Both Bladed's standard fully integrated approach and the “superelement” approach use the Craig-Bampton method to calculate reduced mass and stiffness matrices (corresponding to support structure deflection mode shapes) in order to reduce simulation time. The standard method has some advantages over the superelement approach. The superelement method is sometimes invaluable however, in dealing with cases where the support structure cannot be created in Bladed or is kept confidential. See the document "Superelement Beta: User guide for Bladed v4.8" under User Guides for much more detail on Bladed's superelement feature.
Bladed uses the Craig-Bampton method as a pre-processing step. However, as Bladed also retains the underlying support structure finite element model during the simulation, some modelling refinements compared to the “superelement” approach are possible.
Firstly, the deflected position of each individual member in the support structure can be taken into account at each time step, leading to coupling between structural motion and hydrodynamic loading. Secondly, the structural deflections can be used to provide a better estimate of structural response by calculating the additional stiffness due to structural displacements (called “stress stiffening” or “geometric stiffening”). Finally, retaining the detailed FE model allows Bladed to include a non-linear foundation model which can be important to properly evaluate foundation response.
In the “superelement” approach, Craig-Bampton mode shapes are calculated, but the detailed finite element model used to calculate the mode shapes is discarded. A reduced wave load must also be calculated as a pre-processing step, specifying the wave loading on the support structure assuming that the structure is stationary. The effect of structural motion on wave loading would therefore not be accounted for. Additionally, foundation properties must be included in the pre-calculated superelement, meaning that only linear (i.e. constant stiffness) foundation properties can be defined.