1999-2002 OTRC Project: Numerical Prediction of the Nonlinear Hydrodynamic Forces and Responses of Flexible Offshore Structures (VIV)
Objective:
Presently, 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 reveals shortcomings of these models, particularly for the design of deepwater applications. Indeed, the oil industry currently lacks substantial empirical data applicable to deepwater designs and is in great need of a predictive tool for obtaining the hydrodynamic forces and responses of these deepwater structures.
Hence, the motivation of the proposed research is 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.
Employment of existing Navier-Stokes numerical methods has been validated for predictions of the highly nonlinear hydrodynamic forces and responses of rigid and flexible cylinders undergoing vortex-induced vibrations. Such simulations will be extremely valuable for the design of the next-generation of risers intended for deep-water installations and represents a novel approach for addressing the complete flow-structure interaction dynamics of offshore structures.
Up to this point, 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 at cases, and
(2) there is a certain degree of overprediction of the drag coefficient for supercritical Reynolds numbers for cases of perfectly smooth cylinders. This is attributed to the specific turbulence models that have been employed so far. The plan is to test other variations of existing Navier-Stokes methods and turbulence model implementations in order to see (1) if we can obtain a better prediction of the drag coefficient, and (2) if the required computer resources can be reduced.
Approach:
We propose a rigorous testing of the hydro-elastic approach for realistic cases involving large depths, realistic Reynolds numbers, and non-uniform three-dimensional currents. Existing experimental data from the OTRC wave tank, as well as other published data involving risers will be utilized to compare with the numerical predictions. In addition, the required computing resources will be assessed as a function of the length of the riser and the associated Reynolds number.
The general methodology behind the proposed research is to solve the incompressible Navier-Stokes equations numerically using techniques from the area of computational fluid dynamics (CFD). These equations represent the fundamental hydrodynamic equations of motion regarding fluid transport and are solved numerically to obtain the resulting pressure and velocity fields about one or more offshore structures. Note that this Navier-Stokes technology provides copious amounts of pressure and velocity data which can be integrated to obtain all components of the hydrodynamic forces acting on each offshore structure: in-line, transverse, and spanwise forces. To implement the hydro-elastic approach, the developed hydrodynamic solution methodology is to be coupled with a structural analysis code that will provide the resulting deformation of each structure due to the surrounding hydrodynamic forces. Note that both the fluid mechanics and structural dynamics solution procedures will be solved explicitly in the time domain while maintaining a time-accurate solution. The coupling of the two domains in this time-accurate manner allows for the overall solution procedure to correctly capture the nonlinear interaction between the hydrodynamic forces and structural responses.
In addition to addressing the large VIV amplitudes associated with bare riser applications, the proposed research also intends to address the effectiveness of two classes of geometric suppression devices: strakes and fairings. The emphasis will be on studying how different designs affect the overall performance characteristics over a range of current velocities. Note that the main parameters of these designs are: (i) the height and pitch of the helical strakes, and (ii) the thickness-to-length ratio of the fairings.
Anticipated Project Duration: 2 years
Project Plan for Year 1
Scope of Work: The first year of the project will be consumed with additional validation of the hydro-elastic method by comparing the results of numerical simulations with existing experimental data. In addition, it is anticipated that we will determine the required simulation time and corresponding computer resources that will be needed for realistic riser design and analysis. Specific areas of research targeted for the first year are outlined as follows.
Spar platforms undergoing VIV: examine the characteristics of a rigid platform undergoing VIV and examine the use of helical strakes to suppress large VIV motions. The pitch and height of the helical strakes will be varied to isolate particularly effective designs.
Riser Interference: simulate bundles of risers undergoing VIV and again assess the effectiveness of suppression device geometries (fairings in 2D). In particular, it is planned to test multiple fairing geometries over a range of Reynolds number regimes (both subcritical and supercritical).
