Pipeline freespans are at risk from damage by one of two distinct mechanisms: by excessive bending from externally applied hydrodynamic or self weight loading, or by long term fatigue damage, from flow induced vibrations.

The traditional means of ensuring that excessive bending deformation cannot occur is to define a maximum allowable freespan length such that the maximum equivalent stress in the freespan is less than an acceptable fraction of the pipeline steel's yield stress.

Traditionally, Vortex Shedding Induced Vibrations (VIV) of freespans is not allowed to occur at any time during the design life of a pipeline system. In recent years a less stringent approach has become acceptable, in which VIV has been allowed provided it is demonstrated that the allowable fatigue damage is not exceeded.

Spans that are found to be critical with respect to VIV are usually corrected by placing rock berms below the pipe in order to shorten the span lengths and thus increase the natural frequency of the spans. In addition to the cost implication of placing a large number of rock berms on the seabed, the main disadvantage of this approach is that feed in of expansion into the spans will be restricted.

Flowchart for freespan assesment procedure is described below:

Design criteria applicable to different environmental conditions have been defined as follow: