Direct Modal Shape Expansion And Damping Identification Study For Offshore Platforms

The actual modal information acquired in the dynamic response test for offshorestructures is far less than the FEM modal information. One of the reasons is the limitednumber of sensors; besides that in actual test the location of the sensors is also restricted.The modal incompleteness problem is obvious. In health detection and damage diagnose,it’s necessary to establish an accurate FEM model. However, in the course of building upFEM model the damping of the structure is usually ignored or supposed to beproportional damping or liang damping model. Proportional damping couldn’t explain thecomplex modal shape obtained in actual test, and liang damping model which is largelyexplored in engineering assumes that the damping of all the nodes is equal, so theapplication is limited. This paper focuses on the modal shape expansion and dampingcharacteristic identification to solve the two above shortages. In the physical model test,we demonstrate the proposed methods by using a cantilever beam and a jacket platformmodel. And the following works are done.In dealing with modal and spatial incompleteness, a direct modal expansion methodwith a hybid vector is proposed. The vector is constituted with the actual test modal shapecorresponding to the main dofs and the estimated values with regard to the slave dofs.With the actual mode and the information of the FEM model the estimated values arerecalculated, so the spatial complete modal shape can be obtained. In this paper a fivedofs mass and spring model is explored to demonstrate the accuracy of this method, and ajacket model is used to verify the validity and capability for engineering.This paper also expands a damping matrix identification method adoptable fornon-proportional damping. Firstly, the non-zero values of the initial damping matrix isdetermined, then disintegrate the initial damping matrix to many sub-matrixes, after thatcalculate the correction factors of the sub-matrixes by solve the equations which isconstructed with the actual modal information and the FEM information, then theidentified damping matrix is achieved. A cantilever beam and a jacket model platform areutilized to verify the method, and in the numerical stimulation the spatial incompletenessand noise in actual test are both considered, the results prove the method is applicative. Inthe physical model test, a cantilever beam is used to demonstrate the applicability of the proposed method.The direct expansion method of modal shape in this paper doesn’t need Massnormalization and scaling for mode shapes, and allows the FEM model exist certain error,in addition to that the proposed method outperforms traditional expansion method forlower order mode shape expansion. The damping identification method proposed in thispaper is adoptable for non-proportional damping. In the conditions of modal and spatialincompleteness and low noise, the proposed method also could be done.