For both of the above items, a number of different GOM sea conditions are applicable. Hence, proposed sea conditions include:
Loop Currents
Shear Profiles
Reversing Flow (Waves)
Note that the reversing flow sea condition can include a number of variations: wave only, current only, or current plus wave.Project Plan for Year 2
Scope of Work: We propose application of different turbulence and law-of-the-wall formulations for prediction of the drag for supercritical Reynolds numbers. Existing experimental data from the OTRC wave tank, as well as other published data involving risers will be utilized to compare with the numerical predictions. Existing Navier-Stokes methods will be tested regarding their efficiency. The required computing resources will be assessed as a function of the length of the riser and the associated Reynolds number.
Anticipated Results: It is expected to produce data regarding the behavior of risers in deepwater. Time series of the forces and corresponding motions of the risers will be available. It is anticipated to propose modifications to the existing turbulence models that will make them more accurate for VIV problems. Further, conclusions will be drawn if the required computer resources for realistic problems can be reduced and by how much.
Sponsorship: OTRC Industry Sponsors
Principal Investigator and Others Involved: Dr. John Kallinderis, H. Ahn, and Dr. P. Menounou
Date: December 2002
Project Name: Numerical Prediction of the Nonlinear Hydrodynamic Forces and
Responses of Flexible Offshore Structures (VIV)Project Number: 32558-58875 Task Order: 16168
Principal Investigators: Dr. John Kallinderis
Estimated Completion Date: October 2002
Project Description:
Presently, 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 reveals shortcomings of these models, particularly for the design of deepwater applications. Indeed, the oil industry currently lacks substantial empirical data applicable to deepwater designs and is in great need of a predictive tool for obtaining the hydrodynamic forces and responses of these deepwater structures. Hence, the motivation of the proposed research is 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.
Employment of existing Navier-Stokes numerical methods has been validated for predictions of the highly nonlinear hydrodynamic forces and responses of rigid and flexible cylinders undergoing vortex-induced vibrations. Such simulations will be extremely valuable for the design of the next-generation of risers intended for deep-water installations and represents a novel approach for addressing the complete flow-structure interaction dynamics of offshore structures.Up to this point, 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 at cases, and (2) there is a certain degree of over prediction of the drag coefficient for supercritical Reynolds numbers for cases of perfectly smooth cylinders. This is attributed to the specific turbulence models that have been employed so far.
The plan is to test other variations of existing Navier-Stokes methods and turbulence model implementations in order to see (1) if we can obtain a better prediction of the drag coefficient, and (2) if the required computer resources can be reduced.
Progress:
A 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. It is compared to the more established approach of pressure correction. It was found to be about 20% faster than the pressure correction method. However, this speedup depends on the type of mesh that is used in terms of its resolution and its type (structured or unstructured).
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 vibration (VIV) simulations prohibitively expensive in many cases. The issue becomes more serious as the Reynolds number increases.
Implicit timestepping within the framework of the artificial compressibility method was investigated to alleviate the issue of the small timestep. It was found that presence of very small elements does not affect the size of the timestep. This is significant as it results in appreciable savings in computation time. The penalty to be paid though is the increased computation time per timestep. Overall, the implicit method appears to be about half an order of magnitude faster than the explicit ones. However, for cases that require resolution of high frequencies (i.e. small physical timesteps) the speedup offered by the implicit method is diminished, since the size of the timestep is dictated by the flow physics rather than numerical stability considerations.
Reports & Publications:
W. Jester and Y. Kallinderis,
“Numerical Study of Incompressible Flow about fixed Cylinder Pairs”,
J. of Fluids and Structures, Vol. 17(4), 2003W. Jester and Y. Kallinderis,
“Numerical Study of Incompressible Flow about Transversely Oscillating Cylinder Pairs”,
ASME Journal of Offshore Mechanics and Arctic Engineering, in reviewY. Kallinderis,
“The artificial compressibility method for flow structure interaction problems”, in preparation.
