In a recent paper (Impact of Bathymetry on the Mooring Design of an Offshore Floating Unit, OMAE2016-54965), DNV GL authors Jaiswal, Vishnubholta, Cole, Gordon and Sharma examine the effect of using an accurate seabed bathymetry on typical mooring results. Their motivation was to understand potential limitations in the assumption made by most mooring software that slope of the seabed is constant along the direction of each mooring line.
To explore this they consider the case of a hypothetical 8-legged all-chain-moored semi-submersible, located over a simple escarpment (with a constant profile). Consequently the water depth at the anchors varied between c150m and c250m. They considered two different seabed representations: (1) the accurate bathymetry, and (2) an approach equivalent to the constant slope assumption in most mooring software.
OrcaFlex is used for static (including wind, wave & current) and coupled dynamic time domain analysis. For 1st order dynamics waves are included. For 2nd order dynamics, wind and waves are included.
Results for line pre-tensions and the min and max values of vessel offset and line tensions are compared for each seabed representation. They conclude that there are “significant differences” in vessel offset and maximum mooring line tension when using different seabed representations. The authors point out that these results can’t be generalised, but further conclude that where there is uneven seabed bathymetry and marginal safety factors, it may be advisable to use a more accurate bathymetry.
As an aside to the paper summary, please note that we are actively enhancing OrcaFlex functionality for traditional mooring analysis. Wave Frequency Frequency Domain was added in v10.0 (Oct-15), and we’re currently working on a Low Frequency Frequency Domain solver – this should be forthcoming in OrcaFlex v10.1 (cOct-16), unless any last minute hitches postpone it to 10.2.
You can download the paper from the ASME paper here.
The next OMAE conference is to be held in Trondheim, Norway, in June 2017. Orcina are planning to attend and we look forward to seeing you there.