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Preventing Under Scour of Concrete Mattress from Vessel Action
To ensure long-term scour protection, mattress protections must prevent under-scour at the mattress edges. Given the inherent variability in bed materials and vessel berthing actions, the edge detail of any scour protection method should facilitate practical maintenance and preserve the integrity of the protection.
This article will review the options for mattress edge protection and Proserve’s experience and industry guidance on each detail.
Laid flat on Seabed
Why does industry guidance advise that mattresses should be embedded into a trench at the edges?
Avoiding the additional dredging of a trench should make construction cheaper and simpler, so this can be an attractive option. The mattress edges are immediately vulnerable, relying on flexible edges to settle into scour holes.
Flexible mattresses can settle into graduated erosion rather than dropping into localised scour holes common with vessel scour. Trapped flow pressure then builds up under the mattress edge, flipping it.
When positioned flat on the bed, there is not enough depth at the edge of the mattress to place additional rock armour to prevent further under scour if maintenance is required. So the detail has a high likelihood of localised failure and is then difficult to maintain.
The below video shows how bed material can be scoured then filling flexible edges.
Embedded Edge
When the mattress edge is placed into a trench, it facilitates both passive and active edge scour protection. The embedment (passive protection) provides ample time for inspection and maintenance. Should the scour exceed this passive level, the mattress protection then necessitates further ‘active’ protection, featuring an appropriate edge falling detail.
If the scour progresses beyond the designed level, the trench allows for the addition of extra rock armour without compromising the draft depth.
Principles of Edge Protection
1. Capability to drop into localised scour holes, preventing under scour
2. Sufficient deadweight not to be flipped or rolled by trapped flow pressure
3. Be Maintainable
Rock Falling Apron Edge
The edge trench needs to be deep enough to contain rock armour large enough to not be displaced by flow. Without this deadweight the displaced rock armour may roll onto the mattress protection area impacting draft clearance for vessels.
When edge scour occurs, the rock armour should settle into the localised scour holes. Should the scour exceed the designed depth, additional rock armour can be added. If low maintenance is a priority, more rock can be added during the initial construction phase.
Obtaining large enough and cost-effective rock armour can be a driver for using a mattress in the protection area in the first place. Contractors also need to have additional plant on site for the handling of the rock armour. As the armour is placed post mattress installation, construction duration will be elongated.
Mattress Hinged Edge
The hinged edge offers the advantage of a single construction method without the need to place rock armour. Importantly, the hinged edge has two critical properties that set it apart from a flexible mattress: sufficient deadweight and the capability to locally settle into scour holes.
As shown in the image below, each block is able to move independently of its neighbour which allows the HE units to drop to the localised requirement.
The blocks weigh several tonnes each and are sized to remain stable under all flow conditions. The hinged edge reduces the need for initial trench dredging, as the edge blocks can drop a further ~1.0-1.5m into scour holes.
The hinged edge is more suitable for finer bed materials; stiffer clays, for example, do not permit the edge blocks to rotate into scour holes effectively.
Stiff Clay Trench
Erosion tends to be slow in very stiff clays; accordingly, the trench is narrow and filled with concrete bolster to provide maximum initial protection against under-scour. Maintenance involves adding further bolster or tremie concrete as needed before the scour level poses a risk of undermining the rigid mattress.
Maintenance of Edge Details
Its possible for scour to progress beyond the design case in localised sections of the mattress. When scour occurs past design depth, the trench gives the ability for additional rock armour to be placed.
