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Analytical Assessment of the Mooring System Stiffness
Abstract and Figures
This presentations reviews The definition of the mooring systems is one of the most important stages on the design any offshore unit. Some effects associated with it, on the responses of the floating body, are still under investigation. In this context, the full nonlinear high-order hierarchical modeling and the numerical integration of the resulting equations of motion might not be the most cost effective approach for the evaluation of those effects during the early design process. Thus, an expedite analytical formulation to assess the mooring system stiffness, a tool that could help the initial design and analysis. Using classic approaches from Analytical Mechanics, the nonlinear generalized restoring forces associated with the mooring acting on the vessel due to the mooring lines are formulated. The six-degree-of-freedom (DoF) problem is herein addressed. The stiffness matrix is obtained from the linearization of the generalized forces around a generic position. Mooring line characteristic tension function is an input of the method. The closed formulation does not requires a specific line model, although the formulation for a multi-segment mooring line is also derived. The methodology is applied taking the OC4-DeepCwind Floating Wind Turbine semi-submersible platform as a case study. Two Spread Mooring Systems arrangements are studied, in order to demonstrate the use of the presented formulation as a design tool. The calculated mooring system stiffness matrix, evaluated at the trivial equilibrium position, exhibits good agreement with numerical results found in the literature by high hierarchy models. Additionally, the stiffness coefficients are evaluated for other positions than the trivial equilibrium one in the form of colored maps. The natural periods of the motions on the horizontal plane are also mapped. These maps help to understand the effects of the static vessel mean position on the mooring system stiffness and, consequently, on the natural periods associated with the motions on the horizontal plane. Considering the original OC4 mooring system, the effects of the mooring line pre-tensioning are also investigated. Some conclusions on the axial stiffness of catenary cables are also made. The main contributions of the present master dissertation are: (i) the stiffness matrix analytical closed formulation and (ii) the use of colored maps to evaluate the stiffness and the natural periods as functions of the mean offset position. The present master dissertation brings then an innovative closed-form formulation with important practical applications.
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