Dynamic responses of moored installation vessel during lifting operation due to environmental load

摘要

This paper concerns dynamic computational analysis of an installation vessel secured to a symmetrical mooring system, under the effect of environmental loading during offshore lifting operation. The harsh environment is acknowledged as the greatest challenge that should be carefully considered. Experimental studies in shallow water face serious problems due to factors involving costing, duration, scaling and modelling matters. Computational Fluid Dynamic, CFD capable of performing analysis for global performance of moored systems subject to random sea states is utilized. In this paper, the vessel's dynamic motion with and without the effect of the additional load at vessel's main crane is studied. Simulating the coupled dynamics of offshore installation vessel is almost essential where the effect of mooring lines motion, loads arising to the system due to fluid viscosity, external effects such as wind loads exerted on the structure above sea water level, are all coupled with the wave loads acting on the vessel. A software computer program, namely ANSYS AQWA will be used in the analyses. The hydrodynamic analysis of the vessel hull is performed in the frequency domain solution scheme. A fully coupled cable dynamic features enabling mooring line drag and inertial characteristics are included in the vessel motions analysis in time domain solution, which can provide much greater accuracy but at the cost of longer time to run. It introduces a consistent and accurate analytical approach that ensures higher dynamic interaction between the vessel hydrodynamic and mooring responses. By performing the non-linear time domain analysis, evidence is provided that the wave direction and the wave amplitude are major contributing factors on the vessel's motion and mooring lines tension.