Damage Identification For Jackson Platform Based On Operational Modal Analysis And Modal Strain Energy

The focus of fossil energy development and utilization will certainly be transferred to the ocean while the development of human society has led to the depletion of terrestrial energy. As the fundamental infrastructure of marine oil and gas exploitation, the offshore platform is constantly affected by various natural loads during its service life. Fatigue damage has become the main cause for resent accidents according to the survey, so damage identification has become a current research hot spot. The principle of damage identification method which based on vibration test is that dynamic response of the structural will change when damage occurs. However, the manual excitation is not a convenient way to applied in practice both technically and economically for a large structure such as offshore platform. In this paper, a method of fault diagnosis named OMA (operational modal analysis) is proposed, which could reduce the cost and obtain the real dynamic response only by environmental loads. Plenty work is been done in this paper:1. Consider the conditions of coupling of different environmental factors based on the research of common loads such as wind, wave and current. Main DOF of each structure element obtained can be utilized as reference for numerical simulation and sensors arrangement.2. Peak picking and wavelet methods are selected to identity the modal parameters because of the response under environmental loads is small amplitude, random and low SNR.3. All modal data of a giant 3-D structure can hardly stimulated by environmental loads and the obtained mass matrix can’t be normalized. In this paper, MSE (Modal strain energy) method is used as an index for damage identification, a new idea named group method of modal strain energy is be proposed, which divided MSE into two indicators for axial strain and bending strain energy separately (rotational DOFs are abandoned due to the difficulty for testing). This method does not need all the structure modal but only the lower modal response with which the position and extent of the damage could be detected.4. This method has been verified by ANSYS and MATLAB by an offshore platform scale models made by Ocean University of China. A comparison has been made between FEA results and the experimental data. Three cases of single beam, plane truss framework and offshore platform above has been simulated respectively, and in each case the proper index has been chosen to identity the location and extent of the damage.