Structural Modal Parameter Identification And Damage Diagnosis Of Offshore Platforms Under Ambient Excitation

Large civil engineering structures (for example, cffshore platform structures) are prone to be damaged during their service lives, caused by factors such as corrosion, fatigue, impact, and hostile sea environment, so the occurrence of damage during ths life of an offshore structure is unavoidable. Thus, techniques for structural health monitoring in offshore platform structures are essential to ensure safety, reduce the cost of maintenance, and prolong the service life.Nondestructive Damage Detection (NDD) methods based on vibration measurements in large/complex structures were shown to be promising by researchers. However offshore platform structures are very complex, and not easy to excite artifk ially and they are often suffered from natural (ambient) loads that cannot be controlled easily. Overall, modal parameter identification and damage diagnosis are faced with severe challenges. The thesis fo:uses on two key issues for Structural Health Monitoring via vibration in real offshore platform structures.The first issue is to identify modal parameters only utilizing structure out-put dynamic response. Field testing under ambient excitation is less time consuming and reducing cost since special equipment for exciting the structure is not needed. Moreover, it's hard to measure the loading environment on offshore platform; so an automated healtl monitoring system for large civil structures will most likely develop modal parameter identifica ion under operational conditions. Natural Excitation Technique (NExT) is possible to obtain correlation functions from responses to an unknown excitation which may be treated as free visration data. The Eigensystem Realization Algorithm (ERA) was applied in conjunction with the common-based normalized system identification (CBSI) to identify structural modal parameters (natural frequency, damp ratio, mode shape) with limited acceleration information. The proposed out-put only modal parameters identification technique is further verified by numerical model of offshore platform and field testing of CDZX-2 offshore platform. According to the comparison results, the modal parameters identification method is shown to be effective for Structural Health Monitoring system of offshore platform under ambient excitation.The second issue developed herein is damage detection methodology. As we all known, modal parameters identified under ambient excitation can't be mass-normalized; high modal parameter is difficult to be excited under operational conditions; the structural rotational degree of freedoms can't be measured in dynamic tests. To avoid these limitations of the factors mentioned above, the present study will develop a new damage diagnosis method (Modal Strain Energy Decomposition) to localize the damage for a three-dimensional frame structure, specially a template offshore structure. The major concept of the new damage localization method is based on assigning element modal strain energy to two parts and defining two damage detection indicators. One is compression modal strain energy change ratio (CMSECR); another is flexural modal strair energy change ratio (FMSECR). These two damage indicators are obtained by incomplete modal shapes which can be only extracted from dynamic response. Structure Health Monitoring is thus accomplished via monitoring the elemental CMSECR and FMSECR. The proposed method c;.n be combined with modal parameters identification to detect complex frame structures damage and estimate their damage severity. In orderto validate the damage diagnosis method, fifth 3-D frame structure, the finite element models of Benchmark structure and offshore platform structures were developed by finite element method software to simulate several representative damage cases. This study demonstrates that the newly developed method is capable of diagnosis damages for template offshore platform structures under ambient excitation no matter of the damage located either at a vertical pile, a horizontal beam or a slanted brace. Additionally, a new damage detection indicator...