Simulating Deepwater Moorings/Riser Systems In A Model Basin
Jul 5, 2016 News Archive
About 10 years ago, I had the opportunity to do model testing of a dry-tree concept at the Offshore Model Basin (OMB), in Escondido, CA. It was during that model test campaign that I came across their SMART system for simulating deepwater mooring on a basin with relatively shallow water depth.
The SMART (Simple Mooring and Riser Truncation) system was conceived by OMB to reproduce the lateral force/offset curves of deepwater mooring. This system was followed by Extended Range versions (SMARTER and SMARTER-II) which allowed for tuning the vertical stiffness. These systems were the brainchildren of Nick Markov, Mike Lambert, Hatem Elgamiel and Victor Grinious.
Both of these systems were eventually followed by a version that accounted for dynamic effects, dubbed SMARTEST. As seen in Figure 1 below, one of the benefits of the SMARTER system was the fact that it did away with the use of divers when the model needs to be rotated (since the mooring lines are actually anchored above the waterline).
Figure 1 – Sketches of the OMB-developed SMART and SMARTER Systems (Ref 1)
These systems were excellent ways to model the mooring and can be adjusted to simulate flexible risers on semisubmersibles and spars, but they are not intended to be used for modeling tendons on a TLP.
A few months ago, talking with Yusong Cao, we somehow started talking about truncated mooring and how I wished there was something like the SMART/SMARTER systems for modeling the tether effect from a deepwater top-tensioned riser in a shallow water model basin. To my surprise, Yusong told me he had co-authored a paper on the subject (Ref 2). Looking back, I should not have been too surprised since Yusong did work for MARINTEK (although I think of him as a CFD and riser expert).
As depicted in Figures 2 and 3 (Ref 2), the MARINTEK-developed system is based on a computer-controlled anchor (and therefore can be applied to tethers).
Figure 2 – Sketch of the MARINTEK-developed Active Hybrid Testing System (Ref 2)
Figure 3 – Scheme of the MARINTEK-developed Active Hybrid Testing System (Ref 2)
I am not aware if these systems have been used for modeling tethers on TLPs or top-tensioned risers. The norm for these cases is to use a “pit” at the bottom of the model basin, allowing the tethers and risers to extend beyond the bottom of the model basin. I’d be curious to get feedback from anyone knowing if truncated systems have been used for TTRs or tethers. If you have any information on this, please do let me know.
- H. Elgamiel, N. Markov, V. Grinius and M. Lambert. New Mooring Simulation Approaches in Model Testing. ISOPE-2006 : [proceedings of the Sixteenth (2006) International Offshore and Polar Engineering Conference, San Francisco, USA, May 28 – June 2, 2006] ; conference proceedings. Paper No: 2006-CC-03.
- Y. Cao and G. Tahchiev. A study on an Active Hybrid Decomposed Mooring System for Model Testing in Ocean Basin for Offshore Platforms. Proceedings of the ASME 2013 32nd International Conference on Ocean, Offshore and Arctic Engineering OMAE2013. June 9-14, 2013, Nantes, France.