﻿ Line results

# Line results

This section describes the line results that are available for the static and dynamic analyses. These results are available using the results selection form. Results for modal analysis and fatigue analysis are described under their respective topics.

## Selecting which categories of line results are shown

There are a large number of line results available on the results selection form, so we group them into the following categories:

To ease results selection the show controls on the results form allow you to choose which of these categories of variables are shown in the variable list. You can get the full list of available variables by simply selecting all the categories, but there will usually be several categories of variable that you do not currently need, in which case de-selecting them reduces the displayed list of variables to a more manageable size.

For lines that use the analytic catenary representation, only a limited number of these results are available, as well as a few additional results that are not applicable to the finite element representation.

## Specifying the position on the line

For line results you need to specify the position on the line at which you want results reported. This is done by setting the entries in the position table on the results form. You are then offered the available variables for the point in the currently-selected row.

Each row in the table defines one point on the line. There are multiple rows in the table, so you can set up rows specifying a number of different points of interest and then easily switch between them by selecting the row of interest. In a row that you do not want to use you can set the node or arc length to '~', meaning 'unspecified'.

Three rows in the table are dedicated to special arc lengths on the line:

• The first and last rows in the position table are dedicated to the line ends, end A and end B.
• The next to last row in the table is dedicated to the touchdown point. This is defined to be the first node on the seabed (starting from the top end). If the result selected is a segment result (i.e. is available only at mid-segment points and not at nodes) then the value reported for the touchdown point is the mid-segment value in the segment that precedes the touchdown node. When there are no nodes on the seabed then the result is reported as N/A (not available) and the graph shows no value.

### Arc length and node

The arc length value specifies how far along the line the result point is, measured from zero at end A of the full line as defined on the structure page of the line data form. If you set the arc length in the position table, then the adjacent value in the node column is set to the number of the node whose reference arc length is nearest to the value specified.

Alternatively, you can set the node value in the table to the number of a node on the line, and the adjacent arc length value will be given the reference arc length corresponding to that node. The node number must be in the range $1$ (the node at end A) to $N{+}1$ (the node at end B), where $N$ is the total number of segments in the line.

 Note: The actual arc length for which line results are reported may not be exactly the specified arc length. OrcaFlex reports results for the nearest appropriate result point. See result points below.

### R and theta

For some variables (e.g. stress components) you must also specify the position of the result point within the cross section through the specified arc length. Whenever one of these variables is selected in the variables list, two extra columns become visible in the position table. These extra columns specify the polar coordinates (R,theta) of the point within the cross section; see the pipe stress calculation diagram for a definition.

R may be one of inner, outer, mid, corresponding to a radius $r$ based on stress diameters as follows:

• Inner: $r = \frac12 I\!D_\textrm{stress}$
• Outer: $r = \frac12 O\!D_\textrm{stress}$
• Mid: $r = \frac12 (I\!D_\textrm{stress} + O\!D_\textrm{stress})$

## Clearance results

Clearance results can be reported either as clearances from this line to all other lines or from this line to a specified other line, according to the choice you make under clearances are reported as clearances from.

## Result points

OrcaFlex lines use a discretised model and so results are only available at nodes, mid-segment points and line ends; we call these points result points. The available result points depend on which result you request – some results are only available at nodes, some only at mid-segment points and some at mid-segment and line end points. The available result points are documented in the description of each result.

When you ask for a result at a particular arc length, OrcaFlex gives the value for the nearest appropriate result point, by which we mean that OrcaFlex considers the available result points that are in the same segment as the arc length you specified and, from these, chooses the one that is nearest to that arc length. If you use an arc length exactly at the boundary of two segments, OrcaFlex uses the segment that starts at that arc length.

OrcaFlex always labels results with the actual arc length of the result point to which they apply, so you can check that you are getting results at the result point you expect.

### Special considerations for line feeding or have partly inactive lines.

If a line uses line feeding or is partly inactive, this introduces extra complexity when defining result points. The arc lengths specified in the position table are technically reference arc lengths, which are invariant labels ascribed to the nodes; usually, there is no distinction between reference arc length and actual arc length, so the two can be used interchangeably and are generally just referred to as the arc length. When a line uses line feeding or is partly inactive, however, we must be more precise in the meaning of arc length. In this case, asking for results at, e.g. arc length 50m, will give the results at the node whose reference arc length is closest to 50m along the full line. The actual arc length of this node might not necessarily be 50m, and could even vary with time.

When line feeding is in use, then it is possible for certain nodes to become inactive for part or all of the simulation. If this happens, then it will not be possible to extract time history or XY graph results at such times, although the results at other times will still be available. Another complication, is that a node's actual arc length on the full line can vary with time. If this happens, then you can still access time history and XY graph results for that node at such times, but you should be aware that some or all of the results given will be for arc lengths which are not stationary on the full line. The periods in which a node's actual arc length varies will, however, usually be short-lived and confined to the times that the node is at, or next to, a feeding line end.

Selecting the first and last rows in the positions table – those that are dedicated to the line ends, end A and end B – gives special behaviour for lines that use line feeding or are partly inactive. In this case, time history or XY graph results are given for the instantaneous active end node which, over time, moves along the full line. As line is paid out or hauled in, not only does this node's actual arc length vary, but the active end node itself may change; the results will always be given for whichever node is at the active line end as the simulation progresses.

Range graph behaviour is also more complicated for lines that use line feeding or are partly inactive. See the range graphs topic for more information on this.