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Mid segment of the 120m long barrier with speckle pattern on the skirt used for the digital image correlation

Example of a typical deformation of the skirt during wave tests, as determined using DIC

its complex, fluid-structure interaction, or in other words, the interaction between hydrodynamic loading and the response of the flexible structure. As a result, it was important to correctly model the structural properties of the system, in addition to the geometry and metocean parameters. The bending stiffness was the most difficult property to realise, as it scales to the fifth power, therefore the floater core was built up from PVC, in combination with foam elements to meet the structural requirements, while the screen was manufactured from rubber. The resulting scale of the model is 1/10, meaning that the wave makers of the MARIN Offshore Basin were pushed to their limits to test the extreme conditions required.

Digital Image Correlation The measurement of motions and loads on the flexible material of the screen was also very challenging as most standard measurement techniques fail due to their intrusiveness. Therefore, MARIN used a measurement technique called Digital Image Correlation (DIC), which is a novel technique for this type of application. DIC is a full-field image analysis method that uses the image data collected by a stereo camera system to measure displacements and deformations of objects in the three-dimensional space. This optical technique can be used in multiple applications above and below water. The surface of the measured object must present a random speckle pattern which – if not naturally present – can be printed, painted or projected.

Setting up a custom, underwater DIC measurement system able to cope with all the practical challenges of such a complex model test campaign is not trivial. The system included a powerful, underwater illumination system and two, machine vision cameras enclosed in underwater housings with remotely controlled optics to ensure the optimal image settings. One of the main challenges was to design an optical system able to maintain good image quality with sufficient illumination in all the possible locations where the model may shift due to waves and current, especially during the tests with extreme sea states. The optical measurements were focused on a portion of the screen of approximately 15 metres wide at full- scale, which was located at the centre of the barrier.

Detailed quantitative data on the screen motions and deformations were delivered and the screen was tested in operational conditions and in a towing configuration. The measurement system behaved well during the model test campaign and delivered high quality data proving Digital Image Correlation as a valuable technique to measure motions and deformations of flexible structures, especially in situations where conventional sensors are not suitable.

The model test campaign helped The Ocean Cleanup to further prove the concept and refine the design of its pioneering, plastic cleaning system, bringing it closer to the launch of the full-scale system in the great Pacific garbage patch!

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