Modal Analysis Based On The Prototype Measurement Of Offshore Platform Single Point Mooring System

Offshore platform is the main facility of the development of offshore oil and gas, its health status is closely related to the oil development, safety of staffs and production operation. Offshore platform is the large-scale structure, the dynamic analysis through model test in tank will be affected by scale effect, and excitations in the simulation cannot absolutely simulate the real external ones in the ocean environment, which brings difficulty to the analysis and design of the structure. Therefore, the prototype measurement of offshore platform is of great importance to corresponding research.This paper has done research to the prototype measurement of FPSO single point mooring system and during the measurement the full-scale measurement data are collected. In order to ensure the safety production of FPSO, the hull needs to be fixed by the single point mooring system to limit the motion range, but life-time service will cause strain damages to the mooring system. As a result, the health status should be analyzed through the prototype measurement data. One of the objectives is the damage identification of the mooring system and identifying the modal parameters accurately is the key to the structure damage identification.FPSO is under the complex environment excitations in a long-term and the mooring system response has obvious non-stationary characteristics. The non-stationary components in the system response make the random decrement derivate from the exponential decay signal. The mode distribution of the system response is dense and the modes are coupling with each other using the existing method. Aimed at solving the problems mentioned above, a modal identification based on filter-ICA has been proposed in this paper. A stationary filter using moving average is incorporated into ICA, which eliminates the effects of non-stationary components on modal identification. During the modal identification of a multiple degree of freedom simulation system, the identified results are close to the theoretical values. The proposed method is used for modal analysis of FPSO single point mooring system using the measurement data. Compared with the existing identification results, the mode distribution is obvious and the motion tracks of mode shapes present single form of motion more prominently.During the long-term measurement of FPSO, damping ratio reflects the characteristics of the structure from the perspective of energy loss and the changes in the damping ratio reflect the damage status from a certain extent. But among the modal identification of structures using measurement data, especially for the dynamic system with non-stationary, the accuracy of damping ratio is not good enough. Therefore, a modal damping ration identification method based on time-frequency ICA, which eliminates the effect of damping ratio on the accuracy of identified parameters, has been proposed in this paper. The effectiveness of the proposed method is verified in the modal identification of the simulation system. Through the analysis of the prototype measurement data, the modal damping ration of FPSO single point mooring system is identified using the proposed method. The identified results of damping ratio in the same modal frequency range are compared, which can prove that the proposed method is more stable.The research mentioned above has identified the modal parameters in perspective of time invariant system. But when the hull is doing surging motion, the stretch length of the yoke of the mooring system is changing with time and the mooring system can be seen as a time variant system. When the structure damages happen, the modal parameters changes and the modal instantaneous can reflect this kind of changes more intuitively. In this paper, an instantaneous frequency identification method based on the mooring system, called ICA-TVAR, has been proposed. The effectiveness of the proposed method is verified through the instantaneous frequency identification of a time varying simulation system and the six degree of freedom motion platform. Further, the proposed ICA-TVAR method is used to identify the instantaneous frequency of FPSO soft yoke single point mooring system and the results are better than that identified by HHT method.