Vortex–Induced Vibration (VIV) is an important problem that can be seen in offshore structures such as drilling riser. VIV occurred due to synchronization (lock–in) of the vortex shedding and natural frequency of the structure. The lock–in phenomenon is normally observed on the drilling riser at different current directions. This problem can cause failure and the collapse of drilling riser during operation. Therefore, this study aims to investigate the effects of the current incidence angle on VIV by using Computational Fluid Dynamics (CFD). The significant of this incidence angle study is for us to observe the formation of the wake and interferences of the flow over the cylinders. In this paper, the flow that passes a fixed and freely vibrating cylinder are investigated. The focus of the study is more on the characteristics of a riser system with auxiliaries. The riser system was modelled with a main cylinder and six auxiliary lines with low gap ratio (G/D) of 0 to 0.08, incidence angles (θ) of 0° to 50° and constant diameter ratio (d/D) of 0.45. The simulations were conducted in the laminar flow regime at Reynold Number, Re=200. The simulation focused on a single degree of freedom (SDOF) of the fixed and freely vibrating riser. It was found that the freely vibrating riser experienced higher lift and drag forces compared to the fixed riser. This is probably due to higher frequencies of fluid forces fluctuations on the main drilling riser at the incidence angles studied.
Nowadays, the demand for oil and gas industry is increasing rapidly in the world economy. Drilling risers are immerged in the deep sea to connect surface facility to a subsea system. Therefore, researchers are investigating and developing new techniques for a better prediction of Vortex–Induced Vibration (VIV) in order to minimize the damages on the structures. VIV has become a major problem arises during drilling riser operation which will lead to fatigue and failure as shown in Fig. 1. Due to lack of knowledge and understanding of the VIV phenomenon on drilling risers, some effort has been done particularly on the prediction of the VIV on offshore structures. Earlier, VIV of an isolated cylinder in cross–flow had been reviewed and investigated by Sarpkaya (2004) and Williamson& Govardhan (2008). Contributions towards understanding VIV also have been carried out recently using experiments, numerical simulation and analytical modelling. Gonçalves et al. (2020), Rahman et al. (2019) and Lopes et al. (2019) are among the contributors, to name a few.
