An assessment of mooring systems' forces of ships berthed at dolphins

Abstract

In the current practice of structural design for berth structures, the mooring forces (tension in lines and compression in fenders) are typically determined through simplified linear static analyses, and then magnified by dynamic amplification coefficients. It is known, however, that the mooring's stiffness presents nonlinearities (both physical and geometrical) that may at times become relevant, raising the issue of whether linear analyses are really adequate. Also, the environmental loads induce oscillatory behavior whose peak values may not always be satisfactorily estimated through dynamic amplification coefficients, which are far from being well defined in the technical literature. This work presents an investigation on the forces developed on mooring systems of vessels berthed at dolphins, and the consequences of the current design practice's simplifications. Different hierarchical models are defined (linear static, nonlinear static and linear dynamic in the frequency domain) for a case study of a Floating Storage and Regasification Unit (FSRU) permanently berthed at dolphins at the Barcarena's Liquefied Natural Gas (LNG) terminal in Pará, Brazil. The vessel's displacements and their peak values are determined through stochastic and statistical theories. The corresponding forces on the mooring system are computed and the dynamic amplification coefficients therefrom obtained. A thorough discussion on the results is provided. The methodology presented may be a useful tool to ascertain the dynamic amplification coefficients required in the simplified methodologies from the international standards.