Concrete mattresses are concrete blocks linked by ropes or cables into a flexible mat. They are widely used to protect and stabilize subsea pipelines and cables, and in some river and canal works.
Rock bags do similar work with natural rock in a net. They handle uneven seabeds and scour holes well, and they use rock that can often be sourced near the project.
Side by side
| Rock bags | Concrete mattresses | |
|---|---|---|
| Material | Graded rock or crushed concrete in a polyester net | Precast concrete blocks joined by rope or cable |
| Fit to the bed | Conforms to scour holes and irregular surfaces | Flexes along its joints; covers a flat or gently curved area |
| Handling | Single ring lift | Lifted on a frame or spreader beam |
| Manufacture | Filled near the project with local rock | Cast in a yard, then transported |
| Habitat | Gaps between rocks give shelter and collect sediment | Smooth concrete surfaces |
| Typical use | Scour aprons, toe protection, local cover | Pipeline and cable cover over a set length |
When rock bags are the better fit
- The area to protect is an existing scour hole or uneven bed
- Protection is needed at a few local points, such as around a foundation or at a J-tube
- Transport of heavy precast units to site is costly
- Habitat value is part of the design
When mattresses are the better fit
Rock bags and mattresses are also used together, with bags filling scour at the ends of a mattress run. See offshore wind and subsea protection.
- Long, even lengths of pipeline or cable need a defined cover thickness
- The design or the asset owner calls for a mattress system
Questions
- Can rock bags be used over a subsea pipeline?
- Yes, for stabilization, free-span support and local cover. The bag drapes over the pipe; confirm loads and cover requirements with the pipeline owner.
- Are rock bags easier to handle than mattresses?
- Each bag is lifted by a single ring, so no spreader frame is needed. Mattresses cover a larger area per lift.
- Can both be used on one project?
- Yes. Rock bags are often used at the edges and ends of mattress runs, where scour develops.
Published by EVOSNT Trading FZCO, manufacturer of EVO Rock Bags. Product figures are from our technical specifications. General guidance only; the project engineer is responsible for the final design.
