Motion Tangential to the Seabed |
 
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Motion Tangential to the Seabed |
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With regard to motion tangential to the seabed, the program operation in this case depends on whether a smooth seabed or a seabed with friction is specified. If the seabed is smooth, then the section of riser on the seabed is free to displace tangentially to the seabed during the modal analysis. If friction is present on the seabed, frictional effects may be modelled using a fully restrained model or a partially restrained model. Each of these is now described in turn.
In the case of the fully restrained model, if friction is present and both friction coefficients are non-zero, each node on the seabed is completely restrained against motion tangential to the seabed during the modal analysis. This is true for both rigid and elastic seabeds. If one friction coefficient is zero and one non-zero, the program applies a restraint in the direction (transverse or longitudinal) with the non-zero coefficient, while allowing motion in the other direction. This requires the application of boundary conditions in a local seabed axis system rather than in the global axes.
In the partially restrained model, friction is represented as a stiffness. If friction is present, a stiffness matrix is computed for each element on the seabed in a direction (transverse, longitudinal or both) with a non-zero coefficient. In a direction with a zero coefficient, full free motion is allowed during the modal analysis as in the fully restrained model. The longitudinal and transverse ‘friction stiffnesses’ are user specified inputs.
•*SEABED PROPERTIES is used to specify properties such as seabed type (i.e. rigid or elastic) and friction parameters. Specifically, the FRICTION= input is used to specify friction data such as friction coefficients and friction stiffness terms.