Model - Specifying Advanced Options
When you click on the Advanced button located below the left pane of the Model component, the Advanced Options dialog is displayed, as shown in the figure below. The first input in this dialog is called the Schematic Solver Tolerance and its significance is outlined over the next paragraph.
The Model component is built around a simplified spring-mass solver that continually recalculates the model profile as you progressively add components. The Model component solver is iterative. In some situations, the static configuration is not satisfactory and the Model component needs to allow the configuration to settle for a longer period of time in order to determine the true static configuration. To achieve this, you can reduce the value of the Schematic Solver Tolerance input. However, under normal circumstances you should not need to modify this parameter.
The Max Strain Iterations, SCP Adjustment Increment, Shallow Water Ratio and Strain Threshold options on the dialog relate to the automatic calculation of seabed connection point locations. Typically when such locations are calculated by the Model component they are based on ensuring that the angle of the attached pipeline as it leaves the stinger is approximately equal to the angle of the stinger tip itself. Such an approach is fine for deep and moderate water depths; however for shallow water scenarios this approach can result in the touchdown point being positioned too close to the vessel, which in turn induces significant bending and compression in the pipeline sagbend that may lead to convergence difficulties in the initial static analysis. To help overcome such problems, a second calculation method is available to the user. This method is based on ensuring that a certain threshold for bending strain is maintained in the pipeline sagbend when determining where to position the seabed connection point. A general outline of the alternative method is provided over the points below. These points are from the point of view of what the solver does in its calculation.
1.Initially position the seabed connection point as per the typical calculation approach.
2.Decide if the alternative calculation method is needed by seeing if the ratio of the stinger tip depth to the water depth is below the defined Shallow Water Ratio. If it is then proceed with the new calculation steps. If it isn’t then just accept the initial position.
3.Approximate the governing bending strain in the pipeline sagbend and check if it exceeds the specified Strain Threshold. If it doesn’t then accept the seabed connection point position. If it does then more iteration steps are necessary.
4.Move the seabed connection point iteratively away from the vessel, using the SCP Adjustment Increment input while continually checking the governing strain. Repeat until either the Strain Threshold is achieved or the Max Strain Iterations is reached.
It is worth noting that the default values for the inputs in the Advanced Options dialog are sufficient for the majority of scenarios considered by PipeLay and so they should only need changing in certain circumstances.
The final advanced option is Solver Contact Diameter. This is another option for improving the robustness of certain models by providing the ability to assign a uniform initial contact diameter to every component over the vessel and stinger supports in the Model component. This uniform diameter can be based on a representative value for all component diameters along the vessel and stinger region, namely the Average, Weighted Average (accounting for relative component lengths), Minimum or Maximum. Note that the uniform diameter only relates to the Model as the actual individual component diameters take effect during static analysis. Such initial uniformity helps keep everything aligned prior to analysis, which in turn can improve stability in some circumstances.
The model Solver Contact Diameter applies to all related Installation Stages defined on respective Analysis components. If no selection is made then the Individual component diameters are used by default, as per previous versions.
The use of a uniform contact diameter in the Model component is beneficial for scenarios where there are sudden changes in actual diameter across relatively short sections of the pipeline, for example in the area of flanges. If individual diameters are used in such scenarios then local jumps and stretches of the line profile appear at support locations in the Model and these in turn can lead to convergence difficulties, plus inaccuracies in the calculated section lengths. The use of a uniform diameter in these circumstances, in particular the Minimum one, helps keep all sections aligned in the Model initially before the actual diameters can take effect during the initial static analysis. The significance of using a Minimum diameter is that all contact nodes are either resting on or below the support surfaces initially and so are pushed upwards statically during the initial analysis step when the actual larger diameters are introduced. This push up effect is more robust than having nodes fall down on contact surfaces during static analysis.