Sea Technology

SEP 2017

The industry's recognized authority for design, engineering and application of equipment and services in the global ocean community

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30 st / September 2017 www.sea-technology.com result is that ROV/AUV/UAV pilots and divers can conduct such surveys after short training, without any need for ad- ditional personnel or equipment. Data Processing Once the videos are captured, the files can then be sent for data processing. The photogrammetry process includes four main steps: preparation, digitalization, bundling, and 3D measurement or modeling. Preparation consists of extracting relevant images from a video. This can be done automatically (extraction at a certain rate) or manually, depending on the complexity of the survey and the navigation conditions. For digitaliza- tion, common points are identified and measured on all images. This can be done automatically thanks to an image- processing algorithm, yet very often still requires human in- tervention, especially for underwater images. Once points and geometrical features (such as circles or cylinders, for example) are digitalized on all images, the 3D computa- tion is launched. The outputs are the 3D coordinates of all the points and geometrical features, as well as the location and orientation of all camera positions. Depending on the output requirements, it is possible to either produce high- accuracy measurements of distances and angles or a full 3D CAD model. VLS for Anomalies in 3D Mapping When further modifications to mapping are required, creating a 3D as-built CAD model is necessary. Doing this provides the existing geometry and dimensions and allows for the design of new components that will fit in perfectly. It's also an essential tool for ROV/AUV/UAV navigation sim- ulations and can help prepare divers for their subsea opera- tions. But when looking for an anomaly such as a crack or a dent, photogrammetry can only provide a few points and general dimensions (length, width, depth). It's almost impos- sible to measure many points on surfaces simply because the correlation of points between images is impossible; there are no details or texture. This is why DimEye invented the VLS (Video Laser Scan). The idea is to rigidly mount a laser plane projector device with a camera at a certain distance and with a certain angle. Relative positions and orientations of the camera and laser are determined by calibration. Positions and orientations of the camera in space are computed by photogrammetry: On every single image, the laser plane intersects the subsea structure and generates a line that can be triangulated, giv- ing potentially infinite points. When shooting a video from an ROV at slow speed, about 25 images per second are ac- quired, which allows the selection of the required density of laser lines on the object and, therefore, the final density of the point cloud. In terms of operations, adding a laser plane projector is easy: The camera and the laser can be mounted on a small bracket at the end of an ROV arm, or they can be handheld by a diver. Case Studies Spool Metrology. Measuring the relative positions and orientations in space of two objects is a classic application for photogrammetry. Spool metrology involves measuring Unmanned Vehicles and Divers Because photogrammetry doesn't require stability, it's very easy to conduct any survey directly from an ROV. The camera can be mounted either on the pan-and-tilt unit that is usually installed on the skid or on a small bracket at the end of one arm. In the past, shooting still images was quite time consuming and somewhat more complicated than shooting video. When shooting video, the ROV pilot simply needs to fly around or above the region of interest (ROI). There's a lot of flexibility for flight paths, with only a few basic rules to follow, such as being at a certain standoff dis- tance (mainly depending on camera field of view and vis- ibility), never changing the zoom position and being sure to get enough overlap between runs. The photogrammetry rules are very easy to follow for an ROV pilot because they are very similar to what is required for a visual inspection. The rules are exactly the same for divers, AUV and UAV (unmanned aerial vehicle) pilots. The (From top to bottom) Sample pipe-dent 3D CAD model. Sam- ple 3D as-built CAD model (manifold). Sample mooring chain VLS survey and 3D analysis.

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