| In the decades to come, oil and gas will still dominate the globalenergy supplies. With the soaring China’s economy boosting the energydemand, exploiting oil and gas in the deep water region of the SouthChina Sea (SCS) becomes an option to relieve the energy strain. As atypical floating structure for deep water, Spar platform can be an idealtype for the oil and gas exploitation in the SCS, due to its excellentstability, small heave motion, no limitation for water depth and relativelylow cost. Appeared a new type of Spar concept, multi-column Spar,which is much easier for construction than the ordinary single-columnSpar, for the hard tank of this platform consists of four circular columnsarranged in a square shape surrounding the center well.The Vortex-Induced Motions (VIM) become the critical affection forSpar platform, meanwhile the Spar platform has numbers ofadvancements. The hull of the Spar platform is a typical bluff body, andencountering with certain currents can trigger large transvers amplitude, because of flow separation and vortex shedding from the hull. VIM cancause large tension in the mooring lines and risers, add to the fatiguedamage of mooring system and riser system and also shorten the totalfatigue life. Complex flow interference can happen near the columns ofthe multi-column Spar, which is different from any kind of the existingsingle-column Spar platforms or Semi-submersible platforms. So it isvery important and meaningful to carry out the related study on the VIMof it.The flow around the simplified multi-column Spar platform isstudied by numerical method. The main results show that the mean liftforce of the Spar is not zero for the platform is asymmetric to the in-linedirection in the incidence angles of15°and30°. Except for lift and drag,there are some moment acting on the platform. The vortex structures ofeach column converge in the wake and shed form each side in singlevortex street mode.During the numerical study of flow around the Spar, strong flowinterference happened between each column. The vortex structure of thesquare part of the Spar is suppressed by the columns in the downstreamside, and the flow separation pattern of one side of the square is differentfrom flow around single square condition in the incidence angle of30°.The results of four cylinders part of the Spar show that the downstream cylinders are subjected to higher fluctuating forces, and theupstream cylinders experience larger mean drag force than the flowaround single cylinder condition when the incidence angle is small, andexperience smaller mean drag force when the incidence angle is large.A towing tank experiment was carried out to study theVortex-Induced Motions of the multi-column Spar platform for differentheadings and velocities. By analyzing the sway, surge, trajectory of themotion in horizontal plane, yaw, vortex-shedding frequency andcoefficients of hydrodynamic forces, some critical characters of the VIMwere found. The lock-in phenomenon is observed with reduced velocitybetween6up to8(for0°incidence without helical strakes, the lock-inrange is Ur=7~11).The helical strakes do suppress the vortex-inducedmotions, and have the highest efficiency for0°incidence in a relativelyhigher range of current velocity. The trajectories of VIM motions in theXY plane occur primarily along the cross-section diagonals (except in the0°incidence), which is quite different from the existing single-columnSpar platforms. Relatively large yaw motion is also observed in the modeltest. The vortex-shedding frequency of the platform remains the sameinstead of linearly increasing while the velocity rising during the“lock-in” region.The VIM is studied from the aspect of the flow field as well using numerical method. The comparisons of the flow field of fixed Spar andmoored Spar are discussed. The flow structures of the vortex deflect tothe opposite of Spar’s moving direction. The flow separation points andvortex structures of each column are significantly different from fixedconditions, while the character of the flow pattern varied in higherincidence angles. |
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