Is the sea big enough?

3rd April 2024

A project promoter’s perspective on the environmental challenges facing new subsea power cables

The energy transition will rely significantly on the further development of new subsea cables infrastructure, focused on power export cables to connect wind farms to land, interconnector cables between the UK and neighbouring countries, and, more recently, multipurpose cables serving both purposes. In addition, National Grid intends to enhance the UK’s national transmission system by installing marine cables along the coast, rather than overhead lines on land.

However, the sea is not an infinite, unlimited resource that can easily accommodate the planned volume of new infrastructure projects. It has a well-established commercial shipping industry, fishing interests, dredging and military zones, and previous human intervention that has resulted in an extensive existing network of wind farms, power cables, telecommunications cables and oil and gas pipelines. Such anthropological constraints supplement the natural obstacles and limitations imposed on new subsea cables by seabed morphology, metocean conditions and other marine features, as well as sensitive, protected ecosystems and cultural heritage.

Source: Crown Estate Project Listings (June 2023), National Grid interconnectors register (October 2023)

Much of the present energy infrastructure has been developed without long-term planning for the widespread additional development that is now foreseen, meaning that the ‘low-hanging fruit’ of optimum development sites and cable routes have been picked and that space for new projects is restricted and subject to increasing competition from other marine users. For subsea power cables in particular, these constraints are especially evident in the nearshore maritime space, where the options for cable routeing narrow down on the approaches to the points of connection to the onshore grid transmission system. The constraints are exacerbated as the density of other marine uses intensifies, such as commercial shipping funnelling into the navigation channels to major ports, higher numbers of smaller inshore fishing vessels or leisure craft, and an increased concentration of protected sites for nature conservation in estuarine and inter-tidal environments.

As a result, it is increasingly difficult to find a cable route that avoids detrimental effects on every receptor. With more new projects coming forward, there is a real risk that the expanding web of energy infrastructure may represent over-development, because, when taken together, they unacceptably squeeze out pre-existing marine users and/or cause overall environmental impacts that are judged to be significant. A worse-case scenario for a new project promoter is that a well designed project making a major contribution to mitigating climate change could be deemed unacceptable on the grounds of a relatively small individual effect, because it represents the straw that breaks the camel’s back when taken together with existing infrastructure and other new projects.

A further consequence of the current situation is that navigating new developments through environmental impact assessments, public consultations and consenting procedures takes more time, especially for the resolution of conflicts with different stakeholders, ecological surveys (potentially requiring up to two years of monitoring data) and the deliberations by competent authorities and statutory consultees. The increasing complexity of the procedures means that there is a very foreseeable risk of legal challenges to – or even refusal of – marine licences in the future. Such delays and consenting risks directly affect the delivery of new projects.

Therefore, for project promoters to be able to deliver new offshore infrastructure projects quickly, efficiently and with long-term sustainability, the key challenge is to find a way to reduce the tension related to the use of the available maritime space, co-existence with other marine users, and the mitigation of environmental impacts to the satisfaction of the wide range of interested parties.

The density of existing marine users in the Thames Estuary and southern North Sea requires detailed consideration by project promoters

Source: GridLink Interconnector Ltd

Engagement with marine stakeholders

From a project promoter’s perspective, there are typically five key stakeholders, owing to their statutory responsibilities or specific interests in the maritime space, for new subsea cable projects: navigation and port authorities, fishermen (and their representative associations), third-party asset owners, nature conservation authorities and the marine licensing authorities, as supported by statutory consultees and industry bodies. However, this is not intended to minimise other interested parties – such as local communities, local interest groups, non-governmental organisations, and recreational water sports enthusiasts – whose views are also an important consideration in public consultations and consenting procedures.

For a project promoter, navigation and port authorities are a particularly relevant stakeholder, as they are responsible for navigation and shipping associated with the 51 major ports in the UK, including the designation of navigation channels, anchorages and pilot stations. These areas strictly limit subsea cable routeing and marine operations to ensure maritime safety, preserve unhindered navigation, maintain water depths for shipping and avoid any risks to the installed cable from ship anchors.

Fishermen also necessitate special consideration by project promoters, as they have a long cultural history and pre-existing licensed rights to fishing grounds with an active commercial interest that may be disrupted by marine operations and the seabed footprint of an installed cable. Survey, route preparation and cable-lay activities can all damage static fishing gear, such as pots and drift nets, as well as disrupt fishing vessels. If an installed cable is insufficiently buried, it can snag mobile fishing gear, such as trawl nets and shellfish dredgers, thus representing a similar cause of damage/disruption, as well as a serious safety hazard for the fishing vessels and damage risk to the cable.

Coordination with owners of existing cables or pipelines is becoming especially relevant as new subsea cables are laid in proximity to, or cross over, such assets, which is an increasing likelihood, given the widespread development of new offshore infrastructure projects of all kinds. Each situation requires a proximity or crossing agreement with the third-party asset owner to define technical, commercial and insurance provisions, and these agreements can take substantial resources and time to negotiate, with an added complication that there are a large number of historical/out-of-service telecommunications cables, dating back over 100 years with unknown owners, that need to be considered.

