Model Test Experience on Vortex Induced Vibrations of Truss Spars

Abstract

Abstract 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. Introduction Since 1996 Spars have been used as production platforms in the Gulf of Mexico. Vortex Induced Vibrations (VIV) of the Spar are an important consideration in mooring system design. The Vortex Induced Vibrations of Spars are typically reduced by adding helical strakes to the Spar hull. The effectiveness of the strakes must be verified in the design stage of the Spar. At present numerical tools are not capable of accurately predicting VIV-behavior of Spars. Model tests are therefore currently the most practical method to verify and optimize the strake design. A new development in Spar design is the so-called truss-Spar (Refs 1 & 2). In order to evaluate the VIV-behavior of this type of Spars dedicated model tests have been conducted on several truss Spars. Vortex Induced Vibrations A blunt structure placed in a flow (either air or water) will experience an oscillating force due to the shedding of vortices. This phenomenon is studied and discussed extensively (e.g. Ref. 3). If this structure is able to move in the flow Vortex Induced Vibrations (VIV) can occur. The predominant direction of these motions is transverse to the direction of the flow. Large steady state type oscillations occur when the vortex shedding frequency coincides with a natural frequency of the structure. This is known as 'lock-in'. For offshore structures these vortex induced vibrations could add to the fatigue damage of mooring and risers, shortening the total fatigue life and also increase the overall drag on the structure. Experience has shown that in offshore structures cylindrical objects such as risers, calm buoys and Spars are most susceptible to VIV, but also other shapes can exhibit VIV-behavior.