Line types: Limits data

Compression is limited

OrcaFlex has two modes for handling slack segments, i.e. when the distance between two adjacent nodes becomes less than the original unstretched segment length:

The segment Euler load is given by $\pi^2 EI / l_0^2$, where $EI$ is the bending stiffness of the segment and $l_0$ the unstretched segment length. If a segment has been compressed to or beyond the segment Euler load a warning is given on the results form and in the statistics table.

For items such as mooring chain, the bending stiffness is zero and so the segment Euler load is also zero. In this case you should choose that compression is limited – this correctly models a chain or very flexible rope, which cannot support any compression. The segment Euler load warning is then simply a warning that the line has gone slack.

For a line with non-zero bend stiffness you should interpret the Euler load warning as a warning that the segments at that point are too long to accurately model the bending that is occurring. Effectively, bending is occurring at a scale that is less than the segment length, so shorter segments are needed to model it accurately. Using shorter segments in that area will give a larger segment Euler load, and to obtain an accurate solution you should, ideally, use sufficiently short segments that the resulting segment Euler load is not reached. See modelling compression in flexibles for more details.

Allowable tension

The maximum permitted tension for this type of line. This value is used to draw a limit curve on range graphs of tension and to calculate the normalised tension result. It does not limit the tension achieved in the line. To suppress the drawing of this curve, set the allowable tension to '~'.

Minimum bend radii (MBR)

You can specify the minimum permitted radii of curvature for bending about the $x$ and $y$-directions. These values are optional – they are only used to draw allowable curves on range graphs and in the calculation of normalised curvature results. They do not limit the bend radius of the line. To suppress the drawing of either of these curves, set the corresponding MBR to '~'.

The $x$ and $y$ radii will often be the same, and this can be indicated by setting the $y$-radius to '~', meaning "same as $x$-radius".

The given MBR values are used to draw allowable curves on range graphs of $x$ and $y$-curvature, and also (if the $x$ and $y$ MBR values are equal) on the curvature range graph. In addition, they are used (together with the specified bend stiffness) to derive allowable bend moment curves which are drawn on the $x$ and $y$-bend moment range graphs and (again, if the $x$ and $y$ values are equal) on the bend moment range graph. The MBR is also used to calculate normalised curvature.

Notes: MBR can be given as variable data, defined as a table relating MBR to tension – either wall tension or effective tension can be used. A typical example would be a steel core umbilical. A variable MBR is used solely in the calculation of normalised curvature.
The allowable curve may not be visible on the range graph, since it may be outside the range covered by the graph. To see the allowable curve in this case, you will need to modify the graph to increase the range of values covered.