VERIDEEP: Reliable Methods for Laboratory Verification of Mooring and Stationkeeping in Deep Water
Authors: Stansberg C.T. , Øritsland O. , Kleiven G. 
Published: October 2008

Challenges in hydrodynamic model test verification of deep water floating production systems are discussed. Traditionally, tests with models of complete systems including the floater, moorings and risers are preferred if possible. However, for water depths beyond 800m-1000m one meets a depth limitation in laboratory facilites worldwide. A 3-year reaearch project, VERIDEEP, is described where strategies for new or alternative methods are investigated. Extending the model scales down to 1:150 is one philosophy which is tested out through comparative model tests in 1:55, 1:100 and 1:150. A reasonably good correspondence is observed in this case, although the results study cannot necessarily be generalized to all other situations. Another, different solution is combined (hybrid) verification, where model tests at a reduced depth are combined with computer simulations. A specific technique, based upon coupled numerical analysis applied in "copying" reduced tests and extending them to full depth is considered. The results show that the method is a promising tool in future deep-water verification problems.

Introduction

The oil industry is increasingly concentrating their efforts and activities in connection with developing fields in deeper waters (800m – 3000m). Typical areas of such activities include the Norwegian Sea, Gulf of Mexico, offshore Brazil and west of Africa. Depth limitations of laboratories and model scale limitations have traditionally not allowed model test verification of complete systems, including models of floater, mooring lines and risers, for such deep waters. Typical scales have been in the range 1:40 - 1:100, although there have also been examples on even smaller scales. For use of "ultra small scales" 1:100 - 1:200, a thorough investigation on the uncertainties related to such small models is needed. The development and qualification of alternative methods is also needed, such as combinations of simplified model tests and computer simulations, or outdoor testing in fjords or lakes.

In recent years, various methods and efforts have been published in the literature. For a review, see the ITTC Report in Ref. 1. A case study with a 1:170 model testing of an FPSO in 1200m water depth, for Norwegian Sea conditions, has been presented in Ref. 2. It was found that for certain global parameters such as vessel motions and line tensions this is not impossible, although special care must be taken in planning, preparation and execution of such tests. For other types of structures and conditions, other (and probably stronger) limitations would apply. Other works have mainly focused on combined "hybrid" techniques. Examples on this are given in Refs. 3, 4 & 5. While the former focuses on a two-step procedure, with final numerical simulations following as a carefully calibrated numerical extrapolation of tests with truncated set-ups, the latter two references describe active numerical control systems in real-time combined with truncated tests. Results from outdoor model testing have also been presented, see e.g Ref. 6.

In order to contribute to the qualification of methods to face the challenges of deep-water testing in restricted laboratories, an extensive 3-year research project named VERIDEEP has been run.