6 research outputs found
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QALE-FEM for numerical modelling of non-linear interaction between 3D moored floating bodies and steep waves
This paper presents further development of the QALE-FEM (Quasi Arbitrary Lagrangian-Eulerian Finite Element Method) based on a fully nonlinear potential theory to numerically simulate nonlinear responses of 3D moored floating bodies to steep waves. In the QALE-FEM (recently developed by the authors and 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. In 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 Integration Method 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 3D floating 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
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Numerical simulation of fully nonlinear interaction between steep waves and 2D floating bodies using the QALE-FEM method
This paper extends the QALE-FEM (quasi arbitrary Lagrangian–Eulerian finite element method) based on a fully nonlinear potential theory, which was recently developed by the authors [Q.W. Ma, S. Yan, Quasi ALE finite element method for nonlinear water waves, J. Comput. Phys, 212 (2006) 52–72; S. Yan, Q.W. Ma, Application of QALE-FEM to the interaction between nonlinear water waves and periodic bars on the bottom, in: 20th International Workshop on Water Waves and Floating Bodies, Norway, 2005], 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. For some cases, the numerical results are compared with experimental data available in the public domain and good agreement is achieved
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Numerical simulation of nonlinear response of moored floating structures to steep waves
This thesis presents a newly developed Quasi Arbitrary Lagrangian-Eulerian Finite Element Method (QALE-FEM) for numerically simulating wave-body interaction problems based on the fully nonlinear potential theory. The boundary value problem in this model is solved by a finite element method (FEM). The main difference between this method and the conventional FEM is that the complex mesh is generated only once at the beginning of the calculation 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 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. In order to achieve overall high efficiency, some numerical techniques, including the method to move interior nodes, the technique to redistribute the nodes on the free surface, the scheme to calculate velocities, are developed. To overcome the difficulty associated with the force and acceleration of freeresponse floating bodies, an ISITIMFB (Iterative Semi-Implicit Time Integration Method for Floating Bodies) iterative procedure is developed. The developed QALE-FEM method is applied to simulate the waves generated by a wavemaker and their interaction with sandbars on the seabed, waves generated by a floating body in forced motion, the response of a 2D or 3D freely floating body to a steep wave. Some of the results have been validated by analytical solutions, experimental data and numerical results from other methods. Satisfactory agreements are achieved. The convergence properties of this model in cases with or without floating bodies are all investigated. The nonlinearities associated with different cases are investigated. The mesh quality is also investigated using either qualitative or quantitative methods. The results show the mesh quality during long-period simulation is retained. The efficiency of the QALE-FEM method is finally discussed and compared with other methods. It is concluded that the QALE-FEM method is 10 times faster than the conventional FEM method in case with unstructured mesh and at least 7 times faster than the fast BEM methods for the fully nonlinear waves
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Numerical investigations on transient behaviours of two 3-D freely floating structures by using a fully nonlinear method
Two floating structures in close proximity are very commonly seen in offshore engineering. They are often subjected to steep waves and, therefore, the transient effects on their hydrodynamic features are of great concern. This paper uses the quasi arbitrary Lagrangian-Eulerian finite element method (QALE-FEM), based on the fully nonlinear potential theory (FNPT), to numerically investigate the interaction between two three-dimensional (3D) floating structures, which undergoes motions with 6 degrees of freedom (DOFs), and are subjected to waves with different incident angles. The transient behaviours of floating structures, the effect of the accompanied structures and the nonlinearity on the motion of and the wave loads on the structures are the main focuses of the study
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Investigation of interaction between extreme waves and a moored FPSO using FNPT and CFD solvers
To assess the survivability of marine structures, numerical tools that can predict the interaction between extreme waves and structures are needed. Considering the significant nonlinearity associated with the problem, fully nonlinear models, including the fully nonlinear potential theory (FNPT) and general viscous flow theory based on the Navier-Stokes equation (NS) and Continuity equation, are necessary for a reliable prediction. Both methods have relatively higher computational cost compared to the linear or second order wave theories, which are popular in routine design practices. Although the FNPT model generally requires less computational efforts compared to the NS model, its theoretical assumption, i.e. the flow is incompressible, irrotational and inviscid, invalidates its applications to those problems with significant viscous effects and/or breaking waves. It is, therefore, necessary to conduct a comparative study on the accuracy of the FNPT in various problems to quantify its range of application. In this paper, both the Quasi Arbitrary Lagrangian Eulerian Finite Element (QALE-FEM) method based on the FNPT model and the open source Reynolds Average Navier-Stoke (RANS) based code, OpenFOAM, are used to predict the interaction between extreme waves and a moored Floating Production Storage and Offloading (FPSO) model. The extreme waves are generated using the NewWave theory and different wave steepnesses are used. The results, including the wave runup, pressure and force on the FPSO, are compared with the corresponding experimental data obtained from the ocean basin at the COAST Laboratory, University of Plymouth. Satisfactory agreement between the numerical predictions and the experimental measurements are observed. It is also concluded that the differences between the QALE-FEM results and the OpenFOAM results are mainly caused by the effectiveness of the wave generation in the corresponding simulations; the viscous effects may be considerable in the rotational motion of the FPSO when subjected to extreme waves
Nonlinear analysis of sloshing and floating body coupled motion in the time-domain
The aim of this paper is to develop a coupled nonlinear time-domain simulation scheme for nonlinear interactions among sloshing flows and floating body motion for both regular and irregular wave excitation. The contributions of a variety of nonlinear factors, outside waves and inside sloshing induced forces, as well as their influences on body coupled sway and roll motion were investigated. The induced forces are due to the changes in the transient wet surface of the floating body and full nonlinear sloshing. The effects of tank fill ratio and excitation wave height on the nonlinear coupled motion, as well as the relationship between sloshing and floating body nonlinear coupled motion under large wave amplitudes and severe sea conditions were also investigated and the results are presented. Finally, the numerical solutions are compared with existing experimental result. The fully nonlinear sloshing and floating body coupled motion are simulated based on the potential flow theory, with the transient position hydrodynamic assumption. The boundary value problem is solved by the B-spline higher-order panel method. The ISITIMFB (iterative semi-implicit time integration method for floating bodies) is applied to solve for the body's velocity and displacements. The sloshing energy dissipation is modeled by changing the boundary condition on the tank's solid boundaries. An extended principle to determine the energy dissipation coefficient for both regular and irregular cases is extracted. Then, the sloshing and floating body nonlinear coupled motion under large wave amplitudes and severe sea conditions are investigated, and the numerical solutions are compared with existing experimental results. The effects of tank fill ratio and excitation wave height on the nonlinear coupled motion is also investigated