Mild-Slope Wave Modelling for Dynamic Mooring Analyses

Abstract

Moored vessels in harbours around the world are affected by waves penetrating into the port basin. Hydraulic conditions can lead to severe vessel motions that influence the (un)loading efficiency and can even lead to the failure of fenders and mooring lines, creating unsafe situations for operating staff and
A DMA is a chain of numerical models that computes motions of moored vessels due to wind, currents or waves. It is used to model the behaviour of moored ships to evaluate operational conditions and assess the effect of structural or operational measures. The most computational costly model in the DMA chain is the wave penetration model, which computes local wave field, speeding up this part of model chain can reduce time and costs of such DMA's.
In this research, the complex wave model is replaced by a computationally efficient alternative and the main research question reads:
How can a mild-slope wave model be used for the ship motion calculation on ships moored in ports and what are the benefits and limitations?
The research is split up into two parts: 1) Development of the coupling method and evaluating its application based on academic test cases and 2) Applying the coupling to the case study of La Coruña and assessing its suitability.
A practical and efficient method to extract wave components from the wave penetration model is the r-DPRA tool developed by Deltares. The developed method is applied to the case study of the port of La Coruña. Two separate moments in time of the same bulk carrier are modelled: one with moderate offshore wave conditions and one with more severe conditions.
In the measured time series of the surge and sway motions, a clear low frequency can be found. Similarly, in the simulated time series of the surge and sway motions; both in moderate and heavy cases no low frequency wave patterns can be found. A schematic approach is used to include the long waves in the wave model and ship motion model and this approach caused a low frequency wave pattern to arise and leads to more accurate significant motions.
The comparison between the measured motions and modelled motions demonstrated the complexity of modelling real life events. As the surge and sway motions are caused by second order effects that are not included in the applied linear wave model, it is recommended to apply the developed workaround to schematically include this low wave forcing. The result of this research demonstrates the potential for the development method.
Before directly applying the method, it is recommended to test the suitability of the method on a more fundamental case, this reduces uncertainties related to real life measurements and stronger conclusions about the performance of the coupling can be drawn. Moreover, the coupling should be tested for a case where the 2nd order low frequency wave forcing is negligible. As it is confirmed in this research that the developed method is not suitable for modelling non-linear 2nd order low frequency waves.