Proceedings of the Eleventh (2001) International Offshore and Polar Engineering Conference Stavanger, Norway, June 17-22, 2001 Copyright 2001 by TireInternational Society of Offshore and Polar Engineers ISBN 1-880653-51-6 (Set); ISBN 1-880653-52-4 (Vol. I); ISSN 1098-6189 (SeO

Coupled Dynamic Response of Moored FPSO with RisersJ.M. HeurtierInstitut Frangais du P6trole, Rueil-Malmaison, France

P. Le Buhan and E. FontainePrincipia, La Seyne sur Mer, France

C. Le Cunff and F. BiolleyInstitut Fran~;ais du P6trole, Rueil-Malmaison, France

C. BerhaultPrincipia, La Seyne sur Mer, France

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. KEY-WORDS: FPSO, coupled analysis, riser, mooring lines 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 uncoupled analysis consists of two phases. 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. In the first step of the uncoupled analysis, the low

frequency (LF) and wave frequency (WF) part of the ship motion are computed separately. For the added mass and damping coefficients, zero asymptotic values are used in the low frequency computations, while frequency dependent values are chosen to evaluate the wave frequency part of the signal. The so-called memory effect, represented by a convolution integral arising from the passage from the frequency to the time domain is neglected, therefore assuming a clear distinction between the low and wave frequencies. This assumption can be justified for large bodies like a FPSO vessel, but may fail in the case of an offloading buoy which size is comparable to the characteristic wave length. In the ship motion computation, the mooring lines and risers are represented by their corresponding stiffnesses which may depend on the vessel position. These stiffnesses values result from preliminary quasi-static computations. Static equilibrium is computed either using analytical cable elements, or numerically using a Finite Element Method together with a beam element approximation to account for the bending stiffness of the line. Within the framework of the uncoupled approach, the ship motions are not influenced by the mooring lines and risers accelerations. The low frequency drift damping effect due to the lines is also neglected, although taking into account this phenomenon is essential to obtain accurate predictions for the roll motion and the dynamic behaviour in extreme waves (see e.g. Webster, 1995). As far as hydrodynamics loads are concerned, the main nonlinearities of the flow are classically accounted for within the framework of second order potential flow theory, see e.g. Newman (1967) and Molin (1993) for a more recent and complete analysis. Second order radiation - diffraction theory has proven its efficiency to explain the observed nonlinear effects in most cases. For consistency, it would appear natural to solve with the same level of accuracy the motion of the mooring lines and risers, together with their relative influence on the ship motion. Such coupled effects have only recently received considerable attention, due to the large amount of CPU time required for their study. As we shall see, the time step used in the structural computation of the mooring lines is much smaller than the hydrodynamic time step driving the floater

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