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Home > Model Components > Lines > Drilling Risers > Structure

Drilling risers structure

The Structure tab in the drilling riser properties window is composed of three main areas: Sections, Primary points of section, and Composition of segments.

Sections

A new line is initially composed of a single section. Then it can be split into several sections, specifying the position of the intermediate created nodes. (Two sections can also be merged into one, giving complete flexibility in the definition of key points in your model.) Line sections may be split in several sections to introduce several intermediate primary points at once, by using the advanced split feature.

Note

The main interest of splitting a line is to create intermediate nodes to adjust the line shape or to connect them to other model components. However if intermediate nodes are not used, it is preferable to divide one section into several segments instead of creating several sections. Indeed it is much easier to navigate between different segments than between different sections. In particular, the segments list summary of one section can be copied/pasted to an Excel spreadsheet for modification.

Shape: Unlike the generic lines, the shape of each section in a drilling riser cannot be changed and is set to straight.

The number of sections and the line total length are reported at the top of the Structure tab.

Primary points of section

First select the section you wish to edit in the Sections table. Then in the Primary points of section area, enter the positions (in the global coordinate system) of the two section end points. For each section, the primary points of section and the composition of segments have to be defined. Different linked to connections can be defined: free, anchored, or connected (to existing structures, including the Sea & ground); in this last case, the attach point field allows the user to link the line end to one of the existing points on a structure.

Unless the free option is selected, different connection type can be imposed: Pin, Clamp, User defined (with the six degrees of freedom independently chosen free or constraint or clamped), Flexjoint or Spring .

Unless the pin option is selected, elevation and azimuth departure angles must be specified in the impose neutral axis direction fields. Carefully check these imposed departure angles (ENCASD & PSID) at the connected point, to ensure the direction matches the line. See more details...

Warning

Clamp connections may be defined at line ends by specifying the departure angle through its azimuth and elevation. This determines the departure direction only (i.e. tangent direction only but not the 2 principal bending directions associated with the cross-section at the connection point), and the FE engine automatically sets-up the local reference frame associated with the line at the initial stages of the calculation. Initial torsion in the line may arise from inappropriate initial torsion constraints at line ends.

An improved initialization technique is used by default for torsional degrees of freedom along the line which avoids inducing some fictitious torsion in the line.

Another initialization technique allowing prescribing the directions of the principal bending axes (and therefore initial torsion) at line ends can be invoked by ticking the checkbox beside the edit boxes used to input the azimuth and elevation angle.

This makes no difference for the vast majority of cases. By default, it is recommended not to check the "Also impose bending axes" option when defining clamp connections.

Composition of segments

In this part of the window, each selected section can be further divided into segments, and the properties of each segment are defined through drill joint segment type.

Initially there is one segment representing the section selected. The add button will copy the selected segment in the table; the new segment will be added as item 1 in the list, pushing the existing segments down. New segments are assigned a name automatically but this can be edited directly in the table. It is recommended to give segments short but recognisable names.

Alternatively the field seg. nb can be used to set the required number of segments in one quick step.

After creating the number of segments you require, it is necessary to set their length and other properties:

  • Number of joint: This sets the number of joints composing the segment (must be an integer value). The length of the segment is determined from the number of joints and the length of individual joints which is defined in the drill joint segment type.

  • Joint property: Each segment must be assigned an existing drill joint segment type which defines the structural properties to be considered for the line segment. The line segment type must already exist before it is assigned to a line segment. Therefore two short cut buttons are provided to create new drill joint or to edit drill joint property.

  • Partition: Within segments between defined end points, partitioning of the segment is possible. The partition number defines the number of FE element for load, displacement and stress calculations, and thus must be set by the user to achieve optimum balance between run-time (calculation resources) and analysis accuracy. The interest in results at particular regions in the model may require more partition points (and hence smaller elements). Beware that the ratio between the element length set for adjacent sections should never exceed 2. This may mean you need to add intermediate segments simply to create a gradual transition of element length throughout the model.

  • Length: The last column Length is not editable but is an echo of the unstretched length associated with the segment to act as a reminder. The length is derived through the number of joints and the length of individual joints which is defined in the drill joint segment type.