Nonlinear Time-domain Analysis of Floating Space Frame Structures: Floating Offshore Structures
Authors
Kadriu, Arber ; Taha, Alaa Mohammad
Term
4. term
Education
Publication year
2015
Submitted on
2015-06-16
Pages
111
Abstract
Denne afhandling undersøger, hvordan man kan forudsige belastninger og den samlede dynamiske respons for et flydende system bestående af offshore fortøjningsliner, en bøje og en slank rumramme (en let gitterkonstruktion) udsat for opdrift og bølger. Da strukturen kan deformere meget, anvendes en ikke-lineær analyse; konkret en to-dimensionel korotationel bjælkeformulering, som kan håndtere store rotationer og deformationer. Alle dele modelleres som cylindriske bjælkeelementer i to værktøjer: en ny Nonlinear Wave Code udviklet i MATLAB og det kommercielle finite element-program Ansys Workbench, som bruges til at validere den nye kode. Nonlinear Wave Code kan simulere forskellige havtilstande, herunder lineære, ikke-lineære og uregelmæssige bølger. Hydrostatik og hydrodynamiske kræfter beregnes med flere metoder, hvor bølge- og opdriftskræfter bestemmes ud fra, hvor stor en del af hvert medlems tværsnit der er neddykket. Metoderne kontrolleres løbende på simple eksempler med lodrette og vandrette cylindre, før den flydende rumramme analyseres. Rumrammen modelleres med en fin netopdeling for at opnå en tilstrækkelig repræsentation af havbelastninger og dynamisk respons. Der udføres en tidsdomæneanalyse, hvor strukturen påvirkes af en lineær bølge. De beregnede hydrostatiske og hydrodynamiske kræfter samt den dynamiske respons fra Nonlinear Wave Code og Ansys Workbench stemmer overens. Dermed kan Nonlinear Wave Code forudsige den dynamiske respons af en flydende slank rumramme.
This thesis examines how to predict the forces and overall motion (global dynamic response) of a floating system made up of offshore mooring ropes, a buoy, and a slender space frame (a lightweight lattice) under buoyancy and wave loading. Because the structure can experience large deformations, a nonlinear analysis is used; specifically, a two-dimensional corotational beam formulation that handles large rotations and deformations. All components are modeled as cylindrical beam elements in two tools: a new Nonlinear Wave Code developed in MATLAB and the commercial finite element program Ansys Workbench, which is used to validate the new code. The Nonlinear Wave Code can simulate different sea states, including linear, nonlinear, and irregular waves. Hydrostatic and hydrodynamic forces are computed with several methods, where wave and buoyancy loads depend on how much of each member’s cross-section is submerged. The methods are checked throughout using simple cases with vertical and horizontal cylinders before predicting the dynamic behavior of the floating space frame. The space frame is modeled with a fine mesh to capture ocean loads and dynamic response. A time-domain analysis is performed with a linear wave. The hydrostatic and hydrodynamic forces and the dynamic response predicted by the Nonlinear Wave Code and by Ansys Workbench are consistent, indicating that the Nonlinear Wave Code can predict the dynamic response of a floating slender space-frame structure.
[This abstract was generated with the help of AI]