Weather Windows and Operational Limits
A weather window is a continuous period in which every limit for a lift holds for its full reference period.
Significant wave height (Hs) and wave period are the usual limits, set for the vessel, crane, payload, rigging and lift phase. For a weather-restricted operation, DNV-ST-N001 takes the reference period as the planned operation time plus a contingency, and sets the forecast limit at the operational limit times an alpha factor below 1 to cover forecast uncertainty. A calm hour at the start does not make a window: the limits have to hold for the whole period.
An envelope, a window and a campaign result.
Each answers a different question. Moving from one to the next needs more project information.
Operating envelope
Which conditions satisfy the defined response criteria for the lift?
Required data and output
- Required project input
- Vessel, crane, payload, rigging, compensation system and lift phase
- Output
- Case-specific Hs/Tp operating envelope
Executable window
Do conditions remain acceptable for the complete required period?
Required data and output
- Required project input
- Site and season data, required continuous duration, forecast and alpha criteria
- Output
- Executable windows, including persistence
Waiting time & cost
What delay and cost follow from the planned activities and their windows?
Required data and output
- Required project input
- Mobilisation, sequence, downtime logic and contracted day rate
- Output
- Waiting days and cost distribution
Continuity determines which periods are usable.
Both rows meet the criterion in 8 of 12 intervals. This example requires four consecutive intervals.
Illustrative requirement: at least four uninterrupted intervals inside the criterion.
A / Fragmented availability
8 of 12 intervals inside the criterion
- ✓
- ✓
- ×
- ✓
- ✓
- ×
- ✓
- ✓
- ×
- ✓
- ✓
- ×
B / Longer continuous runs
8 of 12 intervals inside the criterion
- ✓
- ✓
- ✓
- ✓
- ×
- ×
- ✓
- ✓
- ✓
- ✓
- ×
- ×
This is an explanatory sequence, not a weather record or a project screen. An actual operation uses its declared reference period and all applicable criteria.
What changing the threshold does in this record.
NDBC 41002 · 12 February–24 March 2023 · continuous windows of at least 24 hours.
What is a weather window?
A weather window is a continuous period during which all criteria required for a specific operation remain acceptable for its required reference period. It is more than a favourable condition at the start of the lift.
Significant wave height (Hs), commonly described as the average height of the highest third of waves, is one planning input. The allowed conditions depend on the vessel, crane, payload, rigging, equipment and operation; wave period and other relevant criteria are part of the declared basis.
Why weather windows matter for project cost
Waiting-on-weather cost depends on the simulated delay and the contracted vessel rate. Establish the campaign assumptions before attaching a cost to a change in operating limits.
The campaign effect, waiting days and vessel cost, comes from a time-series campaign simulation on the project metocean data, with the operation durations, forecast criteria and contracted day rates in it. That simulation, not an annual percentage, is what the commercial case rests on.
Apply the project’s forecast-uncertainty and monitoring rules. A wider analysed envelope does not replace the forecast criteria, decision rules or contingency allowance for the operation.
- Screen both cases. Apply the analysed uncompensated and compensated envelopes to the same project metocean time series.
- Count usable runs. A percentage of samples below a threshold is not enough; each window must remain inside the criteria for the operation’s full reference period.
- Price the simulated delay. If a campaign simulation returned 10–15 fewer days, then USD 50k–500k per day gives USD 0.5–7.5 M. That is arithmetic, not a project prediction.
The compensated Hs limit comes from analysis: a CONSTELLATION lift-and-heave study documents it. The forecast and reference-period mechanics are on the DNV-ST-N001 page.
How heave compensation expands weather windows
A passive heave compensator decouples the payload from the crane tip motion, reducing the dynamic amplification factor (DAF). How much it removes, and how much workable Hs that buys, is a case result for a stated load channel and lift phase, which is exactly what the lift analysis reports.
There is no fixed conversion from load reduction to allowable Hs: vessel response, hydrodynamic loading, stroke and the governing criterion all move with the sea state, so the envelope is recomputed rather than scaled.
For seasonal installation campaigns (particularly in the North Sea, Barents Sea, or offshore West Africa) this capability extension can be the difference between completing the scope in one mobilisation or needing to return the following season. An ANTARES adaptive compensator is specifically designed for campaigns where the operating range must be as wide as possible.
Planning lifts around weather
Document the operational criteria and reference period in the lift procedure. The nominated project basis and reviewer appointments determine the required review and approval scope.
- Operating criteria: the wave-height and period limits and any other applicable conditions for the operation.
- Reference period: the time the criteria must cover, including the planned operation and assessed contingency under the nominated basis.
- Forecast treatment: uncertainty treatment, decision rules and monitoring requirements selected for the operation.
- Review responsibilities: the appointed MWS, client, class or other review scope, as applicable.
Compare campaign cases using documented operating envelopes and task durations. Any change in execution time must be justified in the operation plan, rather than assumed from the presence of a compensator.
Basis and assumptions
The figure uses buoy observations from NOAA NDBC 41002 in the western Atlantic, covering 12 February to 24 March 2023. Windows are continuous runs of at least 24 h below the two illustrated Hs thresholds, counted with the same uptime engine used in a CONSTELLATION study. The 1.5 m and 2.5 m thresholds are scenario inputs, not generic operating limits and not outputs of a compensator design. This six-week screen is not an annual-availability study or a campaign forecast; those require the project site, season, duration, operating envelopes, forecast rules and contract assumptions. A site-specific operability screening supplies that basis.
Weather windows: frequently asked
What is a weather window in offshore lifting?
What Hs limit applies to an offshore lift?
How much does waiting on weather cost?
How does heave compensation widen the weather window?
What weather criteria does the lift procedure define?
Related on Norwegian Dynamics
- Dynamic Amplification Factor (DAF): Formula, worked example and DNV context.
- DNV Standards for Offshore Lifting: ST-0378, ST-N001, RP-N103 and how they apply to compensators.
- Crane Load Chart: Velocity formula, DAF derating, worked example.
Trying to widen your weather window?
Send the operation sequence and available vessel, payload and metocean data. We can define the analysis and weather-window assessment scope for the project.