The importance of the role of nature conservation authorities, in particular Natural England, is generally well established through their statutory responsibility for the preservation of protected sites, habitats and/or species. Such sensitive features are widespread in nearshore waters, with clear statutory requirements and criteria for impact studies (such as habitats regulations assessments), implementation of mitigation measures and compensation for any negative effects. Project promoters need to be aware that they have a significant role not only in determining constraints on cable routeing but also in imposing restrictions on any installation methods that may materially change seabed conditions within protected sites, create suspended sediments or generate underwater noise.

Lastly, engagement with the marine licensing authorities and statutory consultees, led by the Marine Management Organisation, is critical for any project promoter. Ultimately, the success (or failure) of stakeholder engagement is reflected in the marine licensing procedure, which provides the official mechanism for interested parties to comment on a project proposal and influence its approval, as well as determining the conditions and requirements imposed on marine operations, cable installation and future repairs. As granting of consents is a pre-condition for investment decisions on all new projects, project promoters are particularly affected by the time required for the determination process, resource constraints on the authorities and statutory consultees, and the risk of double-jeopardy and delays if licence conditions impose further rounds of studies, surveys and pre-approvals prior to works being authorised to commence.

It should be recognised that project promoters do not find consultations with many stakeholders easy, because a common and understandable first response is a preference that the project either does not happen or it is delivered elsewhere. Therefore, the principal challenge for project promoters is to proactively engage and share information at an early stage, despite the expected reaction, and continue the engagement to seek an amicable outcome or compromise. The latter requires the project promoter to be prepared to adjust aspects of the project design and implementation, such as cable routeing, burial depth, external protection, etc, in response to feedback from the stakeholders. This can be problematic, owing to concerns about incurring additional costs, delays or contractual complexity on a ‘voluntary’ basis outside the statutory procedures.

The secondary challenge for project promoters is that the sensitive issues and/or preferred resolutions or mitigation measures of each of these stakeholders may be contradictory and conflict with each other, so it is impossible to satisfy everybody. Almost all nearshore areas have multiple pre-existing marine users, so cable routeing cannot avoid all sensitive zones. And, although technical solutions such as external protection through rock placement/mattresses at third-party asset crossings may be a necessity, they may be contrary to the preferences of navigation, fishing and/or nature conservation interests, as they may change the character of the seabed or the water depth.

The Second World War Redsands Sea Fort is an example of routeing constraints in the Thames Estuary

How project promoters are reducing the demand for maritime space

To overcome the challenges facing the development of new subsea cables, project promoters are focusing on several key areas:

  • Design optimisation to minimise the consumption of available maritime space
  • Minimisation of the footprint on the seabed
  • Grouping of new subsea cables together.

The number of new cable systems that are required can be minimised by selecting the optimum scale of new subsea cables in terms of power rating, which is achieved by the use of highest possible voltage. The Dutch/German utility company Tennet is leading the way in this respect with its proposals for 14 new offshore wind farms in the Dutch and German North Sea, each of which will use a pair of 525kV export cables with a nominal 2GW power rating. These represent a major step up from most similar previous wind-farm projects, whose export cables operate at up to 320kV and 1GW, by increasing the power transfer capacity of the subsea cable system. Whereas the two-cable circuit at a voltage of 525kV can now carry up to 2GW, the same project using a 320kV solution would require four cables to achieve the same power transfer, thereby potentially increasing the requirements for the cable corridor by 50%.

Another way being promoted to reduce the number of new cable systems is the development of multipurpose subsea cables to connect offshore hubs to two (or more) countries, so that offshore wind farms do not need to install their own export cables to landfall in their home country and then require additional interconnector cables to provide market access to neighbouring countries. Instead, the multipurpose cable system offers a combined solution that gives wind farms connected to the hub options to directly export to either connected country, depending on demand. It could also operate as a conventional interconnector if the wind farms are not generating power.

With respect to the seabed footprint of cable systems, cable installation may use several different techniques to bury the cable in a seabed trench – plough, vertical injector, jet trencher, mechanical trencher – which may affect a section of the seabed from 1m to 5m wide, together with a larger area that will be subject to pre-lay grapnel runs, unexploded ordnance clearance and/or pre-sweeping of sand waves to clear the cable-lay path of obstacles. The main means of reducing the footprint of cable systems is to bundle cables together in a single trench, which reduces the footprint of the cable system, compared with installing each cable individually into its own trench. This can significantly reduce cable footprints in the nearshore and offshore, although it is limited at shore crossings if horizontal directional drilling (HDD) is used, as bundled cables need to be separated for the final kilometre or so of the approach to landfall in order to be pulled into separate HDD ducts.

Lastly, while an understandable extension of the onshore principle of assigning technical corridors for grouping underground infrastructure together, the most problematic solution for project promoters is the laying of new cables in close proximity to existing cables in order to minimise the overall space requirements. Historically, multiple cables that are laid in close proximity to each other tend to serve a single project, such as wind-farm export cables, so, typically, they have a single owner, with the works being carried out by the same installation contractor during the same construction campaign. As a result, all the cables are installed before they are commissioned and become operational. This is a very different situation to the scenario that we are now facing, where a series of new cable systems may be installed next to each other by different project promoters, using different contractors and at different times, when the existing cables are live.

