 2013 shrinks the long dimension by 17% and boosts the measurement rate significantly.
The LiDAR is a flying spot, time-of-flight (TOF) underwater sensor with a capability to accurately map structures to maximum distances of roughly 20 meters and accuracies of a couple millimeters. The maximum distance depends on water clarity. The system has a field of view of a few tens of degrees with a larger field of regard afforded when mounted on a rotation stage. With its significantly greater maximum range and direct TOF measurement capability, area coverage is substantially larger and faster than that afforded by very short range (<3 m), precision laser scanning systems that rely on triangulation to estimate distance (see, for example, 2G Robotic’s offerings in this area). This being said, within their limited operating range, the triangulating laser scanners can provide range accuracies well below 1 mm.
3D at Depth’s LiDAR may be most appropriately compared with more established sonar imaging technologies. Sonar relies on the propagation of acoustic waves and so has the advantage of being far less impacted by turbidity and various constituents in the water, which can dramatically impact the maximum range capability of a laser-based sensor. On the other hand, even with state-of-the-art imaging sonar products such as those offered by BlueView, their achievable spatial resolutions tend to bottom out at the centimeter level. So, it seems that 3D at Depth has found itself a nice performance niche, which they hope proves to be the sweet spot for demanding underwater construction and inspection applications in the oil and gas industry.
Unique Challenges of the Underwater Environment
Of course, underwater operation poses some real challenges for LiDAR. First, water, rather than the atmosphere, is the medium through which the laser must propagate. As is often the case, laser illumination at blue-green wavelengths is preferred. 3D at Depth’s baseline system operates at 532 nm. The amount of attenuation can be significant, ranging from ~0.2 dB/m to 1 dB/m, depending on the water clarity. This means a 20 meter round trip path will experience 9 dB to 40 dB of attenuation. Sufficient performance margin must be built into the design to accommodate this level of signal loss.
A second major challenge is the marine environment. It is extremely corrosive, and so great care must be taken in the design of the optical window especially. A third challenge ties to the extreme pressures experienced at depths of 3000 m, which is the nominal maximum depth quoted for 3D at Depth’s sensors. At these depths, the pressure reaches more than 4400 pounds per square inch. While the main housing is based on an established design, end-cap window enclosure design and manufacturing procedures had to be carefully developed to meet the maximum depth requirement. This required tight coupling of mechanical finite element analysis (FEA) to optical performance simulations in order to achieve a design that maintains the measurement accuracies at these depths.
A less obvious challenge was to develop calibration targets appropriate for deep sea applications including subsea tieback. "Normal range registration targets in use for precision topside measurement would be crushed at these depths. So, we developed special-purpose targets that hold up well at depth while having the desired reflectivity characteristics to support large scene mapping," 3D at Depth’s Emby said.
Freshwater Performance Validation
3D at Depth completed successful performance validation tests the first half of 2012. These entailed integrating one of their units on a Schilling ultra-heavy duty (UHD) work-class ROV and performing precision imaging of test targets in the Schilling ROV test tank in Davis, California. In these tests, a yellow "hot stab panel" target was used and is shown in the upper left portion of Figure 3. The panel measures roughly 2.1 m wide between the two outer support legs and ~0.5 m tall on its right hand side (roughly twice that for the taller left-hand section). LiDAR scans were completed and compared with reference measurements to validate the sensor’s performance. Accuracies better than 3 millimeters were achieved at 8 m distances.
The two range-coded images on the right half of Figure 3 illustrate the high resolution achieved in all three dimensions. The upper right image is a top view and shows the test target in front of the curved back wall of the test pool. The lower right image is the head-on view of the test target with the more distant pool wall. Both LiDAR images are false-color images to depict the longitudinal range information within the point cloud data. Blue pixels are at shorter ranges and red pixels are at more distant ranges.
Figure 4 is a zoomed-in LiDAR scan image of the test target. Because of the shadow cast by the ambient light, the passive video image shown in the upper right can provide some qualitative insight into the dimension of the two fins above the "PIGE" lettering. LiDAR data focused on a rectangular region within the blue circle clearly and quantifiably renders the two fins, which are 20.5 cm apart and 17.8 cm deep. Point cloud data is color-coded and oriented in three different ways in the lower half of Figure 4. The height of the fins and the distance between the fins was verified to within 2 mm. The `PIGE’ label, which has an 8 mm depth, is clearly resolved.
Next Steps
The 2012 verification testing while integrated with the ROV system was a major hurdle for 3D at Depth. In parallel, the team has been working diligently on the product version of the LiDAR scanner. Integration with industry-standard point cloud processing software has been completed, and the team is looking forward to supporting revenue-generating metrology services at actual deep sea construction sites in the latter half of 2013. "The past two years have been hectic and action packed," Embry said. "With the successful tests of 2012, we are excited about our prospects for initial commercial deployment later this year." Time will have to tell, but it seems another Colorado LiDAR star may be on the rise.
More information about 3D at Depth can be found at 3DatDepth.com.
Dr. Stephen Hannon is the principal and founder of Mind the Gap, LLC (MindtheGapLLC.com) where he advises startups and small companies across a range of business and technical issues. Steve was previously with Lockheed Martin Corporation, Coherent Technologies, Inc., and SRI International. He is best known for his founding role establishing the WindTracer Doppler LiDAR product, in use at airports and other facilities around the world. Steve received his Electrical Engineering education at MIT and the University of Illinois. His blog can be found at MindtheGapLLC.com/steves-blog.html.
A 956Kb PDF of this article as it appeared in the magazine complete with images is available by clicking HERE