Fatigue Analysis of Member Joints in a Jacket Platform under Wave Stochasticity

Abstract

Over the years, innovative solutions and advanced materials have significantly enhanced the performance of offshore structures. However, environmental forces, particularly cyclic loading, render these structures vulnerable to fatigue and cumulative damage, often leading to crack formation and a reduced service life. In this context, the present study evaluates the fatigue life of member joints in an offshore jacket-type structure subjected to extreme wave loading. A case study is carried out on a jacket structure located in the Mumbai High Field, focusing on the joint experiencing the highest stress concentration. Stress concentration factors (SCFs) convert the nominal stress into higher stresses known as hot-spot stresses. These SCFs are very sensitive in the computation of the fatigue life of such steel structures. Minor deviations in SCFs can substantially influence the estimated fatigue life of steel structures. Owing to the inherent variability among empirical formulations, different methods often predict distinct SCF values for identical structural components, leading to discrepancies in fatigue life assessments. The present study undertakes a numerical investigation of SCFs in tubular joints with varying geometric configurations using finite-element analysis under prescribed boundary conditions. The numerically obtained results are subsequently benchmarked against SCF values derived from empirical parametric equations to evaluate their consistency and applicability. The jacket was analyzed using a deterministic approach for fatigue conditions. According to the analysis results, all jacket joints provide fatigue lives exceeding the required years. The primary tubular-to-tubular joints of the jacket were evaluated for fatigue damage, with the design criteria and procedure presented in this paper.