There is a growing need for more advanced hydrodynamic analysis of offshore structures. To support this, DNV Software has enhanced its computational engine to include such nonlinearities.
Wasim, now a part of HydroD, was originally developed for the analysis of ships. A main feature of this program is that hydrostatics, inertia and gravity forces are computed exactly. These non-linear effects may be equally important for offshore structures. As a consequence, Wasim has been updated to handle offshore structures such as semisubmersibles, spar buoys and TLPs. The main updates relating directly to offshore applications include:
- More general and flexible geometry modelling and meshing
- Input of additional restoring and damping matrices
The last extension is essential as it allows for a representation of the mooring system. At the same time, HydroD has been upgraded to support Wasim analyses in addition to Wadam analyses. This makes it easy for the user to run both tools on the same model.
A non-linear analysis with a well-selected input will always give better results than a linear analysis, but the importance of non-linear effects is of course casedependent. In general, non-linear effects will be important if the geometry is changing around the waterline. An example of such a problem where non-linear effects can be important, is a semisubmersible at small draft. In the linear analysis of the case, the pontoons come quite far out of the water. This is not the case in the non-linear analysis, which gives a significant reduction of the pitch motion. This difference is due to the fact that in the non-linear analysis the loss of buoyancy when a part of the pontoons gets dry is accounted for.
This gives a large increase in the restoring and a corresponding reduction of the motions. Hence, non-linear analysis can avoid over-conservative designs related to air-gap problems. A very different non-linear effect may occur for spar buoys. For such structures, non-linear effects may dramatically increase responses compared to predictions by linear theory. This is due to a phenomenon known as the Mathieu instability, originating from the non-linear effects. Most important is the non-linear restoring, but non-linear inertia and gravity effects also play an important role. A non-linear analysis is required to make sure that this effect is not present for a selected design.
Comparison of linear (red)
and non-linear (green) pitch
motion for a spar buoy.
To perform a non-linear analysis the user has to input the vessel geometry in the form of a set of curves. HydroD will then create the mesh on both vessel and free surface for the Wasim analysis. The mesh created on the vessel can also be used directly as panel model for Wadam. If anchor/TLP elements and/or a Morison model are used in the Wadam analysis, they can be included in the non-linear Wasim analysis by giving the corresponding restoring and damping matrices (computed by Wadam) as input.