Process-structure-property characterisation of plasticity and fatigue damage in X100 welded joints for steel catenary risers

Abstract

Steel catenary risers are increasingly used for offshore oil and gas production due to their suitability for deep and ultra-deepwater installation, in terms of structural capacity and capital expenditure requirements. However, the fatigue performance of welded connections is a key concern for steel catenary risers due to the extreme dynamic loading associated with the offshore environment. This thesis presents the development of novel experimental and computational methods for characterisation of the fatigue of welded X100, a next-generation, high-strength steel for lightweight steel catenary risers. Particular focus is given to the through-process effects of welding on the inhomogeneity of cyclic plasticity and fatigue response across the parent material weld metal and heat affected zone. A key aim is to develop dislocation mechanics constitutive material models for the complex, microstructure-driven variations of stress-strain response within X100 welded joints. This provides a significant step towards a process-structure-property predictive methodology for fatigue of welded connections which will facilitate the design of high-performance steel catenary risers.

An extensive experimental programme is conducted, including a full-scale girth welding trial on X100, physical-thermal simulation of heat affected zone, microstructural analysis and mechanical characterisation of X100 parent material, weld metal and simulated heat affected zone. The hardness, monotonic strength and cyclic plasticity response of the parent material and simulated heat affected zone materials are shown to vary significantly as a result of microstructural transformation during the simulated welding process. The differences in fatigue life between the parent material, simulated heat affected zone and weld metal indicate that yield strength and cyclic softening behaviour are predominant factors in the variation of fatigue performance among the materials. X100 is shown to exhibit superior fatigue performance to the current state of the art offshore riser steel, X80. A significant reduction in fatigue life is shown for welded specimens and specimens with softened microstructural regions within the gauge length, indicating susceptibility to failure due to heat affected zone softening for matched or over-matched X100 welds.

A new two-stage cyclic damage evolution model is developed to capture the experimentally observed damage evolution for X100 and to predict fatigue life. The model is calibrated and validated against fatigue test data from the experimental programme and is implemented within a non-linear kinematic-isotropic hardening Abaqus user material subroutine for multiaxial application. The subroutine is applied within a hierarchical global-local modelling methodology for dynamic fatigue analysis of a steel catenary riser girth weld, where the interdependency between fatigue damage-induced material degradation and cyclic plasticity at the weld is shown for a range of load cases.

A novel damage-coupled dislocation mechanics based constitutive model is developed for the fatigue analysis of X100 welds. The model is implemented to capture the variation in cyclic deformation behaviour between the parent material and simulated heat affected zone materials based on the initial microstructure, determined during the experimental programme, and the evolution of microstructural characteristics. Bainitic block size and lath width are shown to be the key microstructural features contributing to the experimentally observed differences in monotonic and cyclic deformation behaviour between the parent material and simulated heat affected zone materials. The physicallybased constitutive model represents a method for microstructure-based modelling of the continuous gradient in cyclic plasticity and fatigue response across a full girth weld due to welding-induced effects on microstructure. This provides the key building block for process-structure-property fatigue design of next-generation welded steel structures.