The Branched Steel Catenary Riser Interference Study

A novel riser concept is being investigated. This is the branched riser system (BRS) with different configuration types including the branched steel catenary riser (BSCR), the branched steel lazy wave riser (BSLWR) and the branched lazy wave hybrid riser (BLWHR). As part of the design and operational requirement for riser systems, riser interference or clash checks is important especially in cases when risers are positioned closely from each other and the external excitations including hydrodynamic interactions are significant. This paper presents findings from the investigation of the interference response of the branches of the BSCR. The BSCR, with different half branch angles(β = 0.1deg, 0.5deg and 1deg), are hosted by a floating production system in a water depth of 1500m and subjected to different loading conditions including current loads, vessel offsets, variation in branches’ weight, dynamic loads from ocean wave excitations and drag amplification due to VIV. Under these conditions, the minimum clearance between the BSCR branches were calculated and checked against the minimum clearance requirements. Results from this study showed that the global displacement responses of the BSCR branches are in tandem with each other for non-VIV scenarios and that the minimum clearances between them satisfy the no-clash criteria. This is due to the close drag-to-apparent-weight (DAW) ratio of the branches. Wave load excitations on the BSCR were observed to cause little changes on the branches’ clearance, even when imposed on a combination of worst static conditions. For ‘lockin’ conditions during VIV, it is found that the transverse vibrational amplitude-to-diameter (A/D) ratio of the branches, which impact amplification on the inline drag force can be different for the two branches. This can result in differential deflection of the branches and higher clash possibilities. Under VIV conditions with the current profile typical of Gulf of Mexico (GoM), the BSCR configuration with half branch angle, β = 0.1deg, was found not to satisfy the interference criteria while the BSCR (β=1deg) survived. This indicates that high half branch angle will be required for the BSCR design under high intensity and shearing current profiles acting on the branches. However, all BSCR configurations with half branch angles (β =0.1deg, 0.5deg and 1deg) were found to satisfy the interference requirement when subjected to constant current profiles (slab currents) and current profile typical of West of Africa (WoA), though slab current profile resulted in very high A/D ratio. These results provide positive indications for continuing investigation of other aspects of the BSCR.