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.
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AbstractThe Holstein truss spar is the largest spar hull ever built with a diameter of 45.5m. Holstein's strakes are unique for this and many other reasons. Vortex Induced Motion (VIM) model tests showed that in this area of high loop currents in the Gulf of Mexico, hull strake coverage and size as a percentage of diameter would need to be increased. This resulted in strakes that were closer to the free surface and therefore subject to much larger wave action loads.Typical spar strakes consist of a main top plate and a supporting lower plate that form an enclosed triangular cross section against the hull. Previous spar strakes have large holes for passage of mooring lines and construction openings into this enclosed space. Holstein holes for mooring chain were designed smaller than other spars and construction holes were closed to increase the efficiency of the strake. Furthermore, external strake stiffeners and anodes were moved or removed to clean up the exterior strake surface.New analysis procedures and techniques were found to be necessary to analyze and design the Holstein strakes. As these processes were being developed and implemented, the hull fabrication was well underway, adding to the overall challenge. This paper addresses how new procedures, through model testing and hydrodynamic analyses, were developed and implemented. The paper describes the process of load development for both strength and fatigue cases, and how the resulting loads were applied in a detailed finite element analysis. It outlines the techniques applied to post-processing the detailed stress results from the finite element software, and how fatigue life estimates were obtained using a filtering process to achieve greater efficiency. Finally it gives sample details from the structure that show design enhancements that were required as a result of the analysis.
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