The accurate evaluation of wind loads applied on floating offshore structures is extremely important as they are in specific conditions one of the dimensioning criteria for the mooring design. Nowadays these loads are mainly assessed through wind tunnel tests performed at model scale. Estimating realistic wind loads however, remains a big challenge. The complexity and associated simplification level of FPSO topside structures, the scale effects and the establishment of the atmospheric boundary layer imply that many simplifications are to be made. Typically, the FPSO topside is greatly simplified and equivalent blocs of wired frame are used. Today with the evolution of CFD software, and the increase of the meshing capacity, new scopes open to CFD. Aerodynamic simulations on complex FPSO structures are therefore now possible, but need specific developments and validations that are presented in this paper.The main objective of the work presented is to investigate the ability of CFD to evaluate wind loads on complex FPSOs topsides and to provide information on the impact of model simplifications made in wind tunnels. In a first stage, the numerical model was intensively validated by comparing its results to a wind tunnel test case. The numerical model was developed in order to ensure the quality of the results and enable a relevant comparison that was obtained with grids density up to 30 million cells. For this purpose, the geometric model used corresponds to the one used in wind tunnel. The same Atmospheric Boundary Layer was simulated and a thorough effort was performed to ensure the mesh convergence. In a second stage, more physical aspects of the wind tunnel methodology were investigated. Typically the accuracy of the blockage effect correction was evaluated by performing computations with and without blockage, and results were compared with classical corrections applied in wind tunnel. The impacts of the Atmospheric Boundary Layer on wind loads have also been investigated. Finally, the wind load contribution of each component of the FPSO was evaluated.

The prediction of ship motions in extreme seastates is very complex as it involves strong nonlinearities. It deals with high motions of the ship and implies strong mooring system loads. These seastates are usually modeled in tank tests but an alternative in the near future could be CFD computations.In this article, all required steps to setup and verify the hydrodynamic and numerical model are performed. The setup of the hydrodynamic and numerical model enable us to show that CFD computations of motion RAOS and pitch decay tests provide results in agreement with diffraction-radiation results.Wave only simulations enable us to verify that irregular waves are accurately modelled in the CFD domain. Since the wavemaker motion used in tank tests to generate irregular waves is not available, a process of linear back propagation is set up from the wave elevation on a wave probe in tank tests. High Order Spectral (HOS) simulations are performed to reproduce the seastate measured in tank tests.Finally, a test was performed to model the ship motions in irregular extreme waves with ICARE solver coupled to the computed HOS wave field through Spectral Wave Explicit Navier Stokes Equations (SWENSE).

Abstract Computational Fluid Dynamics (CFD) tools have progressed greatly in the past decades in such a manner that they have become a recurring tool during the workflow of an engineering project. In the specific case of Marine and Offshore Engineering, these tools are more and more used to predict forces on structures with a very detailed level of precision, to optimize hull forms to conceive more efficient and environmentally friendly designs and also to reassure structural engineers with respect to the assumptions made during complex engineering problems. In this article several examples of how these tools and methodologies have been employed in the design process of Marine and Offshore structures will be presented. Validations compared to tank tests and wind tunnels will be presented for damping coefficients, green water impact, aerodynamic coefficients and non-linear wave loading studies. Throughout these examples it will be shown how the need of dedicated tools/methodologies is necessary in order to efficiently use the CFD tools during the Naval and Offshore design process. Very often, distinct phenomena need to be modeled in order to correctly apprehend the complexity of the flows. To do so, coupling distinct approaches is sometimes necessary: CFD solver to a potential one for instance. In the paragraphs below four examples are illustrated: nonlinear wave loading computations, damping coefficients computation, aerodynamics winf loads estimation on complex topside structures, and prediction of green water impact on a FPSO.

An SPH model of monofluid/structure interactions has already been integrated in a parallel solver named SPH-flow, and applied in the context of sloshing impacts (Oger et al., 2009). The developments carried out by HydrOcean and Ecole Centrale Nantes were supported by GTT (Gaztransport & Technigaz) for sloshing applications. The formulation of SPH-flow has been recently extended as a result of this partnership, enabling the treatment of interactions between several fluids. This paper presents the theoretical model of SPH-flow for the twophase formulation. Applications to liquid impacts are given, confirming the strong influence of the gas on the flow evolution and impact pressure peaks. The two first simulations proposed in the paper are a contribution to the numerical comparative study organized within ISOPE 2010: the monodimensional problem of a piston compressing a gas, and the free gravity fall of a bi-dimensional liquid patch through a gas. Finally, the simulation of a breaking wave impacting a rigid wall with a gas pocket entrapped is presented.

