ABSTRACT

Fish cages in the open sea are exposed to cycle wave loads during their service life, and thus the fatigue reliability of mooring chains should be re-assessed to ensure the safety status. A numerical model was adopted to calculate the tension force of mooring chains. A nonlinear contact finite element model was proposed to calculate the breaking strength of the mooring chains considering corrosion effect and this model was validated by the formula for break test loads in requirements concerning for materials and welding. The time variant fatigue reliability was obtained. The results indicate that the proposed finite element model can provide more reasonable results than the simplified method.

INTRODUCTION

The fish farm is recently forced to move into the offshore area (Fredriksson 2001), and the reliability of fish cage structure considering fatigue limit state should be re-evaluated to quantify the safety status. The gravity cage is commonly used in the aquaculture industry in China. The mooring line for gravity cage is the primary component to withstand the wave force. The reliability of mooring line should be estimated and provides guidance for design proposes.

The interaction of fatigue and corrosion on chains, which is an important phenomenon affecting the risk of offshore structures, has been analyzed extensively in previous research. Moan and Ayala-Uraga (2008) established a reliability-based model for existing ship hull structures considering crack growth and corrosion effect in multiple environmental conditions. Wei et al. (2013) proposed a generalized nonlinear superposition theory to incorporate possible nonlinear effect including cycle-dependent fatigue and time-dependent corrosion. Huang et al. (2014) evaluated the fatigue reliability of the welded joint of a web frame accounting for corrosion wastage. Dong and Frangopol (2015) presented a probabilistic approach to provide optimum inspection and repair plans for ship structures considering the uncertainties in damage assessment associated with corrosion and fatigue. Yang et al. (2016) analyzed time-dependent stress concentration factor of welded joint as a function of corrosion deterioration and fatigue damage.

You can access this article if you purchase or spend a download.