Offshore wind turbines are designed and analyzed using comprehensive simulation tools (or codes) that account for the coupled dynamics of the wind inflow, aerodynamics, elasticity, and controls of the turbine, along with the incident waves, sea current, hydrodynamics, mooring dynamics, and foundation dynamics of the support structure. This paper describes the latest findings of the code-to-code verification activities of the Offshore Code Comparison Collaboration Continuation project, which operates under the International Energy Agency Wind Task 30. In the latest phase of the project, participants used an assortment of simulation codes to model the coupled dynamic response of a 5-MW wind turbine installed on a floating semisubmersible in 200 m of water. Code predictions were compared from load case simulations selected to test different model features. The comparisons have resulted in a greater understanding of offshore floating wind turbine dynamics and modeling techniques, and better knowledge of the validity of various approximations. The lessons learned from this exercise have improved the participants’ codes, thus improving the standard of offshore wind turbine modeling.

In 2002, several mooring chains of a deepwater offloading buoy failed prematurely within a very small time frame. These chains were designed according to conventional offshore fatigue assessment using API recommendations. With this first deepwater buoy application, a new mooring chain fatigue mechanism was discovered. High pretension levels combined with significant mooring chain motions caused interlink rotations that generated significant Out of Plane Bending (OPB) fatigue loading. Traditionally, interlink rotations are relatively harmless and generate low bending stresses in the chain links. The intimate mating contact that occurs due to the plastic deformation during the proof loading and the high pretension of the more contemporary mooring designs have been identified as aggravating factors for this phenomenon. A Joint Industry Project (JIP), gathering 26 different companies, was started in 2007 to better understand the Out of Plane Bending (OPB) mooring chain fatigue mechanism and to propose mooring chain fatigue design recommendations. This paper summarizes the computational Finite Element Analysis (FEA) scope of work that provided the understanding and validation of the OPB mechanism through correlation with the test program results on chains. In addition, a multiaxial assessment of the fatigue stresses is studied and the main results are presented in this paper.

Linear and nonlinear spatial developments of two-dimensional wall jets on curved surfaces are computed using pseudospectral-finite difference methods. Inviscid analysis shows that the instability originates from the inner/outer region on a concave/convex wall; the shear layer is thus always unstable regardless of the curvature. This primary instability is a steady spanwise vortex structure similar to Görtler instability in a Blasius flow. In the present study, a perturbation of prescribed wave number α is assumed. In the limit of high Reynolds number (Re) and small curvature (ε), a parabolic set of nonlinear equations describes the spatial evolution of the disturbance. Direct marching simulation of the perturbation and a parabolic stability approach are employed. Both give the same results with different computational efficiencies. For the concave case at low Görtler numbers (G2=ε√Re), perturbations are unstable for small α. Their energy reaches a maximum and then decays. At high G, the most unstable disturbance occurring at larger α will grow exponentially and reach saturation. The convex case is the most unstable situation. But as for the concave case, the most dangerous disturbance moves from small to larger α as G increases. The numerical results are able to capture the primary instability as observed in the experiment of Matsson [Phys. Fluids 7, 3048 (1995)].

In the present paper, a new fully coupled simulator based on DeepLines™ software is described in order to address floating wind turbines dynamic simulation. It allows its user to take into account either separately or together the hydrodynamic and aerodynamic effects on one or several floating wind turbines. This simulator includes a non linear beam finite elements formulation to model the structural components — blades, tower, drivetrain, mooring lines and umbilicals — for both HAWT and VAWT layouts and advanced hydrodynamic capabilities to define all kinds of floating units and complex environmental loadings. The floating supports are defined with complete hydrodynamic databases computed with a seakeeping program. The aerodynamic loads acting on the turbine rotor are dynamically computed by an external aerodynamic library, which first release includes BEM (blade element moment for HAWTs) and SSM (single streamtube method for VAWTs) methods. The integration in time is performed with an implicit Newmark integration scheme.

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.

Solutions of the Navier–Stokes equations for flow in a curved channel have previously been computed [Bottaro, J. Fluid Mech. 251, 627 (1993)] and the spatial development of the Dean flow is available at different supercritical Reynolds number. In this work the viscous instability of local longitudinal vortex structures (obtained from the nonlinear simulations) is investigated, with a focus toward the secondary instability of Dean vortices. Such a secondary instability takes the form of streamwise traveling waves. High-frequency waves are termed twisting waves, low frequency are defined undulating waves. Instead of performing analyses in which the basic flow in the cross section is two dimensional, significant shear profiles along y and z are considered as base flows at constant x before the establishment of a fully developed state. Thus one is able to discover that the twist instability is of shear type and is caused by inflectional spanwise profiles of the streamwise velocity component. Sinuous waves are always preferred to varicose waves, and the latter mode of instability is destabilized only at large Reynolds numbers. Undulating waves are related to normal profiles of the streamwise velocity; this type of secondary instability is of centrifugal origin. Results of the analyses for both types of waves are in good agreement with experiments.

Experiments were carried out at IFREMER Brest to obtain data on the displacement of Steel Catenary Risers (SCR) in the Touch-Down Zone (TDZ) induced by top motion. Measurements were conducted both in the section on the ground (2D motion) and in the section above the touch-down point (3D motion) with an optical tracking system. In the model test, the bottom 1/10 of the riser was represented at a scale of about 1/10. The present paper is focusing on Heave-Induced Lateral Motion (HILM) in the bottom part of the riser. The model riser is based on a full scale case which is first described. Based on Froude’s similitude, the geometrical characteristics of the model are derived. The tracking system and the various instrumentation are then detailed. In the last section, experimental results on one specific case are presented. Numerical calculations with a coupled fluid/structure solver are performed. A comparison for the amplitude and frequencies of HILM is presented. Comments are made on the analogy between the present experiments and some simpler experiments on a one-degree-of-freedom cylinder/spring system sinusoidally excited. Such an analogy should prove itself very fruitful in understanding and quantifying HILM.

Résumé -Vibrations d'un riser soumis à un écoulement : étude théorique, numérique et expérimentale -Les vibrations engendrées par le relâcher tourbillonnaire dans le sillage d'un cylindre soumis à un courant peuvent créer une fatigue importante dans les tubes utilisés par l'industrie offshore pour amener le pétrole ou le gaz du fond de la mer jusqu'à la plate-forme ou le navire de stockage. Ce sujet fait l'objet de très nombreuses études et, à l'Institut français du pétrole, plusieurs modèles sont développés pour prédire la durée de vie de ces tubes. Les méthodes vont d'une simple analyse modale de la réponse de la structure jusqu'à un calcul couplé fluide-structure avec résolution des équations de Navier-Stokes. Au travers du projet Hydlines, des campagnes d'essais sont menées pour valider les différentes approches.