Refinery units require periodic shutdown for inspection, catalyst replacement and repair. A turnaround takes a unit or a whole plant offline for weeks, and it cannot be deferred indefinitely: catalyst degrades, equipment fouls, and regulatory inspection intervals are mandatory.
Turnarounds cluster seasonally, and the reason is economic rather than technical. A refiner wants to be running flat out when margins are best, which in the northern hemisphere means summer for gasoline and winter for heating fuels. That leaves spring and autumn as the shoulder seasons, and that is when the maintenance goes. The result is a reliable twice-yearly dip in runs and a corresponding seasonal signature in crude demand.
Because the timing is partly economic, it is partly responsive. A refiner facing unexpectedly strong margins will defer maintenance if it safely can, and one facing collapsed margins will bring it forward. So a turnaround season is heavier or lighter than usual for reasons that are themselves informative, and reading the deviation is more useful than reading the level.
Unplanned outages are a different object entirely. A fire, a power failure, a hurricane evacuation or a unit upset removes capacity with no notice and for an uncertain duration. The market reaction differs accordingly: a planned turnaround is priced in advance and largely a non-event, while an unplanned outage moves product cracks immediately and crude in the opposite direction.
Hurricanes deserve a specific mention because of geographic concentration. A large share of US refining capacity sits on the Gulf Coast, along with crude production, import terminals and export terminals. A single storm can simultaneously shut in offshore production, close ports, and take refineries offline, and the net price effect depends on which of those dominates. It is one of the few events that can plausibly move crude and products in either direction.