Specification of free displacement space around the pipeline (Input Table)
Through the input data table SLACK a free space is specified along the pipeline axis. Each line of the table represents a separate area, starting and ending at the specified start and end reference points in one of the available coordinate axes.
Nodes on the pipeline with a coordinate between or at start and end point are given the freedom to move without generating soil reactions within the specified SLACK space. SLACK areas may not overlap (in case two SLACK areas connect, specify the connection coordinate between two nodes).
In case the pipeline in a lateral direction does not touch the envelope of the SLACK space, no axial friction reaction will be generated. If the pipeline touches the envelope, the axial displacement at that moment is taken as a minimum friction slack, whether an X-SLACK has been specified or not. If the specified X-SLACK > calculated minimum slack, then the remaining difference is applied as X-SLACK from that point. The envelope of the SLACK space is a cylinder with elliptic cross-section, the elliptic cross-section composed of two half ellipses. The upper half with main axes 2*H-SLACK and 2*VT-SLACK, the lower half with main axes 2*H-SLACK and 2*VS-SLACK.
If VS-SLACK=0 at a node and the vertical component of the displacement vector at that node is directed downward or =0, then H-SLACK is taken =0 as well (no SLACK space). The same applies to VT-SLACK if the vertical component is upward or =0. In case H-SLACK=0 then VS- and VT-SLACK are taken =0.
If a vertical or horizontal 'slot' slack space is required, the slack spaces perpendicular shall be given a small value (but not =0).
In order to prevent unstable pipeline structures, the first soil iteration in Design Function 5 will always be performed without SLACK spaces.
The use of the SLACK space follows the behaviour of the K(..) and R(..) spaces, which means in case of degenerated K or R ellipses (ratio>20) the support axes are fixed to the horizontal and vertical direction. The ratio of axes of the SLACK space has no influence on this transition.
SLACK at an INFIN or FIXED specified START or END node is ignored.
H320202, last changed: 2009-07-16