【作者】 郭峰玮；

【导师】 岳前进；

【作者基本信息】 大连理工大学 ， 工程力学， 2010， 博士

【摘要】 本文首先分析了直立结构挤压冰荷载的物理本质,阐明了直立结构的挤压冰力是一个固体破坏力学和结构力学／动力学的耦合问题。鉴于目前固体破坏力学不能准确描述冰的挤压破碎过程,理论和数值方法的应用非常困难。因此确定了研究直立结构挤压冰荷载的合理技术路线,即首先利用原型实验和模型实验获得冰力、结构的响应等相关实测数据,通过分析实测数据并结合一定的理论假设,建立挤压冰荷载的力学模型和计算方法,进而利用实测数据对模型进行检验和修正。对冰力学的研究、抗冰结构的发展作了简单总结。随后对挤压静冰力和动冰力的研究进展进行了综述,明确了目前直立结构挤压冰荷载需要继续研究的几个问题,即冰在压缩状态下的力学行为、极值挤压静冰力的计算、交变冰力的形成机理和力学模型、以及动冰力模型实验的方法等,这些也是本文主要探讨的问题。由于冰材料的力学行为存在显著的尺寸效应,因此原型实验的实测数据被认为是研究冰荷载的最可靠依据。本文首先简单概括了国内外已经开展的、具有代表性的原型结构实测冰荷载成果,随后详细介绍了在渤海辽东湾的采油平台上建立的监测体系,论述了该原型实验获得的信息对冰荷载研究的有力支持。根据原型结构实测数据的分析,本文把直立结构的挤压冰荷载分为三种不同的模式：准静态冰力、稳态交变冰力以及随机交变冰力。根据三种冰力模式发生在不同冰速范围内这一现象,本文提出了一套完整的物理机理解释,认为三种冰力模式对应着冰在不同加载速率下的力学行为,即低速率下的韧性、中等速率下的韧脆转变以及高速率下的脆性破坏特征,并深入分析了三种冰力模式分别包含的关键力学问题。模型实验是冰荷载研究的另一个重要手段。本文简单总结了国内外已经开展的各种尺度的冰荷载模型实验,分析了其中的相似性问题。根据原型实验的分析结果,认为冰与结构的相对速度是挤压冰荷载相似性的主要控制因素,并给出了直立结构挤压动冰力模型实验的设计原则,基于此设计了一个小尺度的动冰力模型实验系统,并完成了多次实验,成功模拟了直立结构的静冰力、动冰力以及带齿条的自升式平台桩腿冰力。在实测数据分析的基础上,本文对直立结构挤压静冰力的力学模型和计算方法进行了详细论述。现有静冰力公式的主要差异在于对同时／非同时破坏的认识不同,根据原型和模型实验的数据分析,本文证明了对于柔性窄结构的挤压静冰力计算,同时破坏和韧性破坏模型更为合理,所以柔性窄结构的挤压静冰力应该显著高于传统低值冰力公式的计算结果。直立结构的挤压动冰力分为稳态动冰力和随机动冰力两种模式。实测数据分析表明,冰激结构稳态振动属于自激振动,本文结合冰在压缩应变速率进入韧脆转变区时的力学行为,给出了冰致直立结构自激振动的物理机制,证明了冰内微裂纹的生成和扩展是结构运动控制动冰力的关键因素。提出了进一步的研究思路,用以建立自激振动的发生判据和计算方法。基于已有的随机动冰力研究工作,本文分析了影响随机冰力频率成分的主要因素,提出了利用断裂力学的概念描述冰的脆性破碎尺寸,进一步描述随机冰力频率成分的思路。更多还原

【Abstract】 The essential physical process of crushing ice force on vertical structure is analyzed, and it is concluded that crushing ice force is a subject coupled with damage mechanics of solid material and structure mechanics/dynamics. Since modern damage mechanics is not able to describe the crushing process of ice sheet accurately, it is fairly difficult to study crushing ice force using theoretical or numerical methods. Therefore, the crushing ice force problem should be investigated according to the following approach:firstly prototype and model tests are conducted to obtain the real data of ice force, structure’s response and other related information; then the qualitative model of ice crushing and ice force calculation methods are developed based on data analysis; finally the model and calculation of crushing ice force are verified using data from tests.Ice mechanics and development of ice-resistant structures are briefly summarized, and then the research on static and dynamic crushing ice forces is reviewed. The topics on crushing ice force on vertical structures which need further study include:mechanical behavior of ice under compressive state, calculation of peak static crushing ice force, mechanism and model of dynamic ice force, model test on dynamic ice-structure interaction and so on, which are the topics discussed in present thesis.Because of the remarkable scale effect of ice’s mechanical properties, data from prototype tests is the most reliable information for ice force research. Typical prototype tests on ice force study are reviewed firstly, and then the field test system on jacket platforms in Bohai Sea is described in detail. The research of ice force is supported by the field tests information to a great extent.Based on analysis of the field tests data, crushing ice forces on vertical structures are classified into three modes:quasi static ice force, steady state dynamic ice force and stochastic ice force. According to the phenomenon that three ice force modes take place in different ranges of ice velocity, a set of physical mechanism is developed to explain the three ice force modes:the three modes accord with mechanical behavior of ice under different loading rates respectively, which include ductile behavior under low loading rates, ductile-brittle transition behavior under intermediate loading rates and pure brittle failure under high loading rates. Besides, the essential mechanical problems involved in the three modes are discussed thoroughly. Model test is another important way to study ice force. Previous model tests conducted in many countries are reviewed briefly, and problems of similarity and scaling are analyzed. According to the prototype tests results, it is concluded that relative velocity between structure and ice sheet is the dominant parameter for similarity of dynamic crushing ice force. Design principles of model tests which aim to dynamic crushing ice force are given, and a model test system is designed and constructed in the laboratory of DUT. A series of model tests are performed and different crushing ice force modes are obtained successfully, in addition, ice forces on jack-up leg which has gear teeth are also simulated using model tests.Based on ice force data from field and model tests, static crushing ice force calculation is discussed detailedly. The scattered results from existing ice force formulas are due to simultaneous/non simultaneous failure hypothesis. The field and model tests data indicate that ductile failure or so called simultaneous failure hypothesis is valid for static crushing ice force on narrow and compliant vertical structures, accordingly, the static crushing ice force on narrow and compliant structures should be remarkably higher than conventional low level ice force formulas.There are two modes of dynamic crushing ice forces on vertical structure:steady state ice force and random ice force. The field test data indicates that ice induced steady state vibration is a type of self excited vibration, and the physical mechanism is developed to explain ice induced self excitation. It is proved that during self excitation process, compressive strain rate in ice sheet enters ductile-brittle transition range, and the formation and failure of micro cracks in ice controls ice force’s fluctuation and results in lock in phenomenon. In order to develop criterion and mathematical model for ice induced self excitation, further method is presented based on the physical mechanism. Based on previous research work on random dynamic ice force, the dominant factors on frequencies of random ice force are analyzed, and it is suggested that fracture mechanics is applied to describe the brittle failure size of ice sheet, and then the frequency distribution of random ice force could be predicted.更多还原