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Project Number 361
Date of Summary March 16, 2004
Subject VIV CFD Loads and Responses of Tubular Members – Numerical Prediction of the Nonlinear Hydrodynamic Forces and Responses of Flexible Offshore Structures
Performing Activity Texas A&M University, Offshore Technology Research Center (OTRC)
Principal Investigator Dr. John Kallinderis
Contracting Agency Minerals Management Service
Completion October 31, 2002
Description The offshore industry estimates VIV hydrodynamic forces based on empirical measurements or low-order fluid dynamic models calibrated for shallow water applications. Comparisons of the predicted forces and responses of flexible structures with experimental data reveal the shortcomings of these models, especially in the design of deepwater systems. Hence, the motivation of the proposed research to employ flow-structure interaction methods based on solving the Navier-Stokes and structural dynamics equations of motion to provide predictions of the forces and responses of risers and spars. Two issues of the numerical methods have been identified; 1) the computational expense required for full three-dimensional simulations of straked risers and spars can be prohibitive in some cases, and 2) certain degree of over prediction of the drag coefficient for super critical Reynolds numbers for cases of perfectly smooth cylinders.
Progress Method for solving the Navier-Stokes equations, which is based on the artificial compressibility approach, has been tested in terms of accuracy and computing resources requirement. The issue of the allowable timestep size was investigated next. The timestep size for time-accurate simulations is dictated by the size of the smallest element for several of the popular CFD methods. It is quite typical that very small elements exist in most of the meshes employed. As a consequence, the timesteps used are very small which renders vortex-induced vibrations (VIV) simulations prohibitively expensive in many cases studied. This issue becomes more serious as the Reynolds number increases.

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