【作者】 许宁；

【导师】 岳前进；

【作者基本信息】 大连理工大学 ， 工程力学， 2011， 博士

【摘要】 冰荷载是寒区海洋结构的控制荷载。由于海冰的破坏和断裂理论尚未成熟,使得冰荷载研究发展缓慢,迄今未能建立合理的理论和计算模型。对于传统的直立形式抗冰结构,海冰发生挤压破坏,产生最危险的冰荷载,其中极值和交变荷载的机理尚未研究清楚。本文提出在直立结构上安装破冰锥体的概念以降低冰力,其原理是将结构水面位置改造成锥体形式,将海冰的挤压破坏模式转化为弯曲破坏,从而达到降低冰力和冰激振动的目的。自上世纪70年代开始,人们主要针对锥体结构极值静冰力进行研究,而动冰力的研究开展较少。针对动力问题显著的渤海冰区锥体导管架结构,大连理工大学曾开展了完备的海冰现场监测,基于实测数据建立了冰力模型,并已广泛应用于同海域结构的冰振响应预测。但上述结论对于其它海域和结构的应用存在着一定的局限性。海冰对结构作用的破坏过程非常复杂,无法采用经典力学理论或数值方法进行分析与模拟,实验方法成为研究冰荷载的主要手段。本文通过对直接测量冰力和海冰破坏行为的分析,对锥体结构冰荷载进行理论分析并建立了计算表达式：通过开展不同尺寸的原型结构和室内模型的实验研究,揭示了海冰荷载的周期、大小和时程变化的形成机理,建立了动冰力计算模型；提出了交变冰力的发生判据和宽、窄锥体结构的定义,并论证了结论的适用范围。通过冰荷载和实测结构响应的对比分析,论证了安装破冰锥体降低平台冰振的效果。本文的主要研究工作如下：1.针对渤海冰区两座原型结构的现场测量和德国汉堡船模实验室(HSVA)模型实验研究。原型结构现场测量能够为冰荷载研究提供最真实的信息,主要包括冰力和海冰破坏行为：利用JZ20-2MUQ平台上的压力盒冰力测量系统能够直接获取冰力时程曲线,通过同步测量的海冰-结构作用过程说明动冰力形成的物理过程；为避免特定结构测量信息和结论的局限性,针对同海域最大锥体导管架结构进行了冰力测量,即通过结构水下光纤应变测量整体冰力。通过现场测量得到了海冰破坏的基本规律和冰荷载的基本结论。为进一步分析结构形式(尺寸)对海冰破坏和冰荷载的影响,开展了锥体模型的室内实验。通过模型实验更加详细地分析了与锥体作用时的海冰破坏行为(包括正、倒锥体),进而对现场监测的研究结论进行验证。2.基于直接测量的海冰破坏模式分析,统计分析了海冰对锥体结构作用力的周期和幅值,确定了动冰力的形成机制,并建立了相应的计算模型。海冰交变冰力形成的物理机制是冰在锥体前的持续断裂过程：冰力周期的实质在于相邻海冰断裂的时间间隔,取决于海冰断裂长度和运动速度；冰力幅值的本质在于裂纹形成和扩展时的能量释放。二者均受到海冰破坏模式的控制。结合海冰断裂的形成机理分析,即海冰内部裂纹的形成和扩展机制,确定了原型和室内实验中两种主要的海冰破坏模式：楔梁型和板型弯曲破坏。根据实验研究中的参数范围,提出海冰破坏模式的控制要素,其中对锥体结构尺寸的影响进行了重点讨论,为研究结论的适用性奠定基础。基于海冰破坏行为的分析,利用实测冰力数据建立了冰力周期和幅值的计算模型。3.根据海冰与结构作用的物理过程确定了交变冰力的基本形式和发生判据,并给出了宽、窄锥体结构的定义。海冰与结构作用的物理过程是海冰荷载时程变化的机理解释。根据压力盒直接测量的冰力时程和同步获取的冰-锥作用过程,提出了交变冰力的基本形式：三角脉冲(加载、卸载)和完全卸载组成的周期性荷载。完全卸载对应破碎冰在结构前的完全清除,能够保证未破碎冰直接作用于锥体,也是海冰持续弯曲破坏和交变冰力的发生条件和判据。若破碎冰能够完全清除,则定义该结构为窄锥体,此时发生交变冰力；若破碎冰不能及时清除而发生冰堆积,完整冰板不能直接作用于锥体,此时定义该结构为宽锥体。在此基础上确定了交变冰力的基本形式及其适用范围,并建立了窄锥结构的动冰力计算模型。该模型通过了多座原型结构实测响应的验证,可用于锥体海洋结构的冰振预测、评价和结构设计。4.基于原型结构的现场测量,对安装锥体降低冰振危害的效果进行了分析和评价。在直立海洋结构上安装锥体的主要目的是降低冰力和冰振。通过对冰力和结构振动响应的对比分析,说明了加锥减振的可行性和优势。安装锥体后可以从根本上消除直立结构由于海冰挤压破坏而可能引发的频率锁定冰力和恒幅稳态振动现象；通过原型结构的直接冰力测量,证明了锥体结构对冰力具有显著的降低作用,并对加锥降低冰振的效果进行了量化分析,结果表明安装破冰锥体达到了预期的效果。本文系统地开展了锥体结构动冰力形成机理、冰荷载计算模型以及安装锥体抗冰效果评价等方面的研究,从理论上研究了海冰与锥体结构相互作用时的破坏机理及冰力模型,对冰区锥体海洋结构的优化设计和安全评估具有很好的应用价值。更多还原

