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Applying risk and consequence modelling knowledge from the oil and gas industry to urban energy projects

19 August 2018

This year's 30th anniversary of the Piper Alpha disaster offers a timely opportunity to look back at the significant developments in risk and consequence software modelling, says David Price of Gexcon UK. The Public Inquiry into the accident, which claimed 167 lives, triggered increased attention in the assessment of risk from gas leaks and subsequent ignition leading to jet fire and explosion modelling.

Modelling airflow through plant showing areas of high (green) and low (red) ventilation
Modelling airflow through plant showing areas of high (green) and low (red) ventilation

The Public Inquiry led to new and improved models to predict the severity of the consequence and quantify the effects of mitigation measures, and the knowledge developed and lessons learned and applied in the oil and gas industry are now being used in other areas of the energy sector.

Many UK cities are undergoing a rapid programme of urban regeneration with local authorities and their private sector partners under increasing pressure to make cities self-sustainable. A significant challenge facing commercial and residential projects is how to provide affordable energy as well as tackle the issues around sustainability, efficiency and carbon footprint.

Community heating schemes and Energy Centres, which are far more efficient than conventional power stations and provide energy at the point of use, are becoming more popular. This has led to an increase in the demand for consultancy services for explosion, gas dispersion and gas detection optimisation studies.

With a shift away from conventional power stations to more localised hubs, the need to assess risk is greater than ever before, especially when you consider that a Combined Heat and Power (CHP) plant might house some extremely powerful gas fired engines in addition to cooling plants, which in some instances which can contain a toxic gas.

Explosion risk and hazardous materials specialists, architects, designers, surveyors, engineering contractors and installers are all having to work together to keep risks to As Low As Reasonably Practicable (ALARP).

There are no definitive guidelines to follow, partly because traditionally these installations have been located in remote areas, away from the population at large.

Whilst in an ideal world Energy Centres would be located in a totally separate building, this is often not feasible so innovative solutions are required to reduce the risk ensuring many layers of protection are in place to satisfy the ALARP principal.

Unlike in the past, where hazard and safety was often a tick box exercise, stakeholders are now much more focused on ensuring that they have commissioned appropriate risk assessments and are asking 'have we done everything possible to assess the risk?'

The modelling study feeds into an overall risk assessment to challenge ALARP and allow the risk management to be demonstrated using complimentary techniques such as BOWTIE which allows sufficient layers of protection to be assessed and additional measures incorporated as necessary to ensure a robust basis of safety is established and maintained.

A typical scope of work for an Energy Centre would include a comprehensive ventilation, dispersion and explosion analysis, including the following main tasks:
*  Constructing a detailed geometrical model of the energy centre building based on available layouts drawings
*  Carrying out a ventilation study to assess the proposed ventilation and to evaluate the most optimum configuration to reduce the potential size of a flammable gas cloud
*  Gas dispersion modelling to assess the size of the many flammable / toxic gas clouds scenarios and their extent for a range a loss of containment (leak) conditions
*  A gas detection evaluation and optimisation study following potential flammable / toxic releases inside the Energy Centre
*  Gas explosion modelling to assess the maximum internal overpressure inside the building upon ignition of a dimensional equivalent flammable gas cloud
*  The performance of a water-based suppression system, to evaluate the explosion development influence upon the deployment of a water deluge system activated following gas detection.
*  The performance of explosion protection solutions, for example explosion relief, to evaluate the efficiency for the explosion pressure safely dissipate, ensuring that internal pressures are well below the resistance of the structure.

A Gas Detection Optimisation undertaken as a scenario-based study could involve the following:
*  Simulation of a set of accidental gas releases. These simulations provide detailed prediction of gas cloud size and dispersion patterns arising from the different release scenarios.
*  Evaluation of a proposed gas detection systems to detect gas releases, where the ability to detect and the potential time to detection are determined.
*  Establish alternative gas detector layouts. Differences between layouts based on different spatial configuration and number and type of detectors.

Detailed reporting is carried out in the following areas:

Geometry

Experience has shown that incompleteness of detail in the geometry model is one of the main sources of errors in CFD analyses. For this reason, studies tend to be biased toward building a representative model of the installation, paying attention to equipment congested regions, which contribute towards enhancing turbulence levels and mixing, resulting in higher overpressures in explosion scenarios or less effective ventilation and dispersion of gas clouds.

Ventilation Analysis

A detailed prediction of the internal flow patterns helps determine how effective the system is (e.g. in terms on internal air velocity and the injection/extraction of air) and allows any potential dead spots to be identified. Ventilation patterns also provide valuable information to predict the potential future behaviour of some dispersion scenario.

Dispersion Analysis

The objective of the dispersion studies is to simulate a number of accidental gas releases that provide realistic and precise data on how gas might disperse and accumulate in different areas of the energy centre.

Leak scenarios based on those thought most likely to develop large clouds and those most likely to be challenging to the gas detection system. This allows decisions to be made regarding routing of gas lines and connections with equipment to reduce the probability of hazardous gas clouds forming.

Explosion Modelling
Gas explosion simulations are conducted to ascertain the magnitude of the internal overpressures that could be generated if the cloud was ignited by a potential ignition source.

Gas Detection Analysis

The focus of the gas detection analysis is on detecting small leaks arising from the higher probability of small hole size releases.

A practical application of these processes is described in a case study accessible in
Editorial / Case studies on this website.


About the author

David Price is Managing Director and Principal Engineer of Gexcon UK.  He is an explosion consultant with more than 25 years experience within the explosion safety industry. His key competencies include dust and gas explosion safety, DSEAR / ATEX assessments, explosion protection design, hazardous area classification and explosion investigation.
He is a process safety study leader and wrote the chapter on Fire and Explosion Hazards, for the Institute of Civil Engineers’ Manual of Health and Safety.


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