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Results
Numerical Simulations of Vortex-Induced Vibrations of Slender Flexible Offshore Structures
Etienne, S. (Institut Franfais du Pétrole) | Biolley, F. (Institut Franfais du Pétrole) | Fontaine, E. (Institut Franfais du Pétrole) | Le Cunff, C. (Institut Franfais du Pétrole) | Heurtier, J.M. (Institut Franfais du Pétrole)
ABSTRACT Vortex-induced vibrations (VIV) of a slender offshore riser are studied. The coupled fluid- structure problem is solved numerically within the framework of strip theory. In each cross-section, the two-dimensional turbulent flow around the cylindrical riser is computed using a specifically designed numerical method which has been thoroughly validated through systematic comparisons between numerical and experimental results. An algorithm has been developed to couple this computational fluid dynamics (CFD) code with DeepLines, a structural software based on a Finite Element Method. The local displacements of the cross-sections are handled within the two-dimensional flow field computations, while the three-dimensional motion of the overall riser is obtained by resolution of the structural problem. Cross-sections are therefore mutually influenced through the global motion of the riser. This coupled approach is then validated on simple tests cases for which quasi-analytical results are available (CFD and modal approach). Finally, comparisons between numerical results and in-situ measurements are presented. INTRODUCTION Flow-induced vibrations of structures are a phenomenon well known to civil engineers since it may generate structural fatigue or threaten the integrity of the overall structure in extreme situations. For arrays of offshore risers, both vortex-induced vibrations and fluid elastic instabilities, such as wake galloping, have been observed. The continuous increase of depth, from deep to ultra-deep waters, has led the oil industry to introduce new concepts requiring engineering tools able to predict these complex phenomena with reasonable accuracy. The vortex-induced vibrations phenomenon is a resonant interaction which arises when the Strouhal frequency associated with vortex shedding is close to one of the natural frequency of the structure. This fluid-structure interaction occurs therefore for a limited range of the reduced velocity, and the corresponding motions of the structure are sell-limited to an amplitude of about one diameter.
Coupled Dynamic Response of Moored FPSO With Risers
Heurtier, J.M. (Institut Frangais du P6trole) | Le Buhan, P. (Principia, La Seyne sur Mer) | Fontaine, E. (Principia, La Seyne sur Mer) | Le Cunff, C. (Institut Français du Pétrole) | Biolley, F. (Institut Français du Pétrole) | Berhault, C. (Principia, La Seyne sur Mer)
ABSTRACT This paper deals with the dynamic response to environmental sea loads of complex offshore structures, such as ship-based Floating Production and Storage Offloading vessels (FPSO) with mooring lines and risers. Usually, each component is analyzed individually, and sub-system interactions are then accounted for in a simplified way. Intrinsically, such a modelling based on an uncoupled approach remains limited to cases of weak interactions. In the present study, a fully coupled approach is presented wherein the motions of the floater, mooring lines and risers are computed simultaneously in the time domain. Comparisons between coupled and uncoupled results are presented for a moored FPSO in harsh environment. INTRODUCTION Computing the dynamic behaviour of a multi-component offshore structure due to environmental sea loads (wind, waves, current) is a complex problem. In the early phase of a project, it is common practice to design each component of the system individually (see e.g. API-2SK), eventually taking into account subsystem interactions in a simplified way. This type of approach is commonly referred to as an uncoupled analysis, by opposition to a fully coupled method wherein all the system components and their mutual interactions are computed simultaneously. The degree of uncertainty of the uncoupled approach is not clearly defined, nor its domain of validity. In particular, the validity of such an approach remains questionable in harsh environment where the highest accelerations are expected. The goal of the study is to provide more insight into the relative importance of these coupled effects for the case of a FPSO moored in deep water. The ship motion is first computed based on a simplified response of the mooring lines. In the second step, the motion given by the RAO is imposed as top end excitations to study the dynamics of the risers and mooring lines.
- Facilities Design, Construction and Operation > Pipelines, Flowlines and Risers > Risers (1.00)
- Facilities Design, Construction and Operation > Offshore Facilities and Subsea Systems > Mooring systems (1.00)
- Facilities Design, Construction and Operation > Offshore Facilities and Subsea Systems > Floating production systems (1.00)
ABSTRACT Slamming loads during the hydrodynamic impact of a threedimensional body are studied numerically. First, the finite element method is used to solved the fluid part. For the water entry problem of a rigid body, the numerical results are successfully compared with analytical solutions for wedge, cone and the three dimensional case of an elliptic paraboloid shaped bodies. In the second part of the paper, the coupled formulation and the coupling matrix between fluid and structure part which will be used to treat the hydroelasticity impact problem is presented and validated by solving for sloshing in a tank with an elastic wall problem. INTRODUCTION In severe sea conditions, impact loads with high pressure occur when the hull of a ship strikes the water surface. These impulsive loads may generate plastic deformations of the local hull structure. Fractures have also been observed as the result of severe slamming events. In extreme cases, the integrity of the overall structure may be threatened due to a large increase of the global bending stresses. The ability to better predict the structural response of the ship hull to slamming loads, both locally and globally, appears therefore necessary. Reviews on the subject of slamming have been proposed by Korobkin & Pukhnachov (1988), Mizoguchi & Tanizawa (1996) and more recently by Faltinsen (2000) who focus attention on the influence of hydroelasticity effects. From a theoretical point of view, slamming loads have been mostly studied within the framework of potential flow theory, assuming blunt and rigid body, and planar flow. Under these assumptions, first order asymptotic solutions were found for the ease of a wedge with small deadrise angles (Wagner, 1932), a cylinder (Cointe, 1987), and more generally, for arbitrary two-dimensional blunt body shapes (Cointe, 1989, Howison, Ockendon & Wilson, 1991).