OrcaFlex Manual - Orcina

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System Modelling: Data and Results, Lines
need to make some assumptions about how the line was originally laid and friction is then assumed to act towards
this position.
If the Step 1 Statics Method is Prescribed, then this 'originally laid' position is assumed to be the position defined by
the Prescribed track. Otherwise, the 'originally laid' position is defined by specifying the Lay Azimuth and As Laid
Tension values.
Lay Azimuth
This data is only used when seabed friction is included in the static analysis and the Step 1 Statics Method is not
Prescribed. It then defines the position in which the line is assumed to have been originally laid, and friction is then
assumed to act towards this position. When Statics Method is not Prescribed, it is assumed that:
1. The line was originally laid, with the specified As Laid Tension, starting with the Bottom End at its specified
position (or at the point on the seabed directly below, if the Bottom End is not on the seabed).
2. The line was then laid in the Lay Azimuth direction, leading away from the Bottom End position and with the
specified As Laid Tension.
3. The line was laid following the profile of the seabed.
4. The Top End was then moved slowly from that original position to its specified position.
To help set this data item, there is a button on the form marked Set Lay Azimuth. This button sets the Lay Azimuth
value to be the direction from the Bottom End towards the Top End, based on their current positions.
Notes: Whilst the program will accept any Lay Azimuth, we would expect the statics convergence routine
to have increasing difficulty in finding a solution as the angle between the Lay Azimuth direction
and the vertical plane through the line ends increases. For example, if we have a line top at X=0,
Y=0, and anchor at X=100, Y=0, we would expect trouble for a Lay Direction of 90°.
As Laid Tension
The Line Setup Wizard also uses the Lay Azimuth direction.
This data specifies the effective tension with which the line was originally laid. OrcaFlex uses this to determine the
as-laid node positions, which are used as the friction target positions towards which friction acts in the static
analysis. This data is therefore only used if friction is included in statics.
If the Step 1 Statics Method is set to Prescribed starting shape, then the statics friction target positions are laid out
along the prescribed shape with a strain determined by the axial stiffness and this As Laid Effective Tension value.
If the Step 1 Statics Method is not Prescribed, then this data is used as described in the Lay Azimuth section above.
Catenary Convergence
If the Catenary statics method is chosen, then an iterative catenary calculation is used to determine the static
position of the line. This calculation is controlled by a number of convergence parameters which can normally be left
at their default values. However sometimes the calculation can fail to converge. If this happens, first check your data
for errors and check for the following common causes of convergence failure:
� Does the solution have a slack segment? This can happen in lines that touch down on the seabed almost at right
angles or in lines that hang in a very narrow U shape. The catenary calculation cannot handle lines with slack
segments – try increasing the number of segments in the relevant section of the line.
� For lines that touch down on the seabed, is the Lay Azimuth value specified correctly? It is the azimuth direction
leading away from End B and it is easy to get it wrong by 180°.
� Is the line buoyant, either deliberately or by mistake. The catenary calculation has problems with floating lines –
you may need to use the Spline statics method instead.
� Does the line have a surface-piercing buoyant clump attached? If the clump is short then the catenary
calculation is more difficult.
If the calculation still fails to converge, then it is sometimes possible to obtain convergence by changing one or more
of the convergence parameters, as outlined below.