Strength Assessment Method Research For Semi-submersible Platform Based On Non-linear Wave Loads

Semi-submersible platform is a kind of exploiting ocean energy equipment which is widely used and good performance. It's necessary to evaluate the structure strength of the unit in order to ensure its security. The main methods of structure evaluating could be divided into two kinds—one is the Design Wave Method, another is the Design Spectrum Method; the method of motion response analysis include Frequency Domain Linear Analysis Method and Time Domain Non-linear Analysis Method; the method of calculating the wave loads can also be divided into two kinds—Linear Method and Non-linear Method; the Finite Element Analysis method can also be divided into two kinds—Linear Finite Element Method and Non-linear Element Method; the main ship classification society have the different principle of strength assessment. It's necessary to research the method of assessing the semi-submersible platform's strength and non-linear method mentioned above with the constant advance to the deep-sea of the ocean engineering. In this thesis, these issues are discussed in detail, mainly contain the following aspects:1. Calculate the wave loads applied in the platform by the Design Wave method. Determining the design wave parameters by ABS guideline and DNV guideline respectively, then compare the differences of the two methods.2. Use Frequency Domain Linear method and Time Domain Non-linear method to predict the motion responses respectively then compare the different prediction results calculating by the two methods. Taking the mooring system into consideration in the Frequency Domain Linear method, then tell the mooring system's influences to the motion and the structure.3. Calculate the wave loads which are applied in the large dimension structure by the linear diffraction theory and the small dimension structure by Morison equation respectively. Propose a kind of engineering facilitated method of calculating non-linear wave loads; the method is based on the non-linear correction to the linear wave loads.4. Research the different load cases and analyze the entire Finite Element model.5. Research the theory, method and principle of global strength assessment, the detail of yielding and bucking strength. Assess the yielding and bucking strength under the linear and non-linear wave loads respectively.