具有多峰、非高斯响应特征的深海浮式结构系统疲劳损伤累积机理及评估方法研究

The deep-water floating system is the fundamental structure of deep-water resources exploitation, and accurately assessing the fatigue damage of the system is vital important to ensure its safety. However, the conventional spectral based fatigue assessment method, which under the narrowband Gaussian assumption, cannot consider the multi-modal and non-Gaussian broadband characteristics of system’s response adequately, and the related theory or methodology need to be improved. In the present research, the causes and mechanisms of multi-modal and non-Gaussian broadband characteristics in stress/tension response of the system are first investigated in detail, and the statistical characteristics and probability density of system’s response are further studied. Next, the system’s response is decomposed into high-frequency, wave-frequency and low-frequency response based on the rain-flow counting technique. The interaction mechanism of high-frequency, wave-frequency and low-frequency response are analyzed, and the fatigue damage cumulative mechanism of the multi-modal and non-Gaussian broadband response is illustrated. Finally, based on the nonlinear transformations and stochastic optimization technique, the analytic expression of probability density function that describes the multi-modal and non-Gaussian response of the system accurately is established from viewpoint of the non-Gaussian model and Gaussian mixture model, and a novel spectral based fatigue assessment method which is able to consider the effects between wave-frequency and low-frequency response. Based on the results of this research, the theory and methodology of fatigue assessment for deep-water floating system are improved significantly. By means of the research approaches such as theoretical analysis, numerical simulation and physical experiment, the object of the present research project is to develop a systematic methodology of fatigue assessment method which is applicable to deal with the multi-modal, non-Gaussian broadband response. The updated method definitely can provide theoretical guidance and technical support for the fatigue design and fatigue life prediction of the large deep-water structure systems.

深海浮式结构系统是深海资源开发的基础性设施，其疲劳强度的准确评估对保证结构系统的安全至关重要。目前基于窄带高斯假设的传统疲劳评估方法无法准确考虑深海浮式结构系统响应中的多峰、非高斯宽带特性，分析理论存在缺陷。本项目将深入研究深海浮式结构系统应力/张力响应中多峰、非高斯宽带特征的成因，探索系统响应的统计特征和概率分布模式，剖析高、中、低不同频段响应的相互作用机理，揭示多峰、非高斯宽带响应下结构疲劳损伤累积规律，拟从非高斯及混合高斯模型出发，借助于非线性变换和随机优化技术，构建准确描述系统多峰、非高斯响应的概率密度函数解析表达式，发展建立一种考虑高低频相互作用的疲劳评估频域模型，完善现有的疲劳评估分析理论。借助于理论推导、数值模拟、模型试验相结合的方法手段，本项目旨在发展一套适用于多峰、非高斯宽带响应的疲劳评估方法，为深海浮式结构系统的疲劳设计和寿命预测提供理论依据和技术支持。

深海浮式结构系统是海洋资源开发的主力装备，长期遭受复杂严酷海洋环境作用。由于系统的复杂性和动力环境的多变性，目前对深海浮式结构系统多峰非高斯响应特征形成机理认知不清，缺乏科学的表征模型和有效的疲劳评估方法，导致深海浮式结构系统安全设计不足，事故频发。本项目围绕深海浮式结构系统疲劳损伤评估中的关键科学问题，创新了非高斯波浪荷载计算理论，发展了多峰非高斯响应概率分布表征方法，突破了多峰非高斯响应疲劳评估技术，实现了预期科学目标。项目基于正交展开原理构建了非线性转换模型以匹配目标非高斯波浪概率分布，改进了随机波浪分解与重构技术以修正非线性转换引起的能量分布扭曲，创新性的提出了基于谱矩表征的非高斯波浪直接模拟方法，摒弃了传统方法概率分布与能量分布不兼容的缺陷；进一步引入了状态空间方程对非高斯波浪荷载卷积积分进行等效表征，建立了浮式结构系统整体耦合分析模型，解决了非高斯波浪荷载计算频时转换方法不适用、卷积积分效率低的难题；揭示了系统双峰非高斯统计特征的形成机理，首次提出了HRW/CRE双峰非高斯响应幅值概率分布模型，揭示了高、中、低频响应幅值遮蔽影响机制以及不同峰值过程疲劳损伤累积规律，建立了基于混合分布模式的系缆疲劳评估频域模型，打破了传统频域模型窄带高斯假设的限制；阐明了变幅荷载下疲劳裂纹扩展机制，建立了基于临界距离理论的弹塑性疲劳裂纹扩展模型，结合谱分析和断裂力学，发展了疲劳裂纹等效累积损伤模型和剩余寿命预报技术，丰富了浮式结构系统损伤容限设计理论；探明了结构疲劳损伤与输入波浪能量之间的内在联系，首创了基于波能等效的波浪散布图组块划分等效方法，弥补了传统方法理论依据不足、等效过程复杂的不足。项目研究成果在多座半潜式平台、深海网箱设计分析中得到应用，为我国深海浮式结构系统疲劳损伤的高效评估和结构设计提供了理论依据和技术支撑。