High Accuracy Underwater Visual Measurement

Abstract

Abstract Inspection of underwater assets on oil and gas offshore platforms are required to assess asset integrity, and whether intervention is required. These assets include mooring chains, fairleads, anodes and ballast tanks. Typically, calipers are used for taking measurements of these objects. Limitations of this technique include inaccuracy, inefficiency and potential damage to the asset. Compact underwater camera systems, including imaging sensors and lighting, are proposed to solve limitations of caliper measurements. Data processing creates scaled 3D models for measurement analysis and automatic fault detection. This paper presents Lantern Eye-S, a stereo-based visual measurement system developed by the authors. It is a stereo imaging system, which includes a stereo camera pair linked to a set of underwater strobes. It is extremely compact and can be deployed on very small ROVs, down to the 30kg class. Results are available as quickly as 5 minutes for in-situ analysis and assessment. Final results are available in 8 hours for detailed inspection. Calibration accounts for the water environment the images are taken, primarily to account for salinity in the water. Measurements can be automatically extracted, and comparisons are made to as-built drawings. Validation of the measurement is also undertaken with a known ground truth object placed in the water, to ensure integrity in the measurement. Results include offshore integration and deployment on a mini-ROV (30kg class) and working class ROV (multiple tons). An important result from this work is that calipers were found to typically overestimate dimensions on mooring chain diameters. High accuracy measurements of small objects, with a compact camera system, was shown to be possible with the work in this paper. The use of a novel validation apparatus and uncertainty analysis allows appropriate levels of confidence to be attributed to the acquired analytics.