An experimental investigation is carried out on the effect of spacing of risers in a cluster on the drag and vortex shedding. The model of a satellite riser consists of six tubes of diameter d arranged circumferentially around a central tube of diameter 2.4 d. Six spacings between the satellite and central tubes are tested from 4.5 d to 1.7 d when all tubes are in contact. Overall drag and lift forces are measured in a range 50k <: Re <: 85k (k = 103) and for the orientations 0 ≤ α ≤ 30 deg. The force measurements in the wind tunnel are complemented by flow visualisation in an open water channel at Re =3.2k. Two vortex shedding regimes are observed and explained. An optimal arrangement of tubes is found with a minimum overall drag force and suppressed vortex shedding behind most tubes.
Multiple marine risers have been developed to meet the ever increasing demand of offshore oil extraction. The design of marine risers followed the operational requirements and those determined the number and size of tubes. Little consideration was given in the design stage to the grouping of risers into a cluster and its effect on hydrodynamic forces. For example, Demirbilek and Halvorsen (1985) reported the force measurements on a series of dissimilar risers which resulted in a wide variation of forces acting on them. The hydrodynamic loading due to currents strongly depends on the tube grouping in the cluster. For example, when two circular cylinders are arranged in tandem and touch each other, the overall drag of both is only 40% of the drag of one cylinder alone (Zdravkovich (1977)). The flow interference causes the drag force exerted on the downstream cylinder to be negative, i.e. a thrust force.