Concrete mattresses provide a recognised engineering solution for several of the challenges faced in subsea pipeline construction, umbilical deployment and seabed and soil protection/stabilisation. They are cost effective and may be readily deployed using standard handling systems.
Typically, concrete mattresses may be used to provide :-
• Protection from dropped objects
• Added weight and stabilisation
• Protection from Trawl Boards
• Scour prevention
• Crossover support and separation for pipelines and umbilicals
• Supports or foundations for other subsea activities
• Riverbank erosion control
SPS manufacture a range of standard mattresses, but will also design and manufacture mattresses to a client’s specific requirement. Quality and safety are the key issues in all SPS designs and appropriate safety factors are applied to address lifting and handling in an offshore environment.
Concrete Matresses Options
The density of concrete used in the manufacture of standard mattresses, is generally between 2.3 and 2.5 Tonnes per cubic metre. Under certain conditions where there may be a requirement for additional, or less weight in a mattress (or other SPS Products), SPS can design and supply a concrete mix with a higher or lower density than the standard.
Under certain operating conditions SPS clients require a low friction or anti abrasion material between the concrete mattress and the asset, that it may be stabilising or protecting. SPS can provide this by affixing a range of materials to its mattress segments.
Fronded Concrete Mattresses
The Anti Scour Frond Mattress (right) may be deployed directly into its location, or alternatively attached to and deployed together with a concrete mattress. This latter method simplifies deployment and provides additional protection and stabilisation.
SPS engineering and experience is available to address any particular requirements that clients may have for special mattress designs and modifications.
The Fleximat is a standard SPS product. It provides a high degree of flexibility, allowing it to closely follow the contours of a pipeline/umbilical cable and seabed. It is constructed using high strength concrete profiled blocks and U.V. stabilised polypropylene rope. The Fleximat brick pattern matrix, reduces the open line effect, thus offering additional protection and stability. Once installed the Fleximat may scour into the seabed to increase the stability and overtrawlability.
Tapered (Wedged) Fleximats
Extreme seabed, current and wave conditions may determine that additional stability is required beyond that provided by the standard Fleximat. In this case SPS can provide Fleximats with enlarged tapered edge elements. This also offers an improved overtrawlability profile.
The Logmat is a standard SPS product, and is flexible in only one plane. This provides an engineering solution to two particular challenges:-
Where soft soil conditions exist a logmat may provide a foundation base for supporting a pipeline or other subsea construction.
In conditions of extreme environmental loadings, buoyancy forces, or loss of weight coat, subsea pipelines may require additional weight beyond that provided by the Fleximat. Logmats have a greater overall density and are ideal for this.
Logmats are constructed using high strength, reinforced concrete logs and U.V. stabilised polypropylene rope. They can be quickly and accurately deployed using standard SPS lifting equipment. Logmat lengths are supplied to meet a client’s individual requirement.
Anti-Scour Frond Mattress
When a solid object, typically a pipeline, offshore platform structure or bridge pier is put in place on a loose sedimentary sea or river bed, the flow of water around this under certain conditions can cause erosion of the seabed; this is called scour. It may be gradual or rapid but it is cumulative and the end result may be unsafe spans on pipelines or an unsafe structure generally.
Natural seaweed reduces water velocity locally and correspondingly reduces scour. The SPS Frond Mattress replicates this natural phenomenon. Over 1000 buoyant fronds per square metre at nominally one metre high are built into the mattress and properly deployed (either directly onto the seabed or attached to an SPS Fleximat). This will stop scour rapidly.
The Revetment mattress is a fleximat used for protection against river and sea embankment erosion. Its flexible construction ensures that it can be laid to cope with variations in the embankment profile, maintaining maximum protection against erosion.
Revetment mats are suitable for various environments including:
- River banks
- Break waters
- Sea defences
- Reclamation applications
- Environmental applications
- Slope stabilisation