Operability envelope

The operability envelope is the set of sea states, headings, and speeds within which a ship or platform can carry out its mission while staying inside chosen seakeeping limits, expressed as the percentage of time those limits are met against a wave scatter diagram. Limiting criteria include root-mean-square roll and pitch, vertical and lateral accelerations, motion sickness incidence, slamming and deck-wetness rates, and propeller emergence. For each cell of the scatter diagram the linear responses are computed, the criteria checked, and the operable fraction summed over the long-term climate to give the percent-time operability or workability index used to compare hull designs and mission feasibility.