Assessment of pipeline where determining the acceptability of pipeline freespan without effected the structural integrity, which was measure the allowable span of pipeline with respect to certain criteria as per below:
-Static stress cross flow vortex shedding for the maximum design wave data combined with steady current.
-In-line vortex shedding using steady current only (no wave induce current)
A lot of factor that contribute the pipeline span like
To maintain the consistency with other features in pipeline, span length based on static stress considerations are governed by:
-Selfweight and coatings
All method of pipeline spanning analysis is according to DnV 81 as follow:
Dynamic span is to check the allowable span for installation, hydrotest and operation under the steady state current conditions.
Analysis is according to DnV 81 which fluid flow passing a free span that cause unsteady flow patterns due to vortex shedding, which may lead to oscillations of the pipe. vortex shedding frequency may coincide with, or be a multiple of, the natural frequency of the pipe. Harmonic or sub-harmonic excitation of the free span may then result.
SYMETRIC VORTEX SHEDDING
WEAK ALTERNATE VORTEX SHEDDING
STRONG ALTERNATE VORTEX SHEDDING