Analysis of Dynamic Stress Propagation of a Vibrating Subsea Structure in a Pressurized Environment
A Thesis Submitted to the School of Postgraduate Studies, University of Lagos
Offshore pipeline and flow line systems define a variety of subsea architectures associated with Floating Production Storage and Offloading units (FPSOs) or Floating Storage and Offloading units (FSOs) that are usually employed for oil and gas production in deep and ultra deep waters. The design of such transmission facilities, must satisfactorily account for various phenomena such as hydrodynamic wave loading, fluid transport velocity, operating pressure and temperature of the internal fluid as well as limitations imposed by the seabed subsoil layer geotechnical properties. In fact the transverse and longitudinal dynamic responses of these pipeline and flow line systems are strongly modulated by these effects. Subsea pipelines are on the high demand to function at high temperatures and pressures. The natural behavior of a pipeline is to relieve the attendant high axial stress in the pipe-wall by buckling. Such uncontrolled buckling can have serious implication on the integrity of a pipeline. Hence, the usual practice to date, in the industry is to restrain pipelines by trenching and burying, or relieving the stress with inline expansion spools. In this work, the effect of transverse and longitudinal vibrations on the dynamic stresses induced by the fluid flow was studied with special reference to onset of buckling or bursting of such pipes. For this purpose, an offshore pipeline was idealized as a fluid conveying elastic beam on an elastic foundation and the corresponding set of equations governing the transverse and longitudinal motion of the pipe were formulated. Particularly, by employing integral transforms, an analytic solution for the induced stresses was computed and simulated for design applications while comparison with corresponding formulae currently in use in the field was also carried out. Furthermore, the earlier work was extended to capture the effect of deliberate or natural sediment covering of pipe that occurs over a long period of time, by examining the dynamic stress propagation through a partially or fully buried offshore pipeline. For this problem a boundary valued partial differential equation for the fluid- structure- soil interaction mechanics was formulated. In particular, by employing operational methods, the burst and buckling pressure profiles as modulated by the seabed sediment layer history were reported for design analysis and applications. Lastly this research reported an analytic solution for the induced stresses in polar coordinates coupled with von Mises yield criterion in conjunction with the corresponding set of equations governing the transverse and longitudinal motions of an offshore pipeline on an elastic foundation. Interesting results were simulated for practical analysis and applications.