A master thesis from Aalborg University
Nonlinear Time-domain Analysis of Floating Space Frame Structures: Floating Offshore Structures
Author(s)
Term
4. term
Education
Publication year
2015
Submitted on
2015-06-16
Pages
111 pages
Abstract
This report concerns the prediction of the loads and global dynamic response of a floating structure consisting of offshore anchoring ropes, a buoy and a slender space frame structure subjected to buoyancy and wave loading. Nonlinear analysis is required as the floating structure undergoes large deformations, and thus a two-dimensional corotational beam formulation is implemented. All structural components are modelled by cylindrical beam elements in both a produced Nonlinear Wave Code in Matlab and in the commercial finite element program Ansys Workbench which is used as validation source for the Nonlinear Wave Code. Via the Nonlinear Wave Code it is possible to evaluate the response of the floating structure in different sea states, including linear waves, nonlinear waves and irregular waves. The hydrostatics and hydrodynamic forces are modelled by various methods, in which the wave and buoyancy forces are found as a function of the amount of submerged cross-sectional area of the structure. Simple validation examples consisting of vertical and horizontal cylinders are performed continually through the report for the purpose of validating the implemented methods before the dynamic response of the floating space frame structure is predicted. The floating space frame structure is modelled with a fine mesh to obtain a sufficient representation of the ocean loads and the dynamic response of the structure. A time-domain analysis is performed, in which the structure is subjected to a linear wave. The predicted hydrostatics and hydrodynamic forces and the dynamic response in the Nonlinear Wave Code and Ansys Workbench are consistent. The Nonlinear Wave Code is thus able to predict the dynamic response of a floating slender space frame structure.
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