Reliability estimation on dynamic responses of offshore structures

具有大甲板面积的海洋平台可以为开发可用作居民区、机场、发电站等的海洋空间提供巨大的可能性。由于这些结构的恒载非常大，基础上的反力变得很大，需要非常牢固的基础体系。如果能通过浮力型大直径构件等方法来减小反力，那么浮力型大直径构件的大型海洋平台的概念可能会在近海工业中流行起来。在本研究中，重点研究了这种结构在海浪、海流和地震作用下的动力响应。主要研究结果如下：(1)靠近海底的浮力型构件结构的波浪力和相应的响应较大。另一方面，海流作用力对带浮力构件的近海结构的动力响应评价有重要影响。(2)近海结构的动力响应受与波浪力评价和结构特性有关的参数变化的影响。由于这些随机参数也会影响响应值，因此有必要考察它们对响应和可靠性评估的影响。(3)有必要发展合理的方法来评估波浪和地震力作用下海洋结构物的可靠性，这两种力在激励持续时间上具有完全不同的特征。其中一种方法是先计算地震反应，然后将其与等效波浪反应和波浪荷载条件相关联。

Offshore platform with large deck area may provide great possibilities to develop ocean spaces which may be used as resident areas, airport, power stations, etc.. Since these structures have very heavy dead loads, reaction forces on the base foundation become severe and necessitate very firm foundation system. If thses reaction forces can be reduced by some method such as large diameter members of buoyancy-type, the concept of large offshore platforms with the buoyancy type large diameter member may become popular in the offshore industry. In this study, emphasis is placed on examining the dynamic response of such a structure under the action of sea waves, currents and earthquakes. The main results are summarized as follows :(1)The wave forces and the corresponding responses are amaller for the structure which has the buoyancy type member near the sea bottom. On the other hand, the current force gives significant effects on the response evaluations for the offshore structure with buoyancytype member and it is important to evaluate these effects.(2)The dynamic responses of the offshore structure are influenced by the variation of parameters which are related to the wave force evaluation and structural characteristics. Since these random parameters also influence the response values, it is necessary to examine their effects on the responses and the reliability evaluation.(3)It is necessary to develop rational methods to evaluate the reliability of offshore structures subjected to wave and earthquake forces which have entirely different characteristics in terms of excitation duration. One of the methods may be to compute the earthquake response first and then correlate it to the equivalent wave response and the wave loading conditions.