This is the accepted version of the paper.This version of the publication may differ from the final published version. Permanent repository link: AbstractThis paper presents a newly developed quasi arbitrary Lagrangian-Eulerian finite element method (QALE-FEM) for simulating water waves based on fully nonlinear potential theory. The main difference of this method from the conventional finite element method developed by one of authors of this paper and others (see, e.g., [11] and [22]) is that the complex mesh is generated only once at the beginning and is moved at all other time steps in order to conform to the motion of the free surface and structures. This feature allows one to use an unstructured mesh with any degree of complexity without the need of regenerating it every time step, which is generally inevitable and very costly. Due to this feature, the QALE-FEM has high potential in enhancing computational efficiency when applied to problems associated with the complex interaction between large steep waves and structures since the use of an unstructured mesh in such a case is likely to be necessary. To achieve overall high efficiency, the numerical techniques involved in the QALE-FEM are developed, including the method to move interior nodes, technique to re-distribute the nodes on the free surface, scheme to calculate velocities and so on. The model is validated by water waves generated by a wavemaker in a tank and the interaction between water waves and periodic bars on the bed of tank. Satisfactory agreement is achieved with analytical solutions, experimental data and numerical results from other methods.

This is the accepted version of the paper.This version of the publication may differ from the final published version. based on a fully nonlinear potential theory, which was recently developed by the authors ([1], [2]), to deal with the fully nonlinear interaction between steep waves and 2D floating bodies. In the QALE-FEM method, complex unstructured mesh is generated only once at the beginning of calculation and is moved to conform to the motion of boundaries at other time steps, avoiding the necessity of high cost remeshing. In order to tackle challenges associated with floating bodies, several new numerical techniques are developed in this paper. These include the technique for moving mesh near and on body surfaces, the scheme for estimating the velocities and accelerations of bodies as well as the forces on them, the method for evaluating the fluid velocity on the surface of bodies and the technique for shortening the transient period. Using the developed techniques and methods, various cases associated with the nonlinear interaction between waves and floating bodies are numerically simulated. Permanent repository linkFor some cases, the numerical results are compared with experimental data available in the public domain and good agreement is achieved.

The 'New Year Wave' was recorded at the Draupner platform in the North Sea and is a rare high-quality measurement of a 'freak' or 'rogue' wave. The wave has been the subject of much interest and numerous studies. Despite this, the event has still not been satisfactorily explained. One piece of information that was not directly measured at the platform, but which is vital to understanding the nonlinear dynamics is the wave's directional spreading. This paper investigates the directionality of the Draupner wave and concludes it might have resulted from two wave-groups crossing, whose mean wave directions were separated by about 90• or more. This result has been deduced from a set-up of the low-frequency second-order difference waves under the giant wave, which can be explained only if two wave systems are propagating at such an angle. To check whether second-order theory is satisfactory for such a highly nonlinear event, we have run numerical simulations using a fully nonlinear potential flow solver, which confirm the conclusion deduced from the second-order theory. This is backed up by a hindcast from European Centre for Medium-Range Weather Forecasts that shows swell waves propagating at approximately 80• to the wind sea. Other evidence that supports our conclusion are the measured forces on the structure, the magnitude of the second-order sum waves and some other instances of freak waves occurring in crossing sea states.

This is the accepted version of the paper.This version of the publication may differ from the final published version. applied to 2D floating bodies), the complex unstructured mesh is generated only once at the beginning of calculation and is moved to conform to the motion of boundaries at other time steps by using a robust spring analogy method specially suggested for this kind of problems, avoiding the necessity of high cost remeshing. Permanent repository linkIn order to tackle challenges associated with 3D floating bodies, several new numerical techniques are developed in this paper. These include the technique for moving the mesh near body surfaces, the scheme for calculating velocity on 3D body surfaces and the ISITIMFB-M (Iterative Semi-Implicit Time IntegrationMethod for Floating Bodies -Modified) procedure that is more efficient for dealing with the full coupling between waves and bodies. Using the newly developed techniques and methods, various cases for 3Dfloating bodies with motions of up to 6 degrees of freedom (DoFs) are simulated. These include a SPAR platform, a barge-type floating body and one or two Wigley Hulls in head seas or in oblique waves. For some selected cases, the numerical results are compared with experimental data available in the public domain and satisfactory agreements are achieved. Many results presented in this paper have not been found elsewhere to the best knowledge of the authors.

Results from Blind Test Series 1, part of the Collaborative Computational Project in Wave Structure Interaction (CCP-WSI), are presented. Participants, with a range of numerical methods, simulate blindly the interaction between a fixed structure and focused waves ranging in steepness and direction. Numerical results are compared against corresponding physical data. The predictive capability of each method is assessed based on pressure and run-up measurements. In general, all methods perform well in the cases considered, however, there is notable variation in the results (even between similar methods). Recommendations are made for appropriate considerations and analysis in future comparative studies.