In order to evaluate the Vortex Induced Vibration (VIV) response of truss Spars and to optimize their strake configuration several model test programs have been carried out at MARIN. The results show that it is possible to optimize the strake design of Spars to obtain minimum VIV-response. The results of the model tests also suggest that modeling details, such as appendages, can have an influence on the Vortex Induced Vibrations. In order to reliably predict the full-scale VIV-behavior of the prototype Spar these details must therefore be accurately represented on the model. Furthermore, damping of attached structures such as the truss on a truss Spar can significantly contribute to the reduction of VIV. Loads on such structures have been measured in the model tests. An important aspect that needs consideration in VIV model testing is effect of model scale on the Reynolds number. Roughness can be added to the hard tank of the Spar to minimize scale effects. The paper discusses possible scale effects and the effect of hull roughness on model test results. The repeatability of VIV model tests and reliability of these tests in representing the full-scale situation is discussed. The effect of Spar heading with respect to the current direction as well as current speed will be discussed.

Key issues for design of Spar mooring systems in the presence of VIV are discussed. The need to consider interactions between the directional effects of the Spar mooring and VIV response, together with the directional distribution of the loop current environment, is highlighted.For instance, the combination of current speed and varying sway natural periods for environment directions between and in-line with mooring groups give rise to variations in reduced velocity. Furthermore, the Spar response itself could be directional due in part to unsymmetric coverage of the VIV suppression strakes and other appurtenances located on the outside of the Spar. Directional stiffness and drag augmentation due to VIV may also affect maximum offsets and mooring line loads.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThe Holstein truss spar,

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