Numerical Study of Vortex Induced Motion and Vibrations on Risers and Tension Leg Platform for Malikai

Abstract

The operating environment for the Malikai project is strong and persistent current with speed up to 2 m/s for even a return period of 50 years. This could lead to vortex induced vibration (VIV) of the risers and vortex induced motion (VIM) of the floating systems such as Tension Leg Platform (TLP) or Tender Assisted Drilling (TAD) vessel. The VIV and VIM could affect the riser and mooring system strength and subsequent failure. This could lead to unplanned production deferment and riser maintenance. Therefore, the current study investigated the VIV and VIM phenomenon for Malikai riser and floating system respectively.The Malikai project used U shaped AIMS fairing to suppress VIV of the risers. These fairings have low drag, provide operational advantage and faster installation compared to traditional VIV suppression methods. Computational Fluid Dynamics (CFD) simulations were performed for the fairing and riser system and confirmed low drag coefficient (0.4) in Malikai environment. Experiments verified it further.The second part of the study was focussed on investigating VIM of TLP and TAD. The CFD simulations were first carried out to validate the motion of TLP at the lab-scale and established a methodology. The established methods were then used to predict VIM for Malikai TLP and combined motion of TLP-TAD. The drag and lift coefficient for the lab scale TLP at reduced velocity of 10 and heading of 22.5° was 1.33 and 0.13 respectively. The nominal surge, sway and yaw amplitude at this condition was 0.05, 0.12, and 1.8 respectively. These predictions were in well agreement with the experimental data obtained from the University Technology Malaysia (UTM). The sway and yaw frequencies were twice of surge indicating VIM. The motion amplitudes for the Malikai TLP were smaller than the lab scale experimental data at reduced velocity of 5. This could be attributed to difference in Reynolds number and tendon arrangement. There was no evident dominating frequency of motion. It was more of a flow induced motion. The drag and lift coefficients for TLP/TAD at heading of 0° was 1.1/0.28 and 0.014/.001 respectively. The drag and lift coefficients are lower for TAD as it is in the wake of TLP (lower currents around TAD). The motion amplitude for TAD were higher than TLP due to more fluctuations in the drag and lift forces. This could be attributed to TAD having only two pontoons.The motion amplitudes were overall small for all the cases and could be due smaller submersed height of columns above the pontoons. The motion and drag predictions can be directly used in the design of hull, riser and mooring line systems and can be further integrated with structural analysis to obtain the fatigue life. This was an attempt to capture the VIM and has indicated promising results. An exhaustive validation can aid in standardization of CFD methods and make it a potential tool to predict VIM. Accurate predictions can help in lowering and building confidence in design parameters.