深水网箱浮架结构的失效及疲劳性能分析

为了分析台风海况环境与系泊载荷导致深水网箱局部崩塌的现象,分析网箱部件长期承受波浪交变载荷作用而导致的疲劳破坏问题,该文自主开发网箱浮架试样的加工方法,并进行浮架管材的性能参数测定,进一步采用等效载荷有限元模拟、全场景有限元模拟与力学试验对比验证,结合有限元结构仿真分析网箱浮架的失效及疲劳情况。结果表明,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22.12和30.58 MPa;浮架应力集中及疲劳关键节点主要为焊接点、系泊点、管材联接及工字架装配边缘区域,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;浮架单点系泊及均布载荷40 kN时的低周疲劳寿命为734次应力循环,主要是由于此时的结构应力大于疲劳极限3.38 MPa导致疲劳寿命减小。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能。研究结果可为长期和极限作业环境下网箱结构的优化设计提供参考。

Failure and fatigue analysis of floating structure of offshore cage

In order to reduce the structural failure risk of extremely high sea conditions with the strong typhoon impacts, to analyze the yielded phenomenon under excessive wave and mooring loads, and to further study the fatigue problem on the key components of the floating collar caused by long-term bearing of the wave alternating loads, In this paper, the processing method of the floating collar sample is developed, and the performance parameters of the floating collar pipe are measured, and the equivalent load finite element simulation, full scene finite element simulation and mechanical test are carried out, and the failure and fatigue of the floating collar are analyzed by combining the finite element structure simulation in this research. The results show that the fatigue S-N curve of floating collar can be described by S=?1.022ln(N)+22.201, the working stress should be not more than 3.38 MPa if the fatigue life is not less than 108 stress cycles. The mechanical properties of welding joints are lower than that of the respective raw pipe. The minimum tested fatigue life of the dumbbell-shaped specimen is 1.436×105 stress cycles under the load of amplitude 0-1 kN and frequency 10 Hz, while the FEM simulation fatigue life of dumbbell type is 1.505×105 stress cycles. When bending load stress (0-13.465 MPa) is greater than tensile load stress (0-13.129 MPa), whereas bending load life (1×106) is greater than tensile load life (1.884×105), the high-cycle fatigue damage of float under tensile load is larger than that of bending load. The flexural modulus is 513.71 MPa, elastic modulus is 692.69 MPa, the tortuosity is 93.47%, the elongation at break is 340.18%, indicating that, compared to the tensile load, the low-cycle fatigue damage and static damage of bending load is larger. That is, both the tensile and bending loads should be considered at the same time to evaluate the failure of floating collar. During the elastic deformation to plastic deformation, the tensile strength is 22.12 MPa and bending strength is 30.58 MPa, which should be function as the evaluative criterion. During the plastic deformation to failure state, the HDPE pipe will not be destroyed immediately after it reaches yield, the plastic zone will continue to expand to the outer surface and the structural fracture will occur when the elongation at break exceeds 340.18%. The nonlinear behavior of the FEM model of floating collar is mainly caused by geometrical changes in some key areas, and the minimum values of life span and safety factor of hexagon-triangle cage float are mainly in the connection area of the inner hexagon to the outer triangle far away from the mooring point. As the uniform load is 40 kN, the equivalent stress of single point mooring (SPM) and 8 points mooring of C60 floating collart are 38.57 and 3.62 MPa, respectively. More specifically, the stress of 8 points mooring is less than the 1/10 of single point mooring, which shows that increasing the mooring points can effectively improve the safety performance of floating collar. The maximum stress of C60 type floating collar under the condition of SPM and 40 kN uniform load is 38.57 MPa, which is greater than 24.08 MPa of hexagon-triangle type floating collar, under this condition, the minimum number of fatigue stress cycles of hexagon-triangle type floating collar is 734, and its mechanical properties are better than that of circular floating collar structure. The key points of the stress concentration are mainly at the floating collar''s welding points, mooring points and at the edge areas of the braces. Increasing the number of mooring points and braces and reducing the standard size ratio(SDR) of main pipes'' connection areas can improve the ultimate bearing capacity and fatigue reliability of floating collar for the long-term wave and mooring conditions. The results can provide the principle guidelines for the structural optimization design of floating collar under the long-term or extreme environment.