Theoretical And Experimental Analysis Of The Dynamic Properties Of Offshore Platforms

Offshore platforms are designed to withstand various dynamic loadings which are in-duced mainly by surface waves, current, ice and earthquakes, during their service life. Theperiodic/aperiodic detection and safety assessment are very important to make sure the safetyof the structure and to prevent the occurrence of major accidents. Because the continuousageing and subsequent structural deterioration of a structure may cause the change on itsmass and/or stiffness distribution, the modal properties of the structure may alter as well.The estimate of the modal parameters of an offshore platforms can also provide the basis formodal-based damage detection methods.Measured data are inevitably contaminated with noise, it is necessary to identify modalparameters from corrupted response signals. Therefore, noise removal for measured data arevery important in the process of modal analysis. Besides, the number of modes contributingto our measured signal is not known beforehand, and we must first determine this unknownprior to applying the modal identification method.This thesis studies the dynamic properties of offshore platforms, as well as the modalparameter estimation based on measured free vibration response data. Specifically, we uti-lize Prony's method and Eigensystem Realization Algorithm (ERA) to identify multiplemodal parameters from field data of a realistic jacket-type platform in Bohai sea in Chinaand measurements from a physical model of an offshore platform. The main contributionsare as follows:1. Introduces the difference between the Fourier analysis and Prony's method. By adopt-ing singular value decomposition and structured low rank approximation (Cadzow'smethod), we adopt Prony's method to identify modal parameters. We address thatProny's method is very effective in signal processing area. Parameter estimation ofsignals with beat phenomenon has been difficulty. By introducing Prony's method,we accurately identify multiple frequencies and damping ratios from synthesized sig-nals with beat phenomenon.2. We extend Prony's method to identify modal parameters from sea test data of a real-istic offshore jacket-type platform in Bohai sea in China. We address that the modescontributing to the measured signal are related to the initial de?ection of the structureand the direction of the sensors. While using sea test data, since the true values ofthe modal parameters of the test jacket platform were unknown, we judge the perfor- mance of the proposed scheme based on examining the consistency among the modalparameters estimated from three accelerometers mounted at different locations of thetest platform.3. In dealing with noisy measurement data, the Eigensystem Realization Algorithm(ERA) partitions the realized model into signal and noise portions so that the noiseportion can be disregarded. We address that the determination of the dimensions ofthe block Hankel matrix is very important. We show that the signal and noise matricescan be better separated when the number of block-rows and number of block-columnsof the corresponding block Hankel matrix are chosen to be close to each other. Nu-merical studies include both numerical and experimental data.4. We propose a verification procedure to justify that the estimated modal parametersare noise insensitive and thus indeed associated with the true system. The procedureinvolves artificially injecting random noise into the measured signals to create noisy-noisy signals, then comparing the identification results obtained respectively from themeasured and noisy-noisy signals. Using experimental data collected from a plate andjacket-type platform, we demonstrate that the estimated parameters from measuredand noisy-noisy signals are very consistent.