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*Seabed Stiffness

Purpose

To define nonlinear seabed contact stiffness.

Theory

Refer to Embedment for further information on this feature.

Keyword Format

A block of lines that define a force-embedment curve repeated as often as necessary. The block begins with a line defining the curve name. It is followed by as many lines as necessary to define each point on the curve.

Line defining the element set:

CURVE=Curve Name

Line defining a point on a curve:

Embedment Ratio, Seabed Force

Each curve must have at least two points defined. A force-embedment curve may only be specified for an elastic seabed. If a nonlinear seabed stiffness is referenced under the *SEABED PROPERTIES keyword, it must be defined here.

Table Inputs

Notes

(a)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.

(b)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 that you specify here, the tangent stiffness of the curve at that embedment is calculated. This value is used as the vertical seabed stiffness for that element.

(c)You should use as many lines as you need to completely define a force/embedment ratio curve for a particular element set. Simply leave Column 1 blank for second and subsequent lines. For subsequent element sets, put the set name in Column 1 and specify the force/embedment ratio data in the same way. The points defining the force/embedment curve may be specified in any order. Flexcom subsequently sorts the data pairs into ascending order of embedment ratio.

(d)If the computed embedment ratio of an element lies between the specified force/embedment data points that you specify, Flexcom uses linear interpolation to determine the seabed force on the element.

(e)If the computed embedment ratio of an element lies outside the specified range of the force/embedment curve, then Flexcom assumes:

•For embedment ratios greater than the range of the curve, the tangent stiffness of the element is equal to the tangent stiffness between the final two points defined on the curve.

•For embedment ratios less than the range of the curve, the tangent stiffness of the element is equal to the tangent stiffness between the first two points defined on the curve.