【作者】 韩晓双；

【导师】 赵德有； 马骏；

【作者基本信息】 大连理工大学 ， 船舶与海洋结构物设计制造， 2008， 博士

【摘要】 处于海洋环境中的导管架海洋平台,除了承受结构自重以及甲板设备重量等静荷载作用外,还经常受到风、波浪、海流、地震等各种环境动力荷载的作用。其中,波浪载荷和地震载荷是两种比较典型的动力荷载。目前对于波浪荷载作用下平台的动力反应研究得比较多,而对地震作用下平台的动力反应研究得还相对较少。虽然地震荷载没有波浪荷载和风荷载出现的频率高,且发生时持续时间也很短,但是强烈的地震所造成的破坏却是非常严重的。我国渤海、黄海、南海等海域位于大陆板块的边缘,地震活动强烈,海洋平台一旦遭受地震灾害,还将产生严重的次生灾害,特别是造成海洋环境的污染,所以海洋平台抗震分析是十分必要的。相比陆地上的结构,钢质导管架海洋平台是一个复杂的结构-桩-土-水体系,无论是结构型式还是所处的环境都有其自身的特点。而对这一复杂体系的动力分析更是一个十分庞大的课题,所涉及的领域也十分广泛,如流体力学、地震工程学、随机振动理论、土动力学等。在满足精度要求的前提下选择适当的计算模型和分析方法是研究者目前亟待解决的问题。本文对导管架海洋平台地震响应的相关问题进行了研究,具体内容有:(1)开展Ritz法的相关研究。首先简要介绍变分原理和虚拟激励法的基本理论,以及Ritz集的选择方法。然后利用虚拟激励法将平稳随机振动分析转化为简谐振动分析的特点,推导了梁在随机激励作用下系统的泛函表达式。根据变分原理,不将泛函的极值问题转化为求解振动微分方程问题,而是给出所求函数的近似表达式,直接利用泛函极值的必要条件确定表达式中的待定参数,以近似计算结构在随机地震激励作用下的受迫振动响应。利用文中推导的公式,分析了不同Ritz集的维数和类型对计算结果的影响。研究表明,在满足位移边界条件的基础上再满足部分力的边界条件的Ritz集并不一定使计算结果得到改善。Ritz集在选择时主要要求其满足位移边界条件,关于力的边界条件是次要的。最后对评判Ritz集优劣的评比标准进行了研究。根据变分原理,在满足位移边界条件的所有容许函数中,正确解应使系统的泛函取得极值。因此对于整个系统而言,使系统的泛函越接近极值的Ritz集就越好。文中对算例的泛函极值进行了比较。(2)随着大型有限元软件的不断发展和广泛应用,越来越多的实际工程结构采用三维有限元分析。理论上讲计算模型越接近真实情况,计算结果越准确,但这往往大大增加了建模和计算时间,不利于工程实际应用的需要。在保证计算精度的基础上,计算模型和分析方法的科学简化可以显著提高计算效率,是工程实际应用的重要条件。文中提出了一种基于梁理论的导管架海洋平台随机动力响应简化计算模型,将Ritz法和虚拟激励法相结合计算平台的随机响应。分别基于Euler梁和Timoshenko梁理论推导了平台随机地震响应计算公式,编制了计算程序。通过与有限元方法比较,验证了文中计算方法的准确性。此外,该方法也被应用计算了平台在随机波浪作用下的响应。将Ritz法与虚拟激励法相结合,既可不必计算结构振型而直接求出响应量的自谱、互谱及各种谱矩,又充分利用了虚拟激励法的精确性和快速性的特点,在保证计算精度基础上缩短了计算时间。算例表明,本文提出的海洋平台随机动力响应计算模型简便有效。(3)采用不同的计算模型,即土-桩-水相互作用模型和考虑附加水质量的等效桩模型,对三维平台结构在地震作用下的响应结果进行了比较研究。研究表明,考虑土-桩-水相互作用效应使结构体系的自振周期延长,振动速度和加速度减小。分别采用土-桩-水相互作用的三维平台模型和经简化后的一维梁模型计算平台的固有频率,通过比较两种模型的计算结果,对一维梁简化模型进行了频率修正,提出修正系数关系曲线。应用简化的一维梁模型计算平台的振动特性,通过修正系数的修正,即可得到考虑土-桩-水相互作用的三维平台的振动特性,节省计算时间,提高计算精度,为海洋平台的振动计算提供参考。(4)介绍反应谱理论和相关概念,应用该方法对导管架海洋平台地震响应进行谱分析。通过有限元软件分析得到平台的自振频率,根据《海上固定平台入级与建造规范》中规定的地震响应谱计算地震载荷,经过振型组合考虑各振型分量对总响应的贡献,得到平台的地震响应。(5)详细介绍时程分析方法的基本理论,包括参数选取、结构计算模型和计算步骤等方面的内容,系统地给出根据场地土的类别和地震设防烈度选择地震波的准则,包括波的数目、形状、强度等参数的选取,具有一定的参考意义。选用真实的地震波,经过调幅后对平台进行时程响应分析,得到地震响应时程。更多还原

