Deep Water Model Test Methods - Hybrid Verification

(This article was published in MARINTEK Review No. 1 April 2000.)

THE PROBLEM Model test verification has become an important step in the development and hydrodynamic analysis of floating offshore platform systems. Modelling of complete structures including floater, moorings and risers in realistic sea conditions gives a valuable final check of the whole system, when all major complex details and couplings are taken into account. For deep-water fields the challenges are increasing because existing laboratories may become too small to make space for complete systems at "reasonable" model scales. This requires new or improved ideas for hydrodynamic verification.

A case study with model test verification of a moored semisubmersible platform in 3000 m water depth was carried out by MARINTEK in 1998 as part of the Norwegian Deep Water Programme (NDP), and in close cooperation with Norsk Hydro, representing NDP. Due to the extreme depth, a hybrid verification procedure with a truncated set-up was applied. Practical procedures and guidelines for such verification were also worked out.

The tests were conducted in MARINTEK’s Ocean Basin laboratory. The 3000 m depth was impossible to reproduce in a 10 m deep basin, so a truncated set-up was necessary. A method for replacing a full-depth mooring system by a truncated set-up in combination with advanced numerical analysis, denoted as hybrid verification, was applied. The tests were run in ultra-small model scale 1:150, with a mooring system truncated to 1100 m, made as similar as possible to the full-depth system with respect to certain criteria. Numerical reconstruction of the reduced-depth tests were run, as a check and a calibration of the computer program. A coupled-analysis software was used. Final simulations were run in full depth with the calibrated program.  
 
 
As a check of the verification methodology, an additional study was run with the same platform in 1100m, truncated at 550m. In this case, full-depth measurements were also available as a check for the full-depth simulations.

THE CONCLUSIONS

• A general impression is that the hybrid verification technique using coupled analysis technically seems to work fine, especially in moderate waves, but also reasonably well in severe waves. The uncertainty level due to the combination of several steps still has, however, to be quantified, and improvements on the accuracy can still be done, especially in high waves.
• The large water depth generates particularly large effects from the catenary mooring lines, observed through a dominating surge damping contribution from line drag, and very large offset in current.
• Wave drift excitation and damping should be empirically estimated from the truncated set-up tests, especially in high waves. This is partly due to viscous effects. With current present, a potential for improved theoretical formulation is identified. Wave particle velocities should also be included in forces on mooring and risers.
• The ultra-small model scale 1:150 used in these tests is at the limit for use in hybrid verification. A minimum scale around 1:100 – 1:125 should be recommended in the future.
• Heavy FEM computations require time-consuming computations, while future development is expected to speed up the process.

MARINTEK refs.: Carl T. Stansberg / Ola Øritsland