Understanding AOS and MSS for Oil & Gas /Petrochemical EHS Management

*Updated April 2025*

Alternate Operating Scenarios (AOS) and Maintenance, Startup, and Shutdown (MSS) are particularly difficult concepts to distinguish in the Oil & Gas and Petrochemical Industries. But knowing the difference between the two, and understanding how each one affects your emissions tracking and reporting obligations is paramount to keeping your facility in compliance.

Understanding Maintenance, Startup, Shutdown Events

Maintenance, Startup, and Shutdown events are tasks that you undertake in order to ensure the smooth and ongoing operations of your equipment and processes, but are not part of your everyday production tasks. MSS events fall into two categories: planned and unplanned.

Planned MSS: a routine and deliberate activity that occurs on a reliable schedule. For example, you might have a planned MSS activity once a year, after 1,000 hours of operating a piece of machinery, or once per shift. The scheduling of these types of events is flexible and based on your needs. For an MSS activity to be a planned one, you don’t need to know the exact time and date it will occur. Just know the schedule at which to expect it (i.e., The first Monday of every month is an acceptable schedule without having to list the date of each first Monday per month).

Understanding how to write schedules for MSS events is essential since some states now require you to submit an annual schedule for all your MSS events ahead of time. Any MSS activities that were not submitted and approved would be considered to be unplanned instead, even if the owner/operator had scheduled the MSS event internally.

Unplanned MSS: Unplanned MSS activities are those actions you take in response to unpreventable and unanticipated malfunctions or equipment failures. Whereas planned MSS events are designed to prevent malfunctions before they happen, an unplanned MSS event typically takes place to resolve an unpreventable equipment malfunction. Unfortunately, one of the biggest consequences of an unplanned MSS event is a sudden release of uncontrolled air contaminants - which puts your compliance record at risk. EHS Managers should take extra precautions and care during these unexpected malfunctions since you’ll need to know exactly which emissions became fugitive, the quantity of emissions released, the length of the unplanned MSS event, and exactly which responsive actions took place.

Understanding Alternate Operating Scenarios (AOS)

An AOS is a mechanism designed to allow a facility to deviate from its regular operations on the condition that the facility’s emissions do not exceed permit limits and for a temporary period.

The primary reason for an AOS is to reroute the flow of emission-generating materials through your processes so that you can perform maintenance on a piece of equipment without entirely stopping production.

For example, say crude oil is sent to a tank on-site, with emissions being recovered by a Vapor Recovery Unit (VRU) and the crude is sent down a pipeline. If the VRU requires maintenance, the operator can either shut down production, let the emissions become fugitive and compromise regulatory compliance, or reroute the vapors to a flare. Since the first two options are undesirable, the operator would send the vapors to a flare in order to keep emissions within permit levels.

This temporary rerouting is an Alternate Operating Scenario and would be permissible up to the point where air emissions exceed permit limits. Depending on the method and effectiveness of the control technology (or lack thereof) being rerouted, this AOS period will vary in length.

All possible AOSs for your facility must be written into your permit and approved by regulators before you begin operations. They must be revisited any time you want to add a new piece of equipment (i.e., an additional tank to your tank battery), requiring a great deal of forethought.

AOS and MSS: Knowing the Difference

Alternate Operating Scenarios are closely related to MSS events but with critical differences. Until recently, the lines between an AOS and MSS were somewhat blurry, and many businesses incorrectly reported AOS emissions under the MSS umbrella. Now, state regulators have clarified the difference between the two and made clear the compliance expectations for both.

In both AOS and MSS, a piece of equipment is taken offline for maintenance reasons.

In reality, an MSS and AOS typically occur at the same time. In the above example, shutting down the VRU for planned maintenance would be a planned MSS event, and the rerouting to a flare would be an AOS, both occurring simultaneously.

The critical difference is how an EHS Manager is required to track and report those emissions. In the MSS shutdown of the VRU, there would be gas left in the unit that escapes, uncontrolled, into the atmosphere. These emissions are the ones reported as MSS emissions.

In contrast, all of the emissions related to the facility’s operations under the temporary rerouting to a flare - the emissions of the entire process flow, not just air emissions via the flare - are tracked and reported as AOS emissions.

