Towards real-time underwater photogrammetry for subsea metrology applications

High accuracy underwater inspections are getting more and more important in the underwater industry where time and cost optimization represent nowadays the main innovation drivers. The subsea industry is undergoing a digital transformation process and for this reason, methods that can provide real-time accurate 3D digital measurements are increasingly demanded. This paper provides a short review of the main techniques currently used in subsea metrology to then present an experimental study carried out to evaluate the accuracy potential of three vision-based techniques well-known in photogrammetry, namely visual odometry with and without windowed bundle adjustment, and keyframe based simultaneous localization and mapping (SLAM). The accuracy evaluation is done using an ORUS 3D® subsea photogrammetry system using a certified 3D underwater reference test-field available at COMEX facilities, whose spatial coordinates are known with sub-millimetre accuracy. A critical assessment of results is presented against currently set tolerances for the subsea metrology industry.