Analysis of Flow Induced Vibrations Effect on Service Life of Unbonded Flexible Pipe

High frequency vibrations have been reported on flexible pipes for gas export risers installed on platforms offshore in the North Sea. The vibrations are believed to be caused by internal flow vortex shedding as pressured gas passes through the bore at a certain speed. The vibration frequency is typically above 150Hz resulting in approximately 100 billion cycles over the service life. This paper presents analytical models developed to assess the alternating stresses of the strength components induced by the flow-induced carcass vibration, and the consequent influence over their fatigue damage and overall pipe structure integrity. The studied strength layers include the interlocked carcass layer, interlocked hoop strength (pressure armor) and axial tension layers. The models illustrate the behavior of these components inside both the end fitting structure and the free suspended section. The study of a large 15 inch inner diameter gas export riser is presented with test measurements reported in literature.