Tracking AOS emissions can be more difficult for a few reasons:

1. A) There are typically more emissions sources to monitor during an AOS versus an MSS;
2. B) You have to know how long you can run the AOS before you exceed your permit limits;
3. C) A facility will typically have several different AOSs to choose from, each with its own emission rate;
4. D) You must keep careful track of your AOS emissions and incorporate those into your overall annual/monthly/etc. emissions, as AOSs are not considered a special exception to your normal operations in terms of regulatory compliance.

Best Practice for AOS & MSS Management

The best technique for handling AOS and MSS emission tracking and reporting is to use an automated system that is able to do, at a minimum, the following:

1. Collect emissions data automatically from continuous emissions monitoring and continuous parametric monitoring systems (CEMS/CPMS). That way, if either occurs, you’ll have detailed, real-time records of how long the event lasted, what type of emissions were released, and when the equipment was back in compliant condition.
2. Virtual Process Mapping allows you to model out the potential emissions from any of your AOS routes or MSS activities before they occur. This means you’ll be able to anticipate the quantity and type of emissions to report, as well as how long you can be in an AOS before your emissions exceed your permit limits.
3. A Centralized Database System that stores all your information about materials, emissions, control efficiencies, AOS emission rates, and MSS emissions in one place so that you have all the data you need for reports immediately accessible and in one spot.

ERA has developed a comprehensive and free downloadable guide that gives you even more information about MSS and AOS reporting, including important information about how to get AOSs approved and why EPA TANKS software might not be your best option for e-reporting tools.

Taking the Right Approach to AOS & MSS Reporting

Alternate Operating Scenarios (AOS) and Maintenance, Startup, and Shutdown (MSS) events are some of the most discussed topics among Oil & Gas manufacturers and their regulators, and it’s a topic ERA Environmental has covered on its industry advice blog, too.

If you need to learn more about the role of AOS and MSS for your facility, we suggest you start with our quick primer article. Today’s article will focus on some small best practice tips and advice to make managing your AOS / MSS reporting and tracking even easier (and more cost-effective, too).

Don’t Fear the Regulator

Being confused about the difference between an AOS and MSS or not knowing under which category to report emissions from a certain operating scenario is nothing to be ashamed of, and it’s not something your regulators will penalize you for if you come to them for advice. In fact, in many cases where Oil & Gas air regulations are changing, state regulators are in the same boat and would be happy to work with you to create some regulatory clarity.

Unfortunately, many Oil & Gas EHS Managers are afraid to contact their state regulators with questions about MSS and AOS because they fear it will prompt a full investigation into a facility or be used as an admission of guilt for future deviations. These fears are unfounded.

Regulators are a vital resource to manufacturers that want to do good business, stay compliant with air emissions regulations, and put in an honest effort into compliance even when they aren’t 100% sure how. Regulators aren’t out to catch or snare EHS Manufacturers that need help - all regulators want is to help you be in compliance.

That being said, to have a good relationship with your regulators, you do need to have good record-keeping practices and be able to provide all of the information they need to do their jobs. It’s only when gaps in your record-keeping appear that regulators become wary. If you aren’t sure how to categorize air emissions generated from an on-site activity, you had better know which type of emissions and what quantity was released before you ask your regulators for assistance.  

It’s also always better to ask for clarification before taking action than asking a regulator for forgiveness after the fact.

AOS/MSS Deviations: When in Doubt, Report - Accurately

It is no secret that whenever something goes wrong, and you release unauthorized emissions during an AOS or MSS activity, your best choice is to be 100% transparent and report those emissions to your state regulator as quickly as possible. There is never any benefit to sweeping those emissions under the rug. Even if you’re not sure how to categorize the deviation, or if you’re not positive it should be considered a deviation, getting a notification in writing in front of your regulators is the safest and most responsible route. They will be able to help you sort things out after the deviation notification in order to minimize or avoid noncompliance fines.

However, ERA Environmental also advises manufacturers to take the time to ensure the emissions you do report in that deviation notification are as accurate as possible - if you’re using an automated reporting platform, this will only take a few minutes and won’t jeopardize your notification timeline. Submitting an inaccurate deviation notification is not in your best interest in the long term.

