The riser is made up of three sections with the same properties but different segmentation. Look at the segmentation for the line on the ‘Structure’ page. Double click 'Catenary Hose' on the model browser to open its Line Data Form. In this example the area of interest is the touchdown point where curvature is expected to be high. Where the detail is required will depend on what you are modelling. It is best to put detailed segmentation only where it is needed. You want sufficient refinement that the model can capture the response accurately but not so much that runs are slow with no significant improvement in results.
Identifying the line segmentation required is a balancing act. A more detailed explanation is in ‘System Modelling: Data and Results | Lines | Line Ends | End Force and End Force Ez-Angle’ in OrcaFlex Help. To obtain the correct design loads therefore the no-moment direction Also Bend Stiffener design loads need End Force and End Force Ez-Angle, its direction relative to that no-moment direction. The model is then set up ready in case you change to a built-in connection (non-zero connection stiffness) at a later stage. However it is good practice always to set the line End Orientations.
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This means the connection is free to rotate and no end moments will be produced so changing the end orientations will not change the behaviour of the system. In this example, the riser end connections are pinned (zero connection stiffness). the local z-axis direction on which the no-moment direction is based) is into the line at End A and out of the line at End B. The key point is that OrcaFlex adopts an ‘End A to End B’ convention. You can view the local axes, including the line end orientations, by selecting ‘Axes | Local Axes’ in the View dropdown menu (or use the shortcut Ctrl + Y). See an explanation of this in ‘System Modelling: Data and Results | Lines | Line Ends | No-moment Direction’ of OrcaFlex Help. It is therefore called the No-moment direction. If the riser is aligned with that direction at the end fitting then no moment is produced. These specify the directions of the end fittings. The End Orientations have also been set in this model. More details are in ‘System Modelling: Data and Results | Lines | Line Data | Connections’ in OrcaFlex Help.
If you set ‘Height above seabed’ to zero then OrcaFlex offsets the end connection z coordinate by the line contact radius so the bottom edge of the line sits flush to the seabed. If the connection z coordinate was set at zero (centreline directly on the seabed) then the line bottom edge would be placed one radius beneath the seabed. This diameter is found in the Contact page of the Line Types Data Form (note that a line contact diameter of “~” means that the line outer diameter is used as the contact diameter). Seabed contact takes account of the line contact diameter. The Anchored option gives access to an additional data item called ‘Height above seabed’ which is important to get the correct seabed interaction at that connection. The Connection table shows End A is attached to the vessel ‘FPSO’ and End B is Anchored. The lower is a range graph of normal drag coefficient along the catenary length.īuilding the model Open the model browser via the Model dropdown menu (or use the shortcut F6) then double click on ‘Catenary Hose’ to open its Line Data Form. The upper graph is of top connection End Force against End Force Ez-Angle. When you open the simulation file you will see side elevation views of the catenary in both wireframe and shaded format. The example has a single riser in a simple catenary extending from a vessel to the seabed.
Production Risers: A01 Catenary and Wave Systems This is sufficient for most systems of this type to achieve a settled response. The main analysis (Stage 1) is given a duration of five wave cycles. Less than this and the rapid application of the wave could generate unwanted transient loads. The build up (Stage 0), where wave heights are ramped up from zero to the required value, is given a duration of one wave cycle. The Dean Stream non-linear wave theory has been applied because it is accurate over a wide range of water depths. These are repeated waves with the same height and period. Modelling tethers and their clamps.Īll the examples in this set have regular waves applied. Variation of drag coefficient with Reynolds Number. On opening each simulation file, the default Workspace will present wireframe and shaded graphics views of the system. These are a simple catenary, a lazy wave, a steep wave and a pliant wave. A01 Catenary and Wave Systems Introduction In these examples, four types of riser system are shown.