On the dynamics of flexible risers and suspended pipes

This thesis describes a theoretical and numerical study of the static and dynamic behaviour of flexible pipes and risers. Although the effects of conventional loadings due to self-weight, current, waves and surface vessel excitation are included, this work is specifically aimed at identifying the effects of internal and external fluid pressures as well as both constant density and alternating gas-fluid internal flows on the static and dynamic behaviour of risers. Particular emphasis is placed on research and development of advanced numerical analysis methods for solving the non-linear riser equations in the frequency and time domains. Theoretically calculated responses have been compared with results of model tests carried out at Heriot-Watt University and University College London and with theoretical predictions from alternative formulations. Modified forms of the governing equations for flexible risers have been derived from first principles to include the effects of internal and external hydrostatic pressures and a steady internal flow. It is rigorously shown that the conventional derivation of effective tension using a buoyancy analogy is equivalent to that obtained through exact integration of fluid pressures over the curved surface of the riser pipe. It is also demonstrated that the effect of a steady internal flow is analogous to that of hydrostatic pressure and may be included in the governing equations through the effective tension term. The riser governing equations have been solved using a finite element analysis program written specifically for flexible risers and similar pipe geometries. The static analysis of the riser is carried out using an iterative approach that can accommodate general loading conditions using an incremental shifting procedure. Modifications to the standard non-linear static analysis techniques have been proposed and are shown to provide a more accurate representation of the deformation dependence of loading whilst retaining the non-linear influence of tensile forces on pipe geometry. Dynamic analyses have been carried out using both frequency and time domain techniques. The frequency domain approach is a regular wave analysis based on a combined wave and current linearisation whilst the time domain analysis uses the Newmark method and can accommodate regular and irregular sea states as well as geometric non-linearities. Results of this numerical work have been verified by comparison with model tests and analysis results from several sources. Model tests carried out at Heriot-Watt University at 1:50th. scale have been used to verify global predictions for riser responses and tensile forces. Specially designed model tests at University College London have been used to confirm the validity of predicted riser responses to internal flow. Comparisons have also been made with the analysis results of parallel research works. Case studies of typical North Sea flexible risers are presented. The analytical and numerical work demonstrates that it is essential to include the effects of internal and external hydrostatic pressures and internal flow for accurate prediction of the overall response of a flexible riser. In particular, internal flow composed of alternating gas-liquid phases (slug flow) is shown to induce large oscillations in riser tensions at frequencies defined by the flow parameters. These oscillations are comparable to those induced by wave action and have a significant impact on the fatigue life of flexible risers