During the installation phase of a gravity based structure (GBS), just before it lands on the seafloor, the GBS is free floating and moored onsite. This makes it prone to vortex induced motion (VIM), similar to spar floaters, increasing the installation risk and difficulty. Understanding the hydrodynamic response due to VIM and being able to asses it in an early stage is important. This paper reports on the prediction of VIM and hydrodynamic loads of a GBS using computational fluid dynamics (CFD) with detached eddy simulation (DES) turbulence modeling. The response of the GBS to different current speeds is presented and compared to the response of a cylinder, showing that the cylinder is more prone to VIM. Visualizations of the flow around the hull are used to give an insight to the complex hydrodynamics and to gain understanding of the low amplitude response of the GBS.
The phenomenon of vortex-induced vibration (VIV) is associated with body vibration due to unsteady hydrodynamic loading, caused by incident ocean currents. If the oscillating hydrodynamic force has a period close to the natural period of vibration for the body, a state of resonance occurs. This is called lock-in since the hydrodynamic frequency "locks-in" to the natural frequency of the floating body. The result can be significant periodic motion. An extensive description of the phenomenon can be found in references [2] and [5]. VIV is self limiting, meaning the oscillation amplitude does not exceed the value of 1–1.2 diameters for circular cylinders.
VIV is normally associated with tall cylindrical bodies such as risers, tendons, flowlines and other tubular used by the offshore industry. Typical vibration amplitudes for risers reach up to 1 diameter for uniform flow. Among production floaters, Spars are more prone to VIV, referred as vortex-induced motion (VIM), due to their cylindrical shape. Even though strakes are employed to reduce the response, a considerable amount of motion of the order of 0.5 diameters has been observed [1]. Cylindrical members with a high mass ratio, (m*, refer to nomenclature) such as bridge piers during installation, have been known to experience VIM.
The GBS configuration investigated here consists of two low aspect ratio cylinders. It is expected that the response may be low due to the fact that the vortex sheets shed by the two cylinders will be at different frequencies and so will de-correlate the wake. Nevertheless it is important that a proper assessment of the response be performed because it is not obvious that the short and more complex shape of the GBS will preclude VIM. Even bodies of small aspect ratio, such as a tethered sphere, can experience VIV [2,3]. The maximum displacement of a light sphere of m*=0.8 can reach up to 1 diameter while a heavier sphere of m*=28 can exhibit maximum displacements of 0.5 diameters. The vortex shedding pattern in these cases is analogous to that of circular cylinders consisting of a system of streamwise vortex loops.
