Dynamics of long flexible cylinders at high-mode number in uniform and sheared flows

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2007.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections."February 2007."Includes bibliographical references (leaves 81-82).The primary objective of this thesis is to characterize the response of risers at high-mode numbers in sheared and uniform ocean currents. As part of this thesis work, three separate experiments have been planned and executed. The objective of these tests was to create a set of model tests at high-mode numbers, the first test was in uniform currents and the other tests in sheared currents. In the experiments, Vortex-Induced Vibrations (VIV) happened at one frequency at one time, rather than at many frequencies simultaneously. The single VIV frequency varied with time, but the VIV frequencies did not co-exist. The major impact of time-sharing frequencies is that it increases the damage rate and fatigue of the pipe. The high density of the sensors on the pipe allowed for analysis that had not previously been done. Two methodologies are presented to locate the area of the power-in region. Once the region where the vibration originated has been found, the different phenomena that effect the location of the power-in region that were discovered are shown. Four different factors are presented that effect the locations of the power-in region: the incidence angle of the current, the gradient of the current direction, the current profile, and the end effects at high mode number.(cont.) Two dimensionless parameters are presented which help in the prediction of VIV given a current profile. The first is the power-in determination factor which predicts the region where the power-in occurs using a combination of the current velocity and the power-in length. The second parameter, the time sharing parameter, helps to determine whether the riser will respond with a single frequency, or switch between frequencies in time.by Susan B. Swithenbank.Ph.D