A New Coupled Model for the Assessment of Offshore Structures in Non-Gaussian Seas

2020 ◽  
Author(s):  
Griet Decorte ◽  
Alessandro Toffoli ◽  
Geert Lombaert ◽  
Jaak Monbaliu
Keyword(s):  
Coupled Model ◽  
Wave Interaction ◽  
Wave Structure ◽  
Offshore Structures ◽  
Higher Order ◽  
Structure Interaction ◽  
Cfd Models ◽  
Non Gaussian ◽  
Wave Structure Interaction ◽  
Interaction Phenomena

Abstract Although wave-wave interaction phenomena in random seas have shown to lead to a departure from Gaussian statistics and therefore to a higher occurrence of extreme waves, they are usually not taken along in the assessment of the dynamic behaviour of offshore structures. Supported by a rapid increase of computational resources, the use of Computational Fluid Dynamics (CFD) models has become viable for studying the above mentioned wave-structure interaction phenomena. Still, these models remain computationally expensive, which impedes their use for the large domains and the long periods of time necessary for studying non-Gaussian seas. Therefore, a one-way domain decomposition strategy is proposed, which takes advantage of the recent advances in CFD as well as of the computational benefits of the higher-order spectral (HOS) models previously used to assess non-Gaussian seas. The unidirectional non-Gaussian sea obtained by this coupled HOS-CFD model shows excellent agreement with the target wave field generated by the higher-order spectral numerical wave tank. In addition, the wave-structure interaction for a simplified monopile, which is excited by a non-Gaussian sea, seems to be captured well.

scholarly journals On the Use of a Domain Decomposition Strategy in Obtaining Response Statistics in Non-Gaussian Seas

Fluids ◽  
2021 ◽  
Vol 6 (1) ◽  
pp. 28
Author(s):  
Griet Decorte ◽  
Alessandro Toffoli ◽  
Geert Lombaert ◽  
Jaak Monbaliu
Keyword(s):  
Domain Decomposition ◽  
Wave Interaction ◽  
Wave Structure ◽  
Offshore Structures ◽  
Short Term ◽  
Cfd Models ◽  
Highly Nonlinear ◽  
Non Gaussian ◽  
The Impact ◽  
Interaction Phenomena

During recent years, thorough experimental and numerical investigations have led to an improved understanding of dynamic phenomena affecting the fatigue life and survivability of offshore structures, e.g., ringing and springing and extreme wave impacts. However, most of these efforts have focused on modeling either selected extreme events or sequences of highly nonlinear waves impacting offshore structures, possibly overestimating the actual load to be experienced by the structure. Overall, not much has been done regarding short-term statistics. Although clear non-Gaussian statistics and therefore higher probabilities of extreme waves have been observed in random seas due to wave–wave interaction phenomena, which can impact short-term statistics for the structural load, they have not been studied extensively regarding the assessment of the dynamic behavior of offshore structures. Computational fluid dynamics (CFD) models have shown their viability for studying wave–structure interaction phenomena. Despite the continuously increasing computational resources, these models remain too computationally demanding for applications to the large spatial domains and long periods of time necessary for studying short-term statistics of non-Gaussian seas. Higher-order spectral (HOS) models, on the other hand, have been proven to be efficient and adequate in studying non-Gaussian seas. We therefore propose a one-way domain decomposition strategy, which takes full advantage of the recent advances in CFD and of the computational benefits of HOS. When applying this domain decomposition strategy, it appeared to be possible to deduce response statistics regarding the impact of nonlinear wave–wave interactions.

scholarly journals Investigation of Higher-Harmonic Wave Forces and Ringing Using CFD Simulations

2018 ◽  
Author(s):  
Arun Kamath ◽  
Hans Bihs ◽  
Csaba Pakozdi
Keyword(s):  
Structural Response ◽  
Vertical Cylinder ◽  
Wave Interaction ◽  
Wave Structure ◽  
Higher Order ◽  
Wave Forces ◽  
Numerical Wave Tank ◽  
Cfd Model ◽  
Structure Interaction ◽  
Wave Structure Interaction

