The Effect of Appurtenances on the VIM Performance of a TLP for Southeast Asia

Abstract

In Southeast Asia, the characteristic operating environment is strong and persistent currents with small to moderate waves, which encourages vortex induced motion (VIM) in floating platforms. Scaled model tests have been used by the industry to predict VIM in floating platforms. Numerical methods such as Computational Fluid Dynamics (CFD) analysis have the potential to complement model tests for predicting VIM. Hull appurtenances such as tendon porches, SCR porches, caissons, pipes, chains, fairleads and anodes interact with the boundary layer and affect the point of flow separation along the columns which controls the vortex shedding pattern of the hull. In this study, the effect of appurtenances on the VIM performance of a Tension Leg Platform (TLP) for Southeast Asian environment is investigated using a series of model tests and CFD analysis. The findings from this study may be used as a reference to properly account for the effect of VIM. The measured/calculated amplitude of VIM may be used as input for additional load conditions to be considered in the design of the tendon mooring and riser systems. Previous studies by others on the same subject were carried out for spars. Multi-column floating platforms do not have strakes on the columns unlike spars. Thus, significant difference in VIM behaviors between multi-column floating platforms and spars is expected. The wake is expected to remain turbulent in both full scale and model scale. For smooth cylinders, the separation point along the columns is scale-dependent and this may affect the VIM response. If the flow separates from a sharp appurtenance like a pipe, the VIM behaviors should be similar in full scale and in model scale. The results from CFD analysis generally agree with those from model test. The differences in the results from the two methods are discussed and areas for further investigation suggested. Significant VIM can adversely impact the platform mooring and riser systems in terms of both strength and fatigue life requirement and has to be properly accounted for in the platform design. Left unchecked, VIM can lead to loss of production due to unplanned repairs of the risers.