Allowing a second cable system to be installed next to a pre-existing cable is anathema to an existing asset owner, who receives no benefits and only risks from a new cable being installed. The new cable installation works introduce risks of accidental damage to the existing cable from marine operations, unexploded ordnance detonation, scouring of the seabed (especially if rock placement is needed), and it could also interfere with future cable repairs. Furthermore, where anchored cable-lay barges may need to be used, typically in shallower waters inaccessible to dynamic positioning vessels, then the cable installation risks are amplified by the anchor patterns around the barge, with particular concern if an anchor needs to be thrown over the neighbouring cable, adding the risk of an anchor drag towards the pre-existing asset during barge manoeuvring. To add further complexity, such risks could affect the warranties given by contractors, fault-finding and repair contracts and the insurance provisions of the existing asset owner.

Therefore, new and existing subsea cable systems are always likely to require a minimum separation of 100m to 200m to mitigate risks. The minimum separation also needs to take into account the space needed for cable repairs using omega bights, with the rule of thumb that such a bight needs 3x water depth to implement. So two cables in a nominal 50m water depth must have a minimum separation of 150m and up to 300m if they need to allow space for omega bights on both cables.

In relation to these solutions, there is also a new consideration that, if new subsea cables have increased power rating, cables are bundled together and multiple projects are laid in close proximity, catastrophic events such as anchor drags or other external interventions could increase in severity. In the future, an anchor drag from a large ship could theoretically cause damage that renders multiple cable systems inoperable at the same time. Previous anchor drag events have been shown to take place over hundreds of metres, so cables installed in close proximity are at increased risk of simultaneous damage by the same incident as a ‘double-hit’ – the possible loss of two 2GW links to offshore wind farms or interconnectors at the same time would constitute 9% of the UK’s peak demand of 47GW (2022), representing a significant impact on electricity supply, and with a likely repair time of several months.

Working together

If these environmental challenges are to be overcome so that new subsea cables infrastructure can be delivered in a timely manner, substantial collaboration and cooperation is required across the offshore cable industry, marine stakeholders and statutory authorities.

As a central element, the offshore cable industry and individual project promoters have the fundamental responsibility to effectively engage with the relevant stakeholders to fully inform and consult about the strategic direction of the industry, technological advances and limitations, and the planning of new projects. Early consultations in a transparent and organised process are critical to establish trust and build long-term relationships that need to endure through the development phase into construction and then future operations. In particular, such consultations need to be initiated as early as possible – and prior to carrying out offshore surveys – because the survey results set the boundaries for the final cable route and associated construction works (which can only be carried out where the seabed has been sufficiently characterised).

Within the offshore cable industry itself, there are also opportunities for improved coordination and cooperation in order for the use of the seabed to be optimised. Project promoters, financial investors, insurers and cable-lay contractors need to jointly reconsider how cables can be planned and safely laid closer together, with appropriate management and allocation of technical and financial risks. The interactions between project promoters and third-party asset owners would also benefit from the development of industry-standard templates for proximity agreements and crossing agreements for subsea power and telecommunications cables to provide an agreed legal basis and to streamline negotiations. Also, there are difficulties in establishing the owners of subsea cables, especially those that are historic/out-of-service, so the development of a Europe-wide database of subsea assets and their registered owners would help coordination between industry players. This could be complemented by an improved definition of a legal procedure for publicising found assets with unknown owners, to give the opportunity for such owners to come forward, or, if unclaimed, to authorise cutting and recovery as appropriate to enable the installation of new cables.

Most importantly, project promoters need to work with the Marine Management Organisation and statutory consultees to determine how the difficulties in balancing the interests of all marine stakeholders and unavoidable residual impacts should be considered to facilitate the determination of marine licences and underpinning statutory consultations. Timely completion of licensing procedures is paramount for project promoters, but all parties need to be wary of achieving this through the introduction of licence conditions that ‘kick the can down the road’ by requiring additional surveys or information in the future that require sub-approvals before works can start. While this approach may facilitate a faster licence decision, such conditions expose project promoters to a delay risk when time is likely to be more limited as the cable-lay contractor is ready to be deployed, and, ultimately, the limited resources of competent authorities and/or statutory consultees will be put under even greater pressure.

Final thoughts

We all have a shared incentive to expedite the development of new subsea power cables to support the energy transition that will significantly contribute to the mitigation of climate change. However, such development must respect existing marine users and minimise the use of the seabed and maritime space, as well as mitigating environmental effects.

Achieving this objective is going to be difficult, and it will require new approaches and compromises by all the interested parties. This will only grow in importance as the current stream of new offshore projects come to fruition and as further infrastructure is needed in the future.

Offshore survey vessel in the Thames Estuary at sunset

David Barber, MIEMA, MIOSH, MIIRSM, MCIWM, is responsible for the environment and permitting of a new electricity interconnector between the UK and France

Image credit: Shutterstock / GridLink Interconnector Ltd


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