Flexible catenary riser VIV simulation in uniform current
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This paper studied a catenary riser VIV under uniform current using numerical simulation. The riser was a 6" flexible riser installed in a water depth of 1100m. It has an OD of 0.33m (L/D=3300), and mass ratio of 1.8. During the riser installation the riser was suspended at the worktable of the installation vessel, with departure angle approximately 1 deg, and layback distance of 100m. The static top tension was 23Te. In our numerical simulation we modeled the riser section between the worktable and touchdown point (TDP). A uniform current with speed of 0.35m/s was then imposed at the far field. The riser surface and its surrounding fluid were discretized using 1.5 million elements. The flow field was solved using an unsteady Reynolds-Averaged Navier-Stokes (RANS) numerical method in conjunction with a chimera domain decomposition approach with overset grids. The riser was also discretized into 250 segments. Its motion was predicted through a simplified catenary riser motion solver that superposes a catenary equation with a tensioned beam motion equation including structural damping. The external force terms were obtained by integrating viscous and pressure loads on the riser surface. The concerned parameters were then post processed, including riser VIV a/D, vorticity contours, maximum deflections, and effective drag coefficients. The simulation results were studied and compared to the field measurements. It showed that the flexible catenary riser VIV could have much higher cross flow rms a/D than vertical top tensioned risers, and it increases the effective drag coefficients considerably as well. The numerical simulations also showed similar riser behaviors as observed offshore during the riser installation. Therefore, the present CFD approach provides an effective tool for catenary riser VIV study, installation analysis, and engineering practice guideline generation. 2010 by The International Society of Offshore and Polar Engineers (ISOPE).