Free span analysis

Pat Oakley

February 1, 2013

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On a separate but related theme, the Umbilical Manufacturer’s Federation has recently published a guidance note on the subject of free span analysis.

The term ‘free span’ is used to refer to the scenario whereby a section of installed static umbilical is unsupported along its length on the seabed 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 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, eg 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.

The guidance note describes the typical process employed when conducting free span analysis on umbilicals, and describes the primary and secondary parameters utilised in the analysis. The document is intended as a free reference source for all engineers involved in umbilical system specification, design and installation, and can be downloaded from www.umf.as/downloads

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