Fatigue Free Span Analysis

Definition

This section defines typically what free span and free span analysis involves, and also how they specifically relate to umbilical designs. Typically DNV RP F105 is used for free span analysis. The term ‘free span’ is used to refer to the scenario whereby a section of static umbilical is unsupported along its length for a distance, typically greater than 3m.

If the lay route includes free spans longer than the critical length then the umbilical may be subject to Vortex Induced Vibration (VIV) possibly leading to one or more of the following: fatigue, wear, curvature and infringement of local structural capacity at span shoulder.

The reasons for the umbilical being unsupported can be various, e.g. local seabed topology, scouring of the soil from underneath the umbilical, the physical configuration of the subsea terminations, spans between I-tube and seabed, and at pipeline crossings. Umbilicals are significantly less sensitive to the effects of free span compared with rigid pipelines. The reasons are:

• The smaller relative diameter of the umbilical
• The lower bending and axial stiffness of umbilicals compared with pipelines
• Umbilical structural damping characteristics are significantly greater than rigid pipelines

The Free Spanning Analysis (S105) module provides preliminary fatigue screening of observed submarine pipeline span and calculates the allowable spans due to Vortex Induced Vibration (VIV), Fatigue and Ultimate Limit State (ULS), in accordance with DNV-RP-F105 (2006). Following installation or during annual survey of pipeline, spans may be detected between high points on the seabed. The high spots may be natural seabed irregularities, as a result of hard soils encountered during trenching or as a result of scour or sand waves.  Pipeline spans are prone to overstress due to self-weight and hydrodynamic loading, and vibrations induced by vortex shedding from the span.

Static loading on the span is produced by a combination of the self weight of the pipe and horizontal hydrodynamic loads. Although hydrodynamic loads with an oscillatory wave component are strictly dynamic loads, they are usually treated as static loads because the natural frequency of typical pipeline spans is often greater than the frequency of the wave.

Vibrations of the span are caused by periodic vortex shedding. Each vortex shed induces a reaction impulse and consequently a deflection in the pipeline. Where vortices are shed coincidentally, the impulse components perpendicular to the direction of the current cancel and the resultant oscillations are in line with the current. Where the vortices alternate, perpendicular components do not cancel and the resultant oscillations are predominantly perpendicular to the flow direction ('cross flow') but still have some in line component. If the frequency of oscillation matches the natural frequency of the pipeline span, resonance or "lock-on" occurs, resulting in high amplitude oscillations and possible damage to the pipeline or its coating.

The S105 module assesses spans for VIV fatigue onset (VIV fatigue screening criteria), direct wave loading (simplified direct wave fatigue) and ultimate limit states (ULS).

The module S105 is intended for preliminary allowable span evaluation of submarine pipeline span. The module performs the analysis by two distinct calculations as follows:

Provide data for screening of observed span length 

Evaluate allowable span length

The loading on the span includes the self weight, buoyancy and maximum steady state hydrodynamic loading.

Analysis with S105 allows for variation in water depth, current and wave velocity profile data along the pipeline length.

References

• DNV RP F105
• UMF document: “UMF – GN02, Glossary of Terms and Abbreviated Terms for use with Umbilical Systems”
• http://www.penspen.com/Services/Software/Modules/Pages/S105.aspx