Supports: Modelling

The supports facility enables you to model contact between lines and sets of rigid cylinders, called supports, fixed to vessels or 6D buoys (the supports owner). In the main, the supports feature has been designed to facilitate the efficient modelling of pipe lay stingers, but it is also useful in many other applications where contact is important.

Supports contact model

Supports and support cylinders

A single support is composed of one or more support cylinders. The number, geometry and physical properties of the support cylinders comprising a particular support are defined by the support type. The cylinder axis of each support cylinder lies on the support cylinder $z$-axis and in the support axes $zy$-plane; the support cylinder $z$-axis orientation within this plane is defined by the support type. The support cylinder $x$-axis is always parallel to the support's $x$-axis. The whole support cylinder geometry is symmetric about the support's $z$-axis.

Support cylinders are drawn with their support type's finite length, but for the purpose of contact they are treated as being of infinite extent in the cylinder's axial dimension. The support results support off-end contact distance and max support off-end contact distance can be used to check that there is no contact beyond the support type length. This infinite contact length, in conjunction with the desired-side property, makes the convergence of the model to the desired equilibrium very robust and efficient. The figure below illustrates a support whose support type has V-shaped geometry.

Figure: An example of a V-shaped support.

Support cylinder desired side

For the purposes of calculating contact penetration, the support cylinders are treated as more than simple cylinders: each has a desired side associated with it. The desired side is defined, with respect to support cylinder axes, as the region of space above the $xz$-plane in the positive $y$-axis direction. To calculate the contact penetration between a supported line and a support cylinder, a spline is associated with the supported line, in the same way that splines are used in modelling line contact. OrcaFlex calculates the closest approach between the supported line spline and the support cylinder axis. If the point of closest approach on the spline is on the support cylinder's desired side then the contact is handled just as if the support cylinder was a simple cylinder, as shown in the below figure:

Figure: Supported line contact, closest approach on the desired side

However, if the point of closest approach on the spline is not on the support cylinder's desired side then the supported line is assumed to have penetrated to its position through the support cylinder, as shown in the below figure:

Figure: Supported line contact, closest approach not on the desired side

If, then, the point of closest approach on the spline is not on the support cylinder's desired side, and the contact stiffness is not zero, the supported line will feel a reaction force along the line of penetration, even though it appears that the line is not actually in contact with the support cylinder. Such a case is depicted in the above figure.

The support cylinder's desired side is a very useful property. It defines the side of the support cylinder on which the supported line should rest, and so directs static convergence towards your desired equilibrium position. The supports and support cylinders should therefore be arranged such that the support cylinder $y$-axes point in the direction of the expected reaction forces between the supported line and the support cylinders.

Calculating contact force

For each support cylinder and supported line pair, OrcaFlex will calculate only a single point of closest approach. This means that a supported line can contact each support cylinder only once: the supported line cannot be doubled back and laid over a support cylinder multiple times.

Beyond the ends of the supported line, the spline is extrapolated as a straight line in the end nodes' $z$-axis directions. If the point of closest approach on the spline is within the length of the supported line, the line will feel a reaction force equal to the penetration depth multiplied by the contact stiffness. If however the point of closest approach on the spline is outwith the supported line, then the support cylinder and supported line are not in contact and there is no reaction force.

(To be completely precise, the support cylinder and supported line are deemed to be not in contact if the point of closest approach is beyond either end of the line by a distance of more than one support cylinder radius. If this point is outside the supported line length by less than one support cylinder radius, then the contact force is linearly scaled from zero up to the full value over this distance.)

The supports contact model shares many similarities with the line contact model. The major difference is that the supports contact model applies to contact between rigid support cylinders and supported lines, rather than contact between pairs of lines.

Note: The line contact documentation presents details of a situation in which spline contact load distribution may sometimes cause difficult simulation convergence. The same situation can affect supported lines, so we refer the reader to the line contact modelling page for more information.

Defining support geometry

The support cylinder geometry is defined, with respect to the support, by the support type. The position of the supports is defined using the supports data on the owner's (vessel or 6D buoy) data form, using either the simple or explicit geometry specification method.

Using simple geometry

The simple geometry specification is a quick way to set up a basic stinger model, with the supports positioned along a user-defined support path. The support arrangements that can be modelled using the simple method are limited, because individual support orientations cannot be independently specified and the support positions are restricted to the plane of the support path.

To generate a supports model using the simple geometry method, first a support path must be defined. The path starts at the support path origin (defined with respect to the owner's axes), and is initially in the direction of the support path origin's $x$-axis.

The continuation of the support path is defined by the support path sections table. Each row of the table defines a section of the support path by its length and bend radius. These sections are added to the path, starting from the support path origin, in the order they are given in the table. The support path $x$ and $z$ directions, initially aligned with the support path origin axes, track with the path curvature.

Having defined the supports path, the supports themselves are placed along it according to the supports table, which defines their arc length along the path and their z offset from it in the path $z$ direction. For the purposes of reporting results, supports are referred to by their number in the supports table.

Figure: Support placement using the simple geometry specification.

Using explicit geometry

The explicit geometry specification gives you full control over the positions and orientations of individual supports. These are defined with respect to user-defined coordinate systems for complete flexibility: multiple supports can be associated with a single coordinate system, allowing for simple grouping of supports into frames which may then be moved or rotated en bloc.

User-defined coordinate systems are specified with respect to the owner's axes in the coordinate system table. Supports are then added to the support coordinates table, each of which nominates one of these coordinate systems and defines the support position and orientation with respect to this coordinate system.

Figure: Support placement using the explicit geometry specification.