Typical offshore structures are designed as tension-leg platforms or gravity based structures with cylindrical substructures. The interaction of waves with the vertical cylinders in high sea states can result in a resonant response called ringing. Here, the frequency of the structural response is close to the natural frequency of the structure itself and leads to large amplitude motions. This is a case of extreme wave loading in high sea states. This understanding of higher-order wave forces in extreme sea states is an essential parameter for obtaining a safe, reliable and economical design of an offshore structure. The study of such higher-order effects needs detailed near-field modelling of the wave-structure interaction and the associated flow phenomena. In such cases, a Computational Fluid Dynamics (CFD) model that can accurately represent the free surface and further the wave-structure interaction problem can provide important insights into the wave hydrodynamics and the structural response. In this paper, the open source CFD model REEF3D is used to simulate wave interaction with a vertical cylinder and the wave forces on the cylinder are calculated. The harmonic components of the wave force are analysed. The model employs higher-order discretisation schemes such as a fifth-order WENO scheme for convection discretisation and a third-order Runge-Kutta scheme for time advancement on a staggered Cartesian grid. The level set method is used to obtain the free surface, providing a sharp interface between air and water. The relaxation method is used to generate and absorb the waves at the two ends of the numerical wave tank. This method provides good quality wave generation and also the wave reflected from the cylinder are absorbed at the wave generation zone. In this way, the generated waves are not affected by the wave interaction process in the numerical wave tank. This is very essential in the studies of higher-order wave interaction problems which are very sensitive to the incident wave characteristics. The numerical results are compared to experimental results for higher-order forces on a vertical cylinder to validate the numerical model.

Wave Interaction With an Infinite Long Horizontal Elliptical Cylinder

2011 ◽  
Author(s):  
Hao Song ◽  
Longbin Tao
Keyword(s):  
Wave Interaction ◽  
Wave Structure ◽  
Offshore Structures ◽  
Elliptical Cylinder ◽  
Ocean Engineering ◽  
Parallel Side ◽  
Matrix Differential Equations ◽  
Unsteady Loading ◽  
Matrix Differential ◽  
Wave Structure Interaction

Wave-structure interaction in ocean engineering is a major source of unsteady loading and vibration of offshore structures including platforms, risers and long cables. Many efforts focus on vertical structures in which solution procedures can usually be simplified in the plane of mean free surface as the variable in the direction of gravity can be separated. In this paper, wave interaction with an infinite long horizontal elliptical cylinder is solved by a semi-analytical method, namely, the scaled boundary finite-element method (SBFEM). The solution domain is divided into two bounded domains and two unbounded domains with parallel side-faces. The governing partial differential equation (Helmholtz equation) is weakened and transformed to ordinary matrix differential equations in radial direction and are then solved analytically by SBFEM.

scholarly journals Influence of Power Take-Off Modelling on the Far-Field Effects of Wave Energy Converter Farms

Water ◽  
2021 ◽  
Vol 13 (4) ◽  
pp. 429
Author(s):  
Gael Verao Fernandez ◽  
Vasiliki Stratigaki ◽  
Nicolas Quartier ◽  
Peter Troch
Keyword(s):  
Wave Propagation ◽  
Coupled Model ◽  
Wave Structure ◽  
Propagation Model ◽  
Far Field ◽  
Energy Converter ◽  
Coupled Models ◽  
Structure Interaction ◽  
Field Effects ◽  
Wave Structure Interaction

The study of the potential impact of wave energy converter (WEC) farms on the surrounding wave field at long distances from the WEC farm location (also know as “far field” effects) has been a topic of great interest in the past decade. Typically, “far-field” effects have been studied using phase average or phase resolving numerical models using a parametrization of the WEC power absorption using wave transmission coefficients. Most recent studies have focused on using coupled models between a wave-structure interaction solver and a wave-propagation model, which offer a more complex and accurate representation of the WEC hydrodynamics and PTO behaviour. The difference in the results between the two aforementioned approaches has not been studied yet, nor how different ways of modelling the PTO system can affect wave propagation in the lee of the WEC farm. The Coastal Engineering Research Group of Ghent University has developed both a parameterized model using the sponge layer technique in the mild slope wave propagation model MILDwave and a coupled model MILDwave-NEMOH (NEMOH is a boundary element method-based wave-structure interaction solver), for studying the “far-field” effects of WEC farms. The objective of the present study is to perform a comparison between both numerical approaches in terms of performance for obtaining the “far-field” effects of two WEC farms. Results are given for a series of regular wave conditions, demonstrating a better accuracy of the MILDwave-NEMOH coupled model in obtaining the wave disturbance coefficient (Kd) values around the considered WEC farms. Subsequently, the analysis is extended to study the influence of the PTO system modelling technique on the “far-field” effects by considering: (i) a linear optimal, (ii) a linear sub-optimal and (iii) a non-linear hydraulic PTO system. It is shown that modelling a linear optimal PTO system can lead to an unrealistic overestimation of the WEC motions than can heavily affect the wave height at a large distance in the lee of the WEC farm. On the contrary, modelling of a sub-optimal PTO system and of a hydraulic PTO system leads to a similar, yet reduced impact on the “far-field” effects on wave height. The comparison of the PTO systems’ modelling technique shows that when using coupled models, it is necessary to carefully model the WEC hydrodynamics and PTO behaviour as they can introduce substantial inaccuracies into the WECs’ motions and the WEC farm “far-field” effects.