【Abstract】 Ice force is the dominant enviromental force for offshore structures in cold regions. The progress of ice force research is quite slow maily due to the lack of knowledge on sea ice fracture. The reasonal mechanical explanation and generally accepted function have not been found yet. Sea ice crushing failure occurs on traditional vertical structures and induces the most harmful ice force, resulting in the concept of conical offshore structure, so as to reduce the ice force magnitude. The principal idea of sloping or conical structure is to change the ice failure mode from crushing to bending. Research on peak ice force on conical structures has been carried on since 1970’s, whereas there was limited work on dynamic ice force. The team of Dalian University of Technology conducted field measurements on coincal jacket structures, on which distinct dynamic problems under ice action were observed in the Bohai Sea. The ice force models developed by the statistics of full scale test data have been widely used on the offshore structures in this sea area, however limitation still exists when applied to various structure types in other sea areas.Because of the complex sea ice failure process when acting on cone, only the real tests could obtain the accurate and comprehensive information. Based on the directly measured ice force and ice failure process, the mechanism and the model of ice force on conical structure are developed. Based on the field and lab test on cones of different sizes, the mechanism of the main factors of dynamic ice force (period, amplitude and time-varing characteristics) were revealed. The criterion of dynamic ice force and the definition of narrow/wide cone were proposed, which indicates the applicable range of the conclusions from tests. The vibration mitigation effect was validated based on the comparison of ice force and structure response obtained from full scale tests.1. Full scale tests on two jackets in the Bohai Sea and lab tests in Hamburg Ship Model Basin (HSVA) were conducted.Full scale tests could provide the most accurate information, including sea ice failure process and ice force. Ice force directly measured from the load panels installed on JZ20-2MUQ platform contains the time-varing characteristics of ice force. The synchronously observed ice-cone interaction process was the mechanism of dynamic ice force. In order to avoid the limitation that the research is conducted on a specific structure, the full scale tests were conducted on the largest conical jacket structure JZ20-2NW platform, and the global ice forces were measured by the fiber strain sensors installed on underwater structural components. The basic conclusions on sea ice failure and ice force on conical structures were obtained from full scale tests.The lab tests on cone model were carried out, in order to investigate the influence of cone parameters to sea ice failure and ice force. In the lab tests the sea ice failure could be measured in more detail, including the upwards and downwards cones, and thus the conclusions from full scale tests could be proved.2. Combined with the directly measured sea ice failure mode, the ice force period and amplitude were statistically analyzed, and the calculation model of dynamic ice force of conical structure was developed based on the physical mechanism.The mechanism for dynamic ice force is the ice sheet’s continual bending failure on cone. Ice force period is the duration of sea ice failure, which was determined by the sea ice broken length and ice speed. Ice force amplitude was determined by the energy release with sea ice internal cracks formation and propagation, which also determine the sea ice failure modes.Combined with the mechanical analysis of sea ice fracture, mainly two kinds of ice failure modes are determined:wedge typed and plate typed bending failure. The primary influencing parameters on sea ice failure modes are discussed in the range of various parameters during tests, especially the size of cone. Based on the analysis of sea ice failure, which was the basis when considering the applicable range of the conclusions, the caclulation model for ice force period and amplitude are obtained by statistics of measured data in tests.3. The basic form and criterion of dynamic ice force, the definition of narrow and wide conical structures were developed based on the ice-cone interaction process.Ice-cone interaction process is the mechanism for time-varying ice force. Combined with the directly measured ice froce from load panels, the basic form of dyanmic ice force was developed:periodical ice force composed of triangle force (loading and unloading) and total unloading. The total unloading corresponds to the broken ice total clearing in front of the cone, which could ensure the subsequent intact ice sheet directly acting on cone. Only in this occasion, the sea ice continual bending failure and dynamic ice force arise. The definition of narrow and wide cone was brought up:if the broken ice could be totally cleared, the cone is called narrow cone; whereas if the broken ice could not cleared but pile up in front of the cone, the intact sea ice could not directly act on cone, the cone is called wide cone. The dynamic ice force calculation model on narrow cone was founded and validated by real measured structure vibrations.4. The vibration mitigation effect by adding cone was evaluated based on the full scale measurement.The purpose of adding cone on vertical offshore strucuture was to reduce the ice force magnitude and ice induced strong vibration. The feasibility and effect of adding cone were proved by compare of directly measured ice force and structure response on structures with and without cone. The locked-in ice force and steady state vibration caused by ice crushing failure on vertical structures could be avoided basically. The ice force is obviously reduced based on the directly measured ice force from field tests, and the compare results from structure vibration measured before and after adding cone showed the quantitative effect.This paper investigates the mechanism and calculation model of dynamic ice force of conical structures, and also the effect of adding cone on vertical offshore structure. The dynamic ice force function was founded based on the sea ice failure when acting on cone. The conclusions are quite helpful for the vibration prediction, safety evaluation and optimized design of offshore structure in cold regions.更多还原