Detailed knowledge of motion and seakeeping behaviour in an early design stage is indispensable in modern layout of marine offshore structures. Therefore, numerical methods are used to calculate the Response Amplitude Operators (RAO), which are generally based on potential theory or the Reynolds-Averaged-Navier-Stokes-Equation (RANSE).

Calculations with potential-codes are commonly used, well established and time-saving. Main disadvantages are the neglect of viscous effects and the hull structure above the still water level. By using RANSE-methods, these nonlinear effects can be investigated in detail, but at the price of calculation time and extensive grid generation. To achieve sufficient RAOs in frequency domain, time-consuming and intensive calculations would be necessary with these CFD-methods, using seastate applications with regular or irregular waves only. Therefore, these methods are not convenient for standard motion analysis by now.

Transient Wave Packets (TWP) represent an approved method at model tests, revealing the entire RAO for any offshore structure within one single, short test run. Main advantage of this technique is the accurate predictability and short superposition in space and time. Containing all elementary wavelengths of the generated initial wave spectra, the TWP-method could be used in RANSE-methods, implementing all necessary initial conditions to the CFD-solver. To reduce the calculation effort to a minimum in space and time, the superimposed wave train is generated near the investigated offshore structure by using modified, linear wave theory in spatial domain.

To present this method by means of a practical example, the motion and sloshing behaviour of an offshore LNG-carrier (LNGC) are investigated in detail. For validation purpose, all results are compared to model tests, conducted in the seakeeping basin at Technische Universität Berlin (TUB), as well as numerical results of the potential theory solver WAMIT.

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