Floating offshore platforms mostly consist of a single cylinder or multi-cylinder, which is susceptible to vortex-induced motions (VIM) in the ocean current. In this paper, numerical simulations are performed to study the VIM of four rigidly connected cylinder at a large reduced velocity range, which represent the four-column structure of a Tension Leg Platform (TLP). The flow field around the cylinders was calculated by the Fluent software, and the differential motion equation was solved by fourth order Runge-Kutta method which was manually written into the user-defined functions (UDF), then dynamic mesh technique was adopted to update the flow field. The range of Reynolds number covered is 5000<Re<15000, and the mass ratio (m*) are fixed as 0.96 which is the same as the TLP model test..The results showed that the numerical simulation and experimental results of VIM were in good agreement. It can be divided into three typical phases: initial branch, lock-in region and unlocked lower branch. The results also found that the lift coefficient of downstream column are much greater than the upstream column.
Bluff body structures can be used in a broad variety of industrial applications, especially in the field of offshore engineering. Vortex-induced vibration (VIV) is one of the major concerns in design-problems of offshore structures because of the damage it will be caused. Blevins (1979), Jauvtis et al. (2004) and Sun et al (2019) discovered VIV phenomenon can be traced back to the last century, and many investigations. For large offshore structures such as offshore platforms which have long periods and large amplitudes of motion under the action of currents, we refer to the excitation motion generated as a vortex-induced motion (VIM).
When a non-streamlined cylinder located in an incoming flow at a given velocity, as the boundary layer produces unstable separation, the downstream sides of the cylinder bring about periodic and anti-symmetrically aligned vortex shedding. This periodic symmetrical or asymmetrical vortex alternately shedding can excite a drag force at in-line direction (IL) and a lift force in the cross-flow direction (CF). The cylinder vibrates under this periodic excitation force, which in turn to changes the shape of its own wake field.
