STABILITY BASED DESIGN OF TWO-POINT MOORING SYSTEMS WITH HYDRODYNAMIC MEMORY
JIN-SUG CIIUNG, Ph.D., Research Engineer, Research & Development Department, American Bureau of Shipping and Affiliated Company, Two World Trade Center, 106th Floor, New York New York 10048, USA
ABSTRACT
A systematic method of determining the design parameter values of Two-Point Mooring (TPM) systems is presented. The conclusions of nonlinear stability analysis of horizontal plane slow motion dynamics of TPM systems with hydrodynamic memory are used to guide selection of the design variables. Specifically, within the stable regions of bifurcation diagram, a set of parameter values is chosen systematically to examine the corresponding TPM system. Then, each TPM system is simulated and the results are evaluated in terms of given criteria such as tension level of the mooring line or offset of the moored vessel. The hydrodynamic memory effect due to the oscillatory motions of moored vessel is included in the formulation. The memory effect influences the stability properties of TPM systems and alters the design chart/bifurcation diagram and particularly the static loss and dynamic loss of stability. The significance of hydrodynamic memory on several TPM systems is illustrated by comparing the statistics of the simulations. The design approach implemented in this work shows that nonlinear stability theory can be used effectively in the actual design process of mooring systems to reduce dramatically the number of simulations required. It is also shown that, the often inconclusive nonlinear time simulations can be guided by theoretical nonlinear stability an analysis of system dynamics to produce improved design values of system parameters of TPM systems.
EFFICIENT CALCULATION OF HYDRODYNAMIC PROPERTIES OF OWC-TYPE DEVICES
David V. Evans and Richard Porter, School of Mathematics, University of Bristol, Bristol, B581TW, UK
ABSTRACT
A simple model of an OWC-type wave-energy device is used to illustrate a powerful accurate method for computing the hydrodynamic coefficients when sharp edges are present. The device consists simply of a vertical partially-immersed circular cylinder open at either end, with power being extracted by constricting the flow of the air trapped in the cylinder above the internal free surface. The method involves the use of the theory of pressure distributions for OWC devices and the hydrodynamical coefficients are computed using an accurate Galerkin method which preserves the reciprocity relations.
