Beyond Zonation: Time to Rethink Jack-Up Site-Specific Assessments in Offshore Wind

Written by James Adcock

Introduction 

As offshore wind farms (OWFs) move into more challenging environments, the familiar zonation approach to Site Specific Assessments (SSA) is showing its cracks.

This article discusses the challenges and limitations of zonation approaches to SSAs considering the current challenges in offshore wind as well as showing how taking a new approach can improve project scheduling, improve vessel suitability and reduce risk both commercially and operationally.

Current Challenges of the Zonation Approach

At its core, zonation SSA means grouping turbine locations/jack-up positions across a wind farm into ‘zones’ based on broadly similar jack-up foundation conditions or water depths. The intent is straightforward: minimise the number of SSAs required by allowing a single representative analysis to effectively cover a cluster of turbines. In theory, this reduces workload and accelerates delivery.

Once a pragmatic way of balancing efficiency and confidence, it is becoming clear that the zonation approach has a number of drawbacks.

A zonation approach often introduces a cascade of conservatism and rework. Because zonation is inherently a generalisation, the geotechnical parameters are typically defined conservatively to ensure all locations within a zone are captured. If a subsequent SSA within that zone shows over-utilisation, further subdivision may be required, with engineers slicing the zone into smaller and smaller parts to account for variations in water depth or soil response. Each iteration adds time and complexity, eroding the efficiency that zonation is supposed to deliver while adding to cost uncertainty. An example iterative approach to a zonation SSA approach is shown below.

Marine Warranty Surveyors (MWS) can add another layer of challenge. MWS reviews often bring different perspectives on whether zones have been appropriately defined, or whether key variations in soil, water depth, or loading have been overlooked. This can trigger additional benchmarking, alternative assumptions, or requests for standalone analyses of specific turbine locations that fall outside the risk envelope of the defined zones. What begins as a simplified approach can quickly devolve into an iterative loop of checks, comments, and expensive rework, one that feels never-ending, while still leaving behind a trail of conservative assumptions. 

Challenges of Zonation SSA with Industry Challenges

It is well reported that turbine components are scaling faster than the installation vessels built to handle them, rotor diameters, tower heights and nacelle weights continue to balloon, whilst the global fleet of Wind Turbine Installation Vessels (WTIVs) remains largely fixed. Today, there are roughly 50 WTIVs worldwide, but fewer than 20 are capable of installing turbines larger than 10MW.

The result is an industry increasingly reliant on vessels never designed for today’s giants, facing crane reach issues, leg length limits, and capacity shortfalls. The knock-on effect of this are tighter operational envelopes, more weather-restricted operations, and slimmer margins for error, squeezing already tight project schedules.

Using the zonation approach for SSAs could lead to:

• Increasing scope of work and cost during assessment phase

• Greater uncertainty and conservatism

• Increased approval time

• Increased vessel time

• Missed opportunities for vessels at some locations

• Overly restrictive weather limitations

• Restrictive vessel suitability assessments

And this is where the choice of assessment method matters. With existing vessels already operating close to their limits, can assessments justify the inherent generalisations of zonation SSAs? Particularly when per-location SSAs, Leg Penetration Analyses (LPAs), preload optimisation, and risk profiling can now be done rapidly, in parallel, often at similar cost?

Per-location approach utilising New Technology

The technology now exists to do better while still complying with industry standards such as ISO19905-1 and other industry guidelines. With advances in coding, parallel batch analysis and computer processing speeds, make location-specific approaches not only possible, but arguably essential for complex projects. Current projects now have multiple vessel loading conditions, headings and environmental conditions. Every unnecessary metre of penetration, every conservative preload, every layer of rework directly cuts into scarce vessel time and inflates downtime risk.

In fact, in many cases, per-location analysis can deliver shorter project schedules than zonation, while stripping away the conservatism and hidden risks that iterative loops create. By coupling per-location geotechnical and site-specific assessments with operational simulation tools, project teams can run far more realistic schedule models, including weather downtime analysis and vessel availability scenarios.

This enables optimisation not only of preload strategies, but also of the broader installation sequence, identifying critical path constraints, highlighting weather-restricted operations, and testing contingencies before they materialise offshore.

With this knowledge, developers can test alternative sequences, vessel sharing strategies, or load-out scenarios before mobilisation. Contingencies such as standby vessels, alternative jack-up sites, or floating feeder vessel options can be stress-tested virtually rather than at the cost of offshore delays. Instead of relying on generalised assumptions and reacting to problems as they arise, operators can proactively de-risk campaigns, safeguard scarce vessel time, and in many cases unlock installation windows (and even vessels suitability) that zonation-based approaches may have obscured under their conservative assumptions.

In general, my view is that, per location assessments can have significant improvements in the ever-demanding offshore wind sector by:

• Improving scheduling

• Optimising schedules and pre-loading

• Unlocking more suitable vessels

• Providing cost certainty

• Improving MWS and 3rd party approval

• Reducing operational risk

That’s not to say zonation is obsolete. For simpler sites, single vessels, few loading conditions, low geotechnical variability, it can still provide sufficient confidence. But should it remain the default? In an era where turbine growth is stretching vessels, where weather downtime risk is magnified, and where every metre of leg penetration matters, the old zonation method might actually be holding us back.

Final Remarks

So, here’s the contentious question:

If per-location analysis is more accurate and provided at similar costs and delivery times (especially in today’s environment of bigger turbines and tighter installation margins), why are we still clinging to zonation?

I’d be genuinely interested to hear others’ views:

• Is zonation still fit-for-purpose, or is it becoming a relic of a smaller-turbine era?

• How should our approach evolve to balance practicality with precision as vessel limitations and weather downtime risks intensify?

• And are clients, insurers, and MWS’ ready to embrace this shift in methodology?