Line data: Statics

The line statics calculation is one stage of the overall statics analysis in OrcaFlex. While global line statics policies give you model-wide control, there are also specific data which govern the statics calculation behaviour and methods of each individual line.

Included in statics

This switch allows you to exclude a particular line from the statics analysis completely, and may be useful if, when building a model, you find a particular line is not converging. In this situation you might find it helpful to exclude all the other lines from statics (which you can do conveniently via the all objects data form), allowing you to experiment with different statics convergence parameters for the problematic line in isolation.

Note:

Results are not available for lines excluded from statics, and these lines have no influence on other objects in the model. The dynamic simulation will not proceed if any lines are excluded from statics: only the statics analysis is permitted.

Step 1 statics method

This can be quick, analytic catenary, catenary, spline, prescribed or user specified.

The quick method provides a simple approximation that leaves the line in an approximate catenary shape, but neglects many effects that may be important, such as seabed contact and buoyancy.

The analytic catenary method uses an analytic catenary solver to determine the shape of the line. This is more sophisticated than the quick method because it accounts for effects such as seabed contact and buoyancy.

Catenary is the most commonly used method, wherein the static analysis finds the equilibrium catenary position of the line allowing for weight, buoyancy, and drag, but not bend stiffness or interaction with shapes.

While the catenary method is usually effective, some systems, such as those with slack or neutrally buoyant lines, can be troublesome. For such lines you might instead choose the spline solution, whereby the line is set to a 3D spline curve based on user-defined spline control points.

The prescribed method is most suitable for pull-in analysis: you define the starting position of the line as a sequence of straight line or curved sections on the seabed. This option cannot be used in conjuntion with the calculated from end positions length and end orientations option.

User specified allows you to specify the position of each node on the line. No calculations are involved, the nodes are simply placed at the given positions.

Note:

The line statics step 1 policy governs whether or not step 1 statics is calculated. This is a global setting which applies to all lines in the model.

Step 2 statics method (full statics)

This may be either none or full statics.

If none is selected then the position remains that at the end of step 1 statics.

Otherwise, the full statics calculation takes the result of step 1 statics as the starting shape for the line and finds an equilibrium position for the model, taking full account of bend stiffness and interaction with shapes.

Note:

The line statics step 2 policy governs whether or not step 2 statics is calculated. This is a global setting which applies to all lines in the model.

Seabed friction policy

This determines whether or not to include the effects of seabed friction on a line in the statics analysis and, if so, then how to go about applying this friction. There are three options:

None: no seabed friction will be applied to the line during the statics analysis.
As laid: with friction included there is not, in general, a unique static position for the line, since the position it adopts depends on how it was originally laid and its history since then. If friction is included then, in order to define a unique solution, we must make some assumptions about how the line was originally laid; the frictional force then acts towards this originally laid position. If the step 1 statics method is prescribed, the originally laid position is taken to be that defined by the prescribed track; if the step 1 statics method is not prescribed, the originally laid position is defined by the lay azimuth and as laid tension values.
Restart: this option is only valid when restarting from a parent simulation in which the current line was also present. In this case, the parent simulation provides a history from which to initialise the friction calculation.

Note:

The seabed friction policy applies to all sources of seabed tangential resistance, not just friction. In particular, it also applies to user-specified seabed resistance profiles.

Lay azimuth

This value is only used in the static analysis when seabed friction is included 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 towards which friction acts. We assume about this originally laid position that:

The line was originally laid 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).
The line was then laid in the lay azimuth direction, leading away from the bottom end position and with the given as laid tension.
The line was laid following the profile of the seabed.
The top end was then moved slowly from that original position to its given position.

To help you set this data item, the popup menu for lay azimuth has a set lay azimuth option. Clicking this sets the lay azimuth value to be the direction from the bottom end towards the top end, based on their current positions.

Notes:	Whilst OrcaFlex will accept any value for lay azimuth, the statics convergence routine will 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 end at $X{=}0$, $Y{=}0$, and anchor at $X{=}100$, $Y{=}0$, we would expect trouble for a lay azimuth of 90°.
	The line setup wizard also uses the lay azimuth direction.
	Lines that use the analytic catenary representation also use the lay azimuth direction, to determine an effective seabed slope.

As laid tension

This data item defines the effective tension with which the line was originally laid. OrcaFlex uses this to determine the as-laid node positions towards which friction acts in the static analysis. This value is therefore only used if seabed friction is included.

If the step 1 statics method is prescribed, 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 value is used as described under lay azimuth above.