The fatigue assessment of offshore monopile structure considering corrosion

Abstract

Offshore monopile structures are typically employed in shallow water depths. The structures are embedded into the seabed up to certain depths depending on the soil conditions. Fatigue is a critical problem for the lifespan of existing offshore structures. The salinity of seawater provides a corrosive environment for steel materials. It potentially increases the fatigue damage in the structure. The aim of this research is to dem-onstrate the consequences of fatigue behavior of monopile due to corrosion. The analyses have been per-formed by finite element analysis (FEA) using shell elements to simulate the dynamic responses of monopile structures. Soil springs are applied as foundation system. The hydrodynamic force from ocean waves is consi-dered for estimating the force spectrum. A wave scatter diagram is applied for the long-term sea state of mo-nopile structures. The stress spectrum is determined from the spectral analysis. To investigate the effect of corrosion, experimental SN curves from the previous study are implemented. These experimental curves are included with the effect of corrosion. Three different cases are considered: in air (room temperature), seawater (45°C) and seawater (45°C+diffused air). It was observed that higher amounts of diffused air in the seawater provide higher fatigue damage in the structure.