There has been an increasing use of unmanned aerial vehicles (UAVs), or drones, for tank inspections on floating offshore installations over the last five years – and, with seemingly constant advances in the technology, it can be difficult to understand the current capabilities and true potential, writes Tim Walsh, FRINA.
Inspection requirements for floating offshore installations are largely defined by the class societies to the standards laid down by IACS. The main defects are cracking, coating breakdown, corrosion and structural deformation. Therefore, inspection schemes are designed to find and quantify these defects before they become critical.
The basic tank inspection scope for floating installations comprises general visual inspection (GVI), close visual inspection (CVI) and ultrasonic thickness measurements (UTMs) of structural elements. Other non-destructive testing (NDT) techniques can also be used to detect and quantify defects such as cracking in known hotspots.
Tank inspections were traditionally carried out by class society surveyors or owners’ inspectors. Access was always difficult: cargo oil tanks on FPSOs can be 15-25m in height. Over the past few decades, remote inspection techniques (RITs) have been developed as an alternative means of providing inspection data to the surveyor. Initially, this took the form of rope access inspections, sometimes with helmet-mounted cameras, and, more recently, the use of UAVs. The use of all RITs is governed by class society/IACS rules and the required performance standard is clearly defined: the RIT deployed should provide the equivalent standard of survey results “normally obtained by the surveyor”.
To achieve this standard, the task facing all RITs, and UAVs in particular, is twofold: firstly, they must be able to operate effectively in the tank environment; and secondly, they must be able to gather, store and transmit the required inspection data to an equivalent standard to a conventional class survey.
To operate in a tank environment the UAV (and its pilot) must be able to navigate its way around the structure, in a GPS-denied environment, and most of the time out of the direct line of sight of the pilot, known as ‘beyond visual line of sight’ (BVLOS). The UAV must be able to locate the areas for inspection; operate in low and very low ambient light; have a flight duration which is practical; be able to access tight spaces; be able to withstand contact with the vessel structure without damage and without fire risk; and be recoverable, in case of its inability to get ‘back to base’.
It must also be able to perform a full unmanned tank inspection scope (including CVI, structural deformation surveys, coating assessments and thickness measurements) and deal with any local cleaning requirements (such as scale removal). All inspection data must be securely recorded, tagged and stored to the ‘surveyor equivalent’ standard.
These requirements have been around for some time and underpin the traditional approach to integrity assurance and classification. With the continued progress of digital technology, operators and classification societies are developing new approaches based on digital twins and risk-based inspection, so future inspection and monitoring techniques will need to provide more and better data to support this.
(For the full article, see Offshore Marine Technology 1st Quarter 2021)