Motion Normal to the Seabed |
 
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Motion Normal to the Seabed |
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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 modal analysis, as intermittent seabed contact cannot be considered. So nodes which are on the seabed in the initial static solution are restrained there during the modal analysis, and nodes close to the seabed are free to move in all directions regardless of their proximity to the seafloor.
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 modal analysis (modal 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.
•*SEABED PROPERTIES is used to specify properties such as seabed type (i.e. rigid or elastic) and friction parameters.
•*SEABED STIFFNESS is used to define nonlinear seabed contact stiffness.