Pipeline Free Span Mitigation

Pipeline free span is a condition when a pipe segment is not supported by the seabed. This condition can be happened because of some reason such uneven seabed, pipeline crossing, tie-in to subsea structures, sleeper, soil scouring, sand waves, etc. Free span condition causes problems in both static and dynamic aspects. If the free span length is too long, the pipe will be over-stressed by the weight of the pipe plus its contents. The drag force due to near-bottom current also contributes to the static load. To mitigate the static span problem, mid-span supports, such as mechanical legs or sand-cement bags/mattresses, can be used.

Source: https://nonerieska.files.wordpress.com/2013/01/post8-7.jpg

Free spans are also subject to dynamic motions induced by current, which is referred to as a vortex induced vibration (VIV). The vibration starts when the vortex shedding frequency is close to the natural frequency of the pipe span. As the pipe natural frequency is increased, by reducing the span length, the VIV will be diminished and eliminated. Adding VIV suppression devices, such as strakes or hydrofoils, can also prevent the pipe from vibrating under certain conditions. The VIV is an issue even in the deepwater field since there exists severe near-bottom loop currents.

When a free span is not acceptable, mitigating solution should be done. Based on soil conditions, water depth, and span height from the seabed, the appropriate method should be selected. If the span off-bottom height is relatively low, say less than 1 m (3 ft), sand-cement bags or mattresses are recommended. If the span off-bottom height is greater than 1 m (3 ft), clamp-on supports with telescoping legs or auger screw legs are more practical. The table below shows the free span mitigation methods. 

Flexible, articulated mat used for undersea pipeline crossing

Source: http://www.submarwestafrica.com/images/Submar_West_Africa_Mats_ROV_Crossing.png

Clamp-on Supports with Telescoping Legs

Source: http://www.copedi.com.br/media/14738/copedi_-_design_of_subsea_rigid_pipelines_-_the_free_span_problem.pdf

Clamp-on Supports with Auger Screw Legs

Source: http://www.jylpipeline.com/UKC2002.pdf

Alteration of Seabed Terrain (picture showing before and after cutting down high ground elevation using ROV or trencher)

Source: http://www.jylpipeline.com/UKC2002.pdf

Buoyancy module or strakes

Source: http://www.jylpipeline.com/UKC2002.pdf

VIV Suppressions

Source: http://www.trelleborg.com

Rock Dumping

Source: http://www.nordnes.nl/rockinstallation/freespan_correction.jpg

Sources:

http://www.jylpipeline.com/UKC2002.pdf

http://www.copedi.com.br/media/14738/copedi_-_design_of_subsea_rigid_pipelines_-_the_free_span_problem.pdf