Offshore subsea pipelines are to be stable to external loadings which are mostly due to wave and current. To decide the stability of subsea pipeline on seabed, Morison equation is well used which can predict inertia and drag forces. For pipelines in trench the velocity and acceleration of water particle are dramatically reduced which affect on the stability. To predict the stability of pipeline in trench, the reduction factors are often used. However, the known reduction factors are limited to specific conditions with defined slopes and trenched depths. To verify the stability in various trench, the experiment was conducted with the advanced PIV (Particle Image Velocimeter) technology. The circular cylinder model for pipeline is fixed in trench section at various trench depths and slopes. Four different velocities of current are generated at the circulating water channel. The velocities around the cylinder are measured and recorded. Even the current is at constant velocity, there are cyclic circular motions created around the cylinder. The horizontal and vertical velocities in front and back of the cylinder are investigated at each current condition. The results show that the trench depth and slope influence greatly on pipeline stability. The stabilities for various trench conditions are calculated with velocities measured in front of the cylinder. The parametric investigation is also conducted to suggest the reduction factors which can be applied on the stability analysis in pipeline design.
Offshore industry has long recognized the importance of subsea pipelines as a vital link for the transmission of materials between onshore and offshore facilities. Although these subsea pipelines are costly in the installation, maintenance, etc. comparing to the onshore pipelines, they remain the most feasible and therefore the most economical method for transporting materials continuously.
