Fatigue strength analysis of semi-submersible platforms based on wide-band stress

Abstract

As the focus of ocean resource exploitation shifts from shallow sea to ultra-deep sea, semi-submersible platforms are far away from the coastline for a long time. The semi-submersible platform structure is more prone to fatigue damage under complex sea conditions. The stress range and peaks are considered in the conventional frequency domain spectral method to obey the narrow-band distribution, which makes the results of fatigue assessment tend to be conservative. To solve the problems above, a wide-band fatigue assessment method based on Rice distribution is proposed from the perspective of stress distribution. Different methods are used to solve the error functions involved in fatigue calculations to obtain different approximate equivalent stress models. Finally, the broadband fatigue analysis of a semi-submersible platform in the South China Sea was carried out under different models, and the results showed that: The error of the wide-band fatigue calculation based on Rice distribution is related to the selected equivalent stress model, and a suitable model needs to be selected according to the bandwidth coefficient of the structure. The results obtained provide a reference for the safety assessment of pipelines in service. The research provides a reference for the fatigue assessment of semi-submersible platforms under random broadband loads.