Offshore rotation manning: why 9 engineers on duty means 24 on the roster

The fastest way to under-staff an offshore maintenance contract is to plan it like an onshore one. You count the workload, divide by shift hours, get a crew number — and then discover that IOGP-690 §25C duty limits, mandatory days off, and the rotation pattern quietly multiply that number by almost three. Here is the math, with numbers you can reuse.

01The three multipliers onshore plans forget

IOGP-690 §25C caps offshore duty at 12 hours, requires 10 hours of rest between duties, and guarantees 7 days off per month. Each constraint eats availability in a different way:

• Productive fraction: a 12-hour offshore shift yields roughly 9 productive maintenance hours after handovers, breaks, parts runs and deck movements — about 75%, not the 85–90% you might assume in a shore hangar.
• Days-off factor: with 7 days off per month, each engineer works ~23.4 of 30.4 days — availability factor 0.77.
• Rotation factor: on a 4-weeks-on / 4-weeks-off pattern, every seat on site needs a second engineer at home on field break. Roster factor 2.0.

02Worked example: a 4-aircraft AW139 beach base

Scenario: an energy-contract operator flying 4 × AW139 from a coastal base, maintenance demand measured at 80 man-hours per day (line servicing, daily/weekly checks, unscheduled rectification), 7 days a week.

Miss the last two multipliers and you budget for 9 heads — a 62% shortfall that shows up as cancelled leave, duty-limit breaches, or both.

03Certifying staff scale the same way — separately

The multiplier hits each license category independently. Assume the daily plan needs 2 × B1 and 1 × B2 certifying cover per shift:

Note the B2 row: you round up at the days-off step (1.3 → 2), because one-and-a-bit B2s still means one day a month with no avionics signature on site. Rounding down on certifying categories is how offshore contracts end up flying engineers in by charter.

04What the auditor asks for

Under AMC1 145.A.30(d), your man-hour plan must reflect actual availability — and for offshore contracts that means the plan states the rotation pattern, the days-off allowance, and the resulting roster strength, not just the on-duty crew. When the quarterly review compares planned vs. actual hours, offshore operations routinely trigger the deviation discussion first, because leave, training travel and weather days all consume the same 12-hour-limited roster. Model it explicitly and the review is a formality; model it as a flat onshore headcount and you're explaining a 40% gap.