Model Updating And Damage Detection Of Offshore Platforms With Non-proportional Damping

Though most platforms close to their service life, they have to be used continuously even out of their service life for the new exploitation of oil in the nearby areas. In some oil fields, there are portion platforms whose safty and integraty are threatened by damages in some members, which may be caused for lots of reasons, such as improper design and construction, corrosion, fatigue, collision and abominable environment loads, though these platforms have not finished their service life. Then how to evaluate the health level of these platforms become emergent issues. Therefore, to assure the safety of these platforms and to overcome the drawbacks of model updating and damage detection methods at present on damping simulation and spatial incompleteness consideration, foundamental studies on the damping matrix identification, the damage detection and the experiment model updating based on interface modification are performed; and a new method, i.e., dynamically equivalent substructures is proposed, and foundamental studies are performed as well. Then the correctness and feasibility of above methods are demonstrated using a steel cantilever beam. Finally, dynamic test for the CB22B platform is performed, and the feasibility of test scheme and the possibility of obtaining the first few dynamic properties are discussed as well.In theory, this thesis extends the Cross Model Cross Mode (CMCM) method from undamped systems to damped systems employing few spatially incomplete measured data for model updating and damage detection. For damping identification, two methods, i.e., global damping matrix indentification and local damping matrix identification are proposed. The former one can be used for complex structures, the latter one is suitable for simple structures, and the above two damping matrix identification methods can both solve the spatial incompleteness of measured data. For damage detection, using correction coefficientsα_n as damage indicators is available only when the measurement errors are very small, and the robustness of developed indicator, i.e., the magnified idicator MSECI_n is better thanα_n in damage localization; thus the proposed damage detection method, i.e., composite indicator method, can synthesize the advantages of above two indicators, and the achievements can contribute to the application of the CMCM method in engineering. When the required precision of experiment is strict, the thesis proposes experiment model updating method based on interface modification, which can match a few measured modal data. This method is suitable to spatial incompleteness and the initial damping matrix is common, thus engineering meaning is obvious. In addition, the thesis studies the new dynamic substructure method, i.e., dynamically equivalent substructure method, and numerical studies reveal that if the selected substructure is proper, the modified substructure is dynamically equivalent or close to the whole structure in a few measured modal data. From the experiment of a steel cantilever beam, one concludes that the composite indicator method, the experiment model updating method based on interface modification and the dynamically equivalent substructure method display the promising application in engineering. Dynamic test for CB22B platform indicates, the adopted scheme is feasible and universal, and the first few frequencies can be measured reasonably, while the accuracy for mode shapes identification should be improved in the future work.Friswell mentioned in his book that there are three challenges for model updating, that is pairing, scaling between measured and analytical modal, and damping simulation. One side, achievements in this thesis doesn't require pairing and scaling; and on the other hand is that damping properties can be considered. In other words, three challenges are solved at a certain level, thus theoretical innovation is obvious. In addition, the experiment and dynamic test on CB22B platform improved the application of proposed methods in engineering. In general, achievements can be applied not only to offshore platforms, but also to aviation and spaceflight, civil engineering and automobile engineering.