Visual inspections and steel thickness measurements are key elements of renewal surveys. To reduce survey time and cost, DNV GL has developed a new type of drone with ultrasonic technology that can measure steel thickness.
DNV GL has been using camera-equipped drones in surveys since 2016. The concept, the technique and the equipment were developed by a surveyor team in Gdansk, Poland. To also be able to take thickness measurements by drone, in 2018 a flying thickness measurement system with ultrasonic technology was developed. Using a drone for this type of inspection eliminates the need for rafting, roping or staging, thereby saving time and costs. Some inspections can even be done during the voyage when cargo spaces are accessible.
Compact and Universal Drone Design
An attachment frame was engineered in-house by the Gdansk team in an iterative process including repeated practical testing. The new drone design is compact and "universal", allowing surveyors to interchange the close-up inspection camera on the drone for an ultrasonic measurement head for measuring steel thickness at any time, or attach the frame to another drone in case the original one is damaged. This standardisation minimises the amount of equipment surveyors have to carry when travelling to a ship.
DNV GL has tested the new drone technology on a multi-purpose vessel, some bulk carriers and a shuttle tanker.
3D-printed Parts
Key parts of the attachment frame are made by the DNV GL team on a 3D printer. This means any number of identical frames can be manufactured in a repeatable process making it easy to come by spare parts.
Drone Inspection Limitations
Using a drone for thickness measurements does come with some limitations. The coating of the vessel needs to be intact and surfaces have to be clean. It is impossible to take ultrasonic measurements on corroded steel because the measurement head is unable establish direct contact with the steel to get a reading.
DNV GL drone surveyor Leszek Alba points out that 'ultrasonic measurements are suitable to confirm the intact condition of structures. Our drone inspections serve the purpose of verifying the good condition. They are ideal for renewal surveys of younger vessels that are five to ten years old.'
Wherever conditions are detected that warrant further investigation, traditional access using staging is unavoidable. Alba Alba adds 'but when the drone survey shows that everything is in good condition, this expense can be avoided.'
Picture: DNV GL surveyors test a camera-equipped drone to visually evaluate structural components through video streamed to a tablet (by DNV GL).
