Executive Summary
DNV's 2026 update to DNV-RP-0585 formalises seismic design as a primary driver for offshore wind in earthquake-prone regions, and the practical consequence lands squarely on site characterisation. Shear-wave velocity profiling, dynamic soil property testing, liquefaction assessment via CPTu, and active-fault mapping all move from optional to load-bearing. The new emphasis on identifying critical design positions across large arrays means the ground model, not the individual borehole, becomes the deliverable. Survey managers bidding work in Japan, Taiwan, the US West Coast and the Mediterranean should budget for deeper, denser, and more dynamically instrumented investigation programmes.
A standards update that lands on the survey manager’s desk
DNV has released a 2026 edition of DNV-RP-0585, Seismic Design of Wind Power Plants, announced in early July 2026. The recommended practice covers both onshore and offshore wind, sits alongside DNV’s standards for wind turbines, support structures and offshore substations, and is written to support projects designed to the IEC 61400 series. DNV attributes the revision to lessons from recent projects and to the ACE 2-EVOLUTION joint industry project, which brought together more than 20 companies from across the offshore wind value chain.
The visible changes number six. The 2026 edition updates seismic analysis methodologies and modelling requirements, revises recommendations for soil-structure interaction and foundation damping, adds guidance on seismic loading and ground motion selection, introduces recommendations for identifying critical design positions within large wind farms, provides new guidance for installation vessels operating in seismic regions, and appends a summary of local seismic design requirements in Japan. DNV’s Mette Redanz, Vice President for Renewables Certification, framed the update around consistent and transparent methodologies. Marcus Klose, who managed the ACE JIP, pointed to close alignment between turbine, foundation and geotechnical disciplines.
That last point is the one we want to unpack. When earthquake loading becomes a primary design driver rather than a secondary check, the burden shifts onto the ground data that feeds the seismic model. The turbine and support structure engineers can only produce a defensible design if the site characterisation programme delivers the right parameters, to the right depth, at the right spatial density. This is a survey-scope story dressed as a design-code update.
What the revised practice actually asks of your ground data
Seismic design of a fixed offshore foundation runs on a chain of analysis, and every link in that chain is fed by measured soil properties. A probabilistic seismic hazard analysis produces a uniform hazard spectrum at a reference horizon, typically bedrock or a firm datum. That motion then has to be propagated up through the soil column to the mudline and into the foundation, because soft marine sediments amplify and reshape ground motion in ways a bedrock spectrum cannot describe. Site response analysis – one-dimensional equivalent-linear or nonlinear, depending on strain level – is the standard route, and it consumes a shear-wave velocity (Vs) profile from the seabed to engineering bedrock.
The revised attention to soil-structure interaction and foundation damping tightens this further. Radiation and hysteretic damping can materially reduce computed seismic demand on a monopile or jacket, but only if the soil stiffness and damping are justified by data rather than assumed. That means small-strain shear modulus, Gmax, derived from Vs and bulk density, plus the modulus reduction and damping curves that describe how stiffness degrades and damping grows as cyclic shear strain increases. Those curves are not a geophysical output. They come from the laboratory – resonant column, cyclic triaxial and cyclic direct simple shear tests on high-quality samples – and they depend on plasticity index and confining stress.
Ground motion selection and spectral matching complete the picture. Selecting and scaling acceleration time histories to a target spectrum is only meaningful once the site class and the site-specific amplification are established, and both trace back to the Vs structure of the ground. In seismically active regions the design is no longer governed by the fatigue and ultimate limit states that dominate North Sea projects. It is governed by an extreme dynamic event, and the parameters that control that event are the ones our survey scope has historically treated as secondary.
Where the geotechnical and geophysical programme grows
Shear-wave velocity, measured deeper and more often
A conventional offshore wind geotechnical campaign is built around cone penetration testing with pore pressure measurement (CPTu) and sampling to pile-toe depth plus a margin. Seismic design breaks that assumption because the Vs profile must extend well below the foundation to characterise the full amplifying column. Seismic CPT (SCPT) captures Vs at intervals during the cone push and is the efficient default for the upper soils. For depths beyond CPT refusal, P-S suspension logging in a borehole, or downhole and cross-hole seismic where budget allows, becomes necessary to reach the firmer horizons that control the transfer function. Cross-hole gives the most reliable interval velocities but demands multiple cased boreholes at close spacing, which is a real cost and vessel-time decision, not a line item.
Liquefaction and cyclic softening
Where loose to medium-dense sands or sensitive silts are present, liquefaction and cyclic softening assessment moves to the centre of the geotechnical scope. The simplified stress-based procedures compare a cyclic stress ratio driven by peak ground acceleration against a cyclic resistance ratio derived from normalised cone resistance, so CPTu quality and continuity matter directly to the answer. Grain-size distribution, fines content and relative density feed the same evaluation. The consequences are what drive foundation cost: loss of lateral pile support, seismic settlement, downdrag, and reduced axial capacity all follow from a low factor of safety against triggering. Getting this wrong in either direction is expensive – over-conservatism buys unnecessary steel, under-conservatism buys a foundation that cannot carry its design event.
