Experimental charactersation, computational modelling and design tool development for fretting fatigue and wear in flexible marine risers
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Fretting is a key concern in the fatigue design of the pressure armour layer of flexible marine risers, due to the combination of extreme global dynamic loading conditions and the local geometrical-tribological conditions in the nub-groove contact regions. This thesis presents experimental characterisation, global-local computational modelling and the development of a stand-alone analytical design tool for fretting fatigue and wear in the pressure armour layer of flexible marine risers. Extensive experimental testing is carried out to identify tribological and mechanical properties of pressure armour material. The effects of grease-lubricant, contact conformity and loading conditions on fretting behaviour are investigated on cylinder-on-flat specimen configurations. A newly-designed fretting rig provides testing under partial and gross-slip conditions with high contact pressure, low slip conditions, representative of riser nub-groove contact loading conditions. A pre-service flexible marine riser is dissected to extract pressure armour material for monotonic tensile and fatigue testing. A global-local computational methodology for analysis of the pressure armour layer in flexible risers is presented. The methodology consists of a hierarchy of models, including a global riser dynamics model, a three-dimensional riser sub-model, an axisymmetric nub-groove local contact model and cylinder-on-flat fretting contact models of test geometries. This allows, for the first time, quantification of key fretting variables, such as contact pressure, relative slip and sub-surface stresses in this complex geometry, under representative loading conditions. Fatigue lives are calculated using the three-dimensional critical plane Smith-Watson-Topper (SWT) multiaxial fatigue indicator parameter. It is shown that operating pressure and bending-induced axial displacement significantly affects predicted crack initiation. It is also shown that friction coefficient has a significant effect on predicted trailing-edge tensile stresses in the pressure armour layer and, hence on fretting crack initiation in risers. A combined fretting wear-fatigue finite element model has been developed using an adaptive meshing technique and the effect of bending-induced slip is characterised. It is shown that a surface damage parameter combined with a multiaxial fatigue parameter can accurately predict the beneficial effect of fretting wear on fatigue predictions. A design study is conducted using the framework outlined in this thesis. The key functional relationships between global riser variables (running conditions) and local nub-groove fretting variables are identified. This facilitates identification of the critical riser curvatures for minimum predicted numbers of cycles to crack initiation for different riser design geometries. Furthermore, a weight function method for crack propagation is implemented for various riser geometries, to allow prediction of total fretting fatigue life. Running condition fretting maps for different riser geometries are thus developed. Interestingly, the resulting predicted fretting fatigue lives are found to be in the same range as tensile armour layer plain fatigue lives. A stand-alone, computationally efficient, analytical fretting wear-fatigue model is developed. The model solves for frictional contact surface and sub-surface tractions and stresses using quadratic programming and theory of elasticity; fretting fatigue predictions are calculated using the SWT multi-axial fatigue indicator parameter and transient wear is modelled using the Archard wear equation. The analytical solution is validated against finite element incremental wear and crack initiation predictions for fretting in the nub-groove region. This provides a computationally efficient design tool for fretting in the pressure armour layer of flexible marine risers.
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