Study on a Fatigue Analysis for a Vertical Tube Considering Nonlinearity of Morison Force

Abstract

There are two types of offshore structures subjected to Morison force

single pile type and multi pile type which is complex of single piles. Among them, seawater caisson attached to FPSO (Floating Production Storage Offloading), which is vertical single type tube, is selected as the object of this study. Fatigue analysis for the seawater caisson should be performed at the caisson-FPSO hull joint since fatigue failure occurs at the joint, not at the caisson itself in many cases. Local hotspot stress induced by Morison force and global hotspot stress by hull girder load are applied to the supporting structure of the caisson. Therefore, both stresses should be properly combined in the fatigue analysis. In addition, due to relative velocity squared in drag term, nonlinearity of Morison force should be taken into account as well. In actual situation, nonlinearity of Morison force and effects by motion of structure and wave elevation should be included in fatigue analysis for floating offshore structure such as FPSO. In time domain method, all of them can be considered, but it is impossible to consider in frequency domain method. For that reason, time domain method is relatively accurate, but it takes a long time to calculate. Therefore, this study began with the necessity of developing a method that has the merits of both methods. Procedures and in-house codes of a time domain method and two frequency domain methods are developed in this study. Through time domain method, called RFC method, a close approximation to the exact solution can be calculated by considering all factors. In the first frequency domain method, called spectral method, only nonlinearity of Morison force is considered. In the second frequency domain method, called spectral method with stretching, an attempt is made to take into account stretching effect by wave elevation. In time domain method, relative velocities above MWL (Mean Water Level) can be obtained using stretching method

vertical stretching and extrapolation stretching. To keep relations between relative velocities at each depth, superposition is applied. In addition, consideration of time dependent varying submerged range, which has an influence on fatigue damage, is included. In frequency domain method, linearization coefficient is calculated using two stochastic linearization methods

one is for narrow-band wave spectrum and one is for wide-band wave spectrum. Furthermore, wave elevation is taken into account using stretching method. In the results section, there are five categories. The first is verification of in-house code and it is performed using commercial software. Then, using developed code, it is figured out that how well linearization coefficient approximates nonlinear Morison force. In the next category, the results of two linearization coefficient calculation methods are compared. The other two categories are conducted to verify spectral method with stretching. First, which of the two factors, motion effect and stretching effect, has a dominant influence on fatigue damage is found, and then, effectiveness of the suggested method verified.