Stereo Image System for Inspection of Mooring Chain in the Splash Zone

Abstract

Abstract This paper outlines an optical stereo photogrammetric imaging system, called the Lantern Eye Air (LEA), for inspecting the mooring chain above the water in the area known as the splash zone. The system is mounted and deployed by a Rope Access Technician (RAT), and achieves data collection in this difficult to access region for subsequent 3D reconstruction and measurements for inspection. Typically, splash zone mooring chain inspections are carried out using mechanical calipers which are difficult to deploy, provide measurements at discrete locations only and are dependent on the proficiency and judgement of the operator on the field. The LEA system overcomes these problems, providing objective, accurate and repeatable measurements based on reconstructed photorealistic 3D models. Imagery of the chain links is captured systematically which was used to generate 3D reconstructions and establish measurements of bar diameter and inter-grip length. A splash zone inspection was undertaken on an offshore platform. The 3D metrology bar diameter measurements were within 1.7% of caliper measurements based on 2σ (95% confidence). Similarly, the 3D metrology inter-grip length measurements were within 0.9% of caliper measurements based on 2σ (95% confidence). Application of a systematic validation procedure following the inspection placed the expected uncertainty of measurements at the 1% range. Similarly, the uncertainty in caliper measurements has been estimated to be 2% based on the use of a standard handheld caliper and measurement procedure laid out in the ABS Guide for the Certification of Offshore Mooring Chain and the 2020 Life Extension Mooring Platform Chain Inspection. Considering these uncertainty bounds, the 3D metrology measurements can also be deemed consistent with the ground truth dimensions. Based on the results, the LEA and 3D metrology measurements are consistent with current techniques and provide a more systematic and repeatable basis for mooring chain inspection and measurement in the splash zone. This is anticipated to improve the consistency and comparability of inspections and will enable precise change tracking between inspections.