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Title: Hydrodynamic damping and "added mass" for flexible offshore platforms
Authors: Petrauskas, Charles
Keywords: Structural dynamics
Water waves--Hydrodynamics
Ocean waves
Publisher: Coastal Engineering Research Center (U.S.)
Engineer Research and Development Center (U.S.)
Series/Report no.: Technical Paper;no. 76-18
Abstract: Abstract: The dynamic response of deepwater flexible platforms due to wind-generated ocean waves appears to be an important design consideration; therefore, a theoretical and experimental study was made of hydrodynamic damping and"added mass." Classical potential theory with linearized boundary conditions was used to study the hydrodynamic damping due to wavemaking and the coefficient of oscillation frequency, a cylinder diameter, D, water depth, h, and mode shape. The parameters and the mode shape completely defined the nondimensionalized wavemaking and added-mass forces. It was found that for practical purposes the wavemaking force may be assumed to be localized in the near-surface zone and therefore dependent on only the parameter. The coefficient of added mass, in general, depends on both parameters and the mode shape. It also varies with elevation, but for practical purposes a uniform value of one can be used if D/h < 0.01. Damping for a number of proposed platforms was found negligible because the diameters in the nearsurface zone were too small. To attain damping of 2 to 4 percent of critical would require, e.g., diameters greater than 30 feet in a 600-foot water depth for a platform with a natural period of 4 seconds. Experiments were conducted to verify the results of potential theory. Rigid vertical cylinders were oscillated with simple-harmonic motion in calm water. Total forces and radiated waves were measured. They compared very well with theoretical values. Other investigators 1 data also verified the theory. A small experimental study was made in an attempt to verify the hydrodynamic damping implied by the quasi-steady drag-force interaction term of the presently used modified Morison equation to represent the drag force on an oscillating cylinder in waves. Damping was measured for an elastically supported circular cylinder in a steady current. TI1e measured values were up to 4 times lower than the theoretical values. The disagreement appears to be that the experiments were outside the range for which the quasi-steady assumption is valid. Coefficients of added mass were also measured and were found equal to the potential theory value irrespective of the velocity of the current.
Description: Technical Paper
Rights: Approved for public release; distribution is unlimited.
Appears in Collections:Technical Paper

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