The Dynamic Analysis Of Mooring System Of The Spar Platform

Nowadays, the oil storage in the offshore oil fields which are near the earth is limited and it will run out of storage someday. In order to meet the rising needs of the oil energy resources, people are exploring the oil fields in the deep water domain. The Spar platform is one of the most popular and useful platforms considering it merits in oil exploration, exploitation and production, so it has been widely used in the deepwater oil fields.At present, most of the Spar platforms are using the suction anchor, which cause the transformation of the mooringline forms - semi-tension mooringline, which are different from the former ones. Considering this change, the former catenary method is lack of accuracy in calculating the mooringline response, so this article adopts the new model based on the finite element method to form the mooring system stiffness matrix. In the static analysis, wave forces, current forces and weight are taken into account and distributed to the nodes using certain rules. The non-linear equations are solved by the Newton iteration methods, and the displacement and the tension vectors are obtained. In the dynamic analysis, the inertia forces and the added water mass are considered in calculating the forces on the mooringline elements. Considering that on every time point the element is on balance, so the motion equations are formed on every time point. The motion equation of the mooringline is solved by the four-order Runge-Kutta method and the motion equation of the platform is solved by the Houbolt methods. Through the iteration between the mooringline and the platform motion equations, the results of the tension on the mooringline are solved on each time-step. More results are obtained considering the various frequencies of sea waves. The FORTRAN programs are used in calculating the results.Based on the use of the finite element methods this paper provides a simple and useful method in calculating the dynamic response of the mooring systems of the deepwater platform. In this paper some examples are given and the computational results show the reasonableness of the formulations and the effectiveness of the method.