Strain Measurement on Riser Pipe Compared With Model Test and Calculation

Abstract

For pumping up deep ocean water, a SPAR type floating structure with a steel riser pipe was installed in Sagami Bay, Japan, in 2003. The riser pipe, which is 175 m long and 1.0 m in diameter, was connected to the floating structure by two mooring chains. At the middle height of the riser pipe, strain gages were attached on the pipe surface so that they faced to 8 directions at equal angles. The authors measured not only strains but also motions of the floating structure and wave and current conditions from 2003 to 2005. From these measured results in actual sea, it was found that the strains (or stresses) of the riser pipe are mostly concerned with motions of the floating structures, more than wave and current conditions. In order to make clear this phenomenon, the authors carried out forced oscillation tests by using a reduced riser pipe model in the Deep-sea Basin in National Maritime Research Institute, along with some numerical simulations based on FEM. In this paper, we show the results of the model tests and numerical simulations compared with the measured results in actual sea.