Wave motions from vessels moored in ports open to the sea impact the operability of loading operations, and should be assessed as part of any port (re)development plan. When analyzing vessels moored by a quay-side, time domain simulations may show numeric instabilities resulting in unreliable outcomes. The origin of the numerical instability might lie in the hydrodynamic added mass and wave radiation damping typically calculated using potential flow methods. For certain frequencies, these tend to give negative values. This negative added mass is a known phenomenon in the industry. Combined with negative damping, it is believed to cause instability in non-linear (coupled) time domain simulations. In these cases, the vessel seems to generate energy rather than dissipate it. As such, the simulations are unlikely to realistically represent real-life scenarios. This paper describes an engineering method to mitigate numerical instability and derive a working solution to address the operability of loading a vessel subject to wave loading at a quay-side. The work covers a literature study into negative added mass and damping caused by shallow water and vicinity of objects such as a vertical wall. This is followed by an engineering study using 3D diffraction and time domain software.

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