Installation Stages Tab - Upper Section
You type a stage number directly into the Stage Number input field. You do not have to define stages in ascending order by stage number; PipeLay automatically sorts them into ascending order regardless of your specification. You also do not have to assign continuous stage numbers to the installation stages. For example, PipeLay will accept stages numbered 1, 3, 5, 9, 11 and so on.
The Stage Type drop-down list allows you to define a stage as static or dynamic.
The Restart drop-down list enables you to specify if the current analysis stage restarts from a previous stage. The default is No – the stage is independent and stand-alone.
In a restart stage, the structure configuration at the end of the preceding stage becomes the starting configuration for the restart stage. New loads or displacements are ramped on and combined with those in the preceding stage, and PipeLay simulates the structure response to the combined loading.
The major application of this facility in PipeLay is in starting a dynamic analysis from a static configuration that the program has selected to satisfy user-defined criteria. It is important to be clear that a dynamic PipeLay analysis does not in general have to be a restart. In other programs of the Wood Group suite, for example Flexcom, the use of restarts for dynamic analyses is effectively mandatory, and the onus is very much on the Flexcom user to perform the individual static and dynamic steps one by one. This is not the case in PipeLay. If you do a standalone dynamic PipeLay stage, the program automatically picks the optimum sequence of individual static and dynamic steps to ensure the dynamic solution run optimally; no intervention on your part is required.
You can of course perform a dynamic stage in which you define, for example, tension and/or stress criteria. What happens in this case is that the criteria you define are satisfied in the initial static phase of the stage, and the program then computes the dynamic response of your model about this optimised static position. In some cases, it might be more convenient to define individual static and dynamic stages, and to input your criteria for the static stage(s) only. One application might be where you want to look at a range of dynamic stages with different wave and current combinations. If you don’t do the dynamic stages as restarts from an initial static, then you’ll have to define your installation criteria for every dynamic stage, and that might be tedious depending on the number of such stages.
If you define a stage as a restart, the Restart From drop-down list is enabled, and you select the stage number from which the stage should restart. By default, all stages restart from Stage 1.
Restart From Option
The controls shown in the figure below, relating to time, are enabled for all dynamic analyses.
PipeLay has two time-stepping algorithms for dynamic analyses, a fixed step and a variable step. You use the Time Step Time drop-down list on the left to choose between them. When you specify a fixed step analysis (the default), the PipeLay simulation proceeds from the specified start time to the specified end time using the fixed time step or increment you input. In a variable step analysis on the other hand, the choice of time step magnitude is made by the program based on a number of criteria. The time step is continuously monitored and varied as appropriate by the program within user-specified limits, to ensure a stable and convergent solution.
The format of the dialog you get by clicking on the Define button depends on your choice of time-stepping algorithm; time step fixed, time step variable or for a static analysis time fixed static. The entries are mostly self-explanatory. The Ramp Time and Ramp Type entries allow control over the build-up of dynamic loads and displacements. For example, wave loads in a regular wave analysis are typically ramped on over 1-2 wave period, and the solution then proceeds for a further 3-4 wave periods to achieve a steady state solution. Ramp Type can be Linear (the default) or Nonlinear; when a nonlinear ramp is specified, a half cosine ramp function is used.
The Current, Primary Wave and Secondary Wave drop-down lists allow you to associate current and wave data with individual stages by selecting Current & Wave components in the project. These inputs are optional, although the specification of environmental conditions is normal for most stages.
The Transfer Adjustments drop-down list allows you to automatically transfer the model adjustments made during the criteria analysis of a particular stage to all subsequent stages in the analysis. This input is optional and defaults to No, which means that no crtieria adjustments will be transferred from the stage in question to the remaining stages.
If you select Yes on the drop-down list for a stage, then an additional analysis step will be performed on all subsequent stages, in which the changes made during the criteria analysis of the selected stage are applied. Please note that this transfer process does not occur in the case where a Component Length is used as the Iterate On option for the criteria analysis.
The Automated Postprocessing, Tabular Output and Custom Postprocessing options allow you to determine the postprocessing that you want to perform for the individual stages. The Automated Postprocessing and Tabular Output options default to Yes to generate plotted and tabulated output. In the case of Tabular Output, there is a secondary option available, named Tabular Option, which allows you to select the format of the tabulated output; meaning whether it be Stress based or Strain based. The Custom Postprocessing option allows you to augment the standard postprocessing with specific processing of your own.
The Reset option allows you to instruct PipeLay that Parameter changes in a given stage are relative to the original model, rather than the previous stage. Normally a change to the model in one stage is carried forward to all subsequent stages, unless explicitly overridden. For example, if you change the length of a cable in Stage 1 from that specified in the Model component, that cable remains at its new length in Stage 2 and beyond unless you explicitly tell the program otherwise by either changing the length further or by requesting a reset, in which case it goes back to its original value.