VIV of a Composite Riser at Moderate Reynolds Number Using CFD

Abstract

The vibratory response of a long slender riser, made of composite materials and subject to an ocean current, is studied for a range of conditions. The influence of the number of modes of vibration is studied as is the influence of the mass ratio and the value of the damping coefficient. The flow past the riser is represented by a shear flow, ranging from Re = 8000 at the lower end of the riser to Re = 10,000 at the upper end of the riser. The riser vibration is treated as a coupled fluid-flow/vibration problem. The fluid-flow equations are represented by a Large Eddy Simulation model for the wake turbulence present in the flow. Strip theory is used to represent different forcing locations along the length of the riser. Since the composite riser has a variable damping coefficient, which decreases with increasing frequency, its response is different from, say, a steel riser with a constant damping coefficient. The composite riser, with variable damping, has a larger RMS displacement than a riser with constant damping. The vibration amplitude is found to increase with an increase in number of modes.

2 – Way Fluid – Structure Interaction Study for the DIFIS System’s Riser Tube

“…Theoretical vortex shedding frequency is calculated from the dimensionless proportional constant of Strouhal number, the free stream velocity and the cylinder diameter [4]. = * ≅ 0.12 (6) The theoretical vortex shedding frequency is very close to the frequency that was calculated from the FFT analysis. The vortex shedding frequency has a significant difference form the first natural frequency of the riser ( = 0.09 < = 2.1294 ).…”

“…FSI in slender offshore pipes are part of study from many researchers during the recent years especially for VIV phenomenon [6]. The development of the computer's hardware and software was crucial for the realization of these analyses.…”

Fluid – structure interaction study for the DIFIS System’s composite riser tube

“…The FSI analysis was performed in later stage for the comparison between polyethylene RTD and RTComp dynamic response. Many studies [2,[16][17][18] were carried out for the design and structural analysis of composite risers in offshore industry. However, the Riser Tube of DIFIS System differs from the conventional risers in geometry dimensions and operation.…”

“…A 2 -way FSI analysis is used, in order to study the dynamic response of a structure. Vortex Induced Vibrations (VIV) is a common environmental phenomenon in marine environment which can be investigated through a FSI analysis [2].DIFIS system is an innovative and quick deployed structure, which can be used, in maritime disaster in order to eliminate the oil pollution threat. Specifically, DIFIS System limits the oil leakage from the shipwreck to marine environment [3].…”