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Home > Model Components > Lines > Generic Lines > Structure

Generic lines structure

The Structure tab in the generic line properties window is composed of three main areas: Sections, Primary points of section, and Composition of segments.

Sections

Lines are made up of one or several sections. Each section corresponds to a certain extent of line comprised between two primary points. The coordinates of these primary points can be controlled and these points can be connected to other model components.

New lines are initially composed of a single section. The number of sections that make up the line can be controlled by splitting the initial section into several sections or by merging consecutive sections. Line sections may be split in several sections to introduce several intermediate primary points at once, by using the split feature.

Any section can be split into several sections by clicking the Split button once the section has been selected from the sections table. Two consecutive sections can also be merged into one by clicking the Merge button - the section that was selected from the sections table will be merged with the next section.

Note

The main interest of splitting a line is to create intermediate nodes to adjust the line shape or to connect them to other model components. However if intermediate nodes are not used, it is preferable to divide one section into several segments instead of creating several sections. Indeed it is much easier to navigate between different segments than between different sections. In particular, the segments list summary of one section can be copied/pasted to an Excel spreadsheet for modification.

Sections details are listed in the section data table as follows:

  • End1: The name of the first section end node. This cannot be edited straight through the sections table. The name must be defined though the Primary points of section data. Curved abscissa along lines are positive from End1 to End2.

  • End2: The name of the second section end node. This cannot be edited straight through the sections table. The name must be defined though the Primary points of section data. Curved abscissa along lines are positive from End1 to End2.

  • Length: The cumulative length of segments making the section. This cannot be edited straight through the sections table. The non-stretched lengths of segments making the section can be specified through the segments data.

  • Shape: Each section may have a different shape (catenary, straight, upper or lower arc, or prescribed user-defined shape. The user can chose each section easily by clicking it in the summary table and directly adjust its shape via the Shape button.

Hint: By selecting user-defined shape, the user can import a line shape from an external mesh in ASCII format. More details about the ASCII format are available in a specific topic.

  • User-defined shape file: The name of the data file used to define the user-defined shape of the section. This name can be edited only in case the shape was set to user-defined shape.

Reference unstressed configuration

The reference unstressed configuration defines the shape of the line at rest, i.e. when the line is not submitted to any external or internal loadings.

The option defaults to straight pipe which is suitable for most applications unless you wish to include bends along the line.

You may select the As-defined shape option to set the shape of the line at rest similar to the shape shown in the 3D View window of your model. Considering the as-defined shape may alter the stress outputs and is suitable only for lines representing rigid spools or similar applications.

Primary points of section

First select the section you wish to edit in the Sections table. Then in the Primary points of section area, enter the positions (in the global coordinate system) of the two section end points. For each section, the primary points of section and the composition of segments have to be defined. Different linked to connections can be defined: free, anchored, or connected (to existing structures, including the Sea & ground); in this last case, the attach point field allows the user to link the line end to one of the existing points on a structure.

Unless the free option is selected, different connection type can be imposed: Pin, Clamp, User defined (with the six degrees of freedom independently chosen free or constraint or clamped), Flexjoint or Spring .

Unless the pin option is selected, elevation and azimuth departure angles must be specified in the impose neutral axis direction fields. Carefully check these imposed departure angles (ENCASD & PSID) at the connected point, to ensure the direction matches the line. See more details...

Warning

Clamp connections may be defined at line ends by specifying the departure angle through its azimuth and elevation. This determines the departure direction only (i.e. tangent direction only but not the 2 principal bending directions associated with the cross-section at the connection point), and the FE engine automatically sets-up the local reference frame associated with the line at the initial stages of the calculation. Initial torsion in the line may arise from inappropriate initial torsion constraints at line ends.

An improved initialization technique is used by default for torsional degrees of freedom along the line which avoids inducing some fictitious torsion in the line.

Another initialization technique allowing prescribing the directions of the principal bending axes (and therefore initial torsion) at line ends can be invoked by ticking the checkbox beside the edit boxes used to input the azimuth and elevation angle.

This makes no difference for the vast majority of cases. By default, it is recommended not to check the "Also impose bending axes" option when defining clamp connections, unless you wish to the orientation of the bending axes similar to the local X and Y axes of the node the line connects to.

Composition of segments

In this part of the window, each selected section can be further divided into segments, and the properties of each segment are defined through Line segment type.

Initially there is one segment representing the section selected. The add button will copy the selected segment in the table; the new segment will be added as item 1 in the list, pushing the existing segments down. New segments are assigned a name automatically but this can be edited directly in the table. It is recommended to give segments short but recognisable names.

Alternatively the field seg. nb can be used to set the required number of segments in one quick step.

After creating the number of segments you require, it is necessary to set their length and other properties:

  • Name: This is the name that is used to refer to the line segment. Segment names may be used to specify regions along the line when post-processing partial results envelopes graphs. These names are also used to fine tune numerical contact parameters along specific regions of a line (e.g. to request contact being circumscribed to selected regions of a line, or to set optimum density of contact elements along segments).

  • Length: This is the non-stretched length of the line segment. This is set directly in the table, and as you enter a number, the section and line lengths are automatically updated.

  • Target element length: If Target element length is set to "-" then the number of elements along the section is set by Number of elements. Otherwise the number of elements along the segment is chosen based on the Target element length. The Number of elements cannot be edited in this case and is calculated as being the rounded integer value of the segment length over target element length ratio.

Hint: It is advised to set the number of elements by specifying the target element length. This offers better control over the actual length of elements and avoids the need to alter the number of elements in case the length of the segment is modified.

  • Number of elements: This sets the number of finite elements along the segment. The number of finite elements must be set to achieve an optimal balance between runtime (calculation resources) and analysis accuracy. The interest in results at particular regions in the model may require smaller more finite elements. This may be a key reason to split the model up into segments, apart from structural property changes and connections. It is advised that the ratio between the element length set for adjacent sections does not exceed 2.0. This means you need to add some intermediate segments simply to create a gradual transition in element length along the line section.

Note

The Number of elements cannot be edited if the segmentation is set by Target element length. Target element length must be set to "-" if you wish to specify the number of elements.

  • Segment property: Each segment must be assigned an existing Line segment type which defines the structural properties to be considered for the line segment. The line segment type must already exist before it is assigned to a line segment. Therefore two short cut buttons are provided to create new segment type or to edit segment types.

  • Property type: The last column property type is not editable but is an echo of the type associated with the line type you have selected to act as a reminder.