Model Testing for Ultradeep Waters
Authors: Stansberg C.T. , Karlsen S.I. , Ward E.G. , Wichers J.E.W. , Irani M.B. 
Published: October 2008

Model test procedures for global analysis verification of floating production systems in ultradeep waters are reviewed. Due to space limitations in wave basins, a combination of tests at reduced depths with advanced numerical tools is recommended. Guidelines for this approach are presented. The philosophy is that a numerical model of the truncated set-up is validated against the tests, and resulting calibration information is then applied in full-depth verification simulations. Principles for design of truncated systems are given, where the key motivation is to reproduce the full-depth system properties as closely as possible. A priority list with criteria for this purpose is given and discussed. Coupled analysis is recommended for numerical reconstruction of experiments. Key topics in this "model-the-model" procedure, and of the final prototype simulations, are discussed.

Introduction

Scale model tests have been successfully used to study and verify the design responses of moored floating structures for many years. In relatively "shallow" water, a complete scaled model can be tested in a model basin at reasonable model scales (1:50 - 1:100). Test results can be directly scaled using Froude Number scaling to predict the prototype performance. This approach has been successful since the moorings and risers can be modeled to full (scaled) depth, and viscous drag on the mooring and risers has limited influence on the structureâ??s response. For cases with many moorings and risers, the drag could be relatively easily compensated for or handled with simple numerical models. This use of scale model tests to verify a "shallow" water design is shown in Figure 1. As depths and the number of mooring and risers increased, more compensations were required for the testing and interpreting test results, but the scale model approach has been successfully used in many model basins to study Floating Production Systems (FPS's) in depths to 3,000 feet or more.

Testing FP's and validating their predicted design responses in deep (to 6,000 feet) and ultra deep (to 10,000 feet) water depths presents some challenges. Due to the depth and scope of mooring and riser systems, a complete scaled model of the FPS is simply too large to test in present model basins. The modelâ??s mooring and riser systems must be truncated, and a numerical model must be validated and then used to interpret and extend the model test results to predict the prototype FPS responses at the full design water depth. This process is also shown in Figure 1.

In the present paper, the challenges and solutions in deep and ultradeep water model testing of passively moored FP's will be addressed. The main focus is on the basic principles of methods using physical model tests with truncated mooring and riser systems, combined with numerical models to compensate for water depth limitations in wave basins. Examples on such combinations are given in e.g. /1/,/2/,/3/. The deep-water modeling problem has also been discussed by ITTC /4/. In this paper, challenges and actual procedures will be described and discussed. It is mainly based on guidelines recently worked out for DeepStar as a part of a more general guideline study on global analysis of deepwater FPS /5/. An actual truncation design example from a previous case study is also shown for illustration.