【作者】 张琪；

【导师】 郑向远；

【作者基本信息】 清华大学 ， 土木工程， 2021， 博士

【摘要】 随着人类经济社会发展,全球兴建了各种海工结构,如海上风机、跨海大桥、海底管线、水下油气生产系统等。处在地震活动性较强海域的桩基式和坐底式海工结构,将存在遭受海底地震动作用并被损坏的风险,这些海工结构在本世纪以来蓬勃发展,其抗震问题日益凸显。相对于陆地强震观测台网的建设,我国海底强震观测系统的规划部署尚在起步阶段,导致可获取的海底地震动记录十分欠缺,针对实测海底地震动的研究成果相对较少。海工结构抗震设计目前仍参考陆地地震动研究成果和陆地结构抗震设计规范,其合理性有待商榷。基于上述背景,本文开展了海底地震动时频域特性及人工模拟研究,主要研究内容如下:(1)针对目前时域积分无法去除复杂成分的噪音,频域转换方法需人为设定参数,积分结果不够稳定等问题,本文提出了基于沃尔什(Walsh)变换和经验模态分解(EMD)的时程积分方法。该方法利用Walsh函数积分实现地震动加速度到速度和位移信号的重建,利用EMD去除积分中的趋势项误差。通过3个算例的对比讨论,验证了本文提出的时程积分方法具有更高准确性,稳定性和自适应性。(2)对美国SEMS和日本K-NET强震观测系统中筛选收集到的实测海陆地震动记录的工程特性进行了对比研究,并对造成海陆地震动特性差异的原因进行了理论解释。研究结果表明:在同一地震和相同震中距条件下,海底沉积土层的放大效应导致海底水平地震动峰值加速度(PGA)显著高于陆地水平PGA。统计平均结果显示,海底水平PGA约是陆地水平PGA的1.83倍。另外,该放大效应还导致海底地震动中长周期能量进一步被放大,海底地震动中长周期反应谱值大于陆地地震动,尤其是竖向分量。同时,海底地震动90%能量持时也相应增大。(3)为了给海工结构抗震设计和安全评估作参考,基于实测海底地震动反应谱,提出了标准系数谱。通过与陆地规范设计谱对比发现,若使用陆地规范设计谱进行海工结构的抗震设计,将不够保守。(4)基于地震动的非平稳性,从上述实测海陆地震动数据中选取了69组海陆地震动对照组,进一步对比分析了海陆地震动时频域特征的差异。结果发现海陆地震动时频参量以及时间-频率-能量的三维分布特征具有显著差别。(5)提出了基于时变功率谱的海底地震动人工模拟模型。该模型既能反映真实地震动时频非平稳性,又考虑了水深影响。模型模拟结果与实测参数的对比检验表明,该模型能有效合理地对特定海底场地未来可能发生的地震动进行模拟预测。更多还原

【Abstract】 With the development of human economy and society,a wide variety of offshore structures,such as,offshore wind turbines,sea-crossing bridges,underwater pipelines,subsea oil and gas production systems,have been come into being in past decades.However,for bottom-fixed offshore structures located in sea areas with active seismicity,they will inevitably suffer from offshore ground motions and are at risk of being damaged by the earthquake.The seismic problems of these offshore structures have become increasingly prominent since 2000.On the other hand,compared with the fully developed land-based strong motion observation network,the planning and deployment of the seabed strong motion observation system are still in its infancy in China.Therefore,the available offshore ground motion records are rare and research works on the measured offshore ground motion are rather limited.The seismic design of offshore structures still has to resort to the land ground motions and the corresponding land-based seismic design code.However,its reasonability remains to be discussed.In view of these facts,the timefrequency domain characteristics of offshore ground motions are studied in this dissertation,followed by the nonstationary stochastic simulation.The main research contents are as follows:(1)Given that the traditional time-domain integration cannot remove the noise of complex components,and the frequency-domain conversion methods yield unstable and undependable results due to objectively chosen parameters,this paper develops an integral algorithm(WATEBI)based on the Walsh Transform and the Empirical Mode Decomposition(EMD).In this algorithm,the Walsh Transform is adopted to realize the reconstruction of vibration signal.Next,the EMD method is used to remove the trend term error in the integration.Through the comparison and discussion of 3 examples,the higher accuracy,stability and adaptability of this WATEBI algorithm than the traditional methods are verified.(2)A comparative study of the engineering characteristics of the measured offshore and land ground motion records collected in the U.S.SEMS and Japan’s K-NET strong motion observation system is carried out.The reasons for the difference in the characteristics of the offshore and land ground motions are explained theoretically.The comparison results show that the shear wave velocity of the soil layer on the seabed site is lower than that on the land site.Hence,during the same earthquake and by the same epicenter distance,the amplification effect of the seabed sedimentary soil layer causes the peak ground acceleration(PGA)of offshore horizontal component to be significantly higher than that of the land.The statistical average result shows that the offshore PGA is about 1.83 times that of the land PGA.In addition,the medium and long-period energy components of offshore ground motions are further amplified by the soil amplification effect.Their response spectral contents are larger than the land counterparts,especially for the vertical seismic components.The significant duration of offshore ground motion is also increased correspondingly due to soil effect.(3)In order to provide reference for the seismic design and safety assessment of offshore structures,a normalized response spectrum is proposed based on the measured offshore ground motions.Compared with the code design spectrum,it is revealed that the land-based code design spectrum is not sufficiently conservative for the seismic design of offshore structures.(4)In terms of the non-stationarity of ground motions,69 pairs of concurrent offshore and land motions are selected from the above-mentioned collected data.The temporal and spectral characteristics of offshore ground motions are further analyzed.The results show that the temporal and spectral distribution characteristics of offshore and land ground motions have significant differences.(5)An artificial simulation model of offshore ground motion is proposed.It aims at matching the time-frequency power spectrum that is a key reference in this study.This model not only retains the important time-frequency non-stationarity of the measured ground motions,but also involves the effects of water depth.The comparison between the simulated results and the measured records shows that the model is able to effectively and reasonably simulate and predict the possible future ground motions of a specific seabed site.更多还原