All oil and gas facilities have the goal of ensuring the safety of employees, the public and the environment. There is a need to understand the hazards and, through this, reduce any risks in the most effective way. This means that risk assessment is an integral part of any operation throughout its life cycle. For complex offshore installations and onshore terminals, this will extend to undertaking quantified risk assessment (QRA).
The QRA was originally developed as a means of demonstrating risk compliance—that is, the risks are below an intolerable level and may vary from region to region. Since then, understanding and improvements in design and operation have ensured the risk on a typical installation is usually well below the intolerable level. The use of a QRA to demonstrate compliance is a benefit often questioned.
However, many decisions in the design and operation of facilities have risk implications. The QRA has evolved to gain an understanding of potentially high consequence, low frequency, events and is used to support decision-making. This is particularly important when assessing the requirement for further risk reduction measures to ensure that risks are as low as reasonably possible.
The QRA process must be technically robust and delivered quickly enough to keep pace with the decision- making process. The transformation that has occurred in online services introduces new and exciting opportunities in how QRA data can be used to support decisions. In recent years, DNV GL has made a significant investment into digitalizing its QRA service.
Digitalization is a generic term that is very wide in scope and often misinterpreted. It is not about making a process software-based but about using the connectivity and data that exist in the digital world to full effect. A key element that digitalization gives to QRA is the availability of design and engineering data in well-defined databases, enabling automated extraction of information required for a QRA consistently aligned with the facility design and layout throughout its life cycle.
Digitalization provides ready access to cloud-based platforms that provide reduced development costs, increased functionality and flexibility, and simplified access using standard web browsers. It also provides the ability to link data input, analytical applications and user access as separate entities through a cloud database. By having a database at the core and with information properly linked to location and equipment, data can be presented in many ways, ranging from simple location risks through to understanding which part of a safety-critical maintenance backlog should be prioritized.
The MyQRA portal is the first step in this process and has been developed to provide operators direct access to risk data. The portal allows the user to change inputs and re-analyze the results to understand sensitivities. The following case studies illustrate how the QRA process was used to support decision-making.
Offshore personnel transfer systems
An accommodation unit, connected to an offshore installation via a permanent gangway was chartered to support additional personnel required to complete a project scope while the platform was still producing.
In the U.K. the Prevention of Fire Explosion and Emergency Response (PFEER) regulations and its approved code of practice state that, unless shown otherwise by appropriate risk assessment, the totally enclosed motor propelled survival craft (TEMPSC) capacity should be 150% of the personnel on board with 100% readily accessible. During this type of combined operation, credit can be taken for the gangway as an evacuation route provided it is in a safe location.
DNV GL’s QRA models were used to determine the impairment frequency of the gangway at different locations. This information was then considered along with practical/operational restraints to identify an optimal gangway location.
In a similar vein, an operator proposed increasing manning from the PFEER limit to the combined capacity of the three smallest out of the four available TEMPSC. Both manning cases were modeled in the QRA. Because all personnel was guaranteed a TEMPSC seat for all reasonably foreseeable evacuation scenarios (i.e., single TEMPSC failure) for each case, it was demonstrated that the increase in risk was negligible. The study justified the increase in manning and allowed the operator greater flexibility when expediting work scopes.
Riser emergency shutdown valves testing
The final offshore example details the use of QRA to define the frequency of riser emergency shutdown valves testing. Any test has an associated cost, and there is a balance to be made between testing often (greater cost) and not enough (lower valve reliability). Good practice is to not exceed a test interval of two years; however, the operator had set the test interval by default to a year.
A risk-based approach was used by setting the test interval such that the residual potential loss of life was less than 10-4 per year, which often is used to set reliability targets. The required test interval was determined from the potential consequences, ignited event frequency and the average failure rate of the valve. The result was an increase in the test interval from one to two years, which allowed the operator to carry out the tests during the planned shutdown. This presented significant savings.
LNG production trains
The early design stage of an onshore LNG export terminal featured multiple parallel production trains. The trains were originally laid out according to general guidelines developed for other types of plant. DNV GL carried out a QRA of the site that incorporated detailed modeling of the process, allowing consideration of features specific to the design.
This demonstrated that in an accident scenario, the risk of interaction between the units is lower than suggested by standard industry guidelines. Suggestions also were made that allowed alteration of the refrigerant used in the liquefaction process without significant efficiency loss, but with large reductions in the associated explosion consequences, should a release occur. This then allowed the spacing and arrangement of the trains to be optimized to reduce risk to personnel, process escalation and reduce the total land requirements.
Onshore processing site
DNV GL carried out a QRA of an onshore processing site with high-pressure hydrocarbon mixtures with high H2S content, primarily constructed using a series of enclosed modules. The risk assessment software package was modified specifically for the project to allow consideration of these features in a manner that would not be possible with off-the-shelf software. This allowed the provision of fire and blast protection to be optimized, rather than following overly conservative local guidelines.
Approaches for escape and evacuation, including the protective equipment related to the highly toxic release, were defined by the QRA results. Finally, the equipment within the modules was rearranged on the advice of DNV GL to prevent common failures from disabling vulnerable utility equipment that would cause the shutdown of the site, enabling continued operation of the site following a minor incident, which would not have been possible in the original design.
These examples demonstrate the importance of QRA in risk-related decision-making. The ability to utilize QRA data in all phases of a facility’s life cycle in an effective manner will be increased considerably by the digital transformation.
Contact Jennifer Presley at firstname.lastname@example.org for more information.
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