LONG TERM STRESS RANGE DISTRIBUTION OF THE RISER OF AMIRKABIR SEMISUBMERSIBLE PLATFORM UNDER THE EFFECT OF WAVES

Top-tensioned risers, are prone to high amplitude vibrations under the effect of waves, vortex shedding, vessel motion and internal flow. Due to the oscillating nature of forces acting on the riser, it experiences varying stress ranges which leads to fatigue damage. In this research the riser of Amirkabir semisubmersible rig was modeled to estimate the long-term stress range distribution over its length. For this purpose a FEM model of a tensioned Euler-Bernoulli beam was developed for simulation of riser s motions. The vibration of riser under the effect of waves in the long-term was calculated in the time domain for different sea states from the wave scatter diagram. The riser has been analyzed in each sea state ofthe wave scatter diagram by 50 times to reduce the sampling error. Then a rainflow cycle counting method was applied to find the stress ranges and the number oftheir occurrences. Finally a Weibull distribution was fit to the stress ranges to find the distribution of stress ranges in the lifetime of riser