Case Study for Early Design Optimization of Fatigue Sensitive Tubular Connections in Fixed Offshore Platforms

Abstract

Members forming part of structures exposed to cyclic loading (such as offshore oil and gas platforms) are engineered to satisfy various pre-service and in-service loading conditions which include being checked for compliance with fatigue design requirements. In this study, the parameters that affect welded tubular joint fatigue performance are identified to assist design engineers achieve a preliminary design that is more likely to satisfy the fatigue design requirements early in the design cycle with the aim of avoiding later changes in the design. State-of-the-art Finite Element modelling techniques were implemented throughout the course of this study. For a selected typical K-T joint from a 3D support frame ‘Study Model’ initially created in SACS (Structural Analysis Computer Software) was replicated in ANSYS and variations in Stress Concentation Factor (SCF), Hot Spot Stress (HSS) and fatigue life were studied with respect to variable joint properties, namely brace to chord diameter ratio (d/D), brace to chord thickness ratio (t/T) and brace diameter to thickness ratio (d/t). Results obtained from both programs were compared as means for validation of study model. Following the validation process, the same principles were applied to the Case Study. The Study Model - found a good correlation between the trend of variation of results obtained from Nominal Stress Approach and Hot Spot Stress Approach. This serves the purpose of validating the Nominal Stress Approach methodology which shall then be used in the Case Study. Case Study- Fixed Offshore Platform- Fatigue Life predicted longer for joints with d/D in the range 0.41 to 0.44 for constant member combined stress utilization ratio (UCR) and constant t/T. Fatigue Life predicted longer for joints with t/T in the range 0.345 to 0.375 for constant member UCR and constant d/D. Fatigue Life predicted longer for joints with d/t in the range 28 to 36 for constant member UCR, constant d/D and constant t/T. Engineering Consultants and EPC Contractors executing projects involving detailed engineering through to installation of fixed offshore platforms perform structural analysis to define material requirements. Performing fatigue analyses for the final outcome of static in-service analysis may result in re-sizing of members during the course of the design cycle. The joint parameters derived by this paper can be applied at an early stage of the design process with the aim of confidently defining sizing of tubular joints that are likely to satisfy fatigue design requirements. This greatly enhances engineering efficiency by avoiding later design changes in the design cycle.