Investigation On Vortex-Induced-Vibration Of Flexible Risers With Buoyancy Modules

Slender marine structures such as risers, pipelines and marine cables may easily suffer vortex-induced vibration (VIV) by ocean current. Observations of marine riser responses show that current-induced VIV is the main cause of yield failure and fatigue failure. For deepwater risers, due to their own gravity and transverse load by current, enough pre-tension should be available to guarantee stability and strength. However, the risers can only withstand limited top tension since the restriction of strength from the riser near top tensioner and the joint. Therefore, buoyancy modules must be set to provide distributed buoyancy, so as to reduce wet weight of risers as well as rig tension demand. With the development of deepwater exploration and production, vortex-induced vibration of flexible risers with staggered buoyancy is becoming more and more important.Based on the demands above, VIV characteristics of flexible risers with buoyancy modules are investigated in the thesis. Data processing method especially for VIV experimental data is also carefully studied. The main contents and contributions of this thesis can be summarized as follows:1. A comprehensive review on vortex-induced vibration research is carried out first. The basic theory related to VIV is then carefully instructed. Numerical simulation is also conducted in the thesis to study VIV characteristics of riser modules with different coverage rates of buoyancy and different distributions using the software SHEAR 7, which provides numerical basis for experimental design.2. VIV experiments of flexible riser models are designed and carried out in uniform flow. The test riser models include bare riser model, riser with helical strakes, riser with staggered buoyancy as well as risers with both helical strakes and buoyancy modules. The helical strakes are used to study its suppression efficient in uniform flow with a size of 17.5 diameters in pitch and 0.25 diameters in height. In the tests of risers with buoyancy, the buoyancy modules are arranged in three coverage rates, that is 25%, 50% and 100% coverage respectively. The tests with buoyancy modules aim to investigate the VIV characteristics of risers with different coverage of buoyancy. Besides, for the 25% and 50% coverage conditions, the portion without buoyancy modules of the model was also covered with helical strakes of 17.5D pith/0.25D height. The purpose of the strakes here is to evaluate the suppression efficiency of the strakes in conditions of risers with buoyancy.3. VIV data processing method is studied, including modal analysis method, the effects of mode-shape functions, modal aliasing, experimental error analysis as well as damage analysis. The whole data processing procedure is carefully explained with charts and tables using experimental data as input. The validation and reliability of these analysis methods are also investigated.4. Based on the data processing method above, a comprehensive analysis of VIV experimental data are carried out to compare the results of different test conditions. The results show that the responses of bare riser are consistent with the basic VIV character of flexible risers in previous tests. Helical strakes with 17.5D/0.25D have a perfect suppression effect, and the average suppression efficiency is 93.5%. From the tests of risers with different coverage of buoyancy, it can be concluded that the damage rates can be reduced with the arrangement of staggered buoyancy. The addition of helical strakes on the bare portion of riser with buoyancy will reduce the displacement amplitude and excited frequency so as to reduce the damage rate. The average suppression efficiency of strakes in this condition is under 50%, which is much less than the case with 100% coverage of strakes. However, the addition with strakes will compensate the increase of displacement due to the arrangement of buoyancy and the excited frequency will also be reduced, which will finally optimize the damage character.