Note: This study was part of a broader DeepStar Project “Global Performance of Deepwater Structures – Post Model Test Study and Guideline Development.”
Verifying the predicted design responses of FPS’s in deep and ultra deep water depths presents significant challenges. Due to the depth and scope of mooring and riser systems, a completely scaled model of the FPS and its moorings and risers is simply too large to test in present model basin. A combination of physical model testing and numerical modeling as shown in Figure 1 is recommended to verify the responses of prototype FPS designs.
The general approach is to use a scale model of the FPS hull and deck with equivalent (sometimes referred to as “truncated”) mooring and riser systems for the model tests. A numerical model that describes the dynamic responses of the moored FPS with risers to winds, wave, and currents is used to design the equivalent mooring and riser systems, and then used to interpret and extend the model test results to predict the prototype FPS responses at the full design water depth. The numerical model should be carefully calibrated and validated with data from the model tests. The validated numerical model is then adjusted to incorporate the prototype geometry, prototype scale properties and coefficients for drag and damping, and actual (non-truncated and non-simplified) moorings and risers, and then used to predict the design responses of the prototype FPS. A favorable comparison between these predicted and the design responses serves to verify the design.
The robustness of the calibrated and validated numerical model to capture the important physics and complex interactions at both model scale and at prototype scale is critical to this overall process of using model tests and numerical models to validate the design responses of the prototype FPS. Thus the careful calibration and verification of the numerical model with model data is important to the success of this process.
A study was completed to investigate this recommended procedure involving the combination of physical model testing and numerical modeling. The overall results are presented in the final report, and provide a description of the accuracy that can be expected from numerical models that have been calibrated and verified with model tests.
Related Publications: Ward, E. G., Hansen, V. L., Kim, M. H., Wang, L., (2004), “Model the Model: Validating Analysis Models for Deepwater Structures with Model Tests”, Offshore Technology Conference (OTC 16586), Houston, May.
Standsberg, C. T., S. Karlsen, E. G. Ward, J. E. W. Wichers and M. B. Irani, (2004) “Model Testing for Ultra Deep Water”, OTC Paper 16587, Offshore Technology Conference, Houston, May.
Hansen, V. L., Wang, L., Ward, E. G., and Sodahl, N., (2004), “Guidelines on Coupled Analysis of Deepwater Floating Systems” OTC Paper 16588, Offshore Technology Conference, Houston, May.