Coherence study between offshore linear energy infrastructure and other marine users
The North Sea is considered to be one of the busiest seas worldwide. As bordering states expand offshore wind as well as other renewable energy activities, the space requirements are growing in a limited space. Proper ways are needed to integrate this development into the existing marine spatial plans to minimise conflicts between different marine uses and respective users. The space has to be managed efficiently to ensure that economically reasonable uses, both existing and new ones, have sufficient space.
The aim of this chapter is to identify the interplay of components of the offshore linear energy infrastructure and other marine uses that are relevant to marine spatial planners. For this purpose, the spatial overlapping of the different uses was determined. Spatial overlapping assumes that both uses claim the same area simultaneously. Spatial overlaps were qualified with respect to given or potential inherent conflicts. Overlaps were identified for four phases of use.
The chapter deals with grid components from offshore marine renewable energies. Pipelines used for oil and gas have been excluded.
The three-dimensional marine natural space was divided into five horizontal layers comprising:
- space above the water,
- water surface,
- water column,
- seabed and
- underground (subsurface)
In order to specify where the spatial overlapping occurs.
This study differentiates between four phases of the life cycle of the energy grid component:
- Phase 1: Construction,
- Phase 2: Use,
- Phase 3: Maintenance and
- Phase 4: Demolition
The Phases 1 (Construction), 3 (Maintenance) and 4 (Demolition) were grouped, as the grid components have the same space requirements during these phases. The space requirements during the Phase 2 (Use) differ slightly and were analysed separately.
Firstly, the space requirement (by layer) of the components of the offshore grid electricity, as well as the other activities during the phases were determined (table 1). The grid components were then compared with those of other marine uses, also indicating the number of layers affected (table 2). The interplay between the other uses was not analysed yet.
Whenever a use or action is prohibited by law, no spatial overlapping was documented. An examples is the use of restricted areas or the anchoring of boats where cables are located.
It needs to be considered that some uses are temporal and thus do not permanently overlap with grid components.
The analysed grid components included cables and platforms. The grid components were classified according to BSH (2018)
- Inter-array subsea cable defined as a cable that links groups of wind turbines to the transformer substation platform or directly to the converter platform
- HVAC subsea cable defined as a cable that connects the transformer substation platform to the converter platform. The average profile of such a cable is 590 mm².
- HVDC subsea cable defined as a cable that conducts the energy from the converter platform in DC to the shore. The average profile of such a cable is 1,250 mm².
- Interconnector defined as subsea cable systems which run through at least two countries
- Gates defined as corridors where (cross-border) subsea cable systems crossing the border between EEZ’s or to the territorial sea
- Transformer substation platform defined as a platform that bundles the energy produced in one offshore wind farm
- Converter platform defined as a transmission platform on which the power arriving from the offshore wind farms’ substations is bundled, transformed and converted into DC current. Current converter platforms have dimensions of approximately 65 x 105 m but require an area of 100 x 200 m in order to ensure safe installation and reliable operation.
All cables need to be buried in a depth of 1.5 m. Grid components and the affected layers they are touching in the marine can be found in table 1.
With two of the world’s largest ports—Hamburg and Rotterdam—situated on its coasts (OSPAR, 2017), the North sea is an important shipping are. Boats were divided into four categories dependent on their size (IMO, 2018)
- Small crafts (< 85 m): usually maintenance vessels and other smaller boats) that can go around obstacles quite easily.
- Ships (85 – 190 m): most ships in the sea belong to this group. They are bigger, but still quite accessible.
- Panamax vessels (190 – 299 m): Example: A ship of about 200 meters in size needs about 2 NM to turn around.
- Post-Panamax vessels (299 – 400 m): Example: A ship of 400 meters in size needs 5 NM to stop after an emergency break.
Fishing refers to the harvesting of marine fish (Blackhart et al., 2006). Gillnets, trawls and mariculture are the most common commercial fishing techniques in the North Sea (BfN, n.d.-a, ICES, 2017, Animal Welfare Institute, 2018, Gabriel et al., 2005).
- Gillnets can either drift in the water column (Caddell, 2010, Nedelec and Prado, 1990)or be fixed to the ground (BfN, n.d.-c, Blackhart et al., 2006)to harvest fish that entangled themselves in the nets.
