Estimation of the hydrodynamic loads based on strip theory with the Morrison equation provides a fast and inexpensive method for load estimation for the offshore industry. The advantage of this approach is that it requires only the undisturbed wave kinematics along with inertia and viscous force coefficients. Over the recent years, the development in numerical wave tank simulations makes it possible to simulate nonlinear three-hour sea states, with computational times in the order of real time. This provides an opportunity to calculate loads using wave spectrum input in numerical simulations at reasonable computational time and effort. In the current paper, the open-source fully nonlinear potential flow model REEF3D::FNPF is employed for the wave propagation calculations. Here, the Laplace equation for the velocity potential is solved on a sigma-coordinate mesh with the nonlinear free surface boundary conditions to close the system. A technique to calculate the total acceleration on the sigma-coordinate grid is introduced which makes it possible to apply strip theory in a moving grid framework. With the combination of strip theory and three-hour wave simulations, a unique possibility to estimate the hydrodynamic loads in real time for all discrete positions in space within the domain of the numerical wave tank is presented in this paper. The numerical results for inline forces on an offshore wind mono-pile substructure are compared with measurements, and the new approach shows good agreement.

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