Model - Defining Connection Points
In order to add other components to the model, it is necessary to define connection points. There are three types of connection point as follows:
You typically use a fixed connection point to model a constraint at a (known) fixed point on the seabed. The location of the point is a required input for this connection type.
You use a vessel connection point to connect a line to a vessel.
You use a seabed connection point when you want to constrain a line on the seabed, but without specifying the actual location of the constraint on the seabed.
Adding new connection points is very similar to adding a vessel.
To add a fixed connection point, click on the Fixed Connection Points folder in the sub-component list and then click on Add, or else double-click on the Fixed Connection Points folder. This opens the Fixed Connection Point – Add dialog shown in the figure below, allowing you to define the connection point location and to specify the nature of the constraint (if any) you want to apply there.
Add Fixed Connection Point Dialog
Once you add a connection point, a default name is associated with it, but as usual you can change this if you wish. The X, Y and Z coordinates define the connection point location, in the global axes. You specify the nature of the constraint (boundary condition) applied at this connection via the DOF1-DOF6 entries. DOF in this context means ‘Degree Of Freedom’, as defined in Technical Note 4. Constraints or boundary conditions (BCs in shorthand) can be defined in either the global (XYZ) coordinate axes, or in an axis system you nominate; you specify which using the Axis System input. While this offers a good deal of flexibility and generality, specifying BCs at a fixed connection in any axis system other than the global would be unusual. Note that you must explicitly input a value (typically a zero) into any degree of freedom which you wish to constrain – if you leave an input blank, no constraint is applied in that degree of freedom.
You can use the DOF1-DOF6 entries in any combination to define any general constraint. But in reality your specification will frequently be one of two kinds, either i) pinned or ii) fully fixed (‘built-in’). A pinned constraint is modelled by inputting zero in the first three boxes (DOF1-DOF3) while leaving DOF4-DOF6 blank. For a built-in connection on the other hand, all six terms are set to zero. You can of course input a non-zero value for any DOF – in that case a constraint is applied in that DOF and the relevant displacement is applied as a static offset.
Adding a vessel connection point is performed in exactly the same way as adding a fixed connection point. See the ‘Adding a Fixed Connection Point’ above. The inputs on the Add Vessel Connection Point dialog are very similar, as shown in the figure below.
Add Vessel Connection Point Dialog
The first point to note here is that the connection point position is not specified in global co-ordinates but in vessel co-ordinates. In this system, the origin is the vessel origin, and the vessel local-Y axis is aligned with its surge axis. The figure below illustrates this relationship.
You will also notice that there are two additional inputs here, in addition to those provided in the Add Fixed Connection Point menu. The first is the vessel on which the connection point is positioned, and the second is how a connected line is to be attached. If Lay over Stinger is set to Yes, any components connected to this point are placed over the vessel stinger, if one is present.
Often in PipeLay the user needs to connect a line directly to a tensioner location or to the most forward vessel support in a model. In order to do this a vessel connection point is needed with exactly the same coordinates as the point on the centreline of the pipe, which sits directly over the location of the relevant tensioner or support. The manual creation of such a vessel connection point is often tedious and can be error prone in the case of sloping tensioners or supports. With this in mind, the user interface automatically generates appropriate vessel connection points at all tensioner locations in the model as well as at the most forward vessel supports. Note that the Vessel Connection Points folder in the sub-component list is populated automatically with such connection points, as soon as a relevant vessel, with one or more tensioners, is added to the model. The Properties dialog for these automatic vessel connection points is similar in layout to any other vessel connection point. However it is not possible to edit the calculated coordinates and indeed the Name, as they will be directly linked to the associated tensioner. As well as this, the Lay over Stinger input is hardcoded to Yes.
The coordinates for these generated vessel connection points are initially the same as those defined for the corresponding tensioner support on the relevant Vessel component. However as soon as something is connected to these points, their X and Y coordinates are re-calculated as follows:
•X of Connection Point = X of Tensioner or Support + (Rc * Cosφt)
•Y of Connection Point = Y of Tensioner or Support + (Rc * Sinφt)
Note that Rc refers to the contact radius of the component attached to the vessel connection point, while φt corresponds to the angle of the tensioner support whether it be input by the user or calculated by the interface.
Add Seabed Connection Point Dialog
Each seabed connection point must be assigned a unique name. PipeLay provides a default name of SCP(1) for the first seabed connection point, SCP(2) for the second, and so on, but you can override these names. You use the Calculate Position drop-down list to specify whether or not you want PipeLay to automatically calculate the seabed connection point based on the rest of the model data (such as the pipeline length and the location of the vessel connection point). The default of Yes is the normal mode of operation of this facility. However, you can change Calculate Position to No, and directly input values for X Coordinate, Y Coordinate, and Z Coordinate. The most common reason for doing this is where you want to modify slightly the X Coordinate, Y Coordinate, and Z Coordinate values that PipeLay generated automatically for you. You can also specify the nature of the constraint (boundary condition) applied at this connection in a similar fashion to the fixed connection point using the Axis System and DOF1-DOF6 entries. See ‘Adding a Fixed Connection Point’ above. Note that by default this connection type is treated as pinned (DOF1-DOF3 default to 0). Also refer to 'Model Best Practice' for the recommended specification of seabed connection points.