Offshore structures are subject to a wide variety of environmental loads (such as wind, wave and current) all of which exhibit a high degree of statistical uncertainty. The dominant load, however, is normally due to wind-generated random waves. Probabilistic procedures can account for these uncertainties by establishing the statistical properties of loads and responses and hence are necessary for risk-based assessment of these structures. The major obstacle in establishing the probabilistic properties of the response is the nonlinearity of the wave load mechanism and/or the structural system which leads to non-Gaussian distribution for response. The problem is further compounded by current and by intermittent loading on members in the splash zone, which have a significant effect on the statistical properties of response. The most versatile and the most reliable technique for predicting the statistical properties of the response of an offshore structure to random wave loading is the time simulation technique. However, this technique requires very long simulations in order to reduce the sampling variability to acceptable levels. If this technique can be speeded up, then it would be an ideal technique because it does not suffer from any of the inadequacies of the alternative methods and is applicable to all different types of structures. This research is concerned with reducing the computational demand of the conventional time simulation (CTS) method. To this end, a more efficient version of the time simulation technique (ETS) has been introduced which divides the simulated response extreme values into a number of groups based on the magnitude of the extreme values of their associated surface elevation or linear response records. The probability distribution of extreme responses for each group is then calculated individually based on a relatively small number of simulations, and then the total probability theorem is used to derive the probability distribution of response extreme values. In this study, the probability distribution of response extreme values together with the 100-year responses from the ETS models have been compared with corresponding distributions and lOO-year responses from the CTS procedure to examine the accuracy and the efficiency of the efficient time simulation technique. Overall, four different structures, three different sea states and three different current situations have been investigated. This was necessary to ensure that the conclusions of this study are valid for a broad range of conditions. The ETS technique was found to be many times more efficient than the CTS method.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:569889 |
| Date | January 2011 |
| Creators | Abu Husain, Mohd Khairi |
| Publisher | University of Liverpool |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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