PE=(1-s)ns(1-i)n Equ(1)
where,
PE=probability that no action item is overlooked from an MOC, with explicit scoping
S=probability of overlooking an individual supportive action item
ns=number of supportive action items
i = probability of overlooking an individual informational action item
ni=number of informational action items
Values of ns and ni are determined by the case at hand, such as represented by Table 3, but estimating the other parameters is much more difficult, since these are essentially estimates of human error rates.
Various studies have measured human error rates in industrial circumstances[10]. A common case is to measure the error rate of a person who is following a procedure with many steps. Error rates of 0.001 to 0.1 per task appear in the literature, with a value of 0.01/task commonly used by the LOPA community[11]. The comprehensive research of Swain and Guttmann resulted in 0.003/occurrence [10] .
Note that procedures guide the person in his/her thinking and actions. When attempting to create a task list, as is required in explicit MOC scoping, this guidance doesn’t exist, so one would reasonably expect that error rates are higher. Even if the person is creating a new MOC by using the action item list from a prior MOC, the error rate cannot reasonably drop below the guided task range (0.003 – 0.01).
For the purposes of this analysis, the following values are chosen:
s=0.03/occurrence when the person is not guided by the process, as in explicit scoping
i = 0.1/occurrence when the person is not guided by the process
2.2 Guided Scoping
In “guided” scoping, the initiator is provided with a list of many possible action items and must choose which ones are relevant to the change. Often the checklists group similar items to aid the initiator.
Check all documents to update as part of this change
- Electrical one-line diagram
- Instrument loop diagram
- P&ID
- PFD
- Other, please specify
Select all reviewers for this change
- Area Maintenance
- Environmental
- I&E
- Process Safety
- Other, please specify
Select all approvers for this change
- Area Manager
- ES&H Manager
- Maintenance Manager
- Operations Superintenent
- Other, please specify
Table 4. Guided scoping example.
The analysis of guided scoping is similar to the analysis of explicit scoping, except that the beneficial effects of the list, Table 4, need to be accounted for. The beneficial effects of the list only accrue if the item is actually on the list. This can be expressed mathematically as:
s = probability that an action item is omitted
α = list benefit factor. If α = 0, then having the item on the list provides no benefit. If α = 1, then having the item is on the list provides the maximum benefit.
β = probability that the action item is on the checklist
By adapting equation 1, the probability that zero action items are overlooked from an MOC, with guided scoping is:
PG=(1-[1-αβ]s)ns(1-[1-αβ]i)nsEq. 2
Potential values for the parameters are:
α = 0.7
β = 0.95
2.3 sset-Based Scoping
An MOC initiator who is scoping an MOC using the explicit or guided approaches must think about the physics of the change (i.e. the change to be made to the facility), but the expression of those thoughts is entirely in terms of documents, reviewers and approvers. The initiator must be able to reliably translate the change requirements into a set of action items, without much help from the MOC process.
Asset-based scoping overcomes those shortcomings by expressing the checklist items in terms of plant assets. Taking some examples from Matthew’s work[12], and adapting them to the present discussion yields a checklist as indicated in Table 5. Note that the checklist is phrased entirely in terms of plant assets—which even the most inexperienced person understands—and not in terms of the resulting action items. Once the checklist, Table 5, is completed, each “yes” answer triggers the creation of one or more action items. The relationships between checked boxes and action items are contained in a “rule set”, such as the example in Table 6. Presenting the checklist items, and the resulting action items, as demonstrated in Table 6, can make the MOC form very long. While some companies have implemented asset-based scoping using a paper-based system, it is more common to do this in an electronic MOC system.
Instrumentation Changes
Does this change involve…
- Adding safety critical instrumentation?
- Adding, removing or changing safety critical equipment?
- Electrically powered instruments in electrically classified areas?
- Changes to or the addition of a control valve?
Relief System Changes
Does this change involve…
- Addition of a relief valve?
- Piping on the relief valve inlet or outlet?
- Changing hydrocarbon relief valve vent routing (atmosphere/flare/process)?
- Changing hydrocarbon relief valve relief rate to atmosphere?
Table 5. Asset-based scoping example.
In order to make guided scoping more manageable, the checklist items were organized into a number of categories (documents to update, reviewers, approvers). Similarly, asset-based scoping checklists also benefit from organization into categories. However the categories used for asset-based scoping are, as expected, asset-based[12]: