Using QRA to assess risk from Simultaneous Operations (SIMOPS)
- Carsten Stegelmann | Principal Consultant
- May 25
- 5 min read
In this article:
It is often required to perform construction, decommissioning, drilling, etc., next to live hazardous process facilities. The risk management of such Simultaneous Operations (SIMOPS) represents unique challenges which can be assessed by tools such as a SIMOPS QRA.
Introduction
In an industrial setting with hazardous process plants (a COMAH site in the UK or a Seveso III site in the EU), it is often required to construct facilities in the vicinity of the existing process plant. It may also involve decommissioning of facilities next to a live process plant. These are examples of Simultaneous Operations (SIMOPS) that introduce new hazards compared with normal operation.
In an offshore setting, SIMOPS frequently arises during drilling or well-intervention campaigns where a drilling rig or similar unit operates alongside a fixed offshore installation (and may be classified as Combined Operations (COMOPS)).
In this insight, the focus will be on evaluating risks from SIMOPS for construction or decommissioning activities at onshore plants. The focus will be on the SIMOPS hazards introduced by the nearby live process facilities and not ordinary occupational hazards present on virtually all construction sites.
Why is SIMOPS QRA Needed?
Onshore construction and decommissioning campaigns often run for several years, so shutting down neighbouring hazardous facilities for the duration of the works is rarely viable. Decision-makers therefore need a to determine the potential risks and their required management, and QRA is one of the most effective methods for providing that input to the decision-making process.
Compared with normal operation of the process plant, the construction or decommissioning campaign introduces several new hazards that need to be considered. These typically include:
Increased workforce in the vicinity of the hazardous process plant, i.e., the construction workforce may often consist of several hundred workers at the site for prolonged periods, sometimes including accommodation;
The construction/decommissioning campaign will normally introduce a number of additional strong ignition sources:
Hot work, e.g., in the form of welding, grinding, etc.;
Increased vehicle traffic on site;
Increased use of non-Ex equipment on site.
Construction/decommissioning may lead to increased risk of Loss of Containment (LoC) from the operating process plant:
Dropped or swinging loads hitting live process equipment;
Vehicle collisions with live process equipment;
Tie-in to live process equipment or isolation from live process equipment;
Increased levels of vibrations on site from the use of heavy machinery.
Hence, compared to a QRA performed for normal operation of a process plant, the risk may increase significantly for SIMOPS as more personnel are exposed, there is higher risk of ignition of flammable releases, and there is potentially higher risk of LoC.
Risk Metrics and Acceptance Criteria
In a SIMOPS QRA, the primary concern is the risk to the construction or decommissioning workforce. By contrast, a design QRA covering normal operation typically focuses on third-party risk outside the plant boundary. Third-party risk may also be relevant in a SIMOPS QRA, particularly if it increases compared with normal operation or if several adjacent stakeholders are involved.
When performing a SIMOPS QRA, it is therefore often necessary to start by defining Risk Acceptance Criteria (RAC) for the construction/decommissioning personnel, as these often may not have been part of the QRA for normal operation. Here, it is typically relevant to consider both the individual risk (IRPA) of different personnel groups taking part in the construction/decommissioning campaign. In addition, it is relevant to consider the group risk (normally in the form of F-N curves), since a large workforce is typically involved compared to normal operation of a plant.
Applying RAC originally established for third parties to construction and decommissioning groups can be a conservative starting point. As a rule of thumb, the acceptable risk limit for workers and associated groups is often set at least one order of magnitude higher than for third parties.
Fatality rulesets also differ between worker groups and third parties impacted. For third parties, it is typically assumed that exposed people make no successful attempt to escape or protect themselves. Operations and construction or decommissioning personnel, however, are trained for the surrounding hazards and equipped with appropriate work clothing and personal protective equipment (PPE), and so are better able to escape to a safe location when exposed.
Hence, unless a very conservative approach is taken, the risk to people needs to be calculated differently than is normally applied in the design QRA.
Hazard Identification (HAZID)
As with any QRA, a thorough HAZID workshop is essential. In a SIMOPS QRA, the HAZID should focus on the specific SIMOPS-related issues described above.
Depending on the stage of campaign planning, the HAZID may be extended to include other construction or decommissioning hazards for the workforce that are not directly related to the presence of live process equipment.
In many cases, the HAZID feeds into a broader SIMOPS risk assessment that produces matrices of permitted SIMOPS activities or identifies mitigations for specific activities. This is commonly captured in a Matrix of Permitted Operations (MOPO).
QRA modelling for SIMOPS
Traditional QRA modelling is performed based on a specific Assumption Register for the SIMOPS. As in any QRA, the quality of this register determines the quality of the results — “garbage in, garbage out”.
The special aspects for SIMOPS include a much more advanced and detailed ignition model than is typically necessary for a normal design QRA. This involves estimates of the amount and type of hot work, the amount of vehicle traffic, etc.
In addition, it may be necessary to modify frequency contributions to LoC from external impact, e.g., from dropped and swinging loads, etc.
In short, the underlying QRA modelling principles are the same; the SIMOPS QRA is simply more detailed and, in practice, more complex to execute.
Risk Treatment and ALARP Demonstration
As with any QRA, the first step is to compare calculated risk levels against the RAC. If risks exceed the acceptable limit, the planned SIMOPS campaign must be significantly revised or, in the worst case, abandoned.
However, even if the risk levels are below the acceptable limit, this does not necessarily mean the risk is acceptable. Normally, the ALARP principle needs to be followed, and the risk must be reduced further until the cost of further risk reduction becomes disproportionate to the gained risk reduction.
The SIMOPS QRA can be used to pinpoint what drives the risk and thereby aid in identifying effective risk-reducing measures (RRMs). For example, if hot work leads to high ignition probabilities, a risk-reducing measure could be to only perform hot work during a complete or partial shutdown or to perform hot work inside pressurized habitats, etc.
The SIMOPS QRA can also be applied to calculate the safety benefit of proposed RRMs and provide input to a Cost-Benefit Analysis (CBA) evaluating whether it is ALARP to implement the proposed RRM.
Key Takeaways
QRA methodology can be applied to assess SIMOPS risk in relation to live process facilities. The QRA techniques for SIMOPS are largely the same as those used in a normal design QRA for a process plant. However, the construction/decommissioning activities introduce new risks or increased existing risk, meaning that SIMOPS QRAs often become more complex and require more details than design QRAs.
The SIMOPS QRA can aid in deciding if the planned campaigns are acceptable from an overall risk perspective. The SIMOPS QRA can also be used to identify RRMs and evaluate whether the RRMs need to be implemented to have an ALARP campaign.
However, it is important to understand that the SIMOPS QRA is only one tool for assessing if the risk is acceptable and should be treated as an input to the decision-making process and not as the final truth.
