Nonlinear Time-domain Coupled Analysis Of Mooring Systems Of Floating Structures

The ocean platforms used to drill for oil and natural gas in the deep ocean havebecome central issues that academics and engineers in many countries are focusedon. Ocean floating platforms are moored at a certain operation sea area and motionin waves excited by external loads, such as waves, currents and winds. Mooringlines are connected with a floating structure at fairleads, and the floating structure’smotion is effected by mooring force acted on it, while, the structure’s motion alsoeffects the motion of mooring lines. It’s important to study the dynamic andtime-varying coupled interaction between floating structures and mooring systems.This thesis focuses on ocean floating structures and mooring systems, and themethods in frequency domain and indirect time domain are applied to simulatefloating structures’ motion in waves. Elements of slender elastic rod are used tomodel the mooring line segments. Then, the coupled interaction is analyzed in timedomain.Seakeeping qualities of floating structures are studied based on potential flowtheory. Hydrodynamic coefficients and wave exciting force are calculated infrequency domain and indirect time domain. Hydrodynamics in frequency domainare analyzed by using commercial hydrodynamic solver. Deriving the equation offloating structures in indirect time domain is based on Cummins impulse theory. Aprogram used to simulate floating structures’ motion in waves is written in Fortranlanguage, and then a FPSO motion in regular and irregular waves without beingmoored is simulated, and the accuracy of this program is verified.Elements of slender elastic rod are used to model the mooring line segments.Static and dynamic equations are derived and dispersed in finite element method.The static and dynamic calculation programs are written in Fortran language. Thestatic program is used to simulate the common mooring lines and risers, and staticfluid loads and geometric nonlinearity are included. Its accuracy is verified bycomparison with catenary method and references. A buoy mooring system issimulated by using the dynamic program, which includes nonlinear fluid loads. Also,the nonlinear interactions between mooring lines and sea bed are simulated by thisdynamic program. Further, the equations of nonlinear material mooring line arederived, and the programs are used to analyze tension of nonlinear material mooringline, which is excited by a known external motion.Based on the previous two parts, the motion equations of a floating structurewith mooring systems are established in indirect time domain by combination ofmotion equations of a free floating structure and mooring lines. A coupled program is written in Fortran language to simulate a moored structure motion in waves. Thiscoupled program is used to simulate a moored FPSO motion in regular waves, andthe results are compared with the results of hydrodynamic solver to verify itsaccuracy. Then, the coupled program is used to simulate and analyze a mooredsemi-platform motion in irregular waves.