Metrology at Twenty Thousand Leagues Under the Sea

DOF Subsea, together with subsea technology companies Sonardyne, 2G Robotics, and Seatronics (an Acteon company), successfully demonstrated a new underwater surveying technique recently that could significantly shorten the time needed to map underwater structures and offshore sites.

Representatives from across the North Sea energy sector, including senior figures from oil majors, contracting companies and service providers, attended the demonstrations which were held over three days at The Underwater Centre in Fort William.

The event was organised by DOF Subsea to showcase the capabilities of dynamic mobile mapping. The new technique uses a 3D laser scanner fitted to an ROV to create highly detailed, point cloud images of subsea assets and environments. By combining the 3D laser data with precise underwater acoustic and inertial navigation information, it is now possible to generate centimetre resolution engineering models from which accurate measurements can be instantaneously and repeatably captured.

Within the offshore sector, one of the primary applications for the new technology is underwater metrology, a process that requires accurate, precise and robust measurements. These are critical for successful fabrication and installation of spools and jumpers. Not only does the new method save time and money, it also reduces the risk of spool pieces being fabricated incorrectly.

The new laser-acoustic-inertial metrology solution brings together three independent systems, each of which is proven within its respective field.

The 2G Robotics ULS 500 Pro Laser, provided by exclusive distributor Seatronics, is a laser triangulation measurement system that is ideal for capturing high-detail measurements from dynamic platforms such as an ROV or AUV. Sonardyne’s SPRINT inertial navigation sensor makes optimal use of aiding data from transponders deployed on the seabed and from and other acoustic sensors to provide dynamic, centimetric-level navigation for the ULS 500 laser, which is post-processed using Sonardyne’s Janus software. DOF Subsea’s Metro software is configured to use the Sonardyne SPRINT-positioned ULS 500 to undertake standardised metrology calculations offshore, reducing risk in metrology calculations and ensuring approved algorithms are used to generate client deliverables.

The new metrology solution is ROV-mounted and mapping operations are conducted in Dynamic Mode (i.e. with the ROV flying the spool route) in order to simultaneously survey multiple horizontal or vertical flanges. Key operational benefits of the technique include;

  • Non-Intrusive – no prior subsea intervention is required in advance of metrology, and no subsea bracketry, control spheres or metrology aids are required
  • Flexible Deployment – modular system which can be easily fitted to an ROV (for Dynamic data acquisition), or mounted onto a tripod (for static data acquisition
  • Time to Survey – significant reduction in the time (and therefore vessel cost) needed to gather the survey data in comparison to traditional techniques such as pure acoustic LBL; typically less than three hours for a laser metrology.
  • Time to Data – quick delivery of results offshore; typically within six hours of data acquisition
  • Inferred Metrology – with prior dimensional control information, hub position and orientation can be inferred using laser scan data
  • More than Metrology, the 3D model used for metrology serves as an As-Built model of the subsea installation and the surrounding area to provide a baseline for future intervention work