VIM of Dual Buoyancy Can FSHR System

摘要

Free standing hybrid riser (FSHR) system has been used for deepwater Oil & Gas production for over 10 years. Typically FSHR system is tensioned by a single submerged buoyancy can for structural efficiency. However, in a recent FSHR system design exercise, two buoyancy cans were selected to provide the tension setting flexibility and also to lower the offshore installation requirements. This dual buoyancy can design introduced complexity into the system since the relative movement between the two buoyancy cans now becomes a design consideration; especially when subject to strong current flow, and vortex induced motion (VIM) occurs. This paper presents the investigation results of the VIM phenomenon, discloses the characteristics of the relative motions between these two buoyancy cans, and provides technical guidelines to the dual buoyancy can FSHR system design. First, the buoyancy can layout and VIM methodology for a single buoyancy can FSHR were described. Based on that, the methodology for dual buoyancy can FSHR was derived. The riser system global resonance modes and buoyancy can system local resonance modes were also calculated, and the relationship between these resonance modes was studied. Then a series of dynamic simulations were performed for the dual buoyancy can VIM based on different dominant modes of the excitation force, i.e., one of the buoyancy cans dominates the excitation or both buoyancy cans share the excitation. Simulations were also carried out on different tension settings of the buoyancy cans, which can be achieved by flooding or dewatering the buoyancy can compartments. After that the relative motions between the buoyancy cans was investigated, and the possibility of higher order VIM was assessed. Finally, the conclusions were drawn. The results confirmed that the formula in DNV RP F105 is conservative for the prediction of FSHR VIM. It was also found that the buoyancy can separation distance and tension settings have limited influence on the VIM, which confirmed that the dual buoyancy can system functions as a single buoyancy can in most conditions. Higher mode VIM occurs only under uniform current and the amplitude is smaller than that of the first mode VIM.