Graduate Thesis Or Dissertation
 

Coupled response of compliant offshore platforms

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/kk91fp30x

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  • A three dimensional finite element analysis is developed which is capable of simulating the coupled static and dynamic behavior of compliant ocean structures. Nonlinearities which result from large-deflection, reduced or zero stiffness in compression, and the nonconservative fluid loading are considered. The spatial variation of fluid loading is also addressed. The structures are assumed to be in the Morison flow regime. Linear wave theory is used and multidirectional seas may be simulated. A current profile which varies in magnitude and direction with depth may be specified. Concentrated masses and loads as well as foundation properties may also be modelled. The problem is formulated in updated Lagrangian coordinates and a residual feedback incremental-iterative solution scheme is used. Viscous relaxation is used to start the static solution of problems with low initial stiffnesses. The dynamic solution is performed in the time domain and uses the Newmark integration scheme. The beam-column element is a two-noded subparametric element with geometric stiffness. The cable element is a two-noded straight iso-parametric element. Consistent mass matrices are developed for both elements. The directionality of the hydrodynamic added mass is accounted for as is the discontinuity of the mass density for surface piercing elements. A numerical scheme for calculating the equivalent nodal loads due to an arbitrary load profile between nodes is formulated. Numerical examples are presented to validate the solution technique and to demonstrate its use on three types of compliant ocean structures: 1) the articulated tower, 2) the guyed tower, and 3) the tension leg platform. Results from the guyed tower example indicate that a decoupled analysis yields conservative peak guy tension and deck displacement values. However the phase and form of the guy tension trace for the coupled analysis is significantly different from the decoupled solution.
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