Rule-based daily maintenance scheduling

This is better than using only one fixed guiding principle to schedule your work

Any plant that has a significant backlog of maintenance work orders knows the difficulties of deciding what work orders will be scheduled and when to schedule them. Scheduling becomes even more difficult when the same crews complete corrective maintenance jobs as well as take care of the preventive maintenance (PM) work.

Scheduling is even more complicated in the plant that understands the benefits of detailed planning and plans work orders with several jobs. Each job may have more than one resource estimate for skilled technicians, contractor craft workers, special tools, or equipment. It is impossible to effectively schedule by hand a work order with several multi-skilled jobs. This is especially true when there is any kind of interdependence or networking between the jobs.

In many plants, the scheduling function consists of maintenance and operations supervisors deciding on a daily basis which jobs are to be done the following day. The input to this scheduling meeting is a list of backlog work orders with their priorities. General consideration is given to whether the right skills, tools, materials, permits, and equipment are available.

It is impossible to optimize the usage of rental cranes or specialized contractors, with resulting heavy costs for these under-utilized resources. In addition, focusing only on creating a schedule for the next day or even the next week causes some critical targets within the next month to be missed. The manual scheduling is nothing more than a best guess.

Even with the advent of computerized maintenance management systems (CMMS), scheduling is a very basic, non-optimized function. Typically, the CMMS displays the available man-hours by skill and as jobs are scheduled manually, these available man-hours are decremented.

The rule based approach to daily maintenance scheduling proposes that software can have the ability to create effective schedules that lead to significant cost savings through better resource utilization and higher schedule compliance.

An automatic scheduling engine usually creates schedules on a weekly basis with a scheduling horizon of three to four weeks.

Planning defines the scope of work on a work order or PM in terms of a network diagram showing the sequencing of the jobs as well as which jobs can be done in parallel. Planning also describes how long each work order or PM will take and which crafts or individuals and other resources, including parts and material, are required to do that job.

Scheduling objectives
There are at least seven basic objectives that define the requirements for scheduling daily maintenance work. These objectives translate into reduced maintenance and operational costs.

• Schedule the maximum work, safely, with the available resources.
• Schedule according to a set of priority rules--highest priority work orders scheduled first.
• Schedule the maximum number of PM jobs when due.
• Minimize premium time worked on Saturday, Sunday, and night shifts.
• Minimize contract and outside resources required.
• Minimize movement of resources between areas.
• Minimize the wait time for permits, tools, heavy equipment, crews availability to start a scheduled job.

The programs with a scheduling function use only one simple rule for determining the highest priority job--least float to required completion date, or work order priority. This creates schedules that are often far from optimal when measured in terms of actual schedule compliance or in terms of how well the schedule is followed on the job.

An automatic scheduling engine usually creates schedules on a weekly basis with a scheduling horizon of three to four weeks. The schedule combines every PM scheduled for the next three to four weeks with the backlog of corrective work orders. New work orders can be added at any time to the existing schedule.

Priority rules
First, let's look a little closer at the key priority rules that make for better schedules. The scheduling algorithm combines these rules when deciding which job is assigned the available resources first.

By allowing the planner to change the relative importance of the following rules and even to change the way they are combined into a priority function, the scheduling can be fine-tuned to a particular plant. In practice, it may take several months to gain the necessary experience to properly tune the scheduling function.

Work orders work in progress at the time of scheduling are usually given the highest priority. The scaffolding, tools, permits, and special equipment are already in place and the crew is familiar with work requirements.

A simple method of grouping the equipment and specifying the preferred sequence of work overcomes priorities based on a single rigid rule. Consider what happens in real life--if there is a row of equipment to be worked on, the chances are good that a foreman starts the crew at one end and they work their way along. The chances are also very high that a computer program will not schedule the work that way if it is constrained to using only one rule.

Most plants use a simple work order priority system such as E (emergency), 1, 2, 3, & 4. It is also important for the planner to be able to override the preferred sequence of work, for example, if there is either critical equipment or equipment for which the real scope of work is unknown until that equipment
is opened.

The length of time in backlog is another important rule. Within the user specified work order priority, the priority should increase the longer a work order is in the backlog.

Another valuable rule deals with work order type. The rule-based scheduling described here also allows the user to give a relative importance to the type of work order--corrective, PM, shutdown, capital, and so forth.

The work type also affects scheduling. On a work order broken down into several jobs, tasks are ideally scheduled one after another. However, with some of these work orders it makes sense to allow a break after some preparatory work has been done--erecting scaffolding or stripping insulation. Similarly, the removal of the scaffolding and other cleanup work can be scheduled sometime after the last maintenance job.

Accommodating equipment criticality addresses the fact that the failure of different equipment can have significantly different impact on the plant's operations. With an appropriate equipment criticality classification in place, the equipment criticality is one more rule to be factored into the priority calculation.

A super-critical job can be flagged as must start. This means that whether resources are available or not, this job is scheduled at its earliest start time or when its scheduled predecessors are completed.

Scheduling rules
Next, we examine the type of scheduling rules that create practical schedules. With continuous working, if two consecutive jobs use the same crew or even with consecutive jobs on the same work order, it makes most sense to schedule these jobs one after the other. Unfortunately, most scheduling software cannot relate these jobs, other than their correct sequence, and so the second job may be scheduled many hours or shifts later.

The simple rule of minimizing splitting of work over a shift end stops the program from scheduling a four-hour job for the last two hours of one shift plus the first two hours of the next shift. With this rule, the job is delayed until the start of the next working shift on which it can be completed within one shift. The obvious advantages are continuity of work, supervision, permits, and tools.

Some work orders may be waiting for restraints like equipment availability or parts or other material to arrive. These work orders are given a do not schedule before restraint date. Based on the promised date and actual arrival date of material and parts, the program automatically revises this restraint date.

Often the work requester enters a required by date on higher priority work orders. For these work orders, this required by date is a late restraint that is used by scheduling.

Time versus resource constrained scheduling means that if there are insufficient resources to complete all the work on time, either the resource must be over-scheduled or the time extended. This is a user specified scheduling option. One advantage of working with the time-constrained approach is that it gives the planner a good sense of exactly what resources are required to meet the requirements of high priority work orders and PMs being scheduled when required.

Another factor to consider is minimal crew movement and centralized scheduling. A large plant is often divided into distinct planning areas and it is important to avoid moving crews from one area to another. There are also support crafts--inspectors, contract resources, and support equipment such as cranes--required in all areas and must be scheduled without conflict.

The solution is to have two or more levels of resource availability, one for each area for the key crafts and one for the common pool of support resources. Scheduling then balances each area with its own resources, while at the same time scheduling  the support resources over every area.

The manual scheduling is nothing more than a best guess.

Rules should avoid job interruption. A simple rule states that if a high priority job cannot start because of a lack of resources, and if delaying that job would miss a critical required by date, then interrupt the lowest priority job that is in progress that frees the required resources.

Implications
This approach to using a computer program to automatically schedule daily maintenance enables key scheduling objectives to be met. At the same time it creates a schedule that is both practical and workable.

Schedule compliance is the percentage of jobs starting at their scheduled time. High schedule compliance
will only be achieved if the first line supervisor is handed a schedule that makes sense! This means a schedule that contains jobs on equipment he has access to, and he has the crews, the tools,
heavy equipment, necessary permits, and the like to perform those jobs. The result is tighter control of daily maintenance scheduling with significant
cost reductions.