Model Tests to Assess Wave and Current Loads on Ocean Cleanup’s Conceptual Plastic Capturing Barrier

Lot of plastics enters the ocean every day with negative effects on the environment, economy and health. A large portion of the floating plastics accumulate in so-called gyres, where currents converge. One of the aims of the Ocean Cleanup is to develop technologies to extract plastic pollution from the oceans. The idea is to install a flexible barrier in the ocean that is supposed to concentrate the plastic at the Great Pacific Garbage Patch. The design of the barrier is still in the conceptual phase and the model tests described in this report are conducted to assist in the development of the barrier. The model tests were carried out in MARIN’s Offshore Basin and served two main goals: 1) provide loads and displacements for numerical model calibrations and 2) examine the 3D fluid-structure interactions and the barrier performance for three different design concepts. A 360m prototype length barrier was considered that consists of a floater with a diameter of 1.5m and a screen with a height of 2m. To model larger lengths of the barrier, various pretensions were applied at the ends of the barrier. For the secondary mooring concept the barrier was moored each 60m via the bottom of the screen to a submerged tension line. For the low mooring and high mooring concepts, the model was only moored at both ends of the bottom of the screen or bottom of the floater. In general mooring loads were found to be the largest for the low mooring configuration. For this same configuration the mooring loads increment with increasing current velocity was the largest. Mooring load fluctuations seemed to be not strongly influenced by the amount of applied pretension. Vertical screen orientations, which are expected to be beneficial for the plastic capturing efficiency, were mostly observed for the secondary mooring and high mooring configurations. For the low mooring concept the offsets along the flow direction were the largest and the screen was more tilted. Additional ballast for the high mooring concept promoted a more vertical orientation of the screen with as downside increasing mooring loads. Overtopping or bridging (air gap exists below the barrier) negatively impact the plastic capturing efficiency and are important to consider. Both events are most likely to happen for shorter wave conditions and higher current velocities. The number of occurrences of overtopping and bridging was the lowest for the secondary mooring and the high mooring setup with a low pretension. For increasing pretension the number of overtopping and bridging events increased.