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*Lines

Purpose

To define a straight line or a catenary section. Several connecting catenaries, created by weight and buoyancy distributions (e.g. such as a lazy-wave catenary), may also be modelled using *LINES.

*LINE PATHS is better suited to modelling a line with an arbitrary geometrical configuration (e.g. such as a rigid spool).

Theory

Refer to Lines for further information on this feature.

Keyword Format

A block of lines defining an individual line and its associated settings. The block begins with a line defining the line name, and (optionally) its length. This is followed by an optional line defining a default set name for elements within the line which are not part of any sub-sections (if present). The next line defines a node label for the start of the line, along with its coordinates, followed by a similar definition for the end of the line. The next line contains suggested minimum and maximum element lengths for the line to be used during mesh generation. This is followed by an optional line defining undeformed orientation of the elements used to model the line. An optional block consisting of two to three lines defining sub-sections may then follow, and this block may be repeated as often as necessary to define all the required sub-sections in the line. The entire block of lines defining the individual line may then be repeated as often as necessary to define all the required lines in the model.

Line defining the line name, and (optionally) its length. A further option controls whether single or multiple cables are used to model subsections of the line which have different structural properties. Omitting this entry is suitable in the vast majority of circumstances.

LINE=Line Name [, Line Length] [, FORCE=SINGLE]

Optional line defining a default set name for elements within the line which are not part of any sub-sections (if present):

[NON SECTION=Default Set Name]

Line defining a node label for the start of the line, along with its coordinates:

START=Start Node Label, X [,Y] [,Z]

Line defining a node label for the end of the line, along with its coordinates:

END=End Node Label, X [,Y] [,Z]

Line defining suggested minimum and maximum element lengths for the line to be used during mesh generation:

Minimum Element Length (at line start) [, Maximum Element Length] [Minimum Element Length (at line end)] [, RATIO=Aspect Ratio]

Optional line defining undeformed orientation of the elements used to model the line:

[v1, v2, v3, w1, w2, w3]

Optional block consisting of one to three lines defining sub-sections. The first line defines a set name for elements within the sub-section, the start and end distances of the sub-section along the line (positive values denote forward measurement from the start of the line, while negative values denote backward measurement from the end of the line) and optionally the line section group to be repeated across the section. Any Line Section Group Name which is referenced must be defined under *LINE SECTION GROUPS. The optional second line defines an additional set or sets to which the elements within the sub-section are to be added. The optional third line defines suggested minimum and maximum element lengths for the sub-section to be used during mesh generation. This block may be repeated as often as necessary to define all the required sub-sections in the line.

SECTION=Section Set Name, Start Distance [, End Distance] [, REPEAT=Line Section Group Name]

[ADDITIONAL=Set Name, Set Name, Set Name etc.]

Minimum Element Length (at section start) [, Maximum Element Length] [Minimum Element Length (at section end)]

If Line Length is omitted, is equal to the straight line distance between the line’s end points. Y and Z default to zero if omitted.

If the Minimum Element Length (at line or section start) is the only meshing parameter specified, then a uniform mesh density is created based on this length. Aspect Ratio may be STANDARD (default), FINE or SUPERFINE. If the Maximum Element Length is omitted, a uniform mesh density is typically created, based on the specified Minimum Element Length (at start). If the Minimum Element Length (at line end) is omitted, it defaults to the corresponding Minimum Element Length (at line start). If the Minimum Element Length (at section end) is omitted, it defaults to the Minimum Element Length (at section start) for the next section (or the end of the line itself, if it is the last section). This allows the meshing algorithm to automatically blend sections into one another.

The default values of v1, v2, v3, w1, w2, w3 depend on whether the line is modelled internally using a cable or straight section. Refer to Undeformed Versus Initial Positions for a detailed discussion on this topic.

Lines

Purpose

To define a line, by specifying relevant set names, line length, start and end locations, and mesh generation settings.

Table Input

Notes

(a)Refer to Line Mesh Generation for a detailed discussion of the automatic mesh creation facility.

(b)It is important to define meaningful names (for Line Name and Non-Section Set Name entries), as these names will be used by the program to automatically create relevant element sets for you, which you can subsequently reference when you are assigning structural and hydrodynamic properties to your model.

(c)It is important to associate meaningful labels with the line start and end locations, as these labels will be used by the program to automatically create relevant node labels for you, which you can subsequently reference when you are applying boundary conditions to your model.

(d)Based on the length of a line, and the straight line distance between its start and end locations, Flexcom automatically determines whether the line should be modelled internally using a curved (cable) or straight section.

(e)You have control over the distribution of the elements along the line, via the specification of desired maximum and minimum element lengths, and also the division of the line into several subsections if required. The meshing algorithm automatically generates a finite element discretisation based on the guidelines you provide. It also attempts to prevent large changes in relative element length across the finite element mesh by gradually stepping up and down element lengths along the structure, and to avoid over-meshing by using longer elements in the middle of long sections of continuous properties. The specified element lengths are approximate as the automatic line meshing may use slightly different lengths in practice. Refer to Line Mesh Generation for a detailed discussion of the automatic mesh creation facility.

(f)Refer to Lines & Local Undeformed Axes for a discussion on the significance of the V and W vectors (which define local undeformed element axes) in relation to lines.

(g)The meshing algorithm attempts to prevent large changes in relative element length across the finite element mesh by gradually stepping up and down element lengths along the structure. The algorithm ensures that the ratio between the lengths of adjacent elements cannot exceed a certain value. The maximum ratio defaults to 1.5 (Standard option), but may be reduced to 1.25 (Fine option) or 1.1 (Super Fine option). The actual value of the ratio is automatically selected to correctly fill the meshed section with elements.

(h)Flexcom will typically use a separate cable to model each subsection of a line which has different structural properties. In the case of a steep wave riser for example, this is perfectly reasonable. However, in very occasional circumstances, it may be desirable to model several discrete subsections using a single catenary representation. For example, if you are modelling a buoyant section of riser which is comprised of standard material supported by numerous discrete buoyancy modules along its length, a single catenary model is preferable. Otherwise a large number of separate cables will be used internally to model what is essentially a simple catenary, and some or all of the individual cables are likely to violate the basic assumptions associated with catenary equations. In such circumstances, you can use the Force Single Catenary option to explicitly request that an entire line is to modelled using one single catenary/cable.

lines sections

Purpose

To define sections within a line, by specifying relevant set names, start and end distances, and mesh generation settings.

Table Input

Notes

(a)Any Line Name you reference must be defined in the Lines table. Use as many rows as you need to completely define all the sections within a particular line. Simply leave Column 1 blank for the second and subsequent sections. For subsequent lines, put the Line Name in Column 1 and specify the line sections in the same way.

(b)It is important to define meaningful names (for Section Set Name entries, and any Additional Set names if applicable), as these names will be used by the program to automatically create relevant element sets for you, which you can subsequently reference when you are assigning structural and hydrodynamic properties to your model.