Theoretical Prediction Of Cross-Sectional Properties And Fatigue Analysis In The BSR Area For Unbonded Flexible Risers

As the offshore industry advances into deeper waters and harsher environments, the use of unbonded flexible pipes becomes increasingly essential. In many applications where high levels of bending deformations are expected flexible pipes have a distinct advantage over the conventional steel pipes. However, determination of various mechanical characteristics of flexible pipes has never been an easy task due to their complex structure. The presence of helical elements in a flexible pipe multilayered structure raises many uncertainties mainly associated with bending behavior. When flexible pipes are used as risers the upper connection of the riser to the floating production platform is highly affected by cyclic operational loadings and is recognized as one of the most vulnerable parts to failure from excessive bending or from accumulation of fatigue damage. Bend stiffeners (BSRs) are designed for the integrity of flexible risers, providing a gradual stiffness transition at the riser-vessel interface.The objective of this thesis is to build analytical models to predict the structural behavior of the riser-BSR system, which involves cross-sectional analysis of the riser and non-linear analysis of the BSR. In addition, fatigue analysis of flexible risers is also an important issue of this work.The analysis of an unbonded flexible pipe cross-section can be divided into axisymmetric and flexural or bending analysis. Theoretical model is developed for analyzing the response of flexible pipes to combined axisymmetric loads including axial force, torque and internal/external pressures. Equilibrium equations for flexible pipe components are derived and put into a matrix form based on energy approaches. A comprehensive guide for assembling the flexible pipe stiffness matrix is described. Equations for boundary conditions are incorporated into the stiffness matrix and with the help of small iterations the global equilibrium of the entire pipe can be achieved. The procedure automatically detects the tendency of layer separation and adjusts the interlayer contact pressures appropriately. Prediction of gap formations allows the model to account for coupling between the axial and torsional stiffnesses and the direction of load. The difference in construction of rough and smooth bore pipes is also accounted for using a rearranged load vector.In bending, a flexural model is developed separately by superimposing the bending moment functions for each layer. The transition path from the upper (No-Slip) bound to the lower (Full-Slip) bound stiffness of the helical layer is modeled, by treating the helical layer as an equivalent thin tube. Explicit formulations are derived which describe the bending moment-curvature and the bending stiffness-curvature relationships within the transition mode. The influence of local bending and torsion of individual helical elements on the bending behavior of the entire pipe is evaluated. The findings imply that this local behavior significantly influences the full-slip bending stiffness and should be included in the bending analysis. The importance of the initial interlayer pressures is also investigated. The model explicitly determines the bending hysteresis in a pipe undergoing cyclic bending.In the cross-sectional analysis, wherever possible, theoretical results are compared with experimental data from the available literature and encouraging correlations are found.A methodology for the BSR taper design is outlined. The exact expression of the angle at the tip of the BSR required for the design is derived. In the analysis of the riser-BSR system, the formulation is extended taking non-linear properties into account both for the BSR material and for the flexible pipe bending behavior. An analytical procedure is also presented to calculate the bending moment versus curvature relationship for each BSR cross-section, which renders the prediction of the constitutive model much simpler.Fatigue performance of flexible risers is an important design consideration. The top of the riser including the BSR is recognized as a critical region to fatigue failure. The Longuet-Higgins distribution is employed to decompose the wave scatter diagram into an individual regular wave scatter diagram, based on which various analyses are performed to estimate the fatigue life of a flexible riser. The resulting result is compared with that obtained from the stochastic analyses and rainflow counting technique. Comparative analysis shows that the system response should be fully considered in the decomposition process of sea states. Provided that the regular wave bin discretization is performed well, the predicted fatigue life from this method will generally agree well with that from the equivalent rainflow analysis. A parametric study is also conducted to assess the importance of several main aspects in the fatigue response of flexible risers.