Seabed - Rigid Surface
To specify a rigid seabed surface, you select Rigid from the Seabed Type drop-down list, as shown in the figure below.
Rigid Seabed Type
You use the Seabed Modelling drop-down list as shown in the figure below to select whether the seabed is uniform or arbitrary.
You then click on the Seabed Properties button to define the properties of the seabed. The dialog that is displayed varies depending on whether you select Uniform or Arbitrary from the Seabed Modelling drop-down list.
To define a uniform rigid seabed you select Uniform from the Seabed Modelling drop-down list and click on Seabed Properties. The dialog is shown in the figure below.
Uniform Rigid Seabed Properties
The main data inputs are the longitudinal and transverse friction coefficients. Longitudinal refers to a local direction parallel to the pipe axis, and transverse to a direction normal to the pipe axis. A slope may also be included on the seabed. A positive slope defines a seabed sloping upwards in the positive global-Y direction, while a negative slope gives a seabed sloping in the opposite direction. The definition of the maximum longitudinal characteristic length and the maximum transverse characteristic length relates to the operation of the PipeLay seabed friction model, and allows you to control the characteristics of non-linear springs used to model seabed frictional restraint. The torsional stiffness allows control over torsional friction on the seabed. The kinetic friction factor allows you to specify a change (typically a reduction) in the friction coefficients once the limiting or static friction is exceeded. Kinetic friction is a defined as a fraction or ratio of static friction, and is typically (but not necessarily) less than 1 to reflect the fact that there is a reduction in the friction force on a pipe once the pipe has started to move. The Use Pipe Contact Diameter option allows you to specify that the pipe will contact the seabed when the outer pipe surface touches the seabed as opposed to when the pipe centreline touches the seabed, which is the default legacy approach. All of the inputs in this dialog are optional; the default rigid seabed is flat and smooth.
To define a rigid arbitrary seabed you select Arbitrary from the Seabed Modelling drop-down list and click on Seabed Properties. The dialog is shown in the figure below.
Arditrary Rigid Seabed Properties
In this case, the properties dialog is similar to the previous figure except that there is no Slope input.
In addition, in the Seabed component view, the ASF Direction drop-down list and the ASF Name button are now enabled. In PipeLay, ASF stands for Arbitrary Seabed File. When you click on ASF Name, the standard Windows Open dialog is displayed to enable you to browse for the file that contains your arbitrary seabed data. Finally, the ASF Direction drop-down list allows you to specify if the seabed data in your file defines the bathymetry in terms of distance above the zero datum (Ascending) or distance below the Mean Water Line (Descending).