Post-operation safety time

In frePPLe you can define a time gap after the operation has finished. This time gap adds some safety time in your plan, which can absorb variability or unforeseen events.

This feature is typically used for key operations in your plant. This can be at a logical decoupling point in the production process, where you have some controlled work-in-progress buffer stock. Or it can be right before a bottleneck station, where you want to assure the bottleneck station doesn’t go idle because an upstream station has a small production delay.

The solver considers the post-operation time as a soft constraint. This means that we will try to respect it in the plans, but if it is required to avoid planning an order late we will generate a plan that violates the constraint. In that case we’ll have a plan with rushed operations planned to follow each other quicker.

Example

Excel spreadsheet operation-posttime

In this example we model a two-step production process with a decoupling buffer between both steps. A post-operation delay of 3 days is specified for the first step: ideally there is a delay of 3 days between production into the decoupling buffer and the consumption from the decoupling buffer.

There are 4 demands in this model with a different due date.

The first demand is overdue. When you review its delivery plan, you notice that we planned to consume the material from the decoupling buffer as soon as it is produced. We violate the 3 day safety time, because it would only delay the order further. Instead, we rush the order through the different ASAP to minimize the delivery delay.

The second demand has a due date that is later. In its delivery plan you can already see a safety time of 8 hours that we can plan in the decoupling buffer.

The next demands are due even later. We could plan this orders on the requested due date, while fully respecting a safety delay of 3 days in the decoupling buffer.

Note that in this example, there is no inventory planned for the finished product. We plan to ship the product as soon as the second step of the production process is finished. Only the decoupling buffer is planned to carry inventory. In the cookbook recipe on quantity based safety stock we will pick up this very same example and add safety stock targets for the finished product.

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