Dynamic Characteristics And Active Control Of Offshore Platforms

With the growing demand for resources, the exploitation of resources on land can no longer meetthe needs of human development. So under the circumstances of nowadays’ highly developedtechnology and increasing powerful industrial manufacturing, people have turned their eyes to thevast sea field which has more abundant marine resources. As the basic facilities of exploitingoffshore oil and gas resources, and for workers to live and produce, the importance and the safety ofoffshore platform are evident. However, due to the poor working conditions with long-term loadactions of wind, wave, current and tide, it will cause continuous vibration of the offshore platform,discomforting the staff and affecting normal operations of the equipments and machines.Furthermore, it will reduce the reliability of the structural components, causing fatigue failure to theplatform. Therefore, the study of dynamic characteristics and vibration control of the offshoreplatform has important theoretical and practical significance.Marine structures can generally be divided into two categories: fixed structure and movablestructure. At the bottom part of the fixed platform, the platform is directly supported and fixed withthe seabed by pile, expanding footings or other similar structure, and it can be divided into two typesaccording to different supporting case: pile-foundation type and gravitational type. Movableplatform, either floating in the water or supported on the seabed, can move from a well location toanother well location, and it can also be divided into two types according to different supporting case:bottom-supported type and floating type. Compliant structure is a type of movable platform withflexibility, either fixed or movable in deep water. Such structure, with its external force mostlyconsumed by the inertia force, has significant horizontal movement response and geometric nonlineareffects. It mainly including the following structure types: guyed tower platform, tension legplatform (TLP), the SPAR platform and articulated tower platform (ATP/AP). Semi-submersibleplatform is another kind of floating platform, derived from the previous generations of offshoredrilling platforms, mainly composed of the upper structure, submerged body, column and brace, and isrelied on the anchoring positioning or dynamic positioning. This dissertation, funded by the National Natural Science Foundation of China (Grant No.11072146,11272202,11002087), studies dynamic characteristics and active control of two types ofmovable marine structures: compliant offshore platform and semi-submersible offshore platform.The main works are as follows:1. Based on continuum mechanics, a dynamic model of the compliant offshore platform is establishedand is discretizated using finite difference method. The nonlinear dynamic characteristics of theplatform are investigated under three different assumptions: large deflection hypotheses,cross-section rotational angel approximation and small deformation assumption. Firstly based onthe basic mechanics principle, the integro-differential dynamic equation of the system is establishedwith considering external load actions of the gravity, water pressure difference, and wave force.Then this equation is discretized into the form of finite degrees of freedom according to theboundary conditions and using the finite difference discretization method. Finally dynamiccharacteristics of the platform under the conditions of hydrostatic free vibration and forcedvibration with different wave height are studied numerically.2. Active vibration control of the compliant offshore platform is studied using the tuned mass damper(TMD). The parameter determination method of TMD presented. Simulation results indicatethat the control effectiveness using active TMD control is better than that using passive TMDcontrol, and the displacement of the TMD using active TMD control is smaller than that usingpassive TMD control.3. Fatigue reliability issues of semi-submersible offshore platform are studied in this dissertation.The random response spectrum of key nodes from the key parts of the semi-submersible platformunder different wave angel conditions is investigated. The fatigue reliability of these nodes andreliability evaluation index of the platform structure are calculated based on the Miner linearcumulative damage theory, and are used to evaluate the security of the semi-submersible offshoreplatform structures during the designed life.4. This dissertation also studies the dynamic positioning of the semi-submersible offshore platform.The Kalman filter is used to filter high-frequency signal of the system and the linear quadraticregulator (LQR) method is used for controller design. Only low-frequency signal of the system isused for control feedback in this controller. Simulation results show that the control methodpresented can effectively complete the dynamic positioning of the platform.Dynamics of offshore platform is a very challenging research direction, and there are still manyaspects needing further expolration and research. So at the end of the dissertation, a summary of ourstudy is given and possible problems and directions for future research are also discussed.