Chia-Rong Chen

Vertical Riser VIV Simulation in Uniform Current

Recently, some riser vortex-induced vibrations (VIVs) experimental data have been made publicly available (oe.mit.edu/VIV/) including a 10 m riser VIV experiment performed by Marintek, Trondheim, Norway, and donated by ExxonMobil URC, Houston, TX, USA. This paper presents our numerical simulation results for this 10 m riser and the comparisons with the experimental results in uniform current. The riser was made of a 10 m brass pipe with an outer diameter of 0.02 m (L/D=482) and a mass ratio of 1.75. The riser was positioned vertically with top tension of 817 N and pinned at its two ends to the test rig. Rotating the rig in the wave tank would simulate the uniform current. In the present numerical simulation the riser’s ends were pinned to the ground and a uniform far field incoming current was imposed. The riser and its surrounding fluid were discretized using 1.5×106 elements. The flow field is solved using an unsteady Reynolds-averaged Navier–Stokes (RANS) numerical method in conjunction with a chimera domain decomposition approach with overset grids. The riser is also discretized into 250 segments. Its motion is predicted through a tensioned beam motion equation with external force obtained by integrating viscous and pressure loads on the riser surface. Then the critical parameters including riser VIV amplitude (a) to the riser outer diameter (D) ratio (a/D), vorticity contours, and motion trajectories were processed. The same parameters for the experimental data were also processed since these data sets are in “raw time-histories” format. Finally, comparisons are made and conclusions are drawn. The present numerical method predicts similar dominant modes and amplitudes as the experiment. It is also shown that the cross flow VIV in the riser top section is not symmetric to that of the bottom section. One end has considerably higher cross flow vibrations than the other end, which is due to the nondominant modal vibrations in both in-line and cross flow directions. The computational fluid dynamics (CFD) simulation results also agree with the experimental results very well on the riser vibrating pattern and higher harmonics response. The higher harmonics were studied and it is found that they are related to the lift coefficients, hence the vortex shedding patterns. It is concluded that the present CFD approach is able to provide reasonable results and is suitable for 3D riser VIV analysis in deepwater and complex current conditions.

Time-Domain Simulation of Riser VIV in Sheared Current

Riser vortex-induced vibrations (VIV) have attracted significant attentions in recent years in offshore oil and gas industry. There is an increasing interest in using computational fluid dynamics (CFD) approach for deepwater riser VIV time-domain simulations. Our previous study has demonstrated that the long riser (L/D = 1400) VIV response in uniform current can be predicted with reasonable accuracy by time domain simulations with Chimera over-set data grid technique. This paper is to further that study and investigate the riser VIV in sheared current profiles. The riser studied in this paper is a long marine riser with constant tension distribution. Its prototype has an outer diameter (OD) of 0.027m and a mass ratio of 1.6. The fluid domain is discretised using approximately one million elements. A linearly sheared current is imposed in perpendicular to the riser, and the flow field is calculated using an unsteady Reynolds-Averaged Navier-Stokes (RANS) numerical method in conjunction with a Chimera domain decomposition approach with overset grids. The critical parameters including riser VIV root-mean-square (rms) a/D, vorticity, drag and lift coefficients are processed, and compared to those of uniform current and experimental data. The simulation results show that the riser VIV under sheared current behaves differently from uniform current. It is also shown that the presented CFD approach provides reasonable results and is suitable for long riser VIV evaluation in deepwater and complex current conditions.Copyright