Model-the-Model: Validating Analysis Models for Deepwater Structures with Model Tests
Authors: Ward G. , Hansen Vigleik L. , Kim Moo-Hyun , Wang Lihua 
Published: October 2008

Response analysis models are used to predict design responses for floating production systems (FPS's). While these models are the basis for present FPS's, uncertainties exist, there are differences among designers, and the models have not been well validated with prototype data. Model tests have been used to account for these uncertainties in FPS designs. However as depths increase, model tests cannot be done at reasonable scales, and the necessary truncations of mooring and riser systems create uncertainties in test results. These uncertainties impact the design either through conservative design practices or unrecognized risks, and can have safety and/or cost implications for a project.

Several studies have been sponsored by DeepStar to better understand the uncertainties in the predicted responses of deepwater structures design and to provide a basis for reducing these uncertainties. Three theme structures - FPSO, Spar, and a TLP - were selected for these studies. Realistic designs were developed for each structure for water depths ranging from 3,000 to 10,000 feet, and design environments for hurricanes and loop currents were selected. Wave basin experiments of these theme structures were completed, and contractors completed detailed analyses to validate their analytical tools and compare their predictions with the data. This paper presents and discusses the overall comparisons of the predicted and measured design responses, and the uncertainties or scatter between the predicted response and measured values from the model tests.

Introduction

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 (1).

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.