Approximation model is used to construct response surface for the fatigue reliability assessment of a marine riser. Experience of marine riser applications has shown that fatigue is sensitive to varieties of uncertainties which are associated with material behavior, structural modeling, hydromechanics modeling, and fatigue characteristics. With uncertainty occurring in these key parameters, it is vital to evaluate the fatigue failure by probabilistic approach. However, fatigue reliability analysis for marine riser in time-domain is often a very computationally expensive process. An approximation model which requires a low number of experiments with the simulation code was constructed by Response Surface Method (RSM). Approximation model in conjunction with First Order Reliability Method (FORM) and Monte Carlo Simulation (MCS) has been used for the compliant Vertical Access Riser's fatigue reliability estimation. The influence of various random variables on overall probability of failure has been studied through sensitivity analysis. Procedures for constructing accurate response surface approximations used in marine riser fatigue reliability analysis are discussed. It is shown that using approximation model can not only reduce the computational cost significantly but also obtain fatigue reliability of marine riser with a high degree of accuracy.
Existing approaches for marine riser design and analysis are typically based on deterministic methods. Without considering uncertainties and probabilistic quantification, deterministic design with a safety factor may be either unsafe or too conservative (Sheehan, 2006). In the design of marine structures under ocean environmental loads, the fatigue requirement is one of the major design criteria for the structural safety. As the offshore developments in rough environmental conditions now extend to deeper water, marine riser fatigue analysis that accounts for the different uncertainties is needed (Yang and Li, 2009a).Recently several papers are published on various aspects of dynamic and reliability assessment of marine risers.