Design loads for extreme wave events on ships, such as slamming and green water, are hard to define. These events depend on details in the incoming waves, ship motions and structure layout, which requires high-fidelity tools such as CFD or experiments to obtain the correct loads. These tools (presently) do not have the capability to fully resolve the long-term statistics of rare events in all metocean conditions over the ship's lifetime. The idea of ‘screening’ is to use lower-fidelity numerical methods to identify the occurrence of extreme load events based on an indicator. A good indicator has a strong correlation to the design load, but is easier to calculate. A high-fidelity tool can then be used to find the loads in these events. The low-fidelity statistics and the high-fidelity loads can be combined to define a design load and its probability. The present study compares different numerical screening indicators for green water loads on a containership against experiments. The quality of the identification of the critical events and the required computational time served as comparison metrics. This showed that screening both with potential flow tools and with coarse mesh CFD tools is feasible, provided the indicator, grid, time step and wave input settings are well chosen. The results from coarse mesh CFD are slightly better than from potential flow, but the computational costs are much higher. The results also show that the peaks and steepness of the relative wave elevation around the bow are suitable green water load indicators, as well as the undisturbed wave crests at the bow. Fine mesh CFD calculations were done for the identified events based on an example indicator, which resulted in a green water load distribution very close to that of the experiments. This study shows that screening could potentially reduce the required high-fidelity modelling time with up to ∼90% compared to common practice.

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Experimental or numerical analysis of the response of ships and other floating structures starts with correct environmental modelling. The capabilities of numerical tools are rapidly expanding, but presently the evaluation of extreme events in waves (such as slamming, green water, air-gap exceedance) still requires a combination of experiments and different levels of numerical tools. The present paper describes recent efforts within the Maritime Research Institute Netherlands (MARIN) to improve experimental and numerical wave modelling and especially their combination. The ultimate objective is to be able to reproduce any wave condition from a basin or from sea in numerical tools and vice versa, including a sound treatment of basin effects, numerical effects and statistical variability. The aspects that are of importance in both types of wave modelling are first introduced, after which a number of examples of recent projects is discussed. It can be concluded that important steps were made towards linking experimental and numerical wave modelling, but there are some challenges common to all wave reproductions. Some future planned studies focussing on how to deal with them are discussed as well.

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The amount of green water and the associated loads that an ocean-going vessel may encounter during its service life are important aspects to consider in the vessel's design and classification. As green water is typically a highly non-linear phenomenon, commonly the maritime industry relies on model tests to predict green water loads and their occurrence. In recent years, however, a lot of progress with Computation Fluid Dynamics (CFD) has been made in predicting non-linear flows and associated loads at a high level of accuracy. Especially in the field of wave impacts on (moored) offshore structures at zero speed, significant progress has been made and documented using CFD. A natural extension of this progress is to expand the obtained confidence in the applicability of CFD for simulating extreme wave events to applications involving vessels at forward speed. To that end, this paper presents a validation study towards the prediction of green water loading on a (typical) container vessel at forward speed by CFD. For validation, two extreme green water events were selected from a model test campaign carried out at MARIN within the context of the CRS (Cooperative Research Ships) working group 'green water dynamics'. In these tests a KRISO Container Ship (KCS) is sailing in head seas when encountering severe green water. As CFD tool, the Cartesian-grid based Volume-of-Fluid CFD solver ComFLOW was selected. Furthermore, a deterministic approach is taken for the validation, by reconstructing the non-linear incoming wave in a high amount of detail and imposing the 6 degrees of motion of the vessel using the wave basin measurements. Time traces of the green water flow on deck and local- and global impact loads on the breakwater are presented and compared against the experimental data. Detailed visualizations of the CFD results are presented to further illustrate the obtained match with the model test results and emphasize the additional value of complementing model tests with deterministic CFD analysis.

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Experiments with a flat plate in oblique waves at different speeds, wave conditions, headings and drift speed were done to evaluate non-linear wave run-up along a sailing ship. Both the incoming and diffracted part of the run-up were highly nonlinear in all test conditions. The run-up was larger at 135 than at 150 deg heading, the influence of speed was small, wave steepness increased run-up up to the point of breaking and a drift speed decreased the run-up. Most of the observed differences were larger than the seed and basin variability. (Semi-) linear diffraction methods are not sufficient to predict the highest runup crests, but applying them to screen for critical events could be further studied. CFD is able to accurately predict the nonlinear run-up in such selected events. Combining different levels of tools seems the most efficient way to predict extreme wave events such as green water due to run-up.@en

Wave forces can form a serious threat to offshore platforms and ships. The damage produced by these forces of nature jeopardizes their operability as well as the well-being of their crews. Similar remarks apply to coastal defense systems. To develop the knowledge needed to safely design these constructions, in close cooperation with MARIN and the offshore industry the numerical simulation method ComFLOW is being developed. So far, its development was focussed on predicting wave loads (green water, slamming) on fixed structures, and for those applications the method is already being used successfully by the offshore industry. Often, the investigated object (ship, floating platform) is dynamically moving under the influence of these wave forces, and its hydrodynamic loading depends upon the position of the object with respect to the oncoming waves. Predicting the position (and deformation) of the body is an integral part of the (scientific and engineering) problem. The paper will give an overview of the algorithmic developments necessary to describe the above-mentioned physical phenomena. In particular attention will be paid to fluid-solid body and fluid-structure interaction and non-reflecting outflow boundary conditions. Several illustrations including validation, will demonstrate the prediction capabilities of the simulation method.

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