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*Seabed Properties

Purpose

To specify properties such as seabed type (i.e. rigid or elastic) and friction parameters.

Theory

Refer to Seabed Interaction for further information on this feature.

Keyword Format

A block of lines defining the seabed properties.

The block begins with a mandatory line defining the seabed model, whether rigid or elastic.

Mandatory line defining the seabed model:

TYPE=Seabed Type

This is followed by a block of lines beginning with the element set definition. Next comes an optional line defining seabed stiffness, which is only relevant for an elastic seabed. This is followed by various inputs pertaining to seabed friction. The entire block may then be repeated for subsequent element sets so that different seabed properties may be used with different portions of the model.

Mandatory line specifying the element set:

SET=Set Name

Optional line specifying the seabed contact stiffness:

[STIFFNESS=Stiffness Value or Curve Name]

Optional line specifying seabed friction data:

[FRICTION=Longitudinal Coefficient, Transverse Coefficient [, Longitudinal Mobilisation Length] [, 

Transverse Mobilisation Length] [, Constant Friction Force Coefficient] [, Longitudinal Stiffness] [, 

Transverse Stiffness]

Optional line specifying lateral stiffness (relevant to elastic seabed only):

[LATERAL=Lateral Seabed Stiffness] 

Optional line specifying suction zone parameters (relevant to elastic seabed only):

[SUCTION=Suction Stiffness [, Suction Zone Extent]]

Seabed Type may be RIGID or ELASTIC. Only one of these types is allowed in your model. Any given element may only be assigned seabed properties once. In the event that element sets overlap, the latest definition in the keyword file will take precedence. If a nonlinear seabed stiffness is referenced, it must be defined under the *SEABED STIFFNESS keyword. Both Mobilisation Length inputs default to 0.15m or the equivalent in feet. Refer to Seabed Friction for further details on the friction model employed by Flexcom.

The Longitudinal Stiffness and the Longitudinal Stiffness are only used in frequency domain or modal analyses.

Table Inputs

Seabed Properties – Rigid

Seabed Properties – Elastic

Notes

(a)The longitudinal friction coefficient refers to a local direction parallel to the pipe axis, and the transverse coefficient to a direction normal to the pipe axis.

(b)The default friction coefficients give a smooth seabed, with no frictional resistance.

(c)Linear elastic stiffness is input as a stiffness per unit length of riser or pipeline, in units of [Force]/[Distance]/[Distance] or [Force]/[Distance]2. If a nonlinear seabed stiffness is referenced, it must be defined via under the Seabed Stiffness table.

(d)The Mobilisation Length inputs relate to the operation of the Flexcom seabed friction model, and allow user control over the characteristics of non-linear springs used to model seabed frictional restraint. Both Mobilisation Length inputs default to 0.15m or the equivalent in feet. Refer to Seabed Friction for further details on the friction model employed by Flexcom.

(e)With the Constant Friction Force Coefficient input, the total limiting friction, in the transverse direction, is given by:

Ftotal = μN + CFF

where μN is the Coulombian limiting friction force, and CFF is the constant friction force. This equation is applicable only to transverse friction. The constant friction force is evaluated for each seabed contact element, and is given by the element length multiplied by the constant friction force coefficient.

(f)Regarding the Friction Stiffness entries, you can model friction in a frequency domain dynamic analysis using either a total restraint, or a partial restraint modelled using a spring stiffness. For more information, please refer to Seabed Modelling in Frequency Domain Analysis. By default, seabed friction continues to be modelled in the same way as in earlier program versions (the so-called Fully Restrained Model). This corresponds to the situation where both stiffness entries are zero. To apply a partial restraint in either the longitudinal or transverse direction, you input a non-zero stiffness in the appropriate direction. Note however that a non-zero value is immaterial and unused if the corresponding friction coefficient is zero. In that case, free motion in the appropriate direction is permitted.

(g)The Lateral Seabed Stiffness entry specifies a lateral resistance to horizontal motion on the elastic seabed. Refer to Lateral Resistance for further information on this feature.

(h)The suction or restraining force experienced by a riser element in a “suction zone” just above the mudline is modelled with a linear spring resistance similar to that provided against downward vertical motion by the elastic seabed itself. Refer to Suction Zone for further information on this feature.