A study on the fatigue analysis for a vertical caisson on FPSO subjected to the nonlinear wave loading

Abstract

A cylindrical type hull appurtenance attached to the side hull like a seawater caisson or a riser tube is subject to the drag force and inertia force defined by the Morison equation, as well as global hull girder loads. When performing fatigue analysis for a structure subject to Morison load, the nonlinearity of drag term in the Morison equation should be taken into account for an exact structural safety assessment in an appropriate way. This research proposes a time domain approach (Level II method) in order to take into account the nonlinearity, as well as a proper combination of local hotspot stress induced by the Morison load and global hull girder stress. It is based on a representation of irregular wave as a combination of a large number of regular waves. It enables for reflecting phase angle differences of particle velocities along the vertical tube. The phase difference with global stress is also taken into account. The method also includes the vertical motion of the caisson and the wave elevation above the mean water line. The nonlinearity of drag term can be treated by linearizing the drag term in the frequency domain analysis. For the frequency domain approach, Level I method using a linearization coefficient is proposed to yield a fatigue damage equivalent to the time domain analysis. It incorporates the different contributions of horizontal velocities along the vertical tube on the hotspot stress at a connecting bracket on the side hull. The proposed methods are verified through a caisson in FPSO example and various comparative studies. Various comparative studies are performed for a verification of spectral code, an evaluation of rain flow counting effect and the combination of motion and stretching effect, a verification of rain flow counting(RFC) code and linearization coefficient. Through these case studies, it is found that the proposed spectral fatigue analysis with linearized Morison’s force results in a good agreement with time domain analysis with a drastic reduction of computational time. In this paper, a FPSO model with a seawater caisson is used as a verification example. The proposed methods are expected to be applied to many similar tubular structures on the side of floating offshore structure, such as, riser guide tube, protection frame for riser guide tube, collision protector and so on. In addition, jacket or jack-up rig consisting of tubular members could be good applications.