- 1. Seabed irregularities (unevenness) during
installation(residual tension on span creation is closely linked to
thepipe weight). Subsequent scouring(sand wave) and movement.Seabed
topography and composition (type of soil), waveand current action
and pipe properties.
2. Type of Span 3. End Condition Using in Free Span Fixed-
Pinned end condition may be assumed for single spans. Fixed- Fixed
may only be assumed if validated be observed support condition.
"Fixed/Pinned" in this case is assumed to be the average of
"Fixed/Fixed and "Pinned/Pinned" bending moments, on the basis that
the end fixities of a span are somewhere between the two cases but
it is difficult to determine exactly where. When calculating
permissible span lengths, the assumed end conditions have a large
impact on the results. The fixed/pinned assumption may not be
accurate when, for example, a pipeline spans between two rock
ridges. The support conditions might then be closer to
pinned/pinned; though the adjacent sections of pipe will provide
some restraint so that the pipe section is not truly pinned/pinned.
Analytically, it is only possible to accurately determine these
effects with the use of an advanced finite element analysis to
accurately model the span support conditions and axial effects. It
is obviously impractical to perform this type of analysis on every
span along the pipeline route. However, it may be possible to build
a "typical" FE model to determine the magnitude of these effects
and modify the limiting span criteria. 4. A. DNV-1981II. Criteria
for Span Condition Vortex Shedding Static Stress Induced Bar
Buckling Fatigue.Vibration: In Line; Cross Flow; 5. 1. Static
Stress Checked individual stress components, and the total combined
stress condition is also limited to maximum percentage of the
material SMYS (percentage is variable according to pipeline loading
condition).2. Vortex Induced Vibration VIV dependent upon the pipe
and span characteristics,fluid flow around a pipeline span can
result in vorticesoccurring on the wake side of the pipe. If
vortices areof sufficient frequency, they can produce
significantpipeline oscillations. The parameter assessment VIV is
Reduced Velocity(Vr ). 6. 0 < Vr < 2.2 symmetric vortex
shedding producing "In- Line" oscillations, i.e. parallel to fluid
flow. 2.2 < Vr < 3.5 alternate vortex shedding causing
"In-Line" oscillations (unstable); 7. 4.8 < Vr < 12.0
alternate vortex shedding causing CrossFlow" oscillations i.e.
perpendicular to fluid flow.3. Bar Buckling For a restrained
pipeline, the pressure andtemperature induced axial force
(compressive), ifof sufficient magnitude, may lead to beam
modebuckling of the pipeline 8. 4. Fatigue As mentioned previously,
vortex shedding induced span vibrations may be broadly divided into
two categories: In-line; Cross-flow. Cross flow vibrations by their
nature are almost always high amplitude and consequently their
occurrence should be avoided at all costs, while in-line vibrations
are generally of smaller amplitude and may be permissible. The
criteria for permitting in-line vibrations fall within assessment
of the pipeline fatigue and fatigue usage requirements. 9. III.
CalculationPermissible span lengths for a pipelineFor each of these
criteria the permissible are calculated based on each of thespan
length should generally be calculatedfollowing criteria:for each of
the following four load cases:Static stressEmpty Vortex shedding
(in-Water filledline vibrations) Vortexshedding (crossHydrotestflow
vibrations) Bar buckling. Operation 10. III. Calculation2. Static
stress Due to its self-weight and lateral hydrodynamicloading. The
combined stresses should be checked againstthe allowable levels of
stress given in the relevantcodes, i.e.is not to exceed
thepermissible value. .ep = usage factory as defined table below .F
= specified minimum yeild strength What are a and b ? 11. III.
Calculation Is function likes Operation ? How about Functional and
environmental ? 12. III. Calculation 13. III. Calculation 14. III.
Calculation 15. III. Calculation3. Vortex Sheddinga. Cross-Flow
Vortex Shedding 16. III. Calculation The Reduced Velocity (Vr )
parameter see figure below: 17. III. Calculationb. In-Line Vortex
Shedding Stability parameter is controlling the motion , KS 18.
III. Calculation Effective mass (me) is function of Ca (add
masscoefficient). Submerged Weight(Wsub) The relationship between
VR and the stabilityparameter, KS 1 4.7 and Ks