Previous research into floating offshore wind turbines consistently shows some manner of discrepancy between numerical and experimental simulations when it comes to low frequency forces and motions. A part of solving this issue is the generation of accurate data for the calibration and validation of the numerical models, needed because of the coupled analysis introduced due to the relatively large wind force. The goal of this research thus became to generate accurate experimental data focussed on the low natural frequency of the mooring system in surge direction. This data had to be accompanied by an uncertainty assessment and should encompass multiple mooring systems for a good comparison.
In order to achieve these goals experimental model tests were set up using bichromatic wave sets in a long towing tank. The bichromatic wave sets allowed for the creation of beating patterns, targeting the specific low frequencies on and around the surge natural frequency via the difference frequencies. The long towing tank reduced the amount of reflections and clutter in the tank. Together these factors ensured accurate excitation of model at the desired frequencies.
The model used was a 1:96 semi-submersible model with three buoyancy columns supporting a central tower column. The depth in the tank being 1.25 m at model scale ensured a relatively deep testing environment. Two different mooring systems were tested, each with a different surge natural frequency but with the same semi-taut fibre rope-chain lay-out. A comprehensive measurement system consisting of force transducers in the mooring lines and wind force device and a camera tracking system enabled accurate measurement of both the force and motion of the model. Experiments revolved around examining the influence of the difference frequency in the wave excitation on the low frequency behaviour of the mooring system and floater.
From the lead mooring line force time series, it could be observed that during the surge natural frequency test the mooring line had triple the force compared to the wave tests with the monochromatic components, showing a clear effect of the bichromatic beating pattern. From the tests it also became clear that mooring system 1 had rear lines which were slack during wind and wave excitation. These slack lines had a number of effects on the behaviour of the model during the tests, such as increasing the mooring line force and surge response.
The tests thus show that the low frequency response comes from the difference frequency in the bichromatic wave sets, as was the goal. The heightening of the response around the natural frequency also shows that getting close to this frequency amplifies the effect. The comparison between the test results and the results from the OC5 project show that this is likely also the case there. At the same time the surge response does not show this heightened response. Which is likely due to the non-linear stiffness of the mooring system.
Furthermore, it was seen that the slack lines have a very large impact on the force in the mooring lines, especially on the lower frequencies and should thus be avoided at all times. The damping ratio as a parameter for the low frequency response is both physically and experimentally (excluding the slacked lined system) consistent and shows potential for development.

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.

When lifting operations are performed by a heavy lifting vessel, the motions of the vessel and cargo are predicted by conventional six degrees of freedom calculations. It is assumed that the cargo is attached to the crane top in order to decrease the stability of the vessel and perform accurate calculations. While these conventional calculations can be used to calculate the vessel motions and themotions of the crane top, the cargo motions are not taken into account. In conventional calculations the dynamical aspects of the cargo are not taken into account and the forces due to cargo motions are neglected. In several recent projects carried out by BigLift Shipping swell waves have presented BigLift Shipping with challenges. Due to these swell waves the vessel and cargo where excited which caused problems during these lifting operations. Therefore BigLift Shipping is intrested in a method to calculate the vesselmotions in combination with the cargo motions. There are two objectives of the research presented in this report. The first objective is to increase knowledge of the dynamical effects of the cargo on the vessel motions and vice versa. The second objective of this research is to investigate if it is possible to reduce the cargomotions during lifting operations by changing variables in a lifting plan or by controlling the cargo with cargo control systems. To fulfill both objectives a tool is created. The tool that is created is a mathematical model of the vessel and cargo body. In this model the vessel and cargo are modelled as a multi-body system and is based on the frequency domain. A multi-body frequency domain model is able to calculate the responses of both the vessel and cargo motions quickly. For the input of the model the vessel constraints of the vessel and cargo body with each other and the outside world has to be described in a mathematical way. The constraints can be divided over four different modules. The first module of constraints follow from the vessel hydrodynamics. The connection of the cargo to the vessel via the crane cable are described in the second module. The influences of mooring lines on the multi-body system are described in the third module. Lastly the influences of cargo control systems are described in the fourth module. With the different modules the influence of variables of a lifting operation on the vessel and cargo motions can be studied. The output of the tool consists of the vessel RAOs. Together with wave spectra calculations these RAOs can be translated to response spectra and most probable maximum values. The tool that is created is used to fulfill the two research objectives. First a set of calculations are done to determine the baseline responses of the multi-body system. Different variables of a lifting operation are varied and the influence of the cargo mass, crane cable length, crane top position, vessel draught and vessel metacentric height are studied. This study showed that these variables can have a large influence on the responses and natural frequencies of the multi-body system. Cargo control systems are also implemented in the multi-body model. Calculations that included cargo control systems showed that the increased stiffness of such systems can vary the natural frequencies of the different degrees of freedom in the multi-body model. When damping is applied via cargo control systems the most probable maximum values of the motions can be reduced with maximumof 55.3%.