Mapping the ground using lasers from the air (known as LiDAR) has revolutionised the efficiency of on land and shallow water survey projects, both in terms of the speed, accuracy and coverage that can be attained. The advances made here are now coming to a subsea metrology project near you! Using complementary technologies such as Doppler Velocity Logs (DVL), Long BaseLine (LBL) and other complex tools, users can map underwater to a degree of accuracy that just hasn’t been possible until now.
How does it work?
Subsea Elevator used in the trial
There are two ways we can map using laser sensors underwater; static scanning and mobile mapping. Static scanning involves placing a sensor on the seafloor (e.g. on a tripod), and mechanically rotating it to scan the local area. This method is most suited to confined areas as no navigation is required. Scanning of horizontal and elevated features is difficult since the sensor is tied to the seafloor. The drawback to this method however, is that increased turbidity can reduce the effectiveness of the laser sensors and you may not achieve your planned coverage. Attempting to map a wide area is difficult. The process is complex and time consuming and will need multiple data sets merging from a number of locations. Mobile mapping is similar to multi-beam echo sounder surveying, but provides a much greater resolution. Mobile mapping is faster than static scanning and can cover a much wider area. This method is less affected by turbidity as the laser is moved along the optimal path and close to the structure/ object. This is hugely advantageous when measuring hub/ flange orientations for metrology.
Navigating to higher performance
Being able to take full advantage of laser sensor resolution in mobile mapping has always been constrained by the accuracy of navigation. The next generation of Acoustically Aided Inertial Navigation (AAINS) systems, such as our INS integrated with DVL, will unlock the full potential of the laser sensors. The boost in relative dynamic accuracy enables fast, contactless measurement of target orientation to tiny fractions of a degree.
Mobile mapping and contactless metrology
Subsea metrology is the post-installation measurement of relative position and orientation differences between the hubs/ flanges of two or more subsea structures. The results are used for manufacturing connecting pipes, which must be within tight tolerances. Modern LBL acoustics currently provides the best accuracy and quality control and therefore is the benchmark all other processes are measured against. ‘Contactless’ AAINS mobile mapping inherits the fundamental accuracy of LBL acoustics but dispenses with the need for precision ROV handling of equipment on structures, translating to a much faster process.
In practice: Shooting lasers in California
We headed out to Monterey Canyon, California, on board MBARI’s (Monterey Bay Aquarium Research Institute) R/V Western Flyer, in order to put our equipment to the test. The objective: to laser map our submerged Compatt 6 acoustic transponder.
MBARI’s ROV Doc Ricketts was equipped with our SPRINT 700 AAINS, Syrinx DVL, ROVNav 6 and a precision pressure sensor. Laser hardware came in the form of a 2G Robotics ULS-500 and 3D real-time visualisation from Eiva NaviSuite software. The operation was set up for metrology speed, rather than image clarity.
In total, six metrology baselines were mapped during the voyage. Amazingly, all six baselines were mapped by the ROV within a single 1 hour 45 minute time frame. With realistic streamlining for commercial operations, AAINS mobile mapping technology will support single dive, contactless metrology in considerably less time than any other known method. The image below shows the result of a single pass over, at around 20 seconds at a depth of 1,850 meters.
Contactless Metrology Final Scan
Highly time efficient mobile mapping with reliability, accuracy and resolution proven to metrology standards is generically valuable for a host of other subsea survey, inspection and construction applications. We’re excited to see what the future holds for subsea laser mapping, and you can be sure to see our name leading the way.
This Article was first featured in Baseline issue 15, to read the article in full click here.