Abstract
The objective in this work is to assess fatigue damages on local components of a semi-submersible platform under combined actions of wind and wave loads in time domain. Some improvements are provided in the present study to improve the efficiency and accuracy of the whole evaluating process. Firstly, a combined wind and wave relationship as well as an innovative mixture simulation method are used to generate time series of random wind and waves. Moreover, an m-block division method is proposed to compress the number of the whole short-term sea states in the wind-wave scatter diagram. Then, with an improved multiple interpolation sub-model method, the structural stress responses of the local structural components are calculated as is in the whole model analysis. Finally, a modified rain-flow counting method is provided and validated to count the stress cycles efficiently and accurately. Thus, the short- and long-term fatigue damages are computed based on the S-N curve approach and the cumulative fatigue damage rule. In relative agreement with the numerical results by the traditional time-domain method and existing experimental data, these proposed improved methods are demonstrated to be applicable and efficient methods for fatigue damage analyses. All the fatigue damages on local components satisfy the specification requirements and the minimum value appears under the up wind-wave state, which is the proper working condition for a semi-submersible platform.
Highlights
A semi-submersible offshore platform is utilized for the explorations and productions of offshore energy resources in more hostile and deeper ocean environments, under which circumstances the highly flexible platform is very sensitive to dynamics, such as wind, waves, currents and earthquakes [1]
The local components in the present study focus on the connections between the columns and horizontal braces
The improvements of the improved TDA (ITDA) method include combined wind and waves relationship, the mixture simulation (MS) method, the -block division method and the modified rain-flow counting (MRFC) method used in the loading conditions and the improved multiple interpolation sub-model (IMISM) method used in the modeling process
Summary
A semi-submersible offshore platform is utilized for the explorations and productions of offshore energy resources in more hostile and deeper ocean environments, under which circumstances the highly flexible platform is very sensitive to dynamics, such as wind, waves, currents and earthquakes [1]. Waves and wind play a major role in fatigue damage accumulations due to their continuity in time and in sequence, as being tiny, moderate, and sometimes catastrophic They produce stochastic fluctuating stress responses in offshore structural responses [8]. A modified rain-flow counting method is proposed to count the stress cycles more efficiently with no accuracy loss according to the previous study [6] These three sub-methods in the ITDA method are applied and investigated in the present work to analyze the fatigue damages of local components of a semi-submersible platform. The objective in the present study is to generate the wind and wave loads accurately and efficiently and propose an improved TDA (ITDA) method to better predict the fatigue damage of local components of the offshore structures.
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