ABSTRACT:

A time-domain numerical" method for predicting in-line dynamics of flexible deep seawater risers with internal flow is developed. The analysis method is based on, a swing-by-swing technique with symmetrical vortices· model. The effects of internal flow are considered by adding the momentum change of the internal flow, the frictional force on the inner wall of the pipe, the reduction of the internal pressure, and the reaction force of the intake flow. To validate the present dynamic analysis method, motion measurements in steady current conditions and forced oscillation conditions in still water are performed. The results in steady current conditions show that the horizontal displacement of the pipe with internal flow is larger than that without internal flow due to the reaction force of the intake flow. The results in forced oscillation conditions demonstrate that the amplitude the motion increases when the internal flow exists.

INTRODUCTION

Since 70s, many researches on economics of deep seawater upwelling facilities, mostly ocean thermal energy conversion (OTEC) plants, were carried out (Avery and Wu, 1994). Some of them suggested that the floating-type system is more cost effective than landbased, because the length of the deep seawater riser (so-called cold water pipe, CWP), whose cost is dominant in the total capital cost, for the floating-type system is much smaller than that for land-based (Vega, 1992). The authors (Bando et al., 1998) evaluated capital costs of landbased and floating-type deep seawater upwelling systems. As a matter of course, this cost evaluation is on the assumption that the large diameter CWP can be constructed with low cost and high reliability. This would point out that the accurate dynamic analysis of the CWP is one of the most, important technical subjects for developing commercial size deep seawater upwelling systems.

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