Dynamic Behavior of Flexible vs Rigid Spools During Seismic Loading Events

Abstract

In regions of high seismic activity, the most severe loading experienced by subsea production systems during earthquake events can be significantly greater than normal static design loads. The major components of such systems comprising pipelines, manifolds and wells linked by tie-in spools and jumpers all respond differently when excited by seabed accelerations. Individual components may resonate or move out of phase with each other. It is of vital importance that the design of the system accounts for the dynamic loading at interfaces such as flanges, connectors and pipe supports. High dynamic loads due to a severe earthquake may damage the connection and cause leakage of hydrocarbons to the marine environment. Tie-in spools and well jumpers provide flexibility in the system but layouts and physical properties that satisfy static loads may not be as suitable in a dynamic environment. Where earthquake loads are severe, the dynamic response of the entire system needs to be considered, including the structure and connected spool response together as they interact individually with the soil movement. A comparative study of the seismic responses of flexible and rigid spools has been performed using 3D non-linear finite element simulation in the time domain. The system analyzed consists of a flexible or a rigid spool tied-in to a subsea manifold via a mechanical connector. The FE analysis model of the spool configurations includes the free-span at the structure and seabed contact with both static and dynamic pipe-soil interaction. The effects of spool metrology and fabrication tolerances are included as well as complete static load histories due to displacements from installation to operation phases as the starting point for the seismic simulations. The flexible spool was found to provide significantly lower of dynamic interface stresses compared to a rigid spool with similar geometry. The load stress variance was also found to be reduced, indicating that flexibles may also offer more predictable behavior when subject to the highly variable spectral energy of extreme earthquakes.