Model riser test for vortex induced vibration in a sheared current

Zhu, Lin (2006) Model riser test for vortex induced vibration in a sheared current. Masters thesis, Memorial University of Newfoundland.

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Abstract

This thesis describes a laboratory investigation and a method for analysis of multi-mode vortex induced vibration of a flexible slender riser model which was subjected to a sheared current and a uniform current. The Reynolds number of the tests was in the region from 2127 to 40,171 which belonged to the subcritical range. -- The model riser was made of an 8.5 m long and 0.047 m diameter rubber hose. Twenty pairs of accelerometers were spaced inside the rubber hose to monitor the in-line and crossflow vortex induced vibration responses which will be abbreviated as VIV responses later. The simplified shear flow was produced by a 4 m or 3.5 m long and 38 cm diameter PVC pipe. The model riser segment covered by the PVC pipe stayed in still water and the other part was subjected to a uniform velocity. -- A total of 120 test runs were conducted in the VIV experiment. The test flow velocity range was from 0.1 to 1.0 m/s and increments were 0.1 m/s. The duration of each run was two minutes. Most tests were done twice for the same velocity profile and same pretension in respectively up run test and down run test. The so-called up run test denotes that test current velocities change from low to high. In contrast, the down run test represents that test current velocities change from high to low. In addition, a thirty minute data acquisition sustained test was performed in a sheared current. -- The test analysis was conducted in the time-domain, the frequency-domain and the space-domain. Close attention was paid to investigation of the multi-mode responses, the lock in phenomenon, the difference between the up run test and down run test, and sudden changes occurring in the thirty minute data acquisition sustained test. -- In the thesis, a comparison method is used to help understand the behavior of the VIV responses. The riser VIV test results in uniform currents had been completed before. These test results were familiar to us. Through comparing the test results in the sheared currents with the uniform currents, the test results in sheared currents can be understood better. A rough comparison of the two uniform tests in the Ice Tank and in the Flume Tank is done. Here it was found that reduced velocity V* corresponding to the model VIV maximum amplitude does not change in between these two tests.

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