A pipeline or cable laid on the seabed does not sit on a flat surface. It bridges high points and touches down between them, and wherever it is slightly proud of the bed, water accelerates through the gap underneath.
That acceleration excavates the material below the line. The gap enlarges, more flow goes through it, and the process runs. A free span develops: a length of line unsupported between two touchdown points.
Why the span grows
Three mechanisms working together:
Flow concentration. Any gap under the line is a preferential path. Flow through it is faster than the ambient current, and faster flow carries more material.
Vortex shedding. Current passing a cylindrical body sheds vortices alternately from each side. Those vortices are effective at lifting bed material near the line.
Span-end erosion. At the shoulders where the span meets the bed, flow is disturbed and locally intense. The touchdown points erode, and the span lengthens from both ends.
The process is self-reinforcing. A longer span exposes more line to the current, which increases the disturbance, which erodes the shoulders further.
Why a long span is a problem
Not, usually, because the line cannot carry its own weight over the gap. Pipelines are stiff and a bare span is normally well within structural capacity.
The problem is vortex-induced vibration.
Vortices shedding alternately from either side of the line apply an oscillating force. Every span has a natural frequency that depends on its length, mass, and end conditions. As the span lengthens, that natural frequency falls, and at some length it approaches the vortex shedding frequency for the prevailing current.
When they coincide, the span resonates. It oscillates, and it accumulates fatigue damage in the pipe wall or the cable armouring far faster than the design assumed.
This is why free spans have allowable lengths derived from fatigue analysis rather than strength, and why a span that looks harmless on a survey can be a scheduled intervention. It is also why the allowable length depends on the current regime: the same span in different conditions has a different assessment.
Secondary consequences: loss of thermal or mechanical support, abrasion where the line moves against the bed at the shoulders, and increased vulnerability to interaction with trawl gear or dropped objects.
Two different jobs
This distinction decides the product, and it is the one most often blurred:
Preventing a span from forming. The objective is that the seabed under and around the line stops moving. This is a scour control job. The treatment is armor that stabilises the bed and stays in contact with it as conditions change.
Correcting a span that exists. The objective is to physically support the line at a defined elevation and shorten the unsupported length. This is a support job, and it needs something that bears load: grout bags, mechanical supports, or engineered span correction systems.
Armor placed loosely under an existing span does not support it. Support material placed on a mobile bed does not stop the bed moving.
Rock bags vs grout bags subsea covers that boundary in detail, and it is the single most useful thing to be clear about before specifying anything.
Where bagged armor fits
For the prevention job, and for stabilising the seabed at the shoulders of a span so it stops lengthening.
The properties that matter:
It conforms to the bed. A rigid system on an irregular seabed spans the low points and leaves voids, which is the same problem the pipeline has. A conforming unit sits against what is actually there. Rock bags vs concrete mattresses covers that comparison.
It stays in contact as the bed changes. The seabed does not stop moving because protection was placed on it. Armor that follows the bed keeps covering it.
Placement is per unit, with a defined tolerance. Near a live asset, that matters. The answer bank covers how placement tolerances are established above a subsea cable or pipeline, and how safe separation is confirmed during placement.
Units can be repositioned or recovered. A bag outside tolerance is one bag to move, and the whole installation can be recovered at decommissioning.
Survey and documentation
Free span management is a survey-driven activity. What matters:
- Baseline as-laid survey, establishing where the line is and what the seabed looked like
- Periodic re-survey, since spans develop and lengthen over time
- Span registers, tracking length, gap height, and location against allowable criteria
- As-laid records of any protection placed, verified rather than assumed
The answer bank covers what offshore survey records belong in the final as-laid documentation, and it is worth agreeing that scope before a vessel sails rather than after.
The honest limit
Our offshore position is cable protection and subsea experience rather than a claim to have solved every duty in this field. When not to use rock bags sets out what our data does not cover, including that we hold no citable physical model test results for some conditions. On a span correction requiring engineered support at a specified elevation, a support product is the answer and we will say so.
Where to go next
- Rock bags vs grout bags subsea for support versus armor
- Rock bags vs concrete mattresses for conforming versus spanning
- Rock bags vs fall-pipe rock dumping for the bulk alternative
- Offshore and subsea applications for placement method
- Oil and gas and offshore wind for the sectors
- Subsea pipeline protection for a project example
Send route survey, span register, current data, and whether the objective is prevention or correction through the quote form.