Riser VIV Research Articles

Numerical study of the vortex-induced vibration of a riser taking into account the variation of the tension

ABSTRACT Previous studies of the marine riser VIV have primarily considered the constant values of the tension, but limited work has been done to consider the variations of the tension with riser vibrations. In addition, previous studies have mainly focused on the effect of the inflow velocity profile on a riser VIV, while a study of a riser VIV at a wide range of velocities is still lacking. This study presents a model that can take into account the variation of the tension with riser vibrations. The VIV characteristics of the riser were investigated based on the fluid-structure coupling methods. It is shown that the model that takes into account the variation of the tension is able to accurately capture the bimodal character of the amplitude-ratio curve and the vortex shedding pattern, and to calculate the vibration frequencies accurately.

Experimental Investigation on Vortex Induced Vibration of Marine Risers with Different Wall Thicknesses

In this paper, the vortex-induced vibration experiment of the risers with different wall thicknesses in wave-flow combined water flume was carried out. The dynamic strain sensing technology was adopted. The sensitivity and micro evolution pattern of riser VIV response relative to wall thickness were examined by varying the wall thickness of various riser models, comparing the dimensionless root mean square of strain, vibration frequencies and trajectories by varying different wall thicknesses of the risers. The results indicate that the wall thickness does not have a large influence on the Strouhal numbers of the vibration of the risers, which are always around 0.18; the larger the wall thickness is, the earlier the lock-in region is jumped out, as wall thickness increases, the outflow velocity corresponding to the occurrence of an “8” or oblique “8” shape vibration trajectory continues to delay, and the velocity range for such trajectories gradually narrows.

Understanding vortex-induced vibration characteristics of a long flexible marine riser by a bidirectional fluid–structure coupling method

A bidirectional fluid–structure coupling method was developed to comprehensively understand the VIV characteristics of a 3D flexible marine riser. Through study VIV characteristics of the 3D riser in different flow conditions and by analysis of the structural characteristics of the riser’s responses, this study successfully captured the phenomena of switching of adjacent-order vibration modes and the characteristics of a “traveling wave” and a “standing wave” at both ends of the riser that cannot be obtained from 2D VIV studies. Analysis of the frequency of the structural vibration response characteristics found that the 3D flexible riser VIV showed multi-frequency vibration phenomena and the vibration response characteristics of a broadband spectrum in a high-speed flow condition. This research distinguished vortex forms at different locations of the riser, i.e., in the middle of the riser, mainly the “2P” or “P + S” forms occurred and at both ends of the riser, mainly the “2S” form appeared. After comparative analysis of the vortex forms of the riser wake in different speed flow conditions, this research identified that under a low flow velocity, the riser showed a 3D effect marginally, whereas under a high flow velocity, the riser showed a strong 3D effect.

Experimental investigation on the suppression of vortex-induced vibration of two interfering risers by control rods

ABSTRACTVortex-induced vibration (VIV) is a common flow–structure interaction phenomenon which occurs when a cylindrical riser in a fluid flow causes an unsteady flow pattern due to vortex shedding. The vibration mechanism is further complicated by wake interference. An experimental investigation was conducted to study the effectiveness of multiple control rods, in different configurations around two risers in suppressing wake interference and riser VIV. Strain responses were measured in the cross-flow and in-line directions. The test results showed a significant reduction in VIV, indicating that the control rods effectively suppressed transverse vibration of the riser. It was found that four control rods suppressed transverse VIV up to 90% more effectively than three control rods. The control rods also mitigated the in-line response.

Cross-flow VIV-induced fatigue damage of deepwater steel catenary riser at touch-down point

A prediction model of the deepwater steel catenary riser VIV is proposed based on the forced oscillation test data, taking into account the riser-seafloor interaction for the cross-flow VIV-induced fatigue damage at touch-down point (TDP). The model will give more reasonable simulation of SCR response near TDP than the previous pinned truncation model. In the present model, the hysteretic riser-soil interaction model is simplified as the linear spring and damper to simulate the seafloor, and the damping is obtained according to the dissipative power during one periodic riser-soil interaction. In order to validate the model, the comparison with the field measurement and the results predicted by Shear 7 program of a full-scale steel catenary riser is carried out. The main induced modes, mode frequencies and response amplitude are in a good agreement. Furthermore, the parametric studies are carried out to broaden the understanding of the fatigue damage sensitivity to the upper end in-plane offset and seabed characteristics. In addition, the fatigue stress comparison at TDP between the truncation riser model and the present full riser model shows that the existence of touch-down zones is very important for the fatigue damage assessment of steel catenary riser at TDP.

Numerical scheme for riser motion calculation during 3-D VIV simulation

This paper presents a numerical scheme for riser motion calculation and its application to riser VIV simulations. The discretisation of the governing differential equation is studied first. The top tensioned risers are simplified as tensioned beams. A centered space and forward time finite difference scheme is derived from the governing equations of motion. Then an implicit method is adopted for better numerical stability. The method meets von Neumann criteria and is shown to be unconditionally stable. The discretized linear algebraic equations are solved using a LU decomposition method. This approach is then applied to a series of benchmark cases with known solutions. The comparisons show good agreement. Finally the method is applied to practical riser VIV simulations. The studied cases cover a wide range of riser VIV problems, i.e. different riser outer diameter, length, tensioning conditions, and current profiles. Reasonable agreement is obtained between the numerical simulations and experimental data on riser motions and cross-flow VIV a/D. These validations and comparisons confirm that the present numerical scheme for riser motion calculation is valid and effective for long riser VIV simulation.