Genesis Spar Risers: Interference Assessment and VIV Model Testing

Abstract Consideration of vortex-induced vibration (VIV) and riser interference for adeep water riser array presents a complex engineering problem which requiresknowledge not only of the behaviour of single risers in open-water current, butalso shielding and VIV effects unique to riser arrays. The Genesis SPAR, located in 2,600ft water depth in Green Canyon 205, accommodates a closelyspaced array of drilling, production and export risers run from a centralmoonpool. This paper describes riser VIV model testing and outlines how riserinterference was assessed using a combination of wake flow modelling, globalinterference analysis and VIV prediction, calibrated to model test results and CFD analysis. Tow-tank model testing of riser pairs, performed at MIT, evaluated theinfluence of the wake field of an upstream riser on both VIV and drag for bothbare and straked risers under different conditions of riser separation. Astate-of-the-art approach to interference analysis was subsequently adoptedwhich evaluated riser global response to loop current in the presence of both VIV drag-amplification on an upstream riser and wake-induced drag reduction ona downstream riser. Model tests showed that the use of helical strakes as VIV mitigation devicesproved effective on single risers exposed to currents involving wide reducedvelocity and Reynolds number regimes. Interference & blockage however, degrade the strakes' VIV suppression performance, as illustrated by results of RANS CFD simulations and tow-tank experiments of riser pairs.1 Interference analysis of several loop current conditions illustrated thatminimum riser separation was not associated with the highest surface currentvelocity, but with a lesser surface velocity which attenuated less withdepth. Model testing clearly illustrates the impact of riser separation on theeffectiveness of strakes. Interference analysis calibrated to such testsprovides an innovative approach from which recommendations may be made aboutthe conditions under which adjacent risers should be operated and mitigationmeasures available to avoid interference.