The purpose of this paper is to present combination of the SWENSE (Spectral Wave Explicit Navier-Stokes Equations – [1]) method — an original method to treat fully nonlinear wave-body interactions — and a free surface RANSE (Reynolds Averaged Navier-Stokes Equations) solver using a single-phase Level Set method to capture the interface. The idea is to be able to simulate wave-body interactions under viscous flow theory with strong deformations of the interface (wave breaking in the vicinity of the body, green water on ship decks…), while keeping the advantages of the SWENSE scheme. The SWENSE approach is based on a physical decomposition by combining incident waves described by a nonlinear spectral scheme based on potential flow theory and an adapted Navier-Stokes solver where only the diffracted part of the flow is solved, incident flow parameters seen as forcing terms. In the single-phase Level Set method [2, 3], the air phase is neglected. Thus, only the liquid phase is solved considering a fluid with uniform properties. The location of the free surface is determined by a Level Set function initialised as the signed distance. The accuracy of simulation depends essentially on the pressure scheme used to impose free surface dynamic boundary condition. Comparisons of numerical results with experimental and numerical data for US navy combatant DTMB 5415 in calm water and in head waves are presented.

The prediction of manoeuvring of underwater vehicles such as UAV or submarines is a key point for their performance and safety. From an experimental point of view, such manoeuvring characteristics can be determined either by captive model tests coupled with a mathematical model or by free model tests. The first method consists in determining linear and non-linear hydrodynamic coefficients from fixed cases (combination of drift or incidence angles, gyrating radius in vertical or horizontal plane, for several appendages angles). It requires a very large number of tests to be efficient. The second method needs more complicated models (autonomous, underwater measurements ...), and very large facilities in which manoeuvres can be performed. Therefore, naval architects and engineers in charge of designing underwater vehicles are interested in having at their disposal accurate numerical alternatives. HydrOcean and Ecole Centrale Nantes are involved in a cooperative work in order to develop and validate numerical solvers and associated methodologies in order to accurately compute submarines manoeuvring performances with acceptable CPU time. The final objective is to perform fully free and self-propelled simulation. This requires a RANSE free surface solver able to accurately predict forces and moments applied along the hull of a submerged body and on its appendages. In that way, this paper presents validations that have been conducted on a fixed model of the DARPA Suboff model for several angles of drift, and different configurations.

We describe the development of a highly interactive approach to simulation of engineering multi-mechanics problems, using the smoothed particle hydrodynamics mesh-free method as the computational engine, for applications including ship survival, medical devices and Pelton turbines.

The study focuses on the in-place hydrodynamic behavior and Flow Induced Response of a novel design of Free Standing Riser (FSR) system tensioned by a 'Flat-Buoy'. The paper presents results review of both Experimental and Numerical approaches initiated in the framework of a comprehensive Research & Development program. Experimental and numerical study conclusions converge on the excellent hydrodynamic Stability of such FSR system.First, wind tunnel campaign, based on Reynolds similitude, has focused on the flow features over the fixed Flat Buoy. No Vortex Shedding has been clearly highlighted. Moreover, results have pointed out a more pronounced dependence of the hydrodynamic coefficients to the flow incidence than to the Reynolds number.Secondly, basin model tests, based on Reduced Velocity similitude, have highlighted the stability of such scaled FSR system concluding to maximum Cross-Flow and In-Line amplitude such as A/D<0.35 (A represents the amplitude and D the Buoy diameter).In parallel, hydrodynamic stability has been investigated Benchmarking Computational Fluid Dynamic (CFD) methods. Preliminary validation steps have pointed out ability of such CFD approaches to globally predict both Wind Tunnel and Basin Test results. Finally, extending CFD and Fluid Structure Interaction (FSI) modeling to full-scale configuration, stability of the FSR tensioned by a Flat-Buoy has been proved.

