Actual external supports
Sometimes external supports are applied at buried pipelines. Anchor points are applied under circumstances, but one should always be aware of the great forces that may result from such fixed points. In general it is advised to leave the pipeline as free as possible in its ground environment. But in some configurations the pipeline runs above ground over a series of sleepers, that in this case act as external support.
There is a linear elastic support facility consisting of translational and rotational elastic springs in the main directions (in the Local or Global coordinate system) and a so-called 'Table' support consisting of a vertical elastic spring connected to a horizontal 'table' on which the pipeline rests. The table may provide friction depending on the vertical support reaction.
Virtual end supports
In general only a smaller section of the pipeline will be analysed. This means the model has to be limited to that part without the end points having influence on the behaviour of the section to be analysed. Beyond the artificial model end point the pipeline will continue. In that case to this end point 'a half infinite long' pipeline is connected. Condition to this end point is that the displacements of the connection (end) node remain within the elastic range of the soil model. The direction of this 'half infinite long pipeline' is the same as the adjacent element the end point is connected to. In fact the end point has the same properties as the end point of a 'half infinite long elastically supported beam' (Hetényi, Beams on elastic foundation, 1946), where the elastic support equals the initial soil stiffnesses (equal to the connected end node).
It should be kept in mind, that lateral displacements and accompanying bending moments damp out quite quickly, but longitudinal displacements and rotations may travel quite a distance, especially if the soil friction is low, as is the case for instance for offshore pipelines laying on the sea bottom.
Special end supports
It may also happen that the end of the pipeline is connected to a rigid point or ends in a cap and thus has no artificial external support, or the end of the pipeline may be connected to an elastically reacting structure.
In the first case the pipeline is rigidly connected:
In case the pipeline just ends, for instance a jacket tube, the end point is free to displace:
In case the end point is connected to an object with elastic properties, the end point shall be defined as "FREE" with an elastic support attached.
ExternalSupport, last changed: 14/09/2016