Principia (France)

Abstract Real-time hybrid modelling refers to the use, in real-time, of multiple modelling approaches to improve the overall accuracy of the representation of a complex system. In hydrodynamics, numerical models may not be able to capture non-linear effects such as viscous effects, greenwater, slamming, second or higher order loads and usually require calibration against basin model tests. On the other hand, the downscaling needed for the model test is difficult to achieve for a system submitted to both hydrodynamic and aerodynamic loadings. For some applications, couplings between the individual system components are so large that testing separately each component is not representative of the global system’s behaviour. For such applications, the real-time hybrid modelling, mixing in real-time (i.e. in the basin itself) numerical simulations and basin model tests, can solve this issue. This paper presents the application of such technique to a floating offshore wind turbine. The first basin tests were performed with a single fan and a turbine numerical model with rigid blades. For the second set of tests, the following improvements were made: on the experimental side, a dual rotating fans system was built and on the software side, the influence of blade flexibility was incorporated. Comparisons between the various modeling assumptions and experimental setups are presented. This is useful to define the required level of turbine representation based on the objectives of the basin tests.

International audience

Beside a validation step of numerical models regarding academic tsunami test cases, one of the topic of the TANDEM project aims to qualify long distance propagation models on real tsunami events.In this context, the Saint-Venant model is used to simulate the 2011 Tohoku tsunami in Japan.The numerical resolution is based on a well-balanced finite volumes solver on unstructured mesh.The results are compared to available near and far shore in-situ data.The numerical accuracy is improved by the use of an adaptive mesh refinement method.The relevancy of a non-dispersive model for this event is then discussed.

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.

A hydrodynamic analysis of so-called partial breakwaters ("digues partielles") is performed.It is shown that a simple rectangular shape indented with two "buckets" ("digue à baquets"), when properly dimensioned, can offer excellent performances as far as minimizing the transmission coefficient over a wide range of wave periods, with the drawback that large wave sloshing motions take place in the buckets.These shortcomings can be mitigated by adding up Jarlan walls inside the buckets, at the lee and weather sides.

The effects of localized imperfections on the collapse strength of plates are studied by means of finite element analysis performed by different authors. First a benchmark study is performed to assess the accuracy of the results obtained using different codes on the same case study. Afterwards, a parametric study on the effect of the localized imperfections on the collapse strength is performed. Finally, the effect of combining local and global imperfections was studied. It is found that the addition of a localized imperfection changes the collapse strength of the plate, which depends on the amplitude and size of the localized imperfection change.

Increasing requirements are coming to be set against ship vibration and noise from considerations of both safety and comfort. It is therefore required to perform, at an early stage of the design, vibration prediction by numerical simulation to identify possible problems and define solutions. Finite element methods are widely accepted as a standard tool for such studies. The problem is of prime importance for cruise ships and also for the new generation of fast Ro–Ro/Ro–Pax ferries and large container carriers. For those complex ships, the propeller is the main source of vibration and noise. Therefore, the first part of the paper will mainly focus on propeller excitation. From the propeller, excitation forces are transmitted into the ship via the shaft line and in the form of pressure pulses acting on the ship’s shell. The second part of the paper focuses on a typical cargo ship (container); identification of main excitation forces, as well as investigation of influence of analyzed excitation on ship vibration level. The following excitation forces have been analyzed: dynamic pressure on the transom deck, propeller-generated hydrodynamic forces, as well as internal and external engine-generated forces. Rigidity and attenuation characteristics of lubricating oil film have been taken into account in the analysis of excitations transmitted by propulsion system bearings (stern bearing, propeller shaft bearing and engine main and thrust bearings). The stiffness dynamic characteristics of the power transmitting system foundations have been also taken into account. Multivariant computations of ship vibration using the formerly identified excitation forces have been carried out. The results of analyzes have been verified by comparing the computation results with measuring tests.

Frequency-domain analysis can be used to evaluate the motions of the FPSO with its mooring and riser. The main assumption of the frequency-domain analysis is that the coupling is essentially linear. Calculations are performed taking into account first order wave loads on the floating structure. Added mass and radiation damping terms are frequency dependent, and can be easily considered in this formulation. The major non-linearity comes from the drag force both on lines and the floating structure. Linearization of the non-linear drag force acting on the lines is applied. The calculations can be extended to derive the low frequency motion of the floating structure. Second order low frequency quadratic transfer function is computed with a diffraction/radiation method. Given a wave spectrum, the second order force spectrum can then be derived. At the same time frequency-domain analysis is used to derive the low frequency motion and wave frequency motion of the floating system. As an example case, an FPSO is employed. Comparison is performed with time domain simulation to show the robustness of the frequency-domain analysis. Some calculations are also performed with either low frequency terms only or wave frequency terms only in order to check the effect of modeling low and wave frequency terms, separately. In the case study it is found that the low frequency motion is reduced by the wave frequency motion while the wave frequency motion is not affected by the low frequency motion.