Fatigue Life Calculation of Deep Water Composite Production Risers by Rain Flow Cycle Counting Method

Abstract

Composite production risers in deep sea condition operate in harsh environment induced by wind and waves together with large axial tension fluctuations. Therefore, a suitable fatigue analysis of the riser stress joints must be undertaken at the structural design stage to ensure their adequate lifespan of 25–30 years. The composite riser, under study, is considered to be a part of Tension Leg Platform (TLP) to be installed at a depth of 2000 m. The probabilistic records of twelve sea states are used to simulate the expected loads for its entire service life. The response time histories are obtained by implicit time domain analysis of the composite riser for each sea state of random nature, duly considering the nonlinearities involved. Each stress time history is decomposed into equivalent stress blocks. A stress range histogram is generated in terms of the number of cycles in each block. The rain flow cycle counting method is hence used to estimate the cumulative fatigue damage. Palmgren-Miner rule is used to calculate the damage caused by stress signals of variable amplitudes. The damage fractions are then summed linearly to give an estimate of the total fatigue life for a particular stress history. The cumulative estimate for its entire service life is hence estimated considering all expected sea states.