The Effect of Shifting Natural Frequency on the Reduction of VortexInduced

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 The Effect of Shifting Natural Frequency on the Reduction of Vortex Induced Vibrations of Marine Risers

Younes Komachi1, Said Mazaheri2*, Mohammad Reza Tabeshpour3

1 Ph.D. Student, National Institute for Oceanography; y_komachi@yahoo.com

2* Corresponding author: Assistant Professor, National Institute for Oceanography; said.mazaheri@inio.ac.ir

3Assistant Professor, Mechanical Engineering Department, Center of Excellence in Hydrodynamics and Dynamics

of Marine Vehicles, Sharif University of Technology, Tehran, Iran; tabeshpour@sharif.edu

 

ABSTRACT

Many procedures suggest for reduction of responses of riser to Vortex Induced

Vibrations (VIV). Natural frequencies of marine risers is an important

parameter that can affect the responses of riser to VIV. Change of riser

properties such as top tension and bending stiffness can alter natural

frequencies. In this study effects of riser specifications on the responses and

fatigue damage of marine risers were investigated analytically and

numerically. For numerically analysis 2D wake-structure coupled model is

used for modeling of VIV of riser in two directions of Cross Flow (CF) and In

Line (IL). The wake dynamics, including IL and CF vibrations, is represented

using a pair of non-linear Van der Pol equations that solved using modified

Euler method. The Palmgren–Miner Rule is used for evaluation of fatigue

damage. Riser of Amir-Kabir semisubmersible placed in Caspian sea is used

for case study. Because VIV is self-limiting, it is showed that lower modes

have lower curvature, that in some cases this is lead to lesser stress and also

fatigue damage. The results show that for tension dominant modes of

vibration, natural frequencies was increased with top tension and for a certain

Strouhal frequency, dominant modes of vibration was reduced which leads to

reduction of stress and fatigue damage. The results show that stress and

fatigue damage increased with module of elasticity of riser and reduction of

this leads to reducing of stress and fatigue damage. Therefore suitable

procedure for reduction of VIV responses of riser should be selected based on

the current velocity.

 

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