Investigation On Deepwater Taut-wire Mooring System Design

With the development of resource exploitation in deep sea, the research of new deepwater mooring systems has become an important domain. In this study, key steps of Taut-wire mooring systems design such as calculation of the ultimate pull-out capacity (UPC) of Vertical Loaded Anchor (VLA), dynamic analysis of mooring lines and coupled calculation of moored floating body are investigated by computations. This provides useful theoretical support for the application of Taut-wire mooring system in deepwater structures.Taut-wire mooring system usually adopts Suction Caissons or VLA as its basis. This study provides the construction, mechanism, installation, recovery and advantages of a new type of VLA—DENNLA. More important, UPC is a key factor to verify whether VLA is out of operation. However, the current calculation of UPC is based on the empirical formula which can hardly provides accurate results. In this study, the UPC of VLA is calculated by a three dimension non-linear finite element method based on Elastic-Plastic theory of mud. In addition, the influence of embedment depth, fluke angle and action angle of mooring force on UPC is analyzed by calculating different numerical models.In the dynamic analysis of taut-wire mooring lines, the non-linear deformation effect of a mooring line cannot be neglected under the condition of large deformation and large pretension. Based on the theory of the elastic rod, this study obtains static and dynamic equations of mooring lines by analyzing the force conditions on them. Then, non-linear finite element method is adopted to solve the equations. In the static analysis of mooring lines, Newton-Raphson iterative method is adopted to obtain the relation between tension and offset of mooring lines. In the dynamic analysis, the inertia forces and the added water mass are considered in calculating the forces on the mooring line elements. Considering that on every time point the elements are on balance, Adam-Bashforth method is adopted to solve the dynamic equation of mooring lines' motion and obtain the line tension. Furthermore, based on different numerical models, the tension on different part of mooring lines and the influence of lines' segments on tension force are investigated in this study.Finally, this study provides a basic design method of Taut-wire mooring systems. In the coupled calculation of moored body, the motion of floating body is obtained by GREEN function method in time-domain, and then coupled with non-linear finite element method which is used to analyze the dynamic characters of mooring lines to obtain the motion of moored body. Based on a numerical example, the feasibility and effectiveness of adopting Taut-wire mooring systems for deepwater semi-submersible platform positioning are verified by the results.