深水网箱结构弹塑性分析与疲劳寿命评估研究

The aquaculture deep-water net cage, served as a modern breeding facility to adjust structure and transfer mode of the marine aquaculture as well as to expand the cultivation space of deep-far sea, have significant advantages in large aquaculture capacity and high breeding efficiency, yet it also faces the risk of structural failure caused by the impacts of powerful typhoon, especially under the complicated deep-far sea conditions, this occurs from time to time that the cage’s floating collar has been deformed plastically and even collapsed due to the excessive mooring load of the cage. Moreover, we couldn’t ignore the problems of the key components of the cage being fatigued and damaged by the long-term bearing of the alternating load of the marine environment. Besides, there is no domestic uniformed assessment criteria on the durability of the cage.. This study combines structural mechanics simulation and material model test of cages for visualizing damage responses in multiple levels and perspectives based on previous research data of wave flow and mooring load. The finite element numerical model is first established based on nonlinear mechanical properties of the cage materials, the distribution modes of structural deformation and stress as well as mechanical variation rules of cages are analyzed and the ultimate bearing force on the cages at the moment of plastic fracture is evaluated. Based on S-N curves obtained by the combination of structural mechanics simulation and material model test, fatigue aging behaviors of the cages under long-term wave dynamic loads are further examined. The method can provide a new thought of the optimization of structural strength and fatigue performances and can design the key parameters of fatigue life scientifically. The present research results not only lay data foundation for the plastic fracture failure of the cages as well as safety and reliability analysis, but also provide theoretical supports for formulating life expectancy of the cages under the combined action of wave flow and mooring load, which are indicative of great theoretical values and practical significance. This study can enhance the security evaluation criteria of cages and promote standardized development of structural design.

深水网箱作为我国海洋养殖调结构转方式、大幅拓展深远海养殖空间的现代化养殖装备，具有养殖容量大、养殖效益高的显著优势，但也面临着强台风冲击的结构失效风险，尤其是深远海复杂的高海况环境下因网箱系泊载荷过大造成网箱框架发生塑性变形而崩塌的现象也时有发生。此外，网箱关键部件因长期承受海洋环境交变载荷作用而导致疲劳老化破坏问题不容忽视，在网箱耐用性方面国内也无统一评估标准。. 本项目基于前期研究的波浪流及系泊载荷数据，采用结构力学仿真结合材料模型试验多层次、多角度分析网箱破坏响应。基于网箱非线性力学方法建立有限元数学模型，评估塑性断裂的应力分布模式和承载力极限，进而研究S-N曲线条件下长期交变载荷的疲劳损伤机理，科学指导结构疲劳的关键参数设计。应用上为网箱结构强度及预期寿命研究提供新思路和理论支撑，有助于提升网箱安全可靠性的评估标准及促进结构设计的规范化发展。

本项目评估了HDPE网箱海上作业的结构失效风险，研究了极端海况环境与系泊载荷导致网箱局部崩塌的现象，分析了网箱构件长期承受波浪交变载荷作用而导致的疲劳破坏问题。由力学试验可得，C80网箱浮架在正常海况和极端海况条件下的锚泊载荷分别为27.41kN和86.13kN；HDPE浮架材料的疲劳S–N曲线为S=-11.93ln(N)+29.076，疲劳极限为4.3MPa；网箱浮架承受拉伸及弯曲载荷时弹性变形至塑性变形的临界屈服强度为22.12MPa和30.58MPa，塑性区扩展至表面且断裂伸长率大于340.18%时发生结构断裂。由有限元仿真分析可得，当施加水平载荷3000kN或波高23.4m时C80网箱浮架失效；当施加四点位移载荷扭曲5m时，浮架剪切失效；网箱浮架失效和疲劳主要发生在系泊点，应力集中主要在裂纹区域；增大系泊面积和工字架数量、减少焊接接头、降低联接管材的SDR系数可有效提高网箱浮架的可靠性。优化和设计HDPE超大型网箱及钢制桁架深远海网箱，加强网箱浮架规格标准，增大圆弧曲率和装配间隙，海上测试时养殖效果良好，项目取得的研究成果将对我国深水网箱工程设计与应用提供重要数据和理论参考。