Mitigation of Current-Driven, Vortex-Induced Vibrations of a Spar Platform via “SMART” Thrusters

The possibility of using active-control technology to mitigate long-period (100–300 s) motions of spar platforms was investigated. In particular, the technical feasibility and cost-effectiveness of using a thruster-based active-control system was examined. Only vortex-induced vibrations (VIV) of a moored spar were considered for this study, although it is believed that similar active-control systems can contend equally well with low-frequency wave-drift and wind-induced motions. VIV has been selected for this demonstration due to the considerable attention it has received for spars, because of the high cost for passive VIV-mitigation means. The investigation employed numerical simulations of the hydrodynamics around the spar in two and three dimensions. In particular, for this study, Navier-Stokes methods were used to compute the strongly nonlinear interactions between the current-flow field and the spar hull. The fluid forces (hull-integrated pressures) were then used to compute the unsteady motions of the moored spar. The effect of the motion-controlling thrusters was included as an additional external force—that also varies with time. Of the several different control strategies that were examined, it was found that the most effective one is “$D$-Control” wherein thruster forces are proportional to spar velocities. Using such control, spar motions can be kept below 35% of the spar diameter, whereas uncontrolled responses were computed to be as high as 80% of the spar diameter. Similar uncontrolled motions have actually been observed in current-tank model tests of spars. As part of this study, discussions were held with engineers from three different thruster companies to ascertain the number and type of thrusters that would be required to achieve desirable VIV mitigation—given the computed thruster-forces, and to provide prudent reliability. Associated costs were also very much of interest. For the spar system examined here, which is representative of those introduced into or being considered for the Gulf of Mexico, it was found that the cost of the thrusters and their electric motors is around $8 million—a cost that is significantly less than that estimated for passive VIV-control devices such as helical strakes, or discontinuous hull geometries. Furthermore, the proposed thruster system could also be employed to contend with (to mitigate) other undesirable, large-amplitude, near-resonant spar motions, e.g., low-frequency drift, for which there are no alternative remedies at present.