When the critical path moves
The critical path on most offshore campaigns is computed once, presented at kick-off, and then referred to only when something has already gone wrong. By that point it is usually the wrong critical path – not because the planner made an arithmetic error, but because the schedule was built on a weather assumption that treats every offshore activity as equally exposed.
They are not equally exposed. And the difference decides which path actually binds.
The flat allowance problem
Most campaign schedules apply a single weather allowance across all offshore work – 15%, 20%, whatever the last project used. It is a reasonable-looking number, and it is applied uniformly because applying it any other way requires a workability analysis that takes time nobody has budgeted at tender stage.
But weather does not delay activities in proportion to their duration. It delays them in proportion to how often the sea state exceeds their own operating limit. A multibeam acquisition line and a CPT deployment do not stop at the same significant wave height, and in a marginal season the gap between those two thresholds is where the schedule actually fails.
A worked example
The figures below are synthetic – built to illustrate the method, not taken from real projects – but the structure is one that recurs.
A combined site investigation campaign: a geophysical spread and a geotechnical spread, running in parallel on separate vessels, both feeding a single milestone – an integrated ground model handed to the foundation designer.
As planned at kick-off, with a flat 15% weather allowance applied to both offshore activities:
| Path A – Geophysical | Days |
|---|---|
| Mobilisation | 4 |
| Calibration & patch test | 2 |
| Transit | 1 |
| Acquisition (22 + 15%) | 25 |
| Processing | 10 |
| Interpretation & integration | 3 |
| Total | 45 |
| Path B – Geotechnical | Days |
|---|---|
| Mobilisation | 5 |
| Transit | 1 |
| Sampling & CPT (14 + 15%) | 16 |
| Laboratory testing | 14 |
| Factual reporting | 5 |
| Total | 41 |
Path A is critical at 45 days. Path B carries 4 days of float. The handover milestone sits at day 45.
Everything in the mitigation plan follows from that reading: a standby clause in the survey vessel contract, fixing in advance what an idle day costs, a spare MBES transducer head shipped to the mobilisation port, an extra processor on the team to compress the 10-day processing block if acquisition overruns. All of it protects Path A.
The same schedule, with workability applied
Now replace the flat allowance with each activity's own operating limit, assessed against metocean hindcast statistics for the acquisition window at that site:
| Activity | Operating limit | Workable proportion of window | Productive days required | Calendar days needed |
|---|---|---|---|---|
| Geophysical acquisition | Hs ≤ 2.5 m | 84% | 22 | 26 |
| Geotechnical sampling & CPT | Hs ≤ 1.5 m | 62% | 14 | 23 |
Tap the chart to enlarge it
The geophysical acquisition needed 25 calendar days under the flat allowance and needs 26 – the assumption was close to right. The geotechnical sampling needed 16 and needs 23. The flat allowance under-provisioned it by seven days.
Recomputed:
| Planned | Recomputed | Change | |
|---|---|---|---|
| Path A – Geophysical | 45 | 46 | +1 |
| Path B – Geotechnical | 41 | 48 | +7 |
Path B is now critical at 48 days. Path A has 2 days of float.
Tap the chart to enlarge it
What actually changed
The milestone slipped three days. That is the visible consequence, and it is the smaller one.
The larger consequence is that every mitigation in the plan is now pointed at the wrong path. The standby clause, the spare transducer, the extra processor – all of it protects a path that is no longer binding and now carries float of its own. The path that actually binds, by contrast, has no protection at all: at kick-off it carried four days of float, so nobody assigned it any.
This is the failure mode worth naming: not that the schedule was late, but that the project's protective measures were allocated against a critical path that stopped being critical the moment a realistic weather model was applied – and nobody recomputed.
Where the recovery lever actually is
Once Path B is critical, the instinct is to protect the sampling operation: extend the vessel charter, add a weather standby day, push the window later. All of these are expensive, and none of them are reliable, because the constraint is the sea state and the sea state is not negotiable.
The recovery lever on Path B is the 14-day laboratory testing block – onshore, weather-independent, and compressible. Expediting lab turnaround from 14 days to 9 pulls Path B back to 43 days, at a fraction of the cost of a vessel standby day and with none of the weather risk.
It does not, however, restore the original milestone. With Path B at 43 days, Path A binds again at 46, and the slip narrows from three days to one. Recovering that last day means going back to the geophysical path and compressing the 10-day processing block – which is precisely what the extra processor in the original mitigation plan was for. That mitigation was not wrong. It was premature: it protected a path that had stopped binding, and became useful again only once the other path had been brought back under control.
This is the same lesson applied twice inside a single example. Compress the binding path, and the ranking changes again.
That option was always available. It was invisible for as long as the plan showed the geotechnical path carrying comfortable float.
What to require in campaign planning
- 01A per-activity weather allowance, derived from operating limits. One number applied across the whole offshore scope is not a weather allowance, it is a placeholder. Each activity's limit assessed against site metocean statistics for the actual window.
- 02The workability basis stated explicitly – which hindcast dataset, which years, which percentile. These assumptions drive the schedule more than any duration estimate, and they are usually the least documented part of the plan.
- 03The critical path recomputed weekly, on actuals. Not the baseline re-presented – recomputed, with real downtime to date and an updated forward workability estimate. The path that binds in week four is often not the one that bound at kick-off.
- 04Mitigation mapped to the current critical path, not the baseline one. If the critical path moves and the mitigation register does not follow it, the project is paying for protection it no longer needs.
- 05A near-critical threshold defined. Any path within, say, five days of critical gets tracked with the same discipline as the critical path itself. On offshore work the ranking changes too easily to monitor only the top item.
The underlying point
A critical path is not a property of the project. It is a property of the assumptions the schedule was built on – and on an offshore campaign, the assumption doing the most work is the one about weather.
Change the weather model from a flat percentage to actual operating limits, and the ranking of the paths changes with it. The schedule that results is not more pessimistic. It is pointed at the right problem.
CLEGAR provides project management and technical assurance for offshore campaigns. If you are planning a survey, or reviewing a schedule you have been given, we are glad to talk it through.