Fatigue analysis of mooring system for net cage under random loads

• Statistical analysis of tension force on mooring line for net cage is conducted.
• The sensitivity of fatigue damage of mooring line to wave height, period is analyzed.
• Application of simplified fatigue analysis for mooring line is evaluated.

Mooring system for net cage plays a pivotal role in the aquaculture industry. Much room exists for further development in relation to the mooring line analysis. A statistical analysis of stress range of mooring line is presented. The fatigue design analysis is based on the time domain analysis of net cage and mooring system in irregular waves. The statistical properties of stress range are analyzed using the Weibull plots, histogram, damage plot and evaluation of its applicability in relation to simplified methods. In addition, the fatigue damage of mooring line is also analyzed using spectrum analysis method and compared with the result from rainflow counting method. A visual inspection of Weibull plots indicates a good fit, and the shape parameter is typically between 0.8 and 1.2. The fatigue damage is underestimated based on the Weibull parameters, while it is predicted accurately based on the Rayleigh distribution. The results also show that the fatigue damage of mooring line can be underestimated 10–30% using the spectrum analysis method.