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Cluttered North Sea demands fresh thinking on wind-CCS colocation

The North Sea is getting crowded.

The arrival of large-scale offshore wind development in a maturing petroleum province earmarked for a rapid ramp-up in carbon storage capacity is creating competition for coveted swathes of marine acreage. New pan-industry research projects are getting underway that aim to resolve the wind-carbon storage colocation challenge – but in the absence of a strategic spatial plan that attacked the issue head-on, there are question marks around how future conflicts will be resolved.

Competition between offshore wind and carbon storage broke into the mainstream consciousness in 2021 when Ørsted and BP went head-to-head over the same patch of the North Sea. The British oil major’s site investigations for the proposed Net Zero Teesside and Zero Carbon Humber carbon capture and storage (CCS) initiatives overlapped with part of the Danish wind leader’s 180-turbine Hornsea Four project.

The pair reached a commercial agreement in June 2023 that allowed BP to remove its objections to the wind farm planning application, but details were not disclosed. The deal cleared the way for Hornsea, but its confidential nature denied other stakeholders a template for resolving other overlap conflicts.

Both schemes were awarded preliminary licences by the UK government more than a decade ago, when it was thought that technological advancements would obviate the overlap problem. But if anything, new seismic capability and the growing footprint of offshore wind megaprojects have worsened, rather than solved, the overcrowding issue.

No space to swing a catamaran

Modern seismic acquisition requires more space than ever to gather high-resolution 3D data needed to analyse the suitability of subsea geology to store captured carbon dioxide.

The technology has grown significantly in density, detail and accuracy in recent years, requiring the use of large vessels towing a series of parallel ‘streamers’ gathering data across a large section of the seabed. Carbon storage will also require 4D data gathered by fixed ‘nodes’ that monitor well integrity over time.

At the end of each line pass the survey vessel undertakes a large slow turn that can take several hours. The very large sweep of the trailing streamers during turn poses a risk of collision with seabed infrastructure that might to the untrained eye seem to be located a long way away.

Seismic survey vessel moving ahead with all towed in sea equipment deployed. Oil and gas exploration. Restricted manoeuvrability.

Undertaking ‘deep’ seismic required for carbon storage site requires as many as 20 streamers fanning out behind the vessel and spaced 50 metres apart, with a typical stream exclusion zone of 2,000 metres in width. Overall, the footprint of the seismic acquisition activity is much larger than the area being targeted.

Pipes, platforms and shipwrecks

The overcrowding issue is acute in the Southern North Sea. The SNS has the highest concentration of upcoming carbon stores within two different reservoir targets within a complex geological environment. The existing seismic database is comparatively old, and new seismic acquisition is difficult and increasingly constrained by other marine users and infrastructure.

The North Sea Transition Authority (NSTA) issued a batch of 21 carbon storage licences in September 2023, most of which lie within the SNS. These licences each span roughly 12,000 square km and are predominantly in a region where competition with planned and active windfarm sites is increasing.

The regulator says up to 100 licences could be needed to deliver UK government targets for CCUS: to capture 20-30 MtCO2 per year by 2030. This figure includes 6 MtCO2 from up to four industrial CCUS clusters, rising to 9MtCO2 by 2035.

The Crown Estate has issued almost 30 offshore wind leases and lease options in the SNS. These sites will deliver the bulk of the capacity needed to hit the UK government’s target for 50 GW of offshore wind by 2030.

Map of existing and potential offshore energy agreements, and large sources of CO2 emissions. Source: The Crown Estate

Future offshore infrastructure in the SNS will be built around a plethora of existing obstacles. The SNS is littered with hundreds of nearshore wreck sites as well as legacy oil and gas infrastructure and other human-made objects.

These include subsea manifolds, pipelines, umbilical control lines, anchoring points, rock dumps and other items that might affect seismic acquisition, depending on water depths. Many such objects have an exclusion zone surrounding them to avoid potential collision events in what is already a busy shipping zone.

Pushing the boat out

If anything, the evolution of the offshore wind industry promises to exacerbate the colocation challenge. On the one hand, the shift towards ever larger turbines of 15 MW or more could lead to an increase in the separation corridor between units, from 1 km currently to as much as 2 km.

However, while this would double the seismic corridor, it would not substantially mitigate collision risk for streamers or deployment risk for nodes, so would be unlikely to enable access within the windfarm area for seismic operations.

Given the urgency of the challenge at hand, the offshore wind industry needs to get up to speed quickly on this topic before the next wind-CCS rivalry bubbles to the surface

Moreover, the move into deeper waters will present new challenges. Floating wind turbines will have up to eight catenary cables and multiple anchor points that increase the footprint and spread of in-water equipment.

Bursting at the seams

The NSTA said in a recent report that full co-location of carbon storage sites within wind farms is in many cases “impossible”, given spatial requirements and the need for ongoing seismic monitoring once CO2 is injected.

The regulator called for seismic operations to be conducted before further wind farm development is undertaken “as future intra-windfarm seismic operations will be complex, difficult and costly”.

“Whilst seabed seismic can help to acquire image closer to the edge of a windfarm, it is unlikely any form of seismic equipment will be able to access within the tight confines of current turbine layouts. Partial overlap is only […] possible with careful design of future CS monitoring area and windfarm design,” it said.

