Cables on Seabed |
 
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Cables on Seabed |
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If part of a cable is expected to lie on the seabed, then the internal operation of the cable pre-static step imposes a minor constraint on the specification of the end nodes for that cable. Specifically, the node at the lower end of the cable must be positioned initially on the seabed. If the actual final position of this node is not on the seabed, but rather some distance above it, this must be achieved by the application of a boundary condition.
Consider for example the simple catenary shown below. It is required that the riser be connected to the PLEM at an elevation of 5m above the seabed. To use a cable in this analysis, you must specify that it extends from the node at the PLEM (Node 1 in this case) to the node at the vessel (Node 61). However, if you locate Node 1 at (5, 0, 0), Node 61 at (140,160,0) and define a cable of length 260m between them, Flexcom may fail to converge. You must instead locate Node 1 at (0,0,0) and apply a vertical displacement of 5m as a boundary condition. Convergence will then be achieved in a small number of iterations.
Cable Specification for Sections with Partial Seabed Contact
The use of the cable pre-static step is of course optional, and should only be used where the cable catenary equations can genuinely provide an initial approximation to the final static configuration. As an extreme example, a pipe or cable lying completely on the seabed cannot be analysed by the cable pre-static step. Note also that the cable pre-static step is automatically run, without user interference, whenever you define a cable or cables.