ABSTRACT

This paper develops practical reliability procedures and techniques applicable to offshore risers for evaluating the probabilistic fatigue damage. The study undertaken implements a methodology for risers' reliability assessment which uses deterministic fatigue results as the starting point in conjunction with analytical reliability techniques. Reliability methods can be acknowledged as an additional tool for determining data which can be used for a relative comparison of the importance of the different parameters and sensitivities to changes in them. The case study involves a riser that is connected to a semisubmersible and interacts with the non-linear seabed. Uncertainty is involved in both the stress and resistance components and is well documented in the literature as characterising the probabilistic nature of reliability-based fatigue analysis. Engineers must know the potential effects of any uncertainty behind their decision. A first order reliability method is used for predicting the fatigue safety index. The main benefit of employing a probabilistic fatigue analysis framework is to increase the confidence in the marine risers fatigue assessment. Therefore, this is seen as a way forward for a rational design of offshore risers with seabed interaction.

In the stress component, uncertainty arises due to the uncertainty associated with the scatter in S-N curve data, wall thickness, eccentricity of the welded joint, SCF, uncertainty in estimation of fatigue stresses and the presence of a random variable which quantifies modelling error. Uncertainty in the strength part of the limit state function is due to the modelling error associated with Miner's rule. The safety index indicates the level of safety and whether the probability of failure is small enough to be acceptable. The limit state function may be extended further to include other uncertainties in the fatigue loading part. There is no limit on the number of uncertainties. This is seen as a way forward for a rational design of SCR with seabed interaction.

The challenges regarding the fatigue damage assessment of a catenary pipeline in the touchdown zone (TDZ) are primarily because of the nonlinear behaviour of pipe-seabed interaction, considerable uncertainty in geotechnical model parameters. This study investigates the sensitivity of fatigue performance to geotechnical parameters through a parametric study. Furthermore, this paper presents the probability of failure associated with fatigue analysis of a catenary pipeline in the TDZ due to the uncertainty in seabed response model and geotechnical parameters. A first order reliability method is used for predicting the fatigue safety index. The fatigue analyses results prove that the confounding results indicated by the previous research studies on the catenary pipeline or steel catenary riser (SCR) in the TDZ are due to different geotechnical parameters imposed with the soil model. The main benefit of employing a reliability-based fatigue analysis framework is to increase the confidence in the SCR analysis with seabed interaction.

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