Active faults and seismic geohazards
The geophysical scope expands in parallel. Ultra-high-resolution seismic and sub-bottom profiling are already standard for stratigraphy and shallow hazards, but seismic regions demand explicit mapping of active faults, their traces, throws and evidence of Quaternary movement. Surface fault rupture and secondary faulting create avoidance zones that reshape array layout, and earthquake-triggered slope instability and submarine landslides become design cases in their own right. This work only holds together when the geophysical interpretation is read against the regional seismotectonic setting, which means the interpreter needs the hazard context before the survey lines are planned, not after.
The ground model as the deliverable
The recommendation to identify critical design positions within large wind farms is, in our reading, the most consequential change for survey planning. A gigawatt-scale array can span kilometres of variable geology, and characterising every turbine location to full seismic depth is neither affordable nor necessary. The alternative is an integrated ground model that ties dense geophysical coverage to a targeted set of geotechnical locations, then uses that model to find the positions where seismic demand governs – soft-soil amplification troughs, liquefiable pockets, near-fault locations, and transitions in stratigraphy. Those become the design-driving cases. This is a ground-model-led philosophy consistent with the direction of SUT OSIG guidance and ISO 19901-8 on marine soil investigations, and it puts a premium on interpretation, not just acquisition.
The gaps and gotchas that will bite
How deep is deep enough for Vs. The RP frames the requirement in performance terms, and there is genuine ambiguity in where a profile can be truncated. Terminating Vs logging at a convenient CPT refusal depth rather than at a demonstrable firm horizon is the classic way to invalidate a site response analysis after the fact. The depth to the reference horizon is a project-specific judgement that should be agreed with the design and certification teams before mobilisation, not defended in a QA meeting afterwards.
Under-sampling spatial variability. Reducing borehole count to control cost is reasonable, but seismic demand is sensitive to exactly the lateral changes that a sparse programme misses. If the geophysics cannot be tied confidently to the geotechnical control points, the ground model’s estimate of the critical positions is unreliable, and the whole rationale for a reduced programme collapses.
Timing between disciplines. Dynamic soil property testing has a long laboratory lead time, and site response and SSI models cannot be finalised without those curves. Where geophysical acquisition, geotechnical sampling and laboratory testing are contracted separately with no integration owner, the design schedule absorbs the slack. The alignment DNV calls for between turbine, foundation and geotechnical disciplines has to be a contractual reality, not a good intention.
Installation vessels are now in scope. The new guidance for installation vessels in seismic regions reaches the jack-up assessment. A jacked-up vessel experiencing a seismic event faces base shear, potential sliding and overturning, and spudcan-soil interaction concerns including punch-through on layered soils. The site-specific assessment under ISO 19905-1 needs geotechnical data at each leg position, and seismic loading during elevated operations is a load case that many temporary-works assessments have not carried. Survey scope for the construction phase, not only the permanent works, has to reflect this.
Combined loading and uncertainty. Seismic events do not arrive in isolation from operational, wind and wave loads, and in Japan tsunami loading enters the combination. Epistemic uncertainty in the hazard and in the soil parameters should be carried through a logic-tree approach rather than buried in a single deterministic value. The Japan appendix reflects local design levels and site classification conventions that differ from the ISO and Eurocode 8 frameworks many teams default to, and treating it as a footnote rather than a governing document is a straightforward way to fail certification.
What to do now
- Rebuild the geotechnical basis of design around Vs. Specify SCPT as standard through the CPT-testable section and add P-S suspension logging or cross-hole to reach a defensible firm horizon. Agree the target reference-horizon depth with the design and certification teams before the survey is scoped, and record the basis for that depth in the design basis document.
- Book dynamic laboratory testing early. Reserve resonant column, cyclic triaxial and cyclic DSS capacity for modulus reduction and damping curves at the start of the campaign, and protect sample quality accordingly. Treat these curves as long-lead items on the design programme.
- Scope liquefaction explicitly. Where sands or sensitive silts are indicated, require continuous CPTu with pore pressure, grain-size and relative-density sampling, and a factor-of-safety-against-triggering assessment using a CPT-based procedure consistent with ISO 19901-2 and DNV-RP-C212.
- Commission a fault and geohazard interpretation, not just a survey. Provide the geophysical interpreter with the seismotectonic setting before line planning, and require explicit deliverables on active-fault traces, avoidance zones and seismically triggered slope instability.
- Make the ground model the contractual deliverable. Appoint a single integration owner for geophysics, geotechnics and laboratory data, and require the model to nominate the critical design positions with quantified confidence rather than a list of boreholes.
- Extend the design basis to the installation phase. Include seismic loading in the jack-up site-specific assessment under ISO 19905-1, and specify per-leg geotechnical data at planned installation locations.
- Read the Japan appendix as a governing document where it applies, and reconcile its site classification and design levels against your ISO 19901-2, IEC 61400-3-1 and Eurocode 8 references at the design-basis stage, not during certification review.
The engineering message of the 2026 edition is consistent with where offshore wind is heading geographically. When the design event is an earthquake, the quality of the answer is set at the survey stage, and no amount of downstream analysis recovers a ground programme that measured the wrong parameters to the wrong depth. The survey scope is now the binding constraint on a defensible seismic design, and it should be priced and planned as such.