- Trawls are funnel- or cone-shaped nets towed behind one or several vessels to retrieve fish (Blackhart et al., 2006, FAO Fisheries and Aquaculture Department, 2014). Bottom trawling drags the net above the seabed and is therefore extremely destructive (BfN, n.d.-b, National Academy of Science, 2002, Bradshaw et al., 2012, Palanques et al., 2001). Pelagic (or midwater) trawling drags the net through the water column (Nedelec and Prado, 1990).
- Mariculture is a marine farming system (aquaculture) for vertebrate and invertebrate animals (fish molluscs and crustaceans) and algae (Blackhart et al., 2006). Usually, cages, pens or longlines are employed to grow these organisms in a regulated environment either in the water column (Buck, 2007)or on the seabed (FAO Fisheries and Aquaculture Department, 2014).
This category summarises all uses related to inorganic resources. Dredging refers to the extraction and relocating mineral resources, mostly sand. The most important purposes of dredging are the recovering of material for commercial purposes and to maintain shipping routes. In case of the latter sand is removed from the seabed of shipping routes in order to maintain a required depth and then released at a different location (for more information see OSPAR, 2015b, European Dredging Association, n.d., IADC, 2018).
Moreover, the North Sea is used for the exploitation of crude oil and natural gas. Once a reservoir is identified, the resources are extracted using platforms and transported to the shore using pipelines. Carbon capture and storage (CCS) is a climate change mitigation strategy which stores carbon dioxide in underground reservoirs. For this purpose, disused oil and gas platforms can be used reservedly. CCS is an emerging technology with only two operational plants in the North Sea in 2015 (in Sleipner and Snohvit, Norway) (OSPAR, 2015a, Strachan et al., 2011)
Apart from restricted areas in which any use is prohibited due to potential dangers originating from unexploded ordnance, the military uses offshore space for different training purposes:
- Torpedo: under water (sub surface)
- Submarine: under water (sub surface)
- Shooting: under water (sub surface) and above water surface
- Mine-hunting: under water (sub surface)
- Flight: above water surface (sea level and higher)
As more detailed information is confidential "above water surface" is defined as layer 1 (above water) and 2 (water surface); and "under water" is defined as layer 3 (water column), 4 (seabed), and 5 (subsurface).
Culture, Recreation and Tourism
Human leisure activities are divided into diving, bathing and surfing as well as recreational boating.
Underwater cultural heritage sites refer to all remnants of human existence that have been partially or totally under water for at least 100 years (UNESCO, 2017). These sites, for instance wrecks, are protected and thus limit other uses (Noordzeeloket, 2017).
In this category, areas for the protection of fish, benthic habitats, birds and marine mammals were distinguished. The definition of these protection areas assumes that the ecosystem with respective species are not influenced by the presence of components of the electricity grid (ICES, 2016). Areas for different species were included as they have varying spatial requests.
Moreover, the use for restoration purposes was included. Restoration is defined as the process of returning degraded ecosystems to their earlier good condition by human actions (see e.g. Smaal et al., 2015, CMCZM & The University of Aberdeen, n.d.).
It needs to be indicated that the analysis only refers to the spatial overlapping of uses. Further implications, like a possible sediment warming or effects of electro-magnetic fields due to subsea cables, were not included.
Spatial requests of grid components
The grid components are divided into the two categories (with different sub-categories) cables and platforms. Within these categories, the grid components have the same spatial request. During the use phase, cables are lying buried in the seabed and thus only use layer 5. During the phases of construction (i.e. cable lying), maintenance and demolition (i.e. cable removal), ships with specific appliances are used to access the cables. Therefore, all five layers are accessed during these times. Platforms are constructions accessible above water which are fixed to the ground. They are using all five layers during all four phases.
Shipping – no matter the size of the ship – does not overlap with cables during the use phase. Platforms on the other hand overlap with the transport sector in the layers 1, 2 and 3 and need to be avoided. During the phase’s construction, maintenance and demolition, the transport sector spatially overlaps with all grid components in the upper three layers. Whether the overlapping indicates an incapability depends on the availability of diversions.
When cables are sufficiently buried, i.e. using only layer 5 and not layer 4, fishing does not interfere with them during the use phase. The different fishing methods overlap with platforms in the upper three or four layers, depending on the method.
During the phase’s construction, maintenance and demolition, fishing spatially overlaps with all grid components in the upper three or four layers, depending on the method. Whether the overlapping indicates an incapability depends on the availability of diversions.