【Abstract】 Jacket platforms located in the sea not only are subjected to static loads, such as gravity, deck and equipment load, but also endure dynamic forces including wind, wave, current, earthquake, and collision, etc. Among those dynamic excitations, wave and earthquake are two typical loads. At present, there are many studies on responses of platforms under wave load, whereas the studies on earthquake responses of platforms are relatively fewer. Although the arisen frequency of earthquake is lower than that of wave, and its standing time is also shorter than that of wave, the destroy level of great earthquake is rather severe. The seismic activity is intense in our country, while the offshore platform is suffered from strong earthquake, the secondary disasters will happened, expecilly the pollution of ocean environment. Therefore, the aseismic analysis of offshore platforms is necessary.Comparing with the onshore structures, steel jacket platforms are complex structure-pile-soil-fliud systems, and both the structures and the circumstances have their characteristics. Therefore, it is a huge work to analyze the dynamic responses of the complex system, which conserns extensive theories, such as hydrodynamics, earthquake engineering, random vibration and soil dynamics, etc. It is a significant work to study appropriate calculational models and analytical methods whose precision are kept to some extend. The paper is concerned with the seismic responses of jacket platforms and some correlative researches as follows:First of all, the Ritz method is studied. Based on the principle of virtual displacement, the formulae of the functional of the present system are derived by using the characteristic for the pseudo-excitation method of reducing the structural stationary random response analysis to the analysis of structural harmonic response, and then the necessary conditions for the functional being the extreme value are applied to determine the coefficients in order to approximate the random seismic responses of structures. In addition, the evaluation criterion of Ritz vectors is proposed in order to analyze the reasons. According to the variational principle, among all the admissible functions which satisfy the essential boundary conditions, the exact solution will make the functional of the system obtain the extreme value. Therefore, for the whole system, it is advisable to consider the functional as the evaluation criterion. The one which makes the functional obtain the extreme value is the best.The success of the Ritz method depends on how well linear combinations of Ritz vectors can approximate the natural modes of vibration. In this paper, the effects of Ritz vectors on the response results are studied and the functional extreme values are compared. Results are presented for several cases, and show that the Ritz vectors which not only satisfy the geometrical boundary conditions but also satisfy the natural boundary conditions do not always lead to good results.Secondly, nowadays, with the widely use of well-known finite element software, interest in dynamic responses of jacket platforms was sparked on three-dimensional model. Theoretically speaking, the more complex and more precise are the calculation models; the more accurate are the computation results. However, these models are not easily accepted and achieved in engineering practice because of their low efficiency and complexity. Therefore, it is a significant work to study simplified structural calculation model whose precision are kept to some extent. In the papar, a simplified calculational model is brought forward for computing the random seismic responses of jacket platforms. Based on the Hamilton theory, the equation of bending motion is developed and solved by the classical Ritz method combined with the pseudo-excitation method, and then a program for computing random seismic responses is produced. Usually, random responses of this continuous structure are obtained by orthogonality of modes, and some normal modes of the structure are needed, causing inconvenience for the analysis of the non-uniform beam whose normal modes are not easy to be obtained. However, if the pseudo-excitation method is extended to calculate random responses by combining it with the classical Ritz method, the responses, such as auto-PSD function, cross-PSD and higher spectral moments, can be solved directly avoiding the calculation of normal modes. In order to illustrate the proposed approach, an example is presented, and three different types of Ritz functions are adopted which results are compared with the ones obtained from the well-known finite element software. It is shown that the present method is effective and useful in aseismic design of platforms.Thirdly, the dynamic responses of three-dimensional platforms subjected to earthquake load are compared by adopting different types of models, that is soil-pile-fliud interaction model and model with attached water and equivalent pile. The results and the numerical comparisons are provided in diagrams and tables. It is shown that the natural period changes long and the responses changes small for the soil-pile-fliud effect is considered. The natural frequencies are calculated by applying there-dimentional model and simplified beam model. Based on the comparison of the results, the frequency of simplified beam model is modified. The frequency modified beam model can be used to compute the vibration characteristics of platforms, which can save the time, promote the accuracy, and provide the reference for the vibration calculation of the jacket platforms. Fourthly, the response spectrum method and some related concepts are expatiated by combining with the Criterion for Classifying and Constructing Fixed Offshore Platforms, and the spectrum analysis is given to a jacket platform.Finally, some main contents on time-history analysis method are explained including the selection of parameter, structure model, and calculation process, especially the selection of earthquake wave which concerns the amount, shape, and intensity of seismic wave. Two illustrations are presented by programming based on the Wilson- 6 method and applying the FEM software.更多还原