Fluid loading and hydro-elastic response of towed pipelines

As oil production moves to deeper water and marginal fields, it is necessary to critically consider the merits and drawbacks of different subsea pipelaying techniques. The pipeline tow method is one of these. The basic concept of the tow method is to tow and lay the pipeline at an off-shore location after joining and testing the system at an on-shore fabrication site. This method, therefore, assures improved production quality of pipelines and can be applied economically and easily to all kinds of pipe. It does, however, pose a greater risk of failure due to ocean wave loads during its towing phase. As a consequence, the tow method requires very careful design to achieve a very small submerged weight and maintain a nearly horizontal catenary that is then directly exposed to wave action. The physical problem of towed pipelines in currents and waves is exceedingly complex. The static component of the problem has a significant structural nonlinearity whereas the dynamic component is complicated by the quadratic nonlinearity of the hydrodynamic drag force and the unusual nature of the inertia force under a high near-tangential flow. Such an inertia force was first identified by Sir James Lighthill as applicable to circular cylinders with near- tangential incident flow in waves. The analysis must also account for the axial dynamics of the pipeline, towlines and the tow vessels. The research described in this thesis is aimed at investigating these issues and to obtain a quantitative understanding of their effects on the responses of typical towed pipelines. Initially, the governing equations for towed pipelines and the analytical solutions for simplified cases are presented. In order to tackle a full, representative problem, the finite element method (FEM) is used both for static and dynamic analysis. For the static analysis, full nonlinear algorithms are implemented for the pipeline with simple support boundary condition at each end. To improve convergence performance during this nonlinear analysis, an enhanced catenary equation, which can consider the effect of a uniform normal component of current load, is used to obtain an initial configuration for the nonlinear analysis. To consider deformation-dependent loading from currents and hydrostatic pressure, an improved version of the direct iteration method is applied using a conventional inner iteration approach with constant external load and an outer iteration to ensure that the residual forces resulting from the deformation-dependent loading are equilibrated. Both tow vessels and towlines are added to the static analysis model to form a dynamic analysis model with the initial stiffness method being employed during a direct integration procedure. The dynamic analysis is done in the time domain for regular waves with either the Morison equation or Lighthill's inertia loading formulation. The results from this complete FEM are verified by comparison with those from another finite element package (ABAQUS) for simplified models. The results show that towed pipelines are highly influenced by external loads because their submerged weight must be kept very small. Therefore, an accurate application of deformation-dependent loading and hydrostatic pressure force is essential during the static analysis. In dynamic cases, high lateral responses are concentrated around the ends of the pipeline due to their proximity to the water surface and the soft constraints from towlines. The dynamic bending moment is mainly influenced by wave forces on the pipeline and increases rapidly as the pipeline approaches the water surface. The axial interaction between tow vessels and a pipeline is highly influenced by the property of the towline. Therefore, too stiff towlines may induce large dynamic axial force and give transient compressive axial force. The Morison formulation gives slightly less inertia force and response than that obtained from Lighthill's approach. Considering this slight difference and the uncertainties in wave kinematics and drag forces, it can be said that the Morison formulation still provides reasonable results for towed pipelines