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*Embedment

Purpose

To define force-embedment curves for an elastic seabed.

Theory

Refer to Embedment for further information on this feature.

Note also that the old *EMBEDMENT keyword has effectively been superseded by the new seabed definition keywords which facilitate the modelling of arbitrary seabed profiles. Refer to *SEABED STIFFNESS for further information.

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 element set to which the curve applies. It is followed by as many lines as necessary to define each point on the curve.

Line defining the element set:

SET=Element Set

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. *EMBEDMENT is required if no elastic seabed stiffness is input. You can combine linear and non-linear seabeds in the same model. For elements in the set for the non-linear seabed, the non-linear curve will be used. The linear stiffness will apply for any element not in the non-linear seabed set. Several non-linear seabeds may also be specified (for different sets) in the same model.

Table Input

Notes

(a)Flexcom provides an option to specify that seabed stiffness varies with the degree of embedment of a riser or pipeline. Furthermore, the facility is available to specify different stiffness characteristics for different element sets.

(b)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.

(c)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.

(d)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.

(e)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, which is why the default element set is all elements. Of course, All in this context means only all elements in contact with the elastic seabed.

(f)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.

(g)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.

(h)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.