Sometimes, EHS Managers grossly overestimate the resulting emissions in order to avoid underreporting and creating more trouble for themselves. However, overestimating too much triggers increased regulator scrutiny, urgency, and increased potential fines. Being conservative and overestimating is still acceptable, but having an accurate report that doesn’t over- or underestimate is even better.

Instead of guessing, have a system in place that can accurately track and report emissions from an AOS or MSS activity on demand. This typically means having an automated EHS platform that is capable of virtual process mapping - modeling out the emission factors and processes of any potential AOS or MSS setup and tapping into Continuous Monitoring System (CMS) data.

When you provide an accurate and realistic report within the required deviation notice timeline, it shows your regulators that you have taken the event seriously, are on top of your emission data, and are trustworthy. It also saves you the time of having to revisit the event afterward to generate the “real” emissions report… you’ll already have the real-world data from the get-go.

Get Authorized MSS and AOS in Writing

If you have a large number of MSS activities to plan for and write into your permit, or if your facility is one that experienced a change in MSS state regulations, you may be unclear which of your MSS activities have been authorized by your regulators.

The simplest solution is to ask your regulator for a document outlining which of your activities have been approved. It doesn’t need to be as complex as your Title V permit, just a bare-bones outline of authorized MSS activities (and their approved schedules) and AOSs (of which there can be many). Keep this document handy and accessible in a centralized file-sharing system so that anyone who might need to can access it.

If you’re in the process of rewriting/modifying your permits, applying for a new permit, or adding new MSS/AOS to your permits, it will be important to know which activities are already authorized.

ERA recommends that you write in as many potential MSS and AOS setups as possible and get them authorized so that you’ll be covered for nearly any situation, and that means it can get more difficult to know what was already authorized in your permits, what has been approved recently, and what is pending approval.

Although your Title V permit will also need to contain these approved activities, it can be much easier to keep track of them if they are also contained in a second, streamlined document. The hours saved from digging through a long operating permit could have a significant impact if you were faced with an unplanned MSS event or other emergency requiring an AOS to be run.

It’s about getting your MSS and AOS information put down in writing in the simplest way possible. Not only will you get a clear picture of your acceptable actions, but it will make the audit process much faster if a regulator comes to check-in.

Document Before, During, and After Any MSS Activity or AOS

Documentation is usually the last thing on an EHS manager’s mind when there’s a malfunction or significant maintenance event happening in their facility. Our first instinct in an EHS crisis is to act quickly to resolve the issue, minimize damage, and cease the release of emissions. And those should all be top priorities.

But documenting your actions, the emissions, and the timeline must also be on that top tier of priorities. No matter how well you manage an emergency on-site, your regulators only have your documentation as proof. You could have done everything right, but without the paperwork to prove it, it’s as if your proper response never happened at all.

So get everything down in writing the moment it happens so that you can have a real-time report of how you handled the MSS activity or ran the AOS according to federal and state regulations.

This real-time report includes the before, during, and after operations of your facility. Each one is essential for demonstrating that unplanned MSS events were unanticipated and unpreventable and that you made the right decisions given the information at hand.

For example, if you have Continuous Monitoring Systems in place that notify you the minute a flare stops operating at a minimum regulated combustion temperature, you should look back on the flare’s record for the hours before the MSS event to determine the event’s history before it required attention.

It’s also essential to record the “Close-Out” of any MSS or AOS: when the event ended, sign off on every action taken, do a damage report, hours worked, and a root cause analysis. Many of these steps get forgotten for non-emergency tasks like planned routine maintenance. Although you may not need to collect the same information for a planned MSS emissions report versus an unplanned MSS deviation report, it’s always better to keep the most comprehensive records possible, even if it’s just for your internal tracking.

Ask an ERA Expert:

Do you have any questions or concerns about the topic covered in this article? Want more insight? Now is your chance to ask one of ERA’s Environmental Specialists. You can schedule a discovery call with one of our project analysts today. Please leave your question or comment below, and we’ll make sure one of our expert scientists responds.