Mitigating spurious low-frequency waves in a model basin with a ramped floor

Low-frequency (LF) content of the waves present in a finite model basin may have multiple sources and associated consequences. Typically the horizontal natural periods of the moored structures are long and in the same range as the LF range. As such, the response of the moored structures in shallow water might be significantly affected by the spurious LF waves. The first goal here was to investigate whether the available numerical simulation tools can capture the underlying physics regarding the spurious LF waves in a basin with a ramped floor. It was seen that even though the heights of the short waves were underestimated when reflected waves from the beach reached the area of interest, the heights and phases of the LF waves were captured with an acceptable accuracy. The second goal was to investigate the approach of generating a correction signal (otherwise known as ”anti-wave”) in order to mitigate the spurious LF wave content in the basin. The results of the CFD simulations showed that it was possible to reduce the amount of spurious LF waves. Following this outcome, the approach was tested in a model basin at MARIN with a ramped floor (the Offshore Basin - OB). The experiments were carried out with and without an LNG tanker in the OB using an irregular wave at one water depth. The measurements showed that the spurious LF content was mitigated with the correction signal. As a result, a reasonable reduction in the low-frequency surge motion of the ship was observed.