DNV-ST-N002: Site-Specific Metocean Assessment for Mobile Offshore Units

1. Scope and purpose of DNV-ST-N002

DNV-ST-N002 — Site specific assessment of mobile offshore units for marine warranty — establishes the methodology for demonstrating that a mobile offshore unit (MOU) can survive and operate safely at a specific location. It is a normative reference within DNV-ST-N001 (the marine warranty standard), which means meeting N001 obligations for jack-up operations automatically requires compliance with N002.

The standard applies primarily to:

• Jack-up drilling rigs and jack-up installation vessels being assessed for a specific operational site
• Semi-submersibles and other MOUs undergoing site-specific fitness-for-purpose review
• Any MOU where the as-designed class criteria do not bound the actual site conditions — requiring a bespoke assessment rather than reliance on the class certificate alone

The core question N002 answers is: "Do the metocean conditions at this specific site, for this specific season, fall within the envelope for which this unit was designed?" If the answer is yes with margin, the MOU can operate under its class approval. If not, a supplemental structural assessment is required.

2. When is site-specific assessment required?

DNV-ST-N001 §8.3 and §8.3.2 are explicit: jack-ups designed and classed for elevated operations in conditions exceeding those at the installation site shall comply with DNV-ST-N002. The trigger conditions are:

• The jack-up is to operate in elevated position at a specific field location
• A Marine Warranty Surveyor (MWS) company is engaged for the operation
• The unit's class design environment is not clearly bounding the site environment in all relevant parameters (wave, wind, current, water depth, soil bearing)
• Operations cannot be classified as weather-restricted (i.e., duration exceeds the reliable forecast window)

For weather-restricted operations of short duration, the MWS may accept an abbreviated assessment based on N001 §3.3 criteria rather than a full N002 site-specific study. However, for any sustained offshore operation — typically more than 72 hours in elevated mode — the full N002 process applies.

3. Metocean data requirements

The foundation of any N002 assessment is the metocean dataset for the site. DNV-ST-N001 §3.4.2 specifies that statistical data used for characteristic environmental criteria must:

• Cover a sufficiently long time period — minimum 3–4 years for meteorological and oceanographic data; longer periods are required for seasonal analysis and for sites with known high interannual variability
• Be verified for known biases — if the dataset has a documented bias in wind or wave statistics for any segment of the operation area, criteria must be adjusted per §3.4.21
• Extend far enough back to capture significant storm events historically relevant to the site

In practice, site-specific metocean studies draw from three primary data sources:

Best practice is to use a hindcast-as-primary, measurement-as-calibration approach: the hindcast provides long-record statistics, while in-situ or satellite data are used to validate and correct the hindcast bias at the specific site. Hindcast data alone, without any measurement validation, is generally not accepted by MWS companies for high-consequence assessments.

4. Return period framework

The applicable return period depends on the nature of the operation and the mooring configuration. DNV-ST-N001 §3.4.3 establishes that return periods for environmental criteria shall be related to the operation reference period.

The key distinction is between weather-unrestricted operations (long-duration, no weather window constraint) and weather-restricted operations (executed within a reliable forecast window). N002 site-specific assessments primarily govern the former.

For temporary phases lasting 3 days to 12 months (e.g., on-bottom stability during installation), a 10-year return period is specified per §7.9.12. For operations of shorter duration within a reliable forecast window, return periods can be reduced accordingly.

5. Joint probability and the Hs-Tp contour

Extreme wave conditions cannot be characterised by significant wave height alone. The wave period (peak period T_p, or mean zero-crossing period T_z) determines whether a structure is in or near resonance, and controls the maximum individual wave height within a sea state. A high Hs paired with an unusually long T_p can be more onerous for a floating unit than a higher Hs with a typical period.

DNV-ST-N001 §3.4.13 addresses this directly:

• For mobile moorings, a single extreme Hs with a range of associated peak periods corresponding to the relevant return period is generally acceptable
• For permanent moorings, multiple Hs-Tp combinations along the 100-year return period contour line shall be considered — not just the single most-probable combination
• If a contour plot is unavailable, a sensitivity analysis across plausible T_p values is required

The contour method works as follows: fit a joint probability model (often FORM — First Order Reliability Method) to the historical Hs-Tp scatter diagram. Extract the iso-probability contour corresponding to a 100-year return period. Each point on this contour represents an Hs-Tp combination with the same joint exceedance probability. The structure is analysed at multiple points along the contour, and the worst response governs design.

N001 §3.4.12 also requires that directional wave spreading be accounted for when a directional short-crested wave spectrum is applied: S(ω,θ) = S(ω) × D(θ,ω), where D(θ,ω) is the directional spreading function satisfying energy conservation. For swell — which DNV-ST-N001 §3.4.14 flags as critical for motion-sensitive operations — directional spreading values of n = 2 to 10 for wind seas apply, with swell treated as nearly unidirectional.

6. Deriving the 100-year extreme values

Extreme value estimation from a long hindcast record typically follows one of two approaches:

Initial Distribution Method (IDM)

Fit a statistical distribution (Weibull, log-normal, or GEV) directly to the storm-peak significant wave heights extracted from the historical record. Extrapolate to the target return period. This is computationally straightforward but sensitive to the fitting method and threshold choice.

Peaks-Over-Threshold (POT) Method

Define a threshold Hs (e.g., the 95th percentile) and model the exceedances using a Generalised Pareto Distribution (GPD). This approach makes more efficient use of the data in the tail of the distribution and is preferred for sites with limited record length.

For the North Sea — the operational area most relevant to Norwegian Continental Shelf jack-up campaigns — indicative 100-year Hs values range from approximately 13–15 m in the central North Sea to below 8 m in sheltered Norwegian fjord and near-coastal locations. Site-specific analysis is essential; global or regional values are not acceptable for N002 purposes.

N001 §3.4.19 notes that the accuracy of the metocean database must be known and verified. Voyage simulation methods — running thousands of synthetic voyages against the historical database to extract exceedance statistics — are an accepted alternative to direct frequency analysis per §3.4.20.

7. N002 vs RP-C205: roles and hierarchy

DNV-RP-C205 (Environmental conditions and environmental loads) is the DNV recommended practice for characterising environmental loads on offshore structures — covering wave kinematics, Morison's equation, diffraction theory, wind load formulations, and current profiling. It is the technical backbone for how environmental conditions are translated into structural loads.

DNV-ST-N002, by contrast, is about which environmental conditions govern a site-specific MOU assessment and how to document that the unit's design envelope is not exceeded. The two documents work together:

8. Decision matrix: eight common scenarios

In practice, the right standard depends on the unit type, operation duration, and whether an MWS company is engaged. The table below maps eight typical offshore scenarios to the governing documents.

9. Using Leide Navigator for related queries

DNV-ST-N002 itself is queued for ingestion as a HIGH-priority item in the Leide knowledge base. In the meantime, the three standards that underpin any N002 assessment are already available:

• DNV-ST-N001 (728 chunks) — the marine warranty standard that normatively references N002; covers OPLIM, return period framework, MWS documentation requirements, weather-restricted vs weather-unrestricted classification
• DNV-RP-C205 (329 chunks) — environmental conditions and loads; JONSWAP spectral parameters, Morison coefficients, wave crest statistics, current profiling, extreme value estimation methodology
• ISO 19902 (324 chunks) — fixed offshore structures; uses site metocean data per the N002/RP-C205 methodology for jacket and gravity-base design