On the Dynamic Stability of OTEC Cold Water Pipe (CWP) under Variations of Force Balance Descriptions at the Free Inlet

Abstract

Ocean Thermal Energy Conversion (OTEC) Cold Water Pipe (CWP) can be addressed as a riser conveying fluid which is predicted to be unstable when the internal velocity exceeds its critical value. However, the numerical analysis shows a momentum change at the free inlet, which may influence the riser stability. Thus, this paper analyzed the stability of OTEC CWP analytically by considering the force balance at the inlet. The modeling of the pipe and the variations in force balance modeling were conducted using the finite element method where the pipe stability behavior was assessed by observing the displacement of the last node of the pipe over time generated via the Newmark time scheme method. The critical velocity on a small-scale fixed joint pipe model, including shear force balance, was 5.06 m/s, while the critical velocity with partial shear force balance and no shear force balance was between 2.3-2.4 m/s. In the full-scale model, the critical velocity with shear force balance was 5 m/s and 4.75 m/s, for fixed joint and pinned joint, respectively. Critical velocity generated by partially and fully excluding shear force balance on the pipe model is 3.66 m/s and 3.68 m/s for the fixed joint and 3.58 and 3.62 for the pinned joint. The results showed that the inclusion of shear force balance increased the critical velocity.