Submarine cables have always played a substantial role in various offshore projects, such as oil and gas, and, most recently, marine renewable-energy systems. That role is growing, and their performance is central to the success of the offshore wind projects.
But despite being part of an advanced technology, submarine cables remain relatively easy to damage. For that reason, they need to be designed and built with sufficient protection to give them the resilience required.
Reliability is the top priority
The biggest requirement of offshore project operators from subsea cable manufacturers is reliability. The sector expects a wide range of high-quality, low-maintenance cables for energy, communication, control and data. Adequate protection is essential to ensure this reliability.
The main functions of submarine cables include:
- Exporting power from offshore wind farms to mainland power grids
- Connecting oil and gas platforms to mainland power grids
- Transmitting data and internet traffic
- Providing high-quality and reliable shore-to-platform and internal platform fibreoptic communications
- Providing oceanographic monitoring and surveillance
The main dangers for submarine cables stem from the tough conditions in which they operate. Heat, cold and saltwater can lead to erosion. Ocean currents and strong loads mean sturdy and robust construction is important.
Fishing, vessel anchoring, gas extraction or the installation and maintenance of renewable-energy projects often cause cable rupture. Hurricanes or earthquakes can also lead to damage. Timely repairs are expensive yet critical, so maintenance of the cables’ protection mechanism should be as straightforward as possible.
Static cables
A static cable will face different conditions and seabed features along its route, requiring different protection systems.
A rocky seabed, shoreline or foundation touchdown point will place severe demands on cables, necessitating dedicated methods of protection.
These must be easily deployable and able to withstand the load and demand during a significant lifetime. A common solution is to resort to cast-iron shells.
The rapid development of the offshore-wind industry requires dynamic cables, robust enough to resist the larger stresses acting on them
Cylindrical half shells or split pipes
Cylindrical half shells or split pipes usually include bend restrictor functions and other connection elements, and also have a protection and potential ballasting function. They tend to form part of a complete cable-protection system and are especially suited for the crossing of the scour protection at the bottom of the platforms. They also work for rocky or challenging seabeds, thanks to their abrasion resistance and stabilising/ballasting abilities.
The shells are the only solution regularly used in both applications — in cable protection systems at the foundation touchdown point, to protect longer cables sections for the shore land, and for rocky or challenging seabeds where burial is not possible. The protecting and ballasting shells are a simple, effective and easy-to-assemble solution. They are positioned along the cable and can also stabilise the installation by providing additional weight.
In the context of its development in the offshore industry, Farinia FMGC has designed a full range of cast-iron shells to provide innovative and efficient solutions, combining protection, stabilisation and bend-restrictor elements in a unique system.
Cast-iron protecting and ballasting shells are especially designed to resist impact, abrasion and corrosion. The mechanical properties of cast iron allow it to withstand the harshness of marine environments. Each shell comes with integrated ballast and bend-restrictor functions, eliminating the risk of overbending or dislocation during installation and maintenance operations, but also in the case of free spans.
The main goal in developing the shells was to combine protection, stabilisation and bend restriction in a single product, able to be installed directly on the deck of the laying vessel. Thanks to the specific design, the need for other dedicated means and additional ships is drastically reduced.
The solution can be easily adapted to various types of cables, from 20mm telecom cable to 250mm export cable.
Dynamic cables
Conventional transmission cables are static and have been designed to operate in conditions that differ a lot from those experienced by cables for floating offshore structures in marine renewable energy industries.
Dynamic cables are subject to greater levels of mechanical and electrical stress due to the platform’s motion and sea conditions. The shift in platform design from fixed to floating necessitates an equivalent change in the accompanying subsea cables.
Moreover, marine conditions can be quite severe and unpredictable between surface and seabed. Not only can this lead to malfunctions but it can also make it more difficult for technicians to reach the damage to repair it. Thermodynamics due to contrasting temperatures can also lead to breakages and performance degradation. Marine growth can also affect the cable by increasing its diameter and weight, resulting in additional loads that can result in serious damage.
Therefore, new floating structures need innovation to produce dynamic power cabling that is robust enough to resist to these greater stresses.
The cabling issues faced by the offshore wind industry mean there is a high demand for reinforcing solutions that can minimise the expense associated with maintenance of dynamic cables.
Cast-iron ballast
Cast-iron ballast is a versatile solution that can be adapted to the needs of various offshore installations and improve their resistance and reliability. It offers a cost-efficient and highly reliable solution for the stabilisation and protection of offshore dynamic cables, resulting in lower project costs.
Subsea cables are needed to carry offshore wind energy back to land, so ensuring their protection and stabilisation is of critical importance.
Anchoring
However, due to marine growth, but also because of shallow waters and large waves, buoyancy is not sufficient to ensure the stability and reliability of dynamic cables. Therefore on some configurations, the addition of an anchoring point (pictured, right) is considered and improves significantly the performance and stability in severe environment.
This anchoring point needs to be carefully designed to meet the exact stabilisation needs in accordance with the specifications of the site and the cables used.
Another critical aspect of dynamic cables is their ability to resist abrasion and repeated friction with the seabed at the foundation touchdown point. To minimise the risk of damage this section needs to be protected. A cable protection system based on cast iron shells is in this case highly recommended since it will provide not only protection but stabilisation of the cable at the touchdown point.
Thanks to collaborations with companies such as Principle Power or Ideol in the floating wind industry, and also with cabling companies such as French electricity-grid operator Enedis and offshore installers such as Jifmar or Bourbon, FMGC has developed expertise in different applications for cast-iron stabilisation and ballasting solutions for dynamic cables.
These solutions can be designed in accordance with customer requirements. All share the same objective: protecting submarine cables from fatigue, overbending and external factors by ensuring cable’s protection, stability, strength and best possible performance.
Ralitsa Peycheva is a technical content manager at Farinia Group
This article was first published in WindMax (incorporating WindStats)
