Identification Method And Experimental Analyses Of Hydrodynamic Coefficients For Ocean Platform

Offshore platform is a structure which provides the place for seafaring and offshore operation for the purpose of ocean exploitation and utilization of resources. Nowadays, ocean exploitation has been rapidly developed due to the more emphasis put by the countries of the world on sea resources. And the need of the offshore platform is increasingly demanded. The demand on the reliability of the design of the offshore platform is getting higher. Wave load is one of the important environmental loads in the design of the offshore platform. Most of the wave loads applied on the offshore platforms are computed through Morison equation in engineering design. The key of computing the wave force through Morison equation lies in the selection of the hydrodynamic coefficients which are basically acquired from the experience or experiments and can't accurately reflect the actual sea condition acting on the offshore platforms. Thus, it is of great significance to identify the hydrodynamic coefficients in the current sea condition for the safety design of the offshore platform. Based on the engineering background, this dissertation is devoted to studying the identification method of the hydrodynamic coefficients of the offshore platform and identifying the coefficients in the sea condition acting on the Xijiang23-1 jacket offshore platform by using real-test data, which could be practically applied to the actual engineering problems.The main researches are as follows1) According to the ill-condition problem of the frequency response function appearing near the natural frequency during load identification in frequency domain, a new load identification method based on the characteristic of frequency spectrum is proposed. This method is applicable to the specific condition when the characteristic of load spectrum is given. And the advantage of this method is subsequently verified by comparison in the numerical example.2) The identification method of hydrodynamic coefficients in frequency domain is presented considering applying white noise to the cylindrical structural response under the circumstance of linear wave. Based on the spectrum analysis method, the effects of phase difference between the foremost and following legs of the linear wave are considered. Accuracy of the two identification methods is also compared between the single frequency and the quadrature about the frequencyωin the sampling frequency. The result shows that the identification method through the quadrature about the frequency has an obvious advantage over the condition that wave spectrum is given.3) The method of model reduction is applied into the identification of the hydrodynamic coefficients of offshore platform. Based on the synchronistic assumption, the structure of offshore platform can be divided into several displacement synchronous parts. Every synchronous part can be substituted by a quasi-rigid-body mode and the full-size models can be simplified. The more accurate reduction model can be acquired through deformation modification. The degrees of freedom of the offshore platform structure are reduced by the modification model. By using the simplified model, the hydrodynamic coefficients are subsequently identified. The ill-conditioned problem in force identification is discussed and both the assessment method of the identification result and the basis for acquiring the measurement data in the identification are given.4)On the basis of the real-test data from actual wave-current field, the hydrodynamic coefficients of offshore platform are identified. Corresponding monitoring equipments are developed for the need of monitoring program. By applying the measured load and data about wave field from Xijiang 23-1 offshore platform, the hydrodynamic coefficients are identified. The structural response of the offshore platform is computed by use of the identification results and data about wave field. The identification results are validated by comparison between the measured response and the computed response. The results prove the hydrodynamic coefficients identified by the real-test data are correct.