Flexcom provides an option to specify that elastic seabed stiffness varies with the degree of embedment of a riser or pipeline. In this case, seabed stiffness is characterised in terms of non-linear force-embedment curves (as opposed to being defined using a single linear stiffness value).
The embedment ratio of an element is defined as the average distance the element centreline lies below the seabed, divided by element external diameter. So embedment ratio is dimensionless. “Average distance” in this context means the average of the distances below the seabed of the two nodes on the element.
How this facility operates is as follows. Flexcom computes the average embedment of an element lying on the elastic seabed at each iteration at each solution time. Then using the embedment curve for the element, the tangent stiffness of the curve at that embedment is calculated. This value is used as the vertical seabed stiffness for that element.
The facility to specify different embedment curves for different element sets is provided for complete generality. However in the majority of applications the same curve will apply everywhere.
If an element in contact with an elastic seabed is not included in any element set defined for embedment, the global elastic seabed stiffness is used for that element. If no elastic seabed stiffness is specified, then no vertical restraint will be applied at that element, and a solution indeterminacy could potentially occur.
•*SEABED STIFFNESS is used to define nonlinear seabed contact stiffness.
Note also that the old *EMBEDMENT keyword has effectively been superseded by the new *SEABED STIFFNESS keyword.