TY - JOUR
TI - Analytical Assessment of the Mooring System Stiffness
AU - Amaral, Giovanni Aiosa Do
AB - 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.
DA - 2020/03//
PY - 2020
DO - 10.13140/RG.2.2.28848.38401
DP - DOI.org (Datacite)
LA - en
UR - http://rgdoi.net/10.13140/RG.2.2.28848.38401
Y2 - 2020/10/19/07:49:37
ER -