As the use of inorganic resources is connected to the use of the seabed or subsurface reservoirs with the aid of ships or platforms, all uses within this category overlap with all grid components: During the use phase they overlap with cables in layer 5 and during the other three phases they overlap with cables in all five layers. Inorganic resources and platforms are overlapping in all five layers during all four phases.
As any use within restricted areas is prohibited, per definition no overlapping of uses occurs in these areas. However, these areas of course need to be considered during the planning process as they exclude all other uses.
The torpedo, submarine and mine-hunting training areas occur in layers 3, 4 and 5 and thus overlap with cables in layer 5 during the use phase and in all other cases in the layers 3, 4 and 5. Shooting training occupies all layers and thus overlaps with cables in layer 5 during the use phase and in all other cases in the layers 1, 2, 3, 4 and 5. Flight training overlaps during the use phase only with platforms in layer 1 and 2. During the other three phases, flight training may interfere with all grid components in layer 1 and 2.
Areas of dumped explosives pose a threat to other activities happening on or in the seabed. They therefore overlap with cables in layer 5 and with platforms in layer 4 and 5 during the use phase; and with all grid components in layer 4 and 5 during the three other phases.
Culture, Recreation and Tourism
Recreational activities overlap with platforms during the use phase and with all grid components during construction, maintenance and demolition in the layers 1, 2 and 3. Diving additionally occurs in layer 4. Cultural heritage sites occupy space in layers 3, 4 and 5.
Areas for the protection of benthic habitats and restoration areas are the only nature conservation uses that overlap with cables in layer 5 during the use phase. All other protection areas only overlap with platforms during the use phase: areas for the protection of fish in layer 3 and 4, area for the protection of benthic habitats in layer 4 and 5; areas for the protection of birds in layers 1, 2, 3 and 4; areas for the protection of marine mammals in layers 2, 3 and 4; and restoration an all layers. The overlapping during construction, maintenance and demolition equals for all grid components the overlapping with platforms during the use phase.
Layer 1: above water
All uses in the categories of transport, fishing and inorganic resources as well as shooting and flight training areas, recreational activities, areas for the protection of birds and restoration occur in layer one where they overlap with platforms during the use phase and with cables and platforms during the three other phases.
Layer 2: water surface
All uses in the categories of transport, fishing and inorganic resources as well as shooting and flight training areas, recreational activities, areas for the protection of birds and marine mammals and restoration occur in layer two where they overlap with platforms during the use phase and with cables and platforms during the three other phases.
Layer 3: water column
All uses in the categories of transport, fishing, inorganic resources and culture, recreation and tourism as well as torpedo, submarine, shooting and mine-hunting training areas, areas for the protection of fish, birds and marine mammals and restoration occur in layer three where they overlap with platforms during the use phase and with cables and platforms during the three other phases.
Layer 4: seabed
All uses in the categories of inorganic resources and nature conservation as well as trawling and mariculture, torpedo, submarine, shooting and mine-hunting training areas, areas for dumped explosives, diving and cultural heritage sites occur in layer four where they overlap with platforms during the use phase and with cables and platforms during the three other phases.
Layer 5: subsurface
All uses of inorganic resources, torpedo, submarine, shooting and mine-hunting training areas, areas for dumped explosives, cultural heritage sites, areas for the protection of benthic habitats a d restoration occur in layer five where they overlap with cables and platforms during all phases.
During the use phase, transport and fishing is not overlapping with cables, but with platforms. In case of the latter enough space for diversions needs to be assured around each platform.
The use of inorganic resources is always overlapping with the grid components.
Apart from flight training areas which are only above water, all military uses overlap with all grid components in all four phases. (The only exception are restricted areas as any other use is forbidden in these areas.)
Recreational activities overlap with platforms, but not with cables during the use phase. Cultural heritage sites also overlap with cables during the use phase.
During the phases of construction, maintenance and demolition, all other uses overlap with grid components at least in one layer. (The only exception are restricted areas as any other use is forbidden in these areas.) Whether this overlapping equals an incompatibility of the different uses needs to be determined for each situation.
The water column (layer 3) is the layer where most uses overlap with grid components during all four phases. Subsurface space (layer 5) is occupied by the least other uses during all four phases.
During the planning process, not only spatial overlapping within one area should be regarded. Blue corridors and habitat connectivity need to be considered as well to ensure the protection of marine ecosystems. Natural conditions or the availability of resources that influence the implementation of some uses, such as suitable depth for transport or fish stocks, need to be considered when assigning spaces to different uses as well.