Global Boundary Conditions |
 
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Global Boundary Conditions |
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Theory
Rigid seabed contact is modelled in Flexcom by checking the positions of all nodes at every solution iteration. If a node is found to have penetrated the seabed, it is brought back to the mudline, and a boundary condition is applied in the direction normal to the surface to prevent further penetration. This operation is straight forward for flat seabed surfaces as the local seabed axis system coincides with the global axis system.
This situation is a little more complex for sloping or arbitrary seabeds, where the solution axis system for any contact node is temporarily transformed to local, before responses are transferred back to the global system post-solution. A difficulty arises when a seabed node (e.g. the end of a pipeline) is assigned a constraint (i.e. via the *BOUNDARY keyword) in either DOF1 or DOF2, but not both. Flexcom cannot apply a constraint along an axis which does not coincide with the solution axis system (the seabed axis system in this case), so it simply constrains the node is both local DOFs in such circumstances. Where an additional constraint is applied, the user is notified via an appropriate warning message for transparency.
Note also that where the constrained node lies on a flat section of an arbitrary seabed, the additional constraint is not applied, as it is not actually necessary. For example, this would allow the end of a line to slide horizontally along a flat section of seabed, subject to frictional constraints, if a global constraint was specified in DOF1.
•*BOUNDARY is used to define boundary conditions.