Laying the cable
Installing a submarine transmission cable is a costly and challenging activity. The lifetime of a submarine cable might be tens of years and the technical interventions for its repairing in case of faults are also costly and difficult.
Therefore the cable route must be carefully surveyed and selected in order to minimize the environmental impact and maximize the cable protection.
Laying down the transmission cable on the seafloor is done by specialized vessels (Fig. 1). The most active vessels used for such operations are: Skagerrak (owned by Nexans), Giulio Verne (Prysmian), Team Installer (Topaz Energy and Marine) and C.S. Sovereign (Global Marine Systems Ltd). They are all equipped with a turntable for at least 4000 tons of cable and have the appropriate gear to handle it.
Installing a submarine transmission cable involves a series of actions:
- Selection of the provisional path;
- Obtaining permission from the relevant authorities;
- Survey of the path;
- Designing the cable system in order to meet the conditions of the selected path;
- Laying the cable, including burial in appropriate areas;
- A post-lay inspection may be necessary in some cases;
- Notification of cable position to other marine users.
The complexity of laying down the cable requires a coordinated work of many specialists in different fields. Path selection is done by power system engineers together with marine specialists. The survey is performed by geologists, geophysicists and oceanographers.
Laying the cable on the seafloor is executed by special structures engineers.
The vessel represents just a part of the required gear needed for laying down the cable. It carries the cable and stands for the command centre. But once the cable is in the water other submersible equipment performs the task of settling the cable on its path.
For shallow waters divers might be employed to assist the installation while for deep water Remotely Operated Vehicles (ROVs) are manipulated (Fig. 2).
The work is done with the help of acoustic instruments such as echo-sounders and accurate Global Positioning System (GPS) and differential GPS.
The ROVs dig the trench in which the transmission cable is laid (Fig. 3 and Fig. 4), fix the cable on the right route and cover the cable with sediment. Burying the cable in the seabed is a slowly and costly operation but it is paid back by its reliability and extended lifetime.
Cables are buried in the seabed in shallow waters in order to minimize the risks for damages. The trenches in which the cables are placed are dug by a submarine plough and covered by sediment or rocks.
When it is not possible to use sediment as a cover other solutions are applied like using rocks or concrete mattresses (Fig. 5) as cover or using articulated pipes.
The rate at which the cable is laid-down depends on the type of the cable, the complexity of the cable configuration, the depth and properties of the seafloor (heterogeneous bathymetry and geology). In the case of communication cables a laying rate of 100-150 km/day, for new types even 200 km/day, is expectable.
Due to the size of the transmission cable and the volume of work, which is usually bigger, power cables are installed at a lower rate. For power cables the average burial speed is about 0.2 km/h and depends largely on the seabed conditions.
The depth of the trench is usually of 1 m, only exceptionally more, up to 10 m. With increasing size of ships channels will have to be dredged or deepened.
So in places the cables must be protected for such future works they have to be buried at a safe depth in the sediment. This is the case of the 30 km power transmission cable that connects the Malaysian island of Pulau Ketam with Port Klang which was buried 14 m under the seabed.
The cables are buried in the seabed sediment up to depths of 400-600 m, below this depth they are simply laid down on the bottom of the sea. In places with strong sea currents or steep slopes they are fastened to the seabed.
In order to check the cable security periodic surveys are envisaged.
Submarine power cables must be physically protected against natural hazards or human activity. Since a fault in the good functioning of a cable might have major implications in securing the power supply serious measures are devised.
Submarine cables are the subject of several international treaties which regulate their status. These documents establish norms regarding the rights and obligations for states that wish to lay such cables as well as for states whose territorial waters are crossed by the cables.
The main documents dealing with submarine cables are:
- The International Convention for the Protection of Submarine Cables (1884);
- The Geneva Conventions of the Continental Shelf and High Seas (1958);
- United Nations Convention on Law of the Sea – UNCLOS (1982).
These treaties ensure:
- The freedom to install submarine cables on the high seas beyond the continental shelf and to repair existing cables without impediment or prejudice;
- The freedom to install and maintain submarine cables on the continental shelf, subject to reasonable measures for the exploration of the continental shelf and the exploitation of its natural resources;
- The freedom to install and maintain submarine cables in the exclusive economic zone of all states;
- The ability to install submarine cables in a state’s territory or territorial sea subject to conditions and exercise of national jurisdiction;
- The freedom to maintain existing submarine cables passing through the waters of an archipelagic state without making landfall.
They may differ in size depending on the national rules/laws and the local conditions (e.g. naval traffic). For example around HVDC Inter-Island power cable in New Zealand a seven-kilometre wide CPZ is established and enforced (Fig. 6).
Vessels are not allowed to anchor or fish in this area and the protection zone is constantly monitored from sea or air. Infringement of these rules attracts a fine up to $100,000. Enforcing this rule led to no faults due to human activity.
An example of good practice is represented by the Kingfisher Information Service – Offshore Renewable & Cable Awareness project (KIS-ORCA), a joint initiative between Subsea Cables UK and Renewable UK, which raises awareness of submarine cable locations among operators in fishing industry for the North Sea and Western border of Europe.
Its website (Fig. 7) includes online updated maps picturing sea infrastructure (pipes, power and telecom submarine cables and accompanying equipment) with contact details in case of incidents.
Industry organizations such as International Cable Protection Committee (ICPC) and North American Submarine Cable Association (NASCA) have drawn a set of recommendations regarding protective measures to be implemented for a safer and longer life of submarine cables (CSRIC, 2014).
Maintenance of sea cables
For an optimal operation the cable must be periodically checked and maintained in order to prevent deterioration. This includes:
- Survey of the cable in order to check for possible tears or wears;
- Survey of the cable path in order to check the stability of the seabed and possible geodynamic processes that can threat the cable integrity;
- Preventive replacement of cable components when signs of wear are present or when they are approaching the lifetime end;
- Enforcing rules and regulations regarding the protection in the CPZ.
The operation is performed by specialized vessels with appropriate equipment. It depends heavily on the weather and sea conditions. In high latitude regions where the sea surface is covered with ice or crossed by floating icebergs these operations require additional care measures and lengthy times.
Reference // HVDC Submarine Power Cables in the World by Joint Research Centre, the European Commission’s in-house science service (authors: Mircea Ardelean, Philip Minnebo)