A container fleet is rarely replaced in one purchase. It is topped up as containers retire, and the top-up rate has to cover two different things: the containers leaving service and any growth in the operation. Planning for one and discovering the other is the usual reason a fleet runs short in the middle of a replacement programme.
Two rates, one number
The retirement rate is the number leaving service per year, from the failure data. The growth rate is the additional containers the operation needs as throughput rises. The order quantity per year is the sum, and where a programme plans only for retirements, the fleet shrinks while the business grows – which shows up as a shortage rather than as a planning error.
Why the steady state takes years to reach
A fleet bought all at once all reaches the end of its life at roughly the same time, so the retirement rate is low for years and then spikes. A fleet bought steadily over years has a smooth retirement curve. Where a large fleet is being established, buying in stages over three or four years produces a much more manageable retirement profile than buying once.
Mixing old and new in service
During a staged replacement the fleet contains two generations, and they have to work together – they stack together, they run on the same equipment, and they have to be identifiable. Where the new containers differ in footprint or stacking geometry, they will not interstack with the old ones, and that is a problem that lasts for the entire transition.
Standardising across the transition
The safest approach is to keep the footprint and the stacking geometry identical between generations and allow everything else to change. That way the two generations are interchangeable in service, containers can be replaced one at a time, and there is no point at which the fleet has to be segregated. The argument for this is the same one made in phased automation rollouts.
When a clean break is better
Where the old fleet is a different design entirely, or is worn out, or no longer suits the process, replacing it in one go is cleaner than a long transition. The capital cost is concentrated in one year and the operating cost is lower afterwards. Where a fleet is old and mixed, a clean break is often the right answer despite the appearance of expense.
Budgeting for it
A replacement budget based on the measured retirement rate is a forecast; one based on an assumed life is a guess. Where a fleet has a year of retirement records, the forecast is straightforward and it usually shows a higher rate than assumed – because loss is almost always underestimated.
What to do with the old containers
Containers being replaced are not necessarily scrap. They may be usable on less demanding lanes, they may be saleable, and they are recyclable. Where a fleet is being replaced, the disposition of the outgoing containers is worth planning before the new ones arrive, because containers with nowhere to go occupy the space the new ones need. The recycling question is covered in what happens to a container at end of life.
The review point
Once a year, compare the actual retirement rate against the plan and adjust. Where the actual rate is higher, either the containers are wearing faster than expected or loss is worse than thought – and both are worth knowing early rather than at the point the fleet runs short.