Effect of corrosion on fatigue assessments of offshore monopile

Abstract

Monopile is the simplest offshore structure that is economically applied in shallow water depth, especially less than 30 meters. Typically, the structure is made of a cylindrical steel tube to reduce the hydrodynamics force. It is penetrated into the seabed up to certain depth depending on the loadings and soil conditions. However, there are some drawbacks of monopile structure because the structure has to be encountered with the stress concentration at fixed support due to the bending moment under environmental loading. Fatigue is a common phenomenon for offshore structures because the structure is subjected to the cyclic loading for long term from wind and wave loads, etc. Salinity of sea water can cause the corrosion and diminish the life span of structures. This is because, it affects the cross sectional area of structural elements and it causes the lessening of fatigue life. This research illustrates the effect of corrosion in fatigue life of the monopile offshore structures. The monopile is modeled by using finite element method with shell elements to perform fatigue analysis. The selected monopile is employed for supporting substation in offshore wind farm. Wave elevation spectra and force spectra are generated to incorporate the analysis. Stress response spectra are obtained from the analysis. Time series of stresses are obtained from the response spectra using inverse fast Fourier transformation. Rainflow counting algorithm is used to count the stress cycles. The corrosion in the fatigue degradation is considered by SN curve with corrosion from previous study. Fatigue damage is determined afterwards considering corrosion loss based on the SN curves. Palmgren-Miner's rule is used to compute the accumulated the fatigue damage. The results demonstrate the decreasing of fatigue life due to corrosion effect of the monopile and is also associated with the effect of temperature and presence of oxygen