In this paper, the free-surface modelling in MARIN’s URANS code FreSCo is analysed and tested. FreSCo is a finite-volume code using a two-phase Volume-Of-Fluid approach to handle free-surfaces. After a description of code and method, attention is paid to wave generation and propagation. More specifically, the objective of this study is to assess if the code can model correctly travelling waves of 2nd order Stokes type and what it requires to do so with a quality needed for engineering purposes. Analysis of the effect of numerical schemes and of discretisation errors (via grid-refinement studies) is reported. Dispersion and diffusion errors are quantified by comparison with analytical solutions. Furthermore, two typical benchmark free-surface problems, viz. a 3D Dam-breaking flow and the Duncan foil test case, are used to illustrate the current capabilities of FreSCo. The results are validated against experiments and numerical data available in the literature. The results here shown indicate that FreSCo comes up to expectation, but also that issues such as compressive convection schemes and non-reflective boundary conditions require further research.

In this paper, the flow around a forced rolling body is analyzed with MARIN in-house CFD code ReFRESCO. The objective is to assess if the code can correctly predict the added mass and damping coefficients of a rolling vessel. After a description of code and numerical methods, the results for the flow computed around a 2D rolling hull section are presented. Sharp and rounded bilges are investigated for three roll amplitudes and three roll periods. The influence of grid and time discretisation and iterative errors are analyzed. The CFD results with Re-FRESCO are compared to experiments and to results obtained with the commercial CFD package CFX. The results shown here indicate that ReFRESCO is capable of accurately predicting the added mass and damping coefficients. However, it is also shown that fine grids and time-steps are required to obtain a grid and time-step converged solution.

This paper presents the initial investigations into the ‘Inverse’ concept for wave energy conversion, based on the maximisation of motions and green water. The ‘Inverse’ concept combines aspects of ‘overtopping’, ‘heaving’ and ‘pitching’ wave energy conversion concepts, but also adds specific aspects such as the use of green water. Instead of reducing the motions and green water as is done in normal offshore hydrodynamics, the ‘Inverse’ concepts tries to maximise the motions and green water to generate energy from the waves. Results are presented of frequency domain calculations for the motion (de-) optimisation. Improved Volume Of Fluid (iVOF) simulations are used to simulate the green water flow on the deck. It is concluded that the potential of the ‘Inverse’ concept is clear. As a result of the double connotation of the word ‘green’, this renewable energy concept could also be called the ‘green water’ concept. Further work needs to be carried out on the further optimisation of the concept.

The Catenary Anchor-Leg Mooring (CALM) is the most popular and widely-used type of offshore loading terminal. A CALM buoy consists of a floating buoy anchored to the seabed by catenary chain legs which are secured to anchors or piles. Due to the small inertia of CALM buoys, the mooring line responses are very sensitive to waves and considerable fatigue risk is introduced to the mooring lines. Extreme waves may even lead to mooring line failure. Therefore it is highly relevant to study the motions of the CALM buoy in (extreme) wave conditions. This paper presents a validation study of a coupled CFD – dynamic mooring model for simulating the response of a shallow water CALM buoy in extreme waves (Figure 1). Simulations of an interactively moving CALM buoy in a horizontal mooring system were performed by coupling a Navier-Stokes based finite-volume, VoF CFD solver with a dynamic mooring model. The CFD results are validated against model tests performed in MARIN’s shallow-water basin during the ComFLOW-2 joint industry project. The validation study concentrates on the correct prediction of the coupled responses of the CALM buoy in extreme, regular shallow-water waves. As an alternative to simulations with a fully coupled dynamic mooring set-up, also CFD simulations are presented in which the mooring system is represented by a linearly equivalent spring matrix, including cross terms. The importance of correctly modelling these cross terms is presented in the paper, and the results obtained with- and without these off-diagonal spring terms are compared.

Nowadays, more and more nearshore LNG terminals are being built as it offers easy access to vessels coming from deep water and mitigates the risk by isolating regasification units from the cities. However, designing these terminals can be challenging in shallow water, as it is exposed to low-frequency waves which can excite the moored vessels at their natural periods. By lack of knowledge and adequate numerical simulation techniques, the effect of these low-frequency waves on the motions of moored vessels are unfortunately often ignored in the design. This is likely to result in an underestimation of the vessel motions and terminal downtime. In this paper, a methodology for the design of terminals in a nearshore wave climate is presented. The methodology consists of six steps which guide the engineer from the definition of the deep-water sea states to the calculation of the vessel motions and terminal downtime. In an initial stage, computational efficient tools are used, with the limitation that several approximations need to be made. In a later stage, more detailed but expensive methods are applied. The objective of this paper is to show how the developed methodology can give insight in the expected downtime due to the low-frequency waves in any nearshore mooring location. As an example, the methodology is applied on a fictive but realistic case, for which the motion response of a LNG carrier moored to a jetty on a sloping bottom is calculated. From seven years of deep-water sea states, the terminal downtime is estimated. The application of the methodology to the design case confirms that the terminal downtime can be significantly underestimated if shallow water effects are not taken into account. So the influence of the water depth, bathymetry, wave directionality and low-frequency waves on the vessel motions should be investigated with care. However, the results obtained in the design case also show that the spectral shape of the low-frequency waves predicted by the wave models are sensitive to the tuning of numerical parameters. Tuning the wave models against model tests or full scale data is therefore highly recommended, because the motion response of a low-damped moored vessel can be dominated by the amount of low-frequency free wave energy at its natural periods.