Alwin van Aggelen, CEO of A-Risc, explains how terminals can deal with new scenario analyses and risk assessment requirements for PGS 29
Storage terminals in the Netherlands must comply with the PGS 29 requirements for the storage of flammable liquids in above ground cylindrical tanks.
Like all Dutch guidelines for hazardous substances, the PGS 29 is currently in the process of being updated using a standardised risk-based approach.
One of the reasons for this risk-based approach has been to create a structure where it is clear why a requirement is there, what the objective of the requirement is and if sufficient risk reduction is provided by the requirements.
It also provides a good foundation to support the review and acceptance of alternatives that might be equivalent in terms of function and level of risk reduction. And, not restricting opportunities for innovation and technology development.
Understanding the risk
The core of the new approach is the use of scenarios describing all potential incidents that can occur when storing flammable liquids in tanks. Utilising the BowTie methodology, scenarios are described from cause to consequence, including the impact of the consequence.
For example; overfilling leading to a loss of containment, followed by the formation of a vapour cloud and, when ignited, a vapour cloud explosion with potential for multiple fatalities/injuries, extensive damage and escalation to nearby installations.
To ensure all credible scenarios are covered, the causes are categorised by category based on the PGS 6, e.g. over pressure, under pressure, internal corrosion, external corrosion, external impact, environmental conditions, human actions, etc. These are validated against tank incidents where available.
Each of the scenarios is assessed using a standardised risk matrix to determine the risk of a scenario and to determine if it should be included in the PGS standard. The scope of all the PGS standards, and thus also the PGS 29, is limited to the medium and high-risk scenarios.
To ensure the PGS 29 covers most of the installations, the scenario analyses has been done based on predefined tank typicals (cone roof tank, cone roof tank with inner float, tank with outer float and tank with outer float and dome), typicals for water- and landside loading and unloading (barge/sea-going vessels, railcars and trucks) and for terminal piping.
Besides scenarios for these typicals, specific scenarios are developed in relation to activities that can be performed within a tank such as mixing, adding additives and butanising.
How the risk should be managed
A second new element for the PGS new style, besides the use of scenarios, is the way the requirements are structured through a combination of goal-based requirements and prescriptive requirements.
First level of requirements are the goal based requirements, the so-called ‘Doelen’ (objectives). Per scenario, the different objectives for managing that scenario are formulated. When translated to the BowTie methodology, the objectives are synonym with the barriers.
Example – Overfilling scenario:
Objective 1 – Ullage control
Ensure remaining capacity within tank is sufficient for the planned manipulation, includes the plan for switching to another tank during manipulation when required.
Objective 2 – Alarms and operator action
Operational level control during filling of the tank to prevent the filling of the tank above the high level.
Objective 3 – Independent overfill protection
Independent overfill protection on high-high level to prevent the overflowing of the tank.
These objectives describe the different barrier functions that should be in place to prevent a scenario from happening or to mitigate the consequences of the scenario when it happens.
The second level of requirements are the so-called ‘maatregelvoorschriften’ (prescriptive requirements). Where the objectives describe the ‘what’, prescriptive requirements are describing the ‘how’. Or in BowTie terms, the barrier elements. Distinction is made between primary requirements describing required items or equipment, and secondary requirements that support the primary requirements.
For example, a primary requirement to prevent escalation of a fire is to have a stationary tank cooling system for certain type of tank/product combinations. Secondary requirements supporting this are requirements for the capacity of the tank cooling system and for testing and maintenance of the tank cooling system.
This structure also provides a framework where alternatives for the (prescriptive) requirements can be acceptable, as long as the objectives for each of the scenarios are met.
Although the PGS 29 new style is still under development, it is already possible to draw the conclusion that the PGS 29 wile become much more relevant for storage terminals.
All requirements within the current PGS 29 are really challenged in relation to the scenario they belong to. It looks like certain requirements will be removed because they are not managing a risk that is relevant for the PGS 29.
Other requirements are going to be rewritten because the scenario was not properly addressed by the requirement, e.g. it is too limited in respect to the scenario, or it needs to be re-worded to obtain a proper linkage from scenario to objective to requirement.
But also, most likely new requirements will be defined for scenarios currently not sufficiently addressed or are not addressed at all.
And finally, the PGS 29 new style will have a much better alignment with current operational risk management practices within storage terminals. Instead of a set of disconnected requirements the PGS 29 new style provides a structured approach for managing the risks. It can be used as starting point for describing the storage terminal specific scenario’s and how these are managed.
van Aggelen will be speaking more about the new structures for PGS 29 on the third day of the StocExpo Europe conference on March 22. For more information, and to register to visit, visit www.stocexpo.com.
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