Construction
OFFSHORE CONVERTER PLATFORMS
The converter platforms are located far from the coast out on the high seas. However, due to their size, they are constructed on land. A number of different methods can be used to install the platforms out at sea. How we proceed with the projects depends on the local conditions as the projects progress.
OFFSHORE CABLE CONNECTIONS
The electricity is transmitted from the offshore platforms to the coast by means of submarine cables. We lay the cables safely and in as environment-friendly a manner as possible in the seabed. Submarine cables are laid using specialised ships. These load large lengths of cable, transport them to the installation site at sea and then lay the cables in the seabed. The ships can carry cables many kilometres long. As a result, only a few connecting pieces – the so-called sleeves. Amprion employs various tried-and-tested techniques to lay submarine cables.
CABLE LAYING IN THE WADDEN SEA
Particularly when working in ecologically sensitive areas such as the Wadden Sea, great care is taken to ensure we apply environment-friendly installation methods. To cross under islands such as Norderney and the dyke on the North Sea coast, horizontal directional drilling (HDD) has established itself as the method of choice. This method also enables us to realise longer crossings without having to remove the soil. Experts refer to this as a “closed” construction method. This means our work has just a minimal impact on nature and the landscape. The only points at which we require space for setting up construction site equipment is at the start and destination points of such a route.
Further information can be found here and here.
ONSHORE CABLE CONNECTIONS
Building underground cable connections that are as cost-effective and soil-friendly as possible is a challenge that sees Amprion collaborating with research institutes, industry experts and trade associations to overcome. We can lay underground cables using a number of different methods. Whether an open or closed construction method is used depends, among other things, on the respective soil and groundwater conditions, as well as the landscape – for example, if rivers or motorways have to be crossed. In addition, we always take environmental aspects into account when choosing the construction method.
OPEN CONSTRUCTION
Especially when underground cables are laid under agricultural land using the open construction method, the soil and its water balance must be handled sensitively and carefully. We have gained valuable experience in this field with our first underground cable project conducted in the alternating current grid in Raesfeld and with our ALEGrO direct current project. In these projects, we successfully laid the underground cables using open construction methods in order to protect the soil, collaborating closely with local farmers and benefitting from extensive scientific support.
Underground cable projects require particularly detailed data on the soil and subsoil conditions. While Amprion analyses the soil conditions at the pylon locations along an overhead line route, i.e. usually every 400 metres, test drillings need to be carried out every 50 to 250 metres during the planning phase for cable routes. Soil scientists then analyse the samples and determine how the different soil layers are to be handled. The more layers there are, the more complex excavation and backfilling of the soil will be. After all, the individual layers have to be carefully separated. The soil expert’s report is incorporated into further planning of the cable system and defines the measures necessary to protect the soil for each section of the route. Independent soil experts monitor the construction work on site throughout.
In open trench construction, excavators first remove the soil layer by layer and deposit these layers separately at the side of the cable trench. At a depth of around 1.5 metres, we then lay empty conduits for the cables, which are later pulled through in stages.
Due to logistics constraints, extra-high voltage cables can currently be delivered to construction sites on land in sections that are just some 1,000 to 1,300 metres long. The individual cable sections are therefore connected up using sleeves during installation.
The cable trench is backfilled in the reverse order of excavation – from the bottom to the top layer. In order to minimise soil settlement and prevent cavities from forming, we backfill the earth as finely grained as possible. It has to be dry for this, and damp weather can delay the construction work. Afterwards, the various layers are almost as they had been prior to being excavated: the original structure of the soil is retained and regeneration is rapid. Soil scientists accompany and monitor the construction and recultivation work.
CLOSED CONSTRUCTION
The open construction method is not the method of choice everywhere. If the underground cable has to cross under bodies of water or roads, the closed construction method – which is much more costly and time-consuming – offers significant advantages. We only need space for setting up the construction site equipment at the start and destination points of such a route. A variety of methods are available, such as pilot tube tunnelling, horizontal wash drilling and microtunnelling. Which method we choose depends on the local geology and the length of the route.
PROTECTED ZONES
Once the construction and recultivation measures have been completed, a protected zone remains above the cable trench and the land in question can be used for agricultural purposes again. However, buildings may not be erected on the protected zone as the cables must be accessible at all times for maintenance work and repairs. In addition, the protected zones must be kept free of deep-rooted trees and shrubs to prevent damage to the cables.
For more information about underground cables, click here.
ONSHORE CONVERTERS
As out at sea, we also need a converter station near the legally defined grid interconnection points in order to convert the incoming direct current back to alternating current. This is necessary because the transmission grid on land generally transmits alternating current and the incoming electricity can therefore be fed into the grid and distributed via the connected substation.
The technical components required are identical to those in the converters located offshore. The only difference is in the structure and arrangement of the individual elements as we generally have more space on land and there is no need to stack the components on top of each other, protected from the wind and weather, as we do at sea.
The core element of a converter station is the converter technology. Because these power electronics need to be protected from wind and weather, they are installed indoors. Construction usually takes around two to three years. You can find even more information here.
Further information can be found at amprion.net.
