Hydroelastic Analysis Coupled with Nonlinear Mooring-Risers for a Moored Deformable Floating Body and Development of Dynamic Positioning System for High Accuracy

Abstract

Two main topics of offshore technologies, hydroelasticity and dynamic positioning system, are investigated in the present research. For the larger dimension of offshore structures in harsher operating environments, comprehensive hydroelastic analysis is developed. The interactions of a moored deformable floating body and waves are solved by the modal expansion method, and the dynamics of the deformable floating body is fully coupled with the nonlinear mooring-riser dynamics. Not only the hydroelastic dynamic motion responses but also the corresponding dynamic stress resultants are achieved for regular waves and random seas. A new methodology is developed for the direct time domain hydroelastic structural analysis for the random seas without infeasible computational fluid dynamics. As case studies, both of a slender structure and a large volume 3D structure are considered. The slender body case adopts the well-known beam theory to represent the deformations and 3D structure case employs the finite element method for modal analysis. Moreover, the twisting effects of the 3D structure and corresponding 3D hydroelastic effects are investigated. The developed numerical tools are robustly verified by cross-correlation between frequency and time domains. Each case study includes the resonance phenomenon of the floating body deformation, and its influences on the dynamic motions and stress resultants are studied. The enhanced resonances due to the nonlinear wave loads are investigated by adding the 2nd order wave loads to the time domain hydroelastic dynamic analyses. Taking advantages of time-domain hydroelastic analysis, a numerical hammer test is conceived and the real wet natural frequencies are identified. For the purpose of float-over-based offshore installment, a high accuracy dynamic positioning system is developed. The optimum control system is designed using a combination of Kalman filter and PD control with the separation theorem. Subsequently, the generalized allocation scheme is employed for faster and more accurate reactions. As an example, an appreciably small watch circle is achieved and the corresponding resultants are clarified in terms of individual thrusts and azimuth angles. Moreover, the developed DP control is embedded into the mooring-riser-hull coupled dynamics, and the benefits and characteristics of the mooring-