VIM of Dual Buoyancy Can FSHR System

摘要

Free standing hybrid riser (FSHR) system has been used for deepwaterOil & Gas production for over 10 years. Typically FSHR system istensioned by a single submerged buoyancy can for structural efficiency.However, in a recent FSHR system design exercise, two buoyancy canswere selected to provide the tension setting flexibility and also to lowerthe offshore installation requirements. This dual buoyancy can designintroduced complexity into the system since the relative movementbetween the two buoyancy cans now becomes a design consideration;especially when subject to strong current flow, and vortex inducedmotion (VIM) occurs. This paper presents the investigation results ofthe VIM phenomenon, discloses the characteristics of the relativemotions between these two buoyancy cans, and provides technicalguidelines to the dual buoyancy can FSHR system design.First, the buoyancy can layout and VIM methodology for a singlebuoyancy can FSHR were described. Based on that, the methodologyfor dual buoyancy can FSHR was derived. The riser system globalresonance modes and buoyancy can system local resonance modeswere also calculated, and the relationship between these resonancemodes was studied. Then a series of dynamic simulations wereperformed for the dual buoyancy can VIM based on different dominantmodes of the excitation force, i.e., one of the buoyancy cans dominatesthe excitation or both buoyancy cans share the excitation. Simulationswere also carried out on different tension settings of the buoyancy cans,which can be achieved by flooding or dewatering the buoyancy cancompartments. After that the relative motions between the buoyancycans was investigated, and the possibility of higher order VIM wasassessed.Finally, the conclusions were drawn. The results confirmed that theformula in DNV RP F105 is conservative for the prediction of FSHRVIM. It was also found that the buoyancy can separation distance andtension settings have limited influence on the VIM, which confirmedthat the dual buoyancy can system functions as a single buoyancy canin most conditions. Higher mode VIM occurs only under uniformcurrent and the amplitude is smaller than that of the first mode VIM.