自升式深海网箱不同工况下结构安全性评估研究

Research on structural safety evaluation of jack-up offshore net cage under different conditions

中文关键词: 深海网箱  自升式  结构安全性  强度分析

英文关键词: Offshore net cage  Jack-up  Structural safety  Strength analysis

中文摘要:

      深海网箱养殖受极端环境影响较大，自升式深海网箱凭借其结构特点在极端天气下可将养殖箱体下潜入水，为更好地应对极端环境、增加养殖效益提供了可能。本文针对某自升式深海网箱，基于大型通用有限元软件ANSYS的Mechanical模块，建立其有限元分析模型，而后考虑正常作业及风暴自存2种作业状态，通过编制不同相位角下桩腿支反力搜索程序确定计算工况，最终求解得到网箱各部件结构变形情况及应力分布，从而完成网箱结构安全性评估。研究表明，自升式深海网箱通过箱体下潜入水可以很好应对极端环境条件；经校核，本研究的自升式深海网箱结构安全性满足要求；桩腿及附近撑杆是自升式网箱的结构薄弱区域，在网箱设计中应予重点考虑。本研究可为自升式深海网箱结构安全分析提供方法参考。

英文摘要:

      To further exploit offshore fishery resources while avoiding coastal pollution issues, offshore net cages are gradually moved towards the deep sea. However, the deep ocean environment is very complex and harsh, which severely threatens the safety of offshore net cage structures. Therefore, developing a deep-water net cage with a strong ability to resist high waves and maintain structural safety is essential. Based on the structural characteristics of the jack-up platform that allows the main body to move up and down along the platform legs, a new offshore net cage has been presented. When extreme weather occurs, the net cage body can quickly submerge along the legs, providing a solution for open-sea aquaculture to resist extreme loading. Research on jack-up platforms is mainly in traditional ocean oil and gas platforms and offshore wind turbine installation facilities. It can provide helpful suggestions for this research. The jack-up net cage developed in this study is composed of the leading steel frame and eight pile legs. The bottom of the pile leg was inserted into the seabed to fix the entire net cage. The cage frame could move up and down along the eight pile legs to meet the requirements of cage operation, survival, and towing conditions. The material of the cage was Q345 steel with a yield strength of 345 MPa. In this study, the safety factor was 1.5, and the allowable stress was 230 MPa. To analyze the structural safety of the jack-up net cage under operating conditions and survival conditions, numerical models of the net cage were first created using the general finite element software ANSYS Mechanical according to the geometric and material parameters of the net cage. The element Pipe59 was used to simulate all pipe structures, including the legs and cage frame. The pile leg sleeves were simulated using Solid45. In addition, node coupling technology was used to simulate the relationship between the legs and leg sleeves. The primary environmental parameters under operating and survival conditions are known based on sea statistics. Then, the wave phase angle search code using the APDL language was developed in this study. The implementation of the proposed code successfully simulated the maximum wave-current coupling force; hence, the most dangerous loading condition under different phase angles was obtained. As the cage structure is symmetrical about the x and y-axes, only three loading directions of 0°, 45°, and 90° were considered in this study. After analyzing the reaction force variation of different pile legs with phase angle, 32 loading conditions were determined. Next, structural response analyses of the jack-up net cage under operating and survival conditions were performed. The entire structural deformation, stress distribution, and maximum von Mises stress location were obtained. When the loading direction was 90°, and the phase angle was 102°, a maximum deformation of 0.115 m was obtained under the operating conditions. At the same time, the maximum equivalent stress of 191 MPa was observed at the No.7 pile leg, which was less than the allowable stress of 230 MPa. Under the survival condition, the net cage had a maximum deformation of 0.097 m when the loading direction was 45°, and the phase angle was 322°. Under loading conditions LC18 (loading direction 0°, phase angle 162°) and LC29 (loading direction 90°, phase angle 95°), maximum equivalent stress of 190 MPa was achieved at the connection between the No.4 pile leg brace and the upper frame and the strong vertical brace of No.6 pile leg, which was also less than the allowable stress of 230 MPa. According to the results above, it can be concluded that the structural safety of the offshore net cage in this study meets the strength and stiffness requirements by analyzing the deformation of the offshore net cage and comparing the maximum stress with the allowable stress. In addition, the maximum equivalent stress was mainly found at the legs or the braces near the legs, regardless of the operating or survival conditions, which indicates that these vulnerable parts can quickly fail and require key consideration in design. Moreover, to indicate the effect of the jack-up net cage on the extreme environmental conditions, the structural deformation and stress of the net cage under operating conditions, under the same loading conditions as the survival conditions, were calculated. The results indicate that the net cage's maximum deformation and equivalent stress under diving conditions were reduced by 55% and 41.4%, respectively, compared with those of the net cage under operating conditions. This proves that this jack-up type net cage can handle extreme environmental conditions by moving down the cage body. This study provides a method for the structural safety evaluation of large-scale offshore net cages. Considering that safety evaluation is systematic work, the fatigue strength, pile leg stability, and dynamic response characteristics under towing conditions may be studied in further work.