Performance Of Offshore Mooring Systems During Installation And In Service

The exploitation of offshore energy demands the use of a large number of offshore floating structures.These structures are positioned within a certain range by mooring systems,which consists of mooring lines and mooring foundations.The study on the performance of mooring system during installation and in service involves both offshore structure and geotechnical engineering.The interaction between mooring foundation and seabed soil,and the dynamic contact between mooring line and seabed soil,are both involved in the service of mooring system.It determines the bearing capcity of the mooring system,and also affects the safty of the floating structure.This dissertation conducts a series of researches on the aforementioned issues,which are addressed in this thesis:1.The dynamic analysis of a mooring line is carried out with consideration of the line-soil interaction.Two kinds of seabed sediments,clay and sand,are simulated by different equations.The configuration and tension of the mooring line are first simulated in a pretension case to verify the dynamic model.The mechanism of progressive dig-in effect of mooring line into seabed under cyclic motion is studied.And its effect on the line tension and configuration is evaluated.2.A quasi-static model is proposed to predict the installation of Drag Embedment Anchor(DEA),based on the progressive penetration model of anchor and seabed.The numerical simulation of the penetration trajectory takes consideration of the development in line configuration.The intergrated model consists of the anchor,mooring line and anchor handling vessel.The differences between vessel moving method and line witching method are compared.This model is used to simulate the effect of mooring line length(or mooring radius),line type,shank-fluke angle,soil sensitivity,strength gradient and current velocity on the installation trajectory and ultimate depth.3.The transient pullout behavior of suction bucket foundation is studied through finite element method.Two soil constitutive models,the classical Mohr-Coulomb model and the advanced boundary surface model are adopted for comparison.The model parameters are calibrated against a series of drained and undrained triaxial and DSS tests on dense sand.It is proved that the model is able to capture the behavior and dilation of dense sand under different stress levels.An FEM model is built based on these constitutive models and the calibrated parameters to simulate the pullout response of a suction bucket under different pullout rates.Comparisons are made between different drainage conditions and stress conditions.4.When the suction bucket is subjected to cyclic loading,excess pore-pressure will be generated aound anchor,and reduce the anchor capacity.By introducing the oscillatory and residual mechanisms,a novel numerical model is proposed to predict the instantaneous variations and accumulations of excess pore-pressures around a suction under vertical cyclic loads.As a result,the decrease in effective soil stress leads to a decrease on the interface friction between the bucket wall and the seabed soil.Detailed parametric studies reveal the effects of load property and soil parameters on the pore-pressure accumulation and friction decrease.5.The intergrated mooring system analysis of a construction platform in South China Sea is studied as an example.Coupled simulation of the floating structure and mooring system is conducted to evaluate the platform motion and positioning performance.Dynamic analysis of the mooring line are used to simulate the development in configuration and tension.The performance of the mooring foundation under mooring line tension with an uplift angle is analysed with consideration of the seabed property.