Offshore Platform Damping Analysis And Device Development

Offshore platforms are main structures for marine petroleum and natural gas exploitation. They are also the platforms of production and life on the sea. Jacket platform are widely used in Bohai bay. When a jacket platform is built in Bohai bay, it will be exposed to the risk of very strong ice and earthquake. Bohai bay is a margin oil field. The production cost of jacket platforms has to be reduced. The application of structural vibration control technique is an effective means for raising design quality and reducing production cost. Thus, it is very important to investigate the methods of structural vibration control for jacket platforms.This thesis studies an energy dissipation method for structural response reduction of jacket platforms. An energy dissipation system is formed by adding some viscoelastic dampers, Magnetorheological (MR) fluid dampers or energy dissipation materials between the tube joints. A finite element model of JZ20-2MUQ jacket platform located in Bohai bay is established and it is taken as an example platform. An investigation is carried out on the effectiveness of incorporating energy dissipation devices into the example platform for response control under ice or earthquake excitations. The major contents of this thesis are summarized as follows:1.Passive control of ice-excited platforms by viscoelastic dampers is studied. The equations of motion of damper-platform system are established first. Dynamic characteristics, in particular the modal damping ratios, of the system are then sought using the complex modal analysis method. The response of the platform with added dampers is finally determined by the pseudo-excitation method in the frequency domain. Extensive parametric studies are performed to assess the effectiveness of dampers and to identify the optimal damper parameters. It is showed that responses of the platform under ice excitation can be substantially reduced if the parameters of dampers are selected appropriately.2.Passive control of earthquake-excited platforms by connection dampers is studied respectively. The connection dampers are formed by placing energy dissipation materials between the tube joints. An equivalent element model is established by idealizing tube joints and energy dissipation materials as rotational spring and damper respectively. The stiffness matrix, mass matrix and damping matrix of the equivalent element are derived using a combination of the finite element method and the direct stiffness method. The complex modal analysis is carried out to determine dynamic characteristics of the jacket platform and the pseudo-excitation method is performed to analyses the responses of the jacket platform subject to earthquake excitation. The effects of connection stiffness and rotational damper on dynamic characteristics and the seismic performance of the jacket platform are discussed. The parametric studies on the example jacket platform with and without the connection dampers show that there is an optimal damper damping coefficient by which the modal damping ratio of the jacket platform can be considerable increased and the seismic responses can be significantly reduced.3.Precise instantaneous optimal control algorithm is presented in this thesis. Instantaneous optimal control algorithm is feasible if excitations are not known a priori, such as ice and earthquake excitations. In order to calculate the exponential of a matrix, instantaneous optimal control algorithm has to solve a complex eigenproblem. However, it is almost impossible to get an accurate result if the eigenproblem is quite large. In such case, the results of the exponential of a matrix are very poor. To overcome this problem, a precise instantaneous optimal control algorithm is proposed in this thesis. Actually, the precise instantaneous optimal control algorithm is a combination of the precise integration method and instantaneous optimal control algorithm. The proposed method does not need to solve complex eigenproblem and provides an accurate result. An example jacket pla...