Seabed - Elastic Surface

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Seabed - Elastic Surface

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To specify an elastic seabed surface, you select Elastic from the Seabed Type drop-down list, as shown in the figure below.

Elastic Seabed Type

Elastic Seabed Type

You use the Seabed Modelling drop-down list shown in the figure below to select whether the seabed is uniform or arbitrary.

Seabed Modelling

Seabed Modelling

In addition to the Seabed Properties button, the Embedment button is now enabled to allow you to define a force-deflection relationship for a non-linear elastic seabed.

The dialog that is displayed when you click on Seabed Properties varies depending on whether you select Uniform or Arbitrary from the Seabed Modelling drop-down list. The properties dialogs are described in the following sections.

Uniform Elastic Seabed

To define a uniform elastic seabed you select Uniform from the Seabed Modelling drop-down list and click on Seabed Properties. The dialog shown in the figure below.

 

Uniform Elastic Seabed Properties

Uniform Elastic Seabed Properties

Most of the entries in this dialog are similar to the entries in the Rigid Seabed dialog shown in the above figure and are described in the ‘Uniform Rigid Seabed Properties’ section. However, this dialog contains six additional entries; Seabed Stiffness, Lateral Gap, Lateral Gap Power, Lateral Seabed Stiffness, Suction Stiffness and Suction Zone Extent. These entries are now described.

Seabed Stiffness is used to specify the stiffness of an elastic seabed with respect to motion normal to the seabed. If your seabed is linear, you input a single value here; if your seabed is non-linear, then you leave this entry blank. The elastic seabed also provides an option to model seabed stiffness in the lateral direction. This lateral stiffness can be used to capture trench conditions or indeed to mimic lateral seabed friction. Note that the specification of both a Lateral Seabed Stiffness value and friction coefficients is invalid as these entries are mutually exclusive. In the case of capturing trench conditions, it is possible to specify a uniform gap in the lateral direction between the pipe outer surface and the trench wall. The Lateral Gap input is the distance over which the pipe can move relatively freely before the full lateral stiffness is applied, or in other words, before the pipe reaches the trench wall. By default no gap is included so the full stiffness is applied without any lateral movement. There is also a Lateral Gap Power input available which can be used to control the rate at which the lateral stiffness is ramped up to it full value, by a power law equation, as the gap between the pipe and trench wall closes. The greater the power the less resistance the pipe will encounter before closing the gap. Note that a power value of 1 effectively equates to a linear stiffness, or zero gap, condition. Also, while there may be a desire to use a relatively large power value this may introduce solution stability issues and so generally the default value of 5 is deemed sufficiently large.

The elastic seabed dialog includes two further entries to enable you to model a suction or restraining force experienced by an element in a suction zone above the mudline. This suction zone is modelled with a linear spring resistance similar to that provided against downward vertical motion by the elastic seabed itself. The Suction Stiffness input defines the suction spring resistance, while the Suction Zone Extent is the height above the mudline to which the suction forces extends. Above the specified height, the suction ‘spring’ is removed.  Note that suction forces are experienced only by a pipe section moving upwards (away from the seabed) through the suction zone; a pipe moving downwards (onto the seabed) through the suction zone does not experience suction forces.

Theoretically, the behaviour of an elastic seabed with an infinitely large stiffness should be identical to that of a corresponding rigid seabed of identical profile. However, the modelling of such an elastic seabed often poses problems in PipeLay due to solution convergence difficulties. Generally though, this tends to occur for a stiffness value which effectively constitutes a rigid seabed and it is advisable to use the rigid seabed contact algorithm when modelling an elastic seabed of high stiffness.

 

Arbitrary Elastic Seabed

To define an elastic arbitrary seabed you select Arbitrary from the Seabed Modelling drop-down list and click on Seabed Properties. The dialog shown in the figure below.

 

Arbitrary Elastic Seabed Properties

Arbitrary Elastic Seabed Properties

In this case, the properties dialog is similar to the previous figure except that there is no Slope input. As in the case of a rigid arbitrary seabed, the ASF Name and ASF Direction options are also activated so you can input the name of the file with the arbitrary seabed data, and define whether the bathymetry values represent distance above the vertical datum or distance below the Mean Water Line.

 

Embedment

PipeLay provides an Embedment dialog to enable you to specify that seabed stiffness for a non-linear elastic surface varies with the degree of embedment of a pipeline. The figure below shows the Embedment dialog.

Embedment Dialog

Embedment Dialog

You use the Embedment dialog to specify embedment ratio values on the curve and the corresponding force exerted by the seabed for that embedment ratio. Embedment ratio in this context is defined as the distance the pipe centreline lies below the seabed, divided by pipe external diameter. So, embedment ratio is dimensionless. This non-linear seabed option operates as follows:

1.PipeLay computes the embedment ratio of a pipe lying on the elastic seabed at each iteration at each solution time.

2.Using the embedment curve for the pipe that you specify here, the tangent stiffness of the curve at that embedment is calculated.

3.This value is used as the vertical seabed stiffness for the pipe.

Note that the use of this dialog and the specification of an elastic seabed stiffness on the Seabed Properties dialog are mutually exclusive. A seabed can be linear or non-linear, but not both. If you specify a seabed stiffness, you cannot input an embedment curve, or if you specify an embedment curve, you cannot define a linear seabed stiffness. PipeLay will generate an error if you violate either of these conditions. Of course, you must specify one option or the other. For example, if you do not specify an embedment curve, a (non-zero) seabed stiffness value is required, and again PipeLay will generate an error if your elastic seabed data specification is insufficient.