Cold-water pipe (CWP) is one of the most novel and challenging devices in the Ocean Thermal Energy Conversion (OTEC) floating structure. When the internal fluid velocity inside the pipe exceeds a certain critical velocity, the CWP may cause flutter or divergence. In this paper, numerical model is developed to study the stability characteristics of CWP with internal flow. First, linear motion equation for small lateral motions of CWP is derived using the Newtonian analytical approach, and then the motion equation is solved by Galerkin method in order to determine its stability characteristics. The numerical study was conducted to investigate the effects of axial flow velocity ratio, velocity angle ratio, and tangential velocity ratio on the dynamic response of CWP aspirating fluid. It was found that tangential velocity ratio ϕ=0 cannot effectively explain the Kuiper' experimental phenomenon that the negative pressurization has little effect on the dynamic instability of the pipe. And the axial velocity ratio and velocity angle ratio have significant impact on the stability of the CWP. Inversely, ϕ ≠ 0 can explain experimental phenomenon of Kuiper, and the impact of axial velocity ratio have marginal impact on the stability of the CWP.
Ocean Thermal Energy Conversion (OTEC) target generating electricity by combining deep-sea cold water with tropical warm surface water. OTEC is appropriate for tropical islands and coastal regions and it requires temperature gradient of at least 20°C (Srinivasan, 2009). The surface water temperature in tropical regions can reach above 25°C, and temperature of the water at a depth of 800-1000m is as low as 4-5°C (Sario, 2017). Rich resources for OTEC in the world are represented by the red area as illustrated in Fig. 1. It is a promising way to generate renewable energy in the future.
There are three types of OTEC plants, namely: On-shore, Shelf-mounted, and Offshore as shown in Fig. 2. Compared with other platforms, the main difference of the OTEC floating structure is the CWP, which is used to transport deep-sea cold water upward. One main concern for offshore OTEC plant is the CWP stability performance due to internal flow (Adiputra and Utsunomiya, 2019).
