A new three-dimensional finite element model of an inextensible elastic rod with equal principal stiffnesses is presented. The model permits large deflections and finite rotations and accounts for tension variation along its length. Its use in static analysis is described and a time integration method for dynamic analysis is developed. Accuracy of the spatial discretization and stability of the time integration method are demonstrated by comparison of numerical results with exact solutions for certain nonlinear problems.

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Fully coupled global analysis of Floating Production Systems, including the vessel, mooring and risers is described. Modeling procedures for each component, including hydrodynamic modeling, characterization of the environment and element length selection for mooring lines and risers, are presented. Simulation requirements for both Frequency Domain and Time Domain analysis are presented. This includes criteria for selection of frequency content and simulation length.Results for two Floating Production Systems are presented. The first is a Gulf of Mexico spar with taut-leg mooring and top-tensioned risers in a water depth of 6,000 ft. The second is a spread moored West Africa FPSO with steel catenary risers in 4,500 ft water depth. Modeling of each is described. Results for vessel motions, riser stresses, and mooring line tensions are presented, including detailed comparisons of Time Domain and Frequency Domain results. INTRODUCTIONDevelopment of deepwater fields requires complex systems for production and export of oil and gas. These can include combinations of steel catenary risers, flexible flowlines, top tensioned risers, and hybrid risers, connected to one or more moored floating structures ("floaters") such as spars, TLPs, semisubmersibles, and ships. The floater may be moored with either catenary or taut legs.Coupled analysis includes detailed models of the floater, the mooring lines and the risers in a single model. A single model can include a floater, 12-16 mooring lines and 20 or more risers, resulting in models with more than 10,000 degrees of freedom. Stochastic analysis is used to calculate the response to wind, wave and current loads. For fatigue analysis, there may be more than 1,000 load cases. Efficient, accurate modeling is needed to produce the results needed for design.Recommendations for developing efficient, accurate models for both Time Domain and Frequency Domain analysis are presented. The criteria have been developed over many years of modeling and analysis of Floating Production Systems. COUPLED ANALYSIS METHODOLOGYThe coupled analysis for the examples was performed using Stress Engineering Services' RAMS (Rational Approach to Marine Systems) software. The modeling criteria are not restricted to RAMS, but are generally applicable.RAMS uses a consistent cubic finite element representation of an elastic rod [1] to model risers, TLP tethers and mooring lines. The element includes axial and bending stiffness. Bending stiffness varies linearly and tension varies quadratically along the axis of the rod. The external load model accounts for weight, buoyancy, internal fluids and hydrodynamic loads. A generalized Morison model is used to compute hydrodynamic loads. Seabed contact is simulated by nonlinear spring and friction models. The finite element method uses a displacement formulation that allows for large element displacements and rotations in three-dimensional space, while strains are assumed to be moderate. Each floater is represented by a six degree of freedom rigid body, which is introd...

Viscous damping due to drag on mooring lines and risers is seastate dependent and significantly affects the motion of a floating platform in deep water, particularly in everyday seastates. This in turn impacts design of the risers, which are typically controlled by fatigue. The dynamic interaction between the platform, mooring and risers cannot be evaluated using conventional uncoupled analysis tools, where each is analyzed separately. Rather, coupled analysis is required to provide a consistent way to model the drag-induced damping from mooring lines and risers. We describe a coupled, frequency domain approach (RAMS – Rational Approach to Marine Systems) for calculating the dynamic response of vessel, mooring and risers. In coupled analysis, the risers and mooring lines are included in the model along with the floater. In this way, damping of the floater motion due to drag on the mooring lines and risers is incorporated directly. It is also valuable to estimate the linear damping factors from the full, coupled analysis results. These damping factors may then, for example, be used in an equivalent linear model of the floating system in which the stiffness and damping effects of the mooring and risers are represented as additions to the floater stiffness and damping matrices. Such a model could be used to efficiently design a subsystem (e.g.; an export riser). We describe a technique to determine the equivalent linear damping factors from the coupled analysis results. This paper also illustrates the use of these methods for a West Africa FPSO. The need for coupled analysis is shown by comparing results from the fully coupled model with those obtained using an uncoupled method in which the mooring line damping is approximated.