Create a sensitivity analytic

The Sensitivity analytic provides the ability to define, run and analyze sensitivity cases. With sensitivity cases, you can vary one or more quantities by defining a number of steps over a range. You can then optimize the case for each step and see the effect of the changes on the objective function.

Sensitivity analyses allow you to take into account likely changes in plant operations or market conditions. This helps reduce the chance that a small deviation from the plan would result in a very large loss in operating margin.

To create a sensitivity analytic follow these steps:

1. Select the case you wish to use as the basis for the analytic.

2. Click Add Analytic > Sensitivity in the Cases group on the Home ribbon tab.

   

Sensitivity items

Sensitivity items are added from watch items. If a sensitivity item's original watch Item is deleted, the sensitivity item is also deleted.

Watch item types and variables that can be used as sensitivity items include:

• Purchases: minimum, maximum, price.

• Sales: minimum, maximum, price.

• Unit constraints: minimum, maximum, fixed.

• Product specifications: minimum, maximum.

• Feedstock properties: fixed.

  Tip: Sensitivity Items are used to set new values for constraints and input data for case definitions. Final case settings can also be modified via the Stack View to allow further editing of the case definition alongside sensitivity items.

Due to a known issue, entering a description in the Details page after adding sensitivity watch items and scenarios to the Sensitivity page causes the items and scenarios to disappear.

Follow either of these workarounds:

• Enter the description before adding watch items and scenarios.

• Enter the description in the Description column of the case list in the Model Explorer, rather than the Details page of the sensitivity analytic.

Editing an existing description does not cause any data loss.

Feedstock property sensitivity

The properties of purchased feedstocks can be varied via the sensitivity analytic. This allows you to assess different feedstock qualities and their effect on the overall plant.

3. Add feedstock properties to a watch set

   Ensure the feedstock properties for the relevant feedstock are in a watch set. To add feedstock properties, you may need to select Properties in the Object Browser Options tab of the Watch window.

   

4. Add feedstock property watch items as sensitivity items

   Add the relevant feedstock property watch items to the list of sensitivity Items.

   

5. Enter new feedstock property values

   Override the original feedstock properties with new values. Either use deltas on the original qualities, or enter new fixed values for each property.

   By default, sensitivity items are run independently in each scenario. You may wish to consider grouping the sensitivity items so that they are all varied together at once. Thus, in a group, each scenario defines a whole new set of properties for the feedstock.

   

6. Run cases

   Review the case stack and run the cases.

   

   Use the sensitivity charts and the stack view to review the solution results. Check that the feedstock properties have been updated to their expected value in each case.

   

Feedstock purchases

7. Add feedstocks to a watch set

   Ensure the feedstock purchases are in a watch set. Add them in the model workspace or via the Stack View.

   

8. Add feedstock sensitivity items

   Add the feedstock watch Items to the Sensitivity list.

   

9. Vary feedstock settings

   Enter the values for the amount of each feedstock. You can enter absolute amounts, minimum, maximum or fixed amounts, or use deltas relative to the parent case solution.

   

   Tip: To find feedstocks with non-zero purchase amounts, sort the solution column.
   You can vary multiple parameters at the same time. For example, you could vary the amount of feedstock and its purchase price. Add the watch item multiple times, set the sensitivity parameter differently for each watch item and group the sensitivity items so they are analyzed in step.

10. Run cases

    Run the cases.

    

    Analyze the solution results using either the stack view or the sensitivity charts.

    

Product specifications

11. Add product properties to watch set

    Ensure that the product and its properties are present as watch items in a watch set. The minimum and maximum property values of the product should be included as watch items from the Products branch.

    

12. Add watch items as sensitivity items

    Add the necessary watch items as sensitivity Items. Configure the new minimum or maximum values for the product specification as scenarios.

    

    If a new minimum and maximum are required, add the watch item twice and configure these separately. Group the items so that they are stepped together.

    

    Consider what should happen to parent case solution values when using sensitivity items. Normally these are free to move, allowing the variables to respond to the sensitivity changes, but this may make comparing sensitivity runs difficult.

    Consider whether some parent case solution values should be locked. This will freeze the values in all sensitivity runs, so that other variables must change according to the sensitivity. For example, it may be useful to freeze the crude purchases when performing an operating parameter sensitivity, to understand how day-to-day unit operating changes would affect the standard crude diet.

    When locking solution values, take care not to lock too many values or inappropriate values according to the sensitivity, as this may cause the solution to become infeasible. For example, you might lock the crude diet and swing cut temperatures and vary the sales of gasoline and diesel. It may not be possible to meet the sales required by the sensitivity changes, as there is not enough upstream freedom within the optimization.

    If you want certain parent case items to remain the same, consider either of these solutions:

    • Lock the items in the parent case.

    • Add the items as sensitivity items, setting these to be the same as the parent case solution and grouping them together.

13. Run optimization

    Run the cases.

    

14. Review results

    Review the sensitivity results. Use the sensitivity charts, crude mix and product mix to compare the scenarios.