OTRC PROJECT PROGRESS REPORT
Project Name: Numerical Prediction of the Nonlinear Hydrodynamic Forces and Responses of Flexible Offshore Structures (VIV)
Task Order: 16168 Project Number: 58875
Principal Investigators: Dr. John Kallinderis
Estimated Completion Date: September 2002
Project Description:
Presently, 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 shortcomings of these models, particularly for the design of deepwater applications. Indeed, the oil industry currently lacks substantial empirical data applicable to deepwater designs and is in great need of a predictive tool for obtaining the hydrodynamic forces and responses of these deepwater structures. Hence, the motivation of the proposed research is 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.
Employment of existing Navier-Stokes numerical methods has been validated for predictions of the highly nonlinear hydrodynamic forces and responses of rigid and flexible cylinders undergoing vortex-induced vibrations. Such simulations will be extremely valuable for the design of the next-generation of risers intended for deep-water installations and represents a novel approach for addressing the complete flow-structure interaction dynamics of offshore structures.
Up to this point, 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 at cases, and (2) there is a certain degree of overprediction of the drag coefficient for supercritical Reynolds numbers for cases of perfectly smooth cylinders. This is attributed to the specific turbulence models that have been employed so far.
The plan is to test other variations of existing Navier-Stokes methods and turbulence model implementations in order to see (1) if we can obtain a better prediction of the drag coefficient, and (2) if the required computer resources can be reduced.
Progress:
A 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. It is compared to the more established approach of pressure correction. So far, it is about 20% faster than the pressure correction method. We are currently working on improving its speed further.
The effect of initial solution on the final forces calculation on the cylinder was studied. Different initial solutions were employed and yielded the same basically periodic solution after a few periods (D/U).
A variety of ODE solvers were tested for the rigid body response calculations that are needed for the coupled flow-structure interaction problems. It was found that the Runge-Kutta method is more accurate compared to linear multi-step methods.
Reports & Publications:
W. Jester and Y. Kallinderis,
“Numerical Study of Incompressible Flow about fixed Cylinder Pairs”,
J. of Fluids and Structures, accepted.Y. Kallinderis,
“The artificial compressibility method for flow structure interaction problems”, in preparation.
OTRC PROJECT STATUS REPORT
Project Name: Numerical Prediction of the Nonlinear Hydrodynamic Forces and Responses of Flexible Offshore Structures (VIV)
Task Order: 16168 Project Number: 58875Principal Investigators: Dr. John Kallinderis
Estimated Completion Date: September 2002
Project Description:
Presently, 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 shortcomings of these models, particularly for the design of deepwater applications. Indeed, the oil industry currently lacks substantial empirical data applicable to deepwater designs and is in great need of a predictive tool for obtaining the hydrodynamic forces and responses of these deepwater structures.
Hence, the motivation of the proposed research is 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.
Employment of existing Navier-Stokes numerical methods has been validated for predictions of the highly nonlinear hydrodynamic forces and responses of rigid and flexible cylinders undergoing vortex-induced vibrations. Such simulations will be extremely valuable for the design of the next-generation of risers intended for deep-water installations and represents a novel approach for addressing the complete flow-structure interaction dynamics of offshore structures.
Up to this point, 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 at cases, and
(2) there is a certain degree of overprediction of the drag coefficient for supercritical Reynolds numbers for cases of perfectly smooth cylinders. This is attributed to the specific turbulence models that have been employed so far.The plan is to test other variations of existing Navier-Stokes methods and turbulence model implementations in order to see (1) if we can obtain a better prediction of the drag coefficient, and (2) if the required computer resources can be reduced.
Progress:
We have tested the Spalart-Allmaras turbulence model for high Reynolds numbers cylinder flows in two dimensions. Further, we have investigated the effect of surface roughness due to the discrete resolution of the circular surface on the drag predictions. The other part of the investigation regards comparison of efficiency between two different classes of Navier-Stokes methods (pressure correction and artificial compressibility).
Reports & Publications:
W. Jester and Y. Kallinderis,
“Numerical Study of Incompressible Flow about fixed Cylinder Pairs”,
J. of Fluids and Structures, accepted.W. Jester and Y. Kallinderis,
“Numerical Study of Incompressible Flow about Transversely Oscillating Cylinder Pairs”,
J. of Fluids and Structures, in review .