Time is of the essence. The North Sea could become the heart of a booming pan-European carbon storage market, but other regions on the continent are vying for a piece of the prize. “Owing to the proliferation of windfarms, there is a limited timeframe to acquire a regionally extensive streamer/hybrid 3D in this difficult and congested seaway,” the NSTA added.

It’s good to talk

There is no easy workaround. One option is for the Crown Estate, the landlord of the UK seabed, to offer more acreage to projects to make room for the sweep of a large seismic vessel – but this won’t help with licences already issued.

Another is to undertake ‘hybrid’ seismic campaigns that combine conventional streamers with ocean bottom nodes (OBNs) that can be deployed in more narrow gaps at strategic locations around the perimeter of a wind lease area. Hybrid seismic might be the most cost-effective solution at the margins of overlap, the NSTA says, but it does not resolve the underlying issue of full site conflict.

There is ample seabed space to accommodate a healthy pipeline of projects in both sectors, but we continue to work closely with the Crown Estate, North Sea Transition Authority and other stakeholders to address possible challenges with co-existence.

Department for Energy Security and Net Zero

Ultimately, wind and CO2 storage developers will need to work together and share data to address potential spatial conflicts as early as possible in the development process.

“[E]arly and open discussion concerning the extent and timing of marine activity will always be the best way to manage the potential of our offshore areas and everybody’s expectations,” the NSTA says. “Traditionally long-term planning has been poor, exacerbated by limited awareness of other user’s needs and limited data sharing across disparate databases.”

Guidance from above, please

Since colocation is a relatively new problem, it was not addressed in the most recent marine spatial plans for England or Scotland. In the absence of such a strategic assessment, it is hard to know how much collaboration will occur.

The Department for Energy Security and Net Zero is, outwardly at least, relaxed about the problem. A DESNZ spokesperson told E-FWD: “Both carbon capture and offshore wind are key to our efforts to reach net zero emissions by 2050, and we have implemented ambitious deployment targets for these technologies.

“There is ample seabed space to accommodate a healthy pipeline of projects in both sectors, but we continue to work closely with the Crown Estate, North Sea Transition Authority and other stakeholders to address possible challenges with co-existence.”

DESNZ declined to say if or when it might develop a new marine spatial plan for the UK Continental Shelf to delineate priority areas for carbon storage and others for offshore wind. The Marine Management Organisation (MMO) is responsible for preparing marine plans in England and the government has established a cross-government Marine Spatial Prioritisation (MSPri) programme “to build our understanding of future demands”, the spokesperson said, without elaborating.

Convening the real estate rivals

DESNZ cited the work being done by the Crown Estate’s Offshore Wind and Carbon Capture and Storage (CCS) Colocation Forum, which brings together wind and carbon storage industry representatives with the regulator and government to coordinate developments.

The forum recently commissioned two research projects designed to inform the best approach to test and demonstrate the colocation of offshore wind and CCS activities in the future. The initiatives – Project Colocate and Project Anemone – build on the Forum’s Spatial Characterisation Report, which identified areas of potential overlap.

Delivered by the University of Aberdeen, Project Colocate will investigate the viability of potential colocation areas in the SNS and Irish Sea and develop bespoke monitoring plans for each area. This will create a pipeline of potential test and demonstration sites. Project Anemone will explore mutually beneficial opportunities arising from colocation, with more details to be released shortly.

“Together, both projects will help pave the way for test and demonstration, as well as facilitating greater collaboration and understanding between these two vital sectors,” said Tom Mallows, head of offshore development for emerging technologies and infrastructure at Crown Estate Scotland.

A new horizon for collaboration

There are numerous opportunities for colocation cooperation, although there is much work to do to explore them further. Collaborating on seismic campaigns by co-surveying a site is the most promising, since subsea characterisation is needed to inform wind turbine siting and foundation design decisions.

Under the current approach to seismic data gathering, opportunities for co-surveying are limited because wind developers have very different requirements to carbon storage, which also differ from conventional hydrocarbon resource seismic. But there is scope for potential future convergence around a ‘single geophysical Earth model’, according to the NSTA.

The long-established approach to acquisition and processing has remained largely unchanged for decades, the NSTA said in its report. “This is a fixed, sequential model, and difficult to make major changes to the workflow(s).” But there is a degree of interest in moving towards a “revolutionary new approach” that incorporates “comprehensive seismic imaging” from the start of the workflow.

In theory, this could span a range of geophysical data types and other datasets under a single workflow “defined by iteration, assisted by technological advances in computer processing and accessible digital storage”. If implemented, this new model could gather data required by all current and future marine seabed users under a consistent standardised approach, thus going a long way to eliminating the colocation problem.

This is a new frontier in seismic research. RenewableUK, the UK wind trade body that represents the offshore industry in the colocation forum, said this concept is so new that it was unable to comment on the NSTA’s suggestions or wind-CCS colocation more generally.

Given the embryonic nature of this idea, and the urgency of the challenge at hand, the offshore wind industry will need to get up to speed quickly on this topic before the next wind-CCS rivalry bubbles to the surface. After all, this is precisely the sort of collaboration that the North Sea energy transition demands.

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