The design of nuclear pressure vessel requires the description of various dynamic effects, among which fluid-structure interaction. In some configurations, gravity effects (in the low frequency range) and compressibility effects (in the high frequency range) are of paramount importance and have therefore to be accounted for. The present paper is concerned with the description of free surface flows with gravity and compressibility effects, using a SPH (Smoothed Particle Hydrodynamics) method in circular confinement, with expected applications to the dynamic analysis of auxiliary nuclear component for naval propulsion. For the system under concern, the range of dynamic solicitation extends from low frequency (for seismic analysis of grounded prototype) to high frequency (for shock analysis of embarked reactors); it is therefore of particle interest to employ a numerical techniques which allows the description of linear and non-linear free surface effects, which can be expected in both cases. SPH method gives promising perspective for simulation of sloshing flows in various configurations; the present paper investigates the use of such a technique in the context of three-dimensional problems with cylindrical confinement.

This paper documents recent advances of the SWENSE (Spectral Wave Explicit Navier-Stokes Equations) approach, a method for simulating fully nonlinear wave-body interactions including viscous effects. The methods efficiently combines a fully nonlinear potential flow description of undisturbed wave systems with a modified set of RANS with free surface equations accounting for the interaction with a ship or marine structure. Arbitrary incident wave systems may be described, including regular, irregular waves, multidirectional waves, focused wave events, etc. The model may be fixed or moving with arbitrary speed and 6 degrees of freedom motion. The extension of the SWENSE method to 6 DOF simulations in irregular waves as well as to manoeuvring simulations in waves are discussed in this paper. Different illlustative simulations are presented and discussed. Results of the present approach compare favorably with available reference results.

Applications of Navier-Stokes with free-surface solvers for applications in naval hydrodynamics started about ten years ago. Those solves show great interest by allowing a notable improvement in computations accuracy, mainly due to non linear free surface conditions and viscous effects. Nowadays, they are mainly used for resistance applications by almost all research institutes or industrial parties. Recent development of those solvers alow their application in other fields of hydrodynamic, such as seakeeping and maneuvering. Thus, the field of application of these tools is incredibly wide, in particular for unsteady simulations, allowing to model the whole behavior of a maneuvering ship. The development of ICARE solver is going in this way, we present in this paper the applications on steady and unsteady maneuvering simulation of ships. To achieve this goal, six degrees of freedom, and rotating appendages capabilities have been developed and are presented. 1 DESCRIPTION DU CODE ICARE Le logiciel de simulation hydrodynamique utilise dans cet article est le logiciel ICARE developpe au Laboratoire de Mecanique des Fluides de l’Ecole Centrale de Nantes ([1], [2], [3], [4] et [5]). Les equations de Navier-Stokes en moyenne de Reynolds sous forme convective sont ecrites en effectuant une transformation partielle de l’espace cartesien (x1,x2,x3) vers un espace curviligne adapte a la surface de la carene et de la surface libre a chaque instant. L’elevation de la surface libre, les trois composantes cartesienne de la vitesse (ui), la pression (p) et l’energie cinetique de la turbulence ( ) sont les inconnues. Les equations de transport pour la quantite de mouvement s’ecrivent : ai represente la base contravariante, gij est le tenseur de la metrique contravariante, , fi sont les fonctions de controle de grille et representent les vitesses de glissement du maillage. Les efforts d’inertie dus a un referentiel non-galileen (giration, acceleration) sont pris en compte dans le terme qi , si necessaire. La conservation de la masse s’exprime a l’aide de l’equation de continuite : Le systeme d’equations est ferme par un modele de turbulence propose par Wilcox [6]. Les equations de transport de l’energie cinetique de la turbulence et du taux de dissipation s’ecrivent :

The classical approach of Velocity Prediction Program is to find the balance of Hydrodynamic and aerodynamic sensors acting on the yacht to determine sailing conditions and associated performance. Usually, this approach is based on the data given by towing tank, wind tunnel or numerical computations. We present in this paper the unsteady coupling between an hydrodynamic RANSE with free surface solver and a panel aerodynamic solver that allow to directly find the sailing condition of the yacht, and its performances, compute bay solving the six degrees of freedom motion equations of the ship in the hydrodynamic solver. The main advantage of this computational method is the decrease of numerical evaluation, compared to the classical interpolation approach to determine performances of a hull, and to directly rank several hills in the term of performances and not only in the term of drag in fixed conditions. Further, improvements will allow to simulate unsteady maneuvering of sailing yacht, and focus will for example on restart behavior after tacking, dynamic behavior in waves.

Labour intensive operations for sugar cane and food crop cultivation have been id entified by the Mechanization Division of the MSIRI in view to carry out research aimed at minimizing the impact of labour shortage and cost. Ways and means of reducing production costs and sustaining productivity are suggested and problems arising from increased mechanization are considered. The strategy adopted to carry out the research programme is developed.