It is expected that future deep sea risers will in many cases have to be fitted with VIV suppression devices, for instance strakes. Typically, there will be strakes on the upper parts of the risers, where a high surface current might otherwise induce unacceptable vibrations and fatigue stresses. Further down, strakes may be unnecessary due to lower current. As explained in the introductory part of the paper, the optimum design of strakes for such risers requires detailed input data on the two-dimensional lift coefficient as a function of amplitude and reduced velocity. Such systematic lift coefficient data are available in the literature for smooth cylinders, but not for cylinders fitted with strakes of different geometries. The main purpose of the paper is to present experimental data on the lift coefficient of risers fitted with strakes of different heights. The data were produced in a test program recently conducted at MARINTEK/NTNU. The tests were done in a pendulum-type test set-up in a towing tank. The pendulum was kept constrained until the towing carriage had reached its stationary speed. Then the pendulum was released. From the rate of amplitude increase during the transient the lift coefficient was determined for the full range of amplitudes up to the eqilibrium state of oscillation. For amplitudes above the equilibrium the negative lift coefficient (damping) was measured in the same manner by starting the pendulum from an amplitude far above the equilibrium. Models with strake heights of 0, 2.5, 5, 10, and 15 percent of the diameter were tested. Each of them was tested at many different speeds. The results are presented in the paper in the form of lift coefficients as functions of amplitude, reduced velocity, and height of the strakes. Amplitudes up to a level where the lift coefficient is well into its negative (damping) range are included. The paper is based on a siv.ing thesis at the Norwegian University of Science and technology (NTNU), Ref. 4.
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