The Wood Thilsted View on Seismic Inversion: Unlocking the true potential of offshore wind farm developments

Offshore wind is championed as a critical renewable energy technology by Governments around the world.   It is a key pillar in the transition to Net Zero and figures prominently in the major energy transition outlooks published by world-leading organisations such as the IEA and DNV.  In the UK, the current Government has stated a target of getting 43-50 GW of offshore wind capacity on the system by 2030 including 5 GW of floating offshore wind.

Despite positive policy support, offshore wind is facing increasingly strong headwinds as sites move into deeper water with more challenging ground conditions, inevitably driving higher costs, while energy majors are reevaluating their energy transition strategies in favour of delivering immediate returns. 

With this backdrop, every opportunity must be taken to create value, reduce the payback period and optimise returns on offshore wind developments.  A new way must be found, starting from the ground up.

The Current Model – Lessons from Onshore Engineering

Offshore wind site characterisation is centred on an intrusive ground investigation-led approach, emphasising soil penetration.  This is hardly surprising given the relatively cost-effective nature of the tried and tested Cone Penetration Test (CPT), following the model of what works most easily onshore is first to be transferred offshore.

The CPT has established itself as the critical technology for ground investigation in the offshore environment.  It involves pushing an instrumented cone into the ground, measuring variables such as cone tip resistance, sleeve friction and pore pressure, inferring relevant engineering soil properties and unitisation.  This provides a linkage to the absolute that few other forms of investigation can match, giving the confidence required for large structures to be built. 

However, with offshore wind sites growing in size, entering deeper water, with more challenging ground conditions including harder and shallower bedrock, coupled with a push for increasingly audacious timelines to support clean energy targets, we need to alleviate the already supply-constrained survey contractors and accelerate the rollout of offshore wind. 

One existing method we can use to accelerate offshore wind farm design is Quantitative Interpretation (QI) – seismic inversion for synthetic CPTs – which paves the way for a more cost and time efficient, data-led approach to the development of complex sites alongside the use of CPTs.

The Advantages of Quantitative Interpretation for Offshore Wind

CPTs will always be needed – they are the critical link from the remotely sensed geophysical data to the ground itself – yet QI has the potential to refine what looks set to be some enormous pre-installation ground investigation (GI) campaigns, for floating sites especially.   

QI has been central to the oil and gas industry for decades and its success can be seen in the scale of today’s global hydrocarbons sector.  It is employed to map and to predict rock properties such as porosity and in-place volumes, giving geoscientists the ability to take in situ GI test data and, in combination with seismic data, predict within set confidence bounds parameters of actionable significance.

To get the most out of QI, we require ultra-high resolution seismic data – 2D or 3D if possible – along with targeted, high quality GI data including PS logging, and skilled geoscientists with an in-depth understanding of the modelling process, the data and the geological ground conditions to be expected on site. Through this, geotechnical parameters at any XY location within a seismic survey can be predicted, delivering hundreds of thousands of synthetic predicted CPTs suitable for design.

With this prediction, we can potentially offset the requirement for an all-anchor GI campaign or at least continue to support onward design through FEED and delay DEVEX expenditure into the CAPEX phase.

Further potential benefits include:

• • A reduction in timeline for entire site characterisation directly translating to a reduction in the time to first power, improving returns and cash flows

• • Ability to better de-risk sites prior to more extensive and expensive work

• • Supports and extends site layout flexibility

• • More accurate and competitive bidding for auction rounds with greater understanding of the Rochdale Envelope for concept selection

• • Ability to better de-risk adjacent workstreams such as cable routing

• • Comparative benefits to the natural environment – less survey time, fewer vessels, more efficient designs.

Ultimately, QI can help projects to manage risk and increase their understanding of ground conditions much earlier in their design programme.  This can therefore be used to inform concept maturation, potentially accelerating deployment and saving significant time and resources. 

Meeting the Challenges of Floating Offshore Wind

The benefits are further pronounced with floating offshore wind.  For floating projects, common standards call for one in situ test per anchor location, with many current concepts having between three and six anchors per floater. For some of the larger sites, this could mean more than a thousand tests being required (and let’s not forget about all that shallow bedrock which may require drilling). The scale of campaigning required, under ever shortening timescales and with an already limited survey vessel capacity, is an uphill battle and a major bottleneck in our pursuit of 2030 targets.

Why is QI not already in use for Offshore Wind?

As with any new to sector technology, there are questions around efficacy.  Is it as accurate as a CPT?  No, but consider the absolute accuracy of traditional data in the light of micrositing or drag embedment anchors. 

As with all measurement, uncertainty will remain – be it with QI or a CPT.  QI’s role is to aid refinement of intrusive works such as identifying areas with remaining uncertainty as focal points for any completion campaign.

A short-term obstacle is that this methodology does not yet provide the basis for a certifiable design.  However, as has been championed by Wood Thilsted for several years, there is growing momentum with the methods being considered strongly enough for them to form a sizable part of the recently created GIFT JIP with DNV. 

Historically, sites were relatively small and technically simple, competition wasn’t so fierce, and the economic outlook was more positive.  There was neither the push nor pull for the well-established QI to enter offshore wind.  However, with much larger and complex sites, coupled with the advent of floating offshore wind, the time for QI is now. 

Objective-based Ground Engineering

The potential for QI is clear.  For QI to be successful, the geophysical and geotechnical surveys need to be carefully designed and considered in partnership with the wider project objectives.  The synergistic relationship between the geophysical and geotechnical surveys is vital.  It cannot be an afterthought, as remediation is never better than doing it right the first time.

Wood Thilsted sees QI as an enabler – a tool to unlock doors and create downstream opportunities for further programme optimisation. It is a powerful tool, but we should ensure that it is grounded in the fundamentals of well-integrated, objective-based G&G.

Quantitative Interpretation – Optimising the Value of Offshore Wind

Quantitative interpretation offers numerous possibilities to accelerate the rollout of offshore wind at scale and with reduced risks. 

Pound for pound QI offers huge savings.  In the context of ground engineering, a refined and objective-based ground engineering campaign yields millions of pounds of potential savings.  In the context of the total project, this is a potential saving in the hundreds of millions due to a better-informed concept selection and a multi-year acceleration in delivery.   

Intrusive works – CPTs in-particular – will always be needed.  Advances of geotechnical approaches augmented by enhanced geophysical surveying using QI and geocellular modelling present a ready-made solution to meet the needs of the industry as it accelerates the rollout of this key technology to meet ambitious decarbonisation goals.