﻿ Line data: Mid-line connections

Line data: Mid-line connections

Mid-line connections are those from child mid-line nodes (i.e. nodes other than end nodes) to other (parent) objects. The data required for each connection are: the arc length of the node, the position and orientation of the connection, and whether the connection is released during the simulation. Mid-line connections are broadly similar to end connections, with a few differences:

• Multiple mid-line connections can be made along the line.
• Mid-line connections cannot be defined at the end nodes: line end connections are defined separately by the end connections data.
• Each node may have at most one mid-line connection.
• Mid-line nodes are rigidly connected to the parent object, whereas end connections allow finite values of connection stiffness and damping. To introduce degrees of freedom, stiffness, damping etc. between a mid-line node and its parent object, use intermediate constraints.
• When torsion is included on the line, all rotational degrees of freedom are determined by the parent object. When torsion is not included, only the bending degrees of freedom are determined by the parent, and it is assumed that there is no twist along the line, even in the presence of mid-line connections.
 Notes: It is not possible to use mid-line connections when the line's length and end orientations are calculated from end positions. It is not possible to use the spline and prescribed step 1 statics methods for a line which has mid-line connections. A line with mid-line connections can only be a supported line or a splined line in a line contact relationship if the line includes torsion.

The mid-line connection data are given in a table, each row of which defines a node on the line and that node's connection data. The data are:

Arc length, relative to

The connection is made from the node whose reference arc length is closest to the specified arc length, measured relative to either end A or end B, as specified.

 Warning: If a line uses line feeding or is partly inactive, then the actual arc length of the node where the connection is made may vary as this node approaches a line end at which feeding occurs.

Node arc length

The reference arc length of the node where the connection is made.

Connection

The parent object to which the mid-line node is connected.

Position, z relative to

The position of the centre of the node. The precise meaning of this item depends on the nature of the connection:

• If the node is connected to another object, then the position is that of the connection point relative to that other object's local axes.
• If the node is connected to another line, in particular, then the position $z$ coordinate represents the arc length along the parent line, measured relative to end A or end B of that parent line, as given by z relative to. The child node will be connected to the nearest node to this arc length on the parent line, with an offset relative to the parent node's axes given by $(x,y,0)$. $x$ and $y$ may only be non-zero if the parent line includes torsion.
• If the node is fixed, the position is that of the fixed point relative to global axes.
• If the node is anchored, the position gives the $x$ and $y$ coordinates of the anchor relative to global axes and the $z$-coordinate relative to the seabed level at that $(x,y)$ position.

Orientation

When a mid-line node is connected to an object, it is rigidly connected to that object and rotates with it. You define the orientation of this connection by giving the child node's azimuth, declination and gamma angles relative to the parent object axes. For fixed or anchored connections, this orientation is given relative to global axes.

The orientation angles are defined and interpreted in exactly the same way as the end connections orientation angles.

Release at start of stage

If desired, the node can be disconnected at the start of a given stage of the simulation. For no disconnection, set this item to '~'.