Genesis Spar Risers: Interference Assessment and VIV Model Testing

Abstract

Consideration of vortex-induced vibration (VIV) and riser interference for a deep water riser array presents a complex engineering problem which requires knowledge not only of the behaviour of single risers in open-water current, but also shielding and VIV effects unique to riser arrays. The Genesis SPAR, located in 2,600ft water depth in Green Canyon 205, accommodates a closely spaced array of drilling, production and export risers run from a central moonpool. This paper describes riser VIV model testing and outlines how riser interference was assessed using a combination of wake flow modelling, global interference analysis and VIV prediction, calibrated to model test results and CFD analysis. Tow-tank model testing of riser pairs, performed at MIT, evaluated the influence of the wake field of an upstream riser on both VIV and drag for both bare and straked risers under different conditions of riser separation. A state-of-the-art approach to interference analysis was subsequently adopted which evaluated riser global response to loop current in the presence of both VIV drag-amplification on an upstream riser and wake-induced drag reduction on a downstream riser.Model tests showed that the use of helical strakes as VIV mitigation devices proved effective on single risers exposed to currents involving wide reduced velocity 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 CFD simulations performed by Professor John KallinderisInterference analysis of several loop current conditions illustrated that minimum riser separation was not associated with the highest surface current velocity, but with a lesser surface velocity which attenuated less with depth.Model testing clearly illustrates the impact of riser separation on the effectiveness of strakes.Interference analysis calibrated to such tests provides an innovative approach from which recommendations may be made about the conditions under which adjacent risers should be operated and mitigation measures available to avoid interference.

Flow Induced Motions on Multi Column Floaters

“…Evaluation of vortex induced motion (VIM) in offshore floating structures has been largely been carried out for spars. Classic spar, truss spar and cell spar hulls are all known to exhibit VIV (VIM) [1], [2], [6], [7], [8], [9], [10], [12], [13], [14], [15], [16], [19], [27], [28], [29]. [31], [32],…”

Design Guideline for Riser Collision

“…the work of Huse [4 & 5], Huse & Kleiven [6] or Kavanagh et al [7], while less information is available regarding collisions due to wave loading; see e.g. Significant effort has been applied to investigate riser collision behaviour in steady current; see e.g.…”

Reliability assessment of the collision between marine risers

“…This set of natural frequency should have an ample margin from that of environmental loadings (Huse, 1993;1994 is to install VIV suppression devices on risers. Kavanagh et al (2000) examined the performance of strakes (a type of suppression devices) in a riser.…”

“…or experimentally. By placing another cylinder upstream, these coefficients are Kavanagh et al (2000) A striking feature of Blevins' model is that force coefficients are given as functions of mentioning. The first one is an undisturbed fluid flow model which neglects the wake…”

“…Basically, the static analysis is treated as a preliminary study on riser interference. If the closest distance between two risers is less than 2D, a further dynamic analysis is highly recommended (Kavanagh et al, 2000). The outcomes of static analysis are mainly the initial positions and stresses which are respectively the mean positions and mean stresses for the subsequent dynamic analysis.…”