Vibration Tests Of An Offshore Platform And The Updating Of The Mass And Load In Vibration

With the advancement of marine resources development process, the vibration problem of offshore platform has been paid more and more attention. The vibration analysis of offshore platform mainly focuses on the environmental load, but the equipment vibration has more important influence on the long-served structure such as W12-1 ocean platform. Vibration of equipment can not only directly affect the served life of the equipment, but also make the noise seriously which threats to the health of workers on the platform. In order to reduce the vibration, it needs to accurately analyze the factors which affect the vibration, such as the structural parameters(mass, stiffness) and load parameters. However, for the long-served offshore platform, the structural stiffness is degenerate, and the equipment parameters get changed, which make the structure different from the original design. The structural parameters are difficult to estimate. And the load parameters are also difficult to accurately measure, which are coupled with structural parameters, because it not only relates to the operation of equipment, but also is affected by the local structural parameters. In order to solve above problems, this paper analyzes vibration characteristics of the platform through comprehensive testing. On the basis of vibration testing, this paper developed the vibration parameter updating method for the platform applying the numerical analysis method, and determined the platform structure parameters and load parameters reasonably. The concrete studies included:1) Aim to the vibration problem of the offshore platform, we took a comprehensive vibration testing on W12-1 platform. According to the structural characteristics of the platform, the measured points layout are around the lower and middle platform and are divided into seven groups, a total of 51 points. Those measured points include different types of structure: the compressor and its peripheral members, the planar structure near the compressor, the planar structure far away from the compressor and the column between layers. During the process and analysis of test signal, we can get the displacement time curve and the FFT frequency curve. And we calculated the key vibration information such as the average vibration amplitude of measured points, the natural frequency of partial structure and the excitation amplitude caused by excitation signal. The measured data shows that each measured point reflects the vibration frequency consistent with the compressor operating frequency. The fact indicates that the vibration source of offshore platform is the compressor. The measurement and analysis of the vibration in the platform can have a comprehensive understand on the vibration distribution characteristics of offshore platform, and also lay the groundwork for the further updating of structural parameters and load parameters.2) According to the original design drawings, we established the space finite element model for offshore platform by using beam and shell elements. In the finite element model, the structural stiffness degradation and uncertainty equipment mass can be equivalent to additional mass; compressor vibration behavior can be equivalent to periodic load; Calculation method for vibration solution is using the ABAQUS/Explicit module. It can accurately obtain the simulation acceleration, displacement time history signal of the measured points, then makes a relevant spectrum analysis so as to compare with the measured signal. We didn’t use the simple modal analysis method, because the offshore platform structure is very complex and has multi degrees of freedom. It is difficult to accurately determine the modal corresponding to each measured point.3) For the characteristics that structural parameters and load parameters are difficult to estimate in the long-served offshore platform. Based on the measured response signal, this paper figured out the updating method of the deck equipment mass in the platform and the value of excitation load. The average amplitude, natural frequency of partial platform, and excitation amplitude of the controlled point, as the target parameter, were used to update the equivalent quality parameters and load parameters of each layer of model. The target values of the modified model has a good fit with the measured values on the displacement time curve and the FFT frequency curve. It can be used for further research and analysis.