Line - Stack Up
You use the Stack Up tab to specify the details of a line stack up. You can add Line Sub-components that are currently defined in the project to the line stack up. You can also add Connections to the line and change the Line Component Type. A sample line stack up is shown in the figure below.
Sample Line Stack Up
To add components to the stack up, you select the component type from the Component Type drop-down list, and then click on the Add Component button. A dialog is then displayed which allows you to select a component of the requested type from all of those defined in the project. You can also select All from the Component Type drop-down list to view a list of all of the Line subcomponents that are defined in the project. You can then select the Line subcomponents that you want to add to the stack up. In the case of Pipe Section and Cable components, you are also asked to specify the length of pipe section or cable to be added to the stack up.
When you add a component to the stack up, the list of components that is displayed in the main part of the tab is updated. The values in the columns other than the Section Name and Length columns are not user inputs. Instead these values are calculated from the component properties and displayed in this list to provide a quick visual check on your specification. Specifically, PipeLay echoes the following component properties:
| Accumulated Length | Bare Weight | Coating Weight | Weight in Air | Weight in Water | Cross-Sectional Area | Moment of Inertia | Bending Stiffness | Specific Gravity |
Depending on the component, some or all of these outputs may not be applicable, in which case n/a is indicated. For example, a Spring component would have none of the values listed above, other than having a certain accumulated length. Since the amount of verification data is too much to display at one time, a horizontal scroll bar is provided to allow you to view all of the values.
To change the properties of a component that is already in the stack up, you select the component in the list, and click on the Component Properties button. The Component Properties button is only enabled when the currently selected component in the stack up is a pipe section, cable or spring. The Component Properties dialog allows you to set the length of the selected component. Double-clicking on a component in the stack up will also open the Component Properties dialog.
When you add a component to the stack up, the new component appears above the currently selected component in the stack up list. You can use the Up () and Down () buttons on the right-hand side of the component view to move a component upwards or downwards in the stack up list. There is also an Invert button to completely reverse the order of the stack-up list. To remove a component from the stack up, you select the component in the stack up list, and click on the Remove Component button. There is an option to duplicate the entire stack-up by clicking the Duplicate button from which a number of copies of the stack-up can be specified by the user. These duplications are then generated and displayed in the stack-up list.
In the Model component, you build up a model by first defining connection points such as Fixed Connection Points, Vessel Connection Points, and Seabed Connection Points and then insert components between these connections, typically Line, Pipe Section or Cable components. However, for complete flexibility in defining installation scenarios, some further options are required for defining connections. To access these additional options, you click on the Connections button in the Stack Up tab of the Line component.
An example of the Connections definition is shown in the figure below where a cable is inserted between a vessel connection point (VCP 2) and an internal connection point defined on the line (W.C.1). The W.C.1 connection is not defined in the Model component; but instead it is specified as part of the Line component, using the Connections – Define dialog.
Sample Model with Internal Connection
The Connections – Define dialog is shown in the figure below.
Connections- Define Dialog
You specify a unique name for each connection in the Name column. You specify the location of the connection in terms of length along the line in the Distance along Line column. You must also associate a Connection component with a connection by selecting a component in the Component column. This allows you to assign weight in air and weight in water values to the connection and also to specify how the connection is modelled, that is whether the connection is free, hinged with non-zero stiffness, or clamped.
The Stack Up tab also contains a drop-down list that enables you to select the line component type. The options available are Standard, Payout or Active Length. The default is Standard.
The Standard option meshes the Line component as normal. The Payout option specifies an initial length of pipe section which is meshed in the normal sense. The remaining section of the Line is meshed using “inactive” elements, which are activated during subsequent pay out installation stages when additional pipeline is added. If you select Payout, you can click on the Payout Properties button to specify the initial length of the pipe section. This Payout option is designed specifically to analyse the self-installation of pipelines and risers from a floating production unit (FPU) and so it could prove problematic applying the option to more general installation scenarios such as S-lay, J-lay, reel lay, etc. For such scenarios the Active Length option may be a better method of modelling actual pay out.
The Active Length option allows you to specify a region along the line which can actively change length during an analysis in accordance with relevant inputs. If you select Active Length, the Active Properties and Timetrace File buttons become enabled. When you click on the Active Properties button the corresponding dialog is displayed, as shown in the figure below. This dialog allows you to position the start of the active length region at a suitable distance along the line, to specify the initial length of the elements in the region as well as the initial length of the region itself, and finally to define the allowable length limits to be applied to the elements during subsequent line length change analyses. The significance of these limits inputs will be explained shortly.
Active Properties Dialog
Pressing the Timetrace File button allows you to select an ASCII file containing a time history of line length change, via a standard Windows Open dialog. The time history in the file is applied to the active length region during a dynamic analysis in accordance with the procedure outlined over the next few paragraphs.
An active line length change can be specified for either a static restart analysis or a dynamic analysis. As alluded to previously, the length change applied during a dynamic analysis comes from a suitable timetrace file, while for a static restart the change is defined directly on the Analysis component, as discussed in a separate article. When a length change is encountered during a particular solution step in an analysis how it is applied is dependent on whether it is a positive change (increase) or a negative change (decrease).
In the case of a positive length change, the length of the active length region is increased by:
1.Cycling through the active region element set and identifying the lowest element which has not yet reached the Max. Active Region Element Length limit.
2.Increasing the length of the selected element until either the overall length change is achieved or the Max. Active Region Element Length limit is reached. In the case of the latter, the steps are repeated again with an element above the selected element considered for an increase in length. Note that should all elements in the active region reach the maximum length threshold then no further length increases will be allowed. This should be an unlikely scenario provided the model and active region are set up correctly.
In the case of a negative length change, the length of the active length region is decreased using a similar procedure as that of a positive length change; however in this instance the highest element that has not yet reached the Min. Active Region Element Length limit is considered.
The telescopic length change behaviour outlined here is designed to keep the changes in element lengths isolated to a specific part of the line even when the overall change required is considerable. Such isolation is desirable when you have areas of relatively high curvature and/or intermittent contact with support or seabed surfaces, which in turn may have frictional effects included. Changes in element lengths in these areas could have a negative impact on solution accuracy, however provided the active length region is positioned correctly the adopted telescopic length change scheme ensures that the necessary element lengths changes are kept away from the specific problem areas.
To elaborate further on what has been discussed above it is best to use a particular example. Say if you wish to simulate pay out in a normal lay scenario where changes in element lengths are unwanted in both the overbend and sagbend areas. In this particular example the active length region should be positioned in the area of inflection between the overbend and sagbend as it is free of contact and low on curvature. This positioning coupled with suitable initial and maximum element length inputs will ensure that the telescopic length change procedure will achieve the full level of payout without adversely affecting solution in the overbend and sagbend. Note the correct positioning of the active length region is somewhat aided by the user interface which prevents you from inappropriately placing the region inside any defined stinger and/or touchdown meshes.
The Active Length option is a powerful feature as it allows you to consider the lay process in a continuous fashion rather than in terms of discrete snapshots, especially when combined with suitable vessel motions. Some possible applications of the option include:
▪Assessments of friction build up on seabed during installation.
▪Laying around curves.
▪High velocity reel lay.
▪Cable winching during abandonment or recovery.
▪Detailed modelling of laying over scarps and embankments.
▪Application of actual offshore project recordings, or time histories, to PipeLay models.
▪Continuous simulation of the installation of structures, such as PLETs, PLEMs, ITAs etc.