Application of an Absorbing Boundary Condition in a Wave-Structure Interaction Problem

2012 ◽  
Author(s):  
Bulent Duz ◽  
Rene H. M. Huijsmans ◽  
Mart J. A. Borsboom ◽  
Peter R. Wellens ◽  
Arthur E. P. Veldman
Keyword(s):  
Boundary Condition ◽  
Traditional Approach ◽  
Wave Structure ◽  
Offshore Structures ◽  
Absorbing Boundary Condition ◽  
Design Stage ◽  
Absorbing Boundary ◽  
Structure Interaction ◽  
Interaction Problem ◽  
Wave Structure Interaction

For the design of offshore structures, an accurate assessment of the ability of the structure to survive in extreme sea conditions is of prime importance. Next to scaled model tests on the structure in waves, also CFD capabilities are at the disposal of the designer. However even with the fastest computers available, it is still a challenge to use CFD in the design stage because of the large computational resources they require. In this study we focus our attention on the implementation of an absorbing boundary condition (ABC) in a wave-structure interaction problem. Unlike the traditional approach where the boundaries are located far from the object to avoid reflection, we gradually locate them closer while at the same time observing the influence of the absorbing boundary condition on the solution. Numerical calculations are performed using the CFD simulation tool ComFLOW which is a volume-of-fluid (VOF) based Navier-Stokes solver. Comparisons with experimental results are also provided and the performance of the ABC is discussed.

scholarly journals WAVE INTERACTION WITH SINGLE AND TWIN VERTICAL AND SLOPED SLOTTED WALLS

2018 ◽  
pp. 40
Author(s):  
Noor Al Anjari ◽  
Mohamad Al Khalidi ◽  
Subramaniam Neelamani
Keyword(s):  
Energy Dissipation ◽  
Wave Energy ◽  
Wave Reflection ◽  
Wave Interaction ◽  
Wave Structure ◽  
Wave Transmission ◽  
Random Wave ◽  
Wave Energy Dissipation ◽  
Structure Interaction ◽  
Wave Structure Interaction

The performance of single and twin slotted walls of varying porosity and slope angles is experimentally investigated, in order to understand the wave-structure interaction and to asses the characteristics of wave transmission, wave reflection, and wave energy dissipation under random wave conditions.

Experimental Investigation of Dynamic Response of Tension Leg Platform With Perforated Members

2013 ◽  
Author(s):  
Vishruth Srinath ◽  
Srinivasan Chandrasekaran
Keyword(s):  
Dynamic Response ◽  
Wave Structure ◽  
Offshore Structures ◽  
Scale Model ◽  
Experimental Investigations ◽  
Cylindrical Structures ◽  
Structure Interaction ◽  
Near Shore ◽  
Outer Cover ◽  
Wave Structure Interaction

Perforated cylindrical structures are extensively used in near-shore breakwaters to reduce wave-structure interaction and scouring; however use of perforated members on floating offshore structures is not widespread. Current study investigates the influence of perforated members on the dynamic response of Tension Leg Platforms (TLP) through model testing. Detailed experimental investigations are carried out on the scale model of TLP with and without porous outer cover, under unidirectional regular waves. Based on studies conducted, it is shown that fluid-structure interaction is reduced in the presence of outer perforated covers; as a result, surge and pitch responses decrease.

scholarly journals Investigation of a Stochastic Inverse Method to Estimate an External Force: Applications to a Wave-Structure Interaction

2012 ◽  
Vol 2012 ◽  
pp. 1-25 ◽  
Author(s):  
S. L. Han ◽  
Takeshi Kinoshita
Keyword(s):  
External Force ◽  
Inverse Method ◽  
Inverse Function ◽  
Wave Structure ◽  
Dynamic Responses ◽  
Structure Interaction ◽  
External Forces ◽  
Interaction Problem ◽  
Wave Structure Interaction

The determination of an external force is a very important task for the purpose of control, monitoring, and analysis of damages on structural system. This paper studies a stochastic inverse method that can be used for determining external forces acting on a nonlinear vibrating system. For the purpose of estimation, a stochastic inverse function is formulated to link an unknown external force to an observable quantity. The external force is then estimated from measurements of dynamic responses through the formulated stochastic inverse model. The applicability of the proposed method was verified with numerical examples and laboratory tests concerning the wave-structure interaction problem. The results showed that the proposed method is reliable to estimate the external force acting on a nonlinear system.

Sign in / Sign up

Export Citation Format

Share Document