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Motion Normal to the Seabed

With regard to motion normal to the seabed, the program operation depends on whether a rigid or elastic seabed is specified.

If the seabed is rigid, then all of the section which the initial static analysis calculates as lying on the seabed is completely restrained in the normal direction during the frequency domain dynamic analysis, as the dynamic phase cannot consider intermittent seabed contact. So nodes which are on the seabed in the initial static solution are restrained there during the dynamic phase, and nodes close to the seabed are free to move in all directions regardless of their proximity to the seafloor. However, during the static analysis phases (initial and full – see Frequency Domain Analysis for further details on frequency domain solution procedure), nodes can move onto or off the seabed. The reaction normal to the seabed at seabed nodes is monitored continuously, and nodes may be released and may move away from the seabed. Likewise nodes which are close to the seabed may move onto the seabed and be restrained there.

If on the other hand, the seabed is elastic, then a seabed stiffness matrix in the direction normal to the seabed is computed in standard fashion for each element calculated by the initial static solution as lying on the seabed. However the nodes on these elements are not restrained in the direction normal to the seabed, but are in fact free to displace.

There is also another additional feature for the case of an elastic seabed. Instead of a constant and single-valued seabed stiffness, you can specify that the stiffness varies as a function of the degree of embedment of a riser or pipeline in the seabed (this is defined in terms of a seabed force/embedment ratio relationship). If you invoke this option, then the seabed will be modelled as a linear elastic contact surface in the frequency domain dynamic analysis (frequency domain analysis is inherently linear by nature), with the stiffness for each contact node being equal to the final contact stiffness attained in the initial static analysis.