If you are analysing an SCR or a flexible riser (e.g. a free hanging, lazy S or lazy wave configuration) in a modal analysis, an important consideration is how the program models the interaction between with the riser and the seabed. There are two aspects to this issue, namely:
(i)Riser motion in the direction normal to the seabed.
(ii)Riser motion tangential to the seabed.
Both aspects are discussed in detail in Seabed Modelling in Modal Analysis and you are referred there for further information.