Continuous Monitoring and Statistical Approach to Ground Gas Risk Assessment at a Substation Site by Atkins
Shortlisted Brownfield Awards Category 6 - Best Project Closure/Verification
The site is an electrical substation compound (approximately 1.02 ha) in the south of England. The site includes a number of electrical substation facilities (400 kV, 132 kV and 33 kV) as shown in Figure 1.
Historical maps indicated that a landfill crossed the site boundary near the 132kV substation. The ground investigations undertaken by Atkins and others at the site had identified elevated concentrations of ground gas (particularly methane and carbon dioxide) in the vicinity of and directly beneath the 132kV relay room.
The concentrations of methane recorded within the ground had been previously assessed as representing an unacceptable explosion risk. Consequently, a gas alarm was installed and was operational for five years from 2014.
Atkins has recently reassessed this site using additional ground gas data (including gas flow) collected using the latest available technology and by applying a statistical approach involving probability density functions (PDFs) of gas data as recommended in the latest guidance. This has reduced the level of conservatism in the previous assessment and increased confidence in the estimated risk level such that the ground gas risk has been demonstrated as acceptably low. As a consequence, the requirement for ongoing gas control and mitigation measures has been shown to be unnecessary enabling closure of the project and the lifting of operational controls.
Conceptual Site Model
The site is underlain by Made Ground (including a historically infilled gravel pit) and Superficial Deposits over the Holywell Nodular Chalk Formation. Gravel pits were identified both on site and extending to the north of the site on historical maps between 1960 - 1964 and were infilled with landfill waste prior to construction of the 132kV substation compound in 1972 -1973.
Biodegradable materials such as domestic refuse, wood and paper which had been identified within the infill materials are capable of generating landfill gas including methane. Therefore, there was potential for ground gas to migrate through the ground and gaps or cracks in the floor and accumulate in the buildings on site. If methane concentrations exceed the Lower Explosive Limit (LEL) of 5 % v/v. within a building or confined space such as an electrical cabinet, then this has the potential to ignite. The resulting explosion could cause damage to the structure and its contents and, if the building is occupied, injury or even loss of life.
The site had been characterised by a series of historical ground investigations and assessments undertaken since 1966. Monitoring locations within the historical landfill had reported methane concentrations between 41.8 and 97.2 % v/v. Boreholes located next to the relay room of the 132kV substation had reported lower methane concentrations up to 16.2 % v/v but which still exceeded the LEL.
Continuous gas monitoring was undertaken for a period of three months under the floor slab of the 132 kV building relay room in 2013. Methane was measured above the LEL, up to 29.3 % v/v, directly beneath the slab. The estimated risk of methane explosion (using 95th percentile site conditions and applying forward modelling method as per CIRIA report 152 ) identified a potential annual frequency of an explosion event to be outside of a ‘tolerable risk range’.
As a result of the potential risk identified from ground gas, a gas alarm was installed within the sub-floor void of the relay room on a precautionary basis and an emergency plan was implemented; this arrangement remained operational for over five years since May 2014. In addition, four venting cowls connected to gas wells constructed in boreholes were installed around the perimeter of the relay room building in 2017 to mitigate the risk from elevated concentrations of ground gas.
Figure 1 - Site Layout
Sub-slab monitoring of the relay room in 2018 reported a maximum methane concentration of 1.7 % v/v, which was significantly lower than reported in 2013. Further, the gas alarm installed above the sub-slab also had not reported methane concentrations above 20% of LEL. Although some of these gas monitoring results indicated that the likelihood of the pollutant linkages being realised was low, the risk from the ground gas was not able to be discounted due to the lack of gas flow monitoring from beneath the building and the inherent conservatism in the conventional risk assessment methodology.
The challenge was to understand better the risks to site users and to reduce or remove the operational costs, disruption and administration complexities involved in maintaining the gas alarm, either through physical remediation or further detailed risk assessment.
Figure 2 – Sub-slab monitoring (Indoor)
Figure 3 – Borehole monitoring (Outdoor)
To resolve the challenge, Atkins undertook a programme of innovative continuous gas and flow monitoring, to inform an unconventional detailed quantitative risk assessment (DQRA).
The continuous monitoring was undertaken using Ambisense GasfluX™, which is the world’s first device able to monitor real time continuous gas and flow including ambient conditions such as barometric pressure, temperature and humidity. The in-built technology of telemetry in the device enables access to the monitoring data remotely to check whether the critical pressure drops is achieved. A total of seven weeks of continuous monitoring was carried out from April to June 2019 at two locations on site (indoor and outdoor as shown in Figure 2 and Figure 3 respectively). The outdoor unit was attached directly to an existing monitoring location (borehole) along the potential ground gas pathway. The indoor unit was attached to the same sub-slab monitoring location (at the receptor) that was used previously for continuous gas concentration monitoring in 2018.
The supplementary investigation also included conventional spot monitoring across the site using a hand-held GA5000 gas analyser unit during the same period as the continuous monitoring was undertaken. Validation of the continuous monitoring data was undertaken by comparison with the spot monitoring data which showed that the two data sets were in good agreement.
Figure 2 presents the continuous gas monitoring results and gas screening values (GSV) calculated as per the modified Wilson and Card methodology for both the outdoor (borehole) and indoor (sub-slab) monitoring locations. The rating of characteristic situation (CS) based on the calculated continuous GSV for new developments demonstrated that at least 12 % of the monitoring period was CS2, which implies low risk but still requires gas protection measures .
The pressure drops that occurred during the indoor (sub slab) monitoring period on different time-scales ranging from one hour (short term) to two days (long term), were also compared against the worst case zone associated with the critical pressure drop presented in the CLAIRE guidance  as shown in Figure 5 . It clearly showed that the worst-case zone was encountered during the latest continuous gas monitoring period as recommended in the latest CLAIRE guidance ; therefore, the continuous monitoring data were considered to be sufficient to support a robust ground gas risk assessment.
Unconventional risk assessment
While the calculated GSVs (shown in Figure 4) are useful for initial screening of potential gas risks, GSVs do not assess quantitatively the risk posed by extreme events which are unlikely to occur in a given monitoring period. Under the acceptability of risk presented in CIRIA report , an event causing total loss of life is considered to be of little concern if it has an annual likelihood of less than 1 x 10-7 (one in 10 million years). Therefore, a robust detailed quantitative risk assessment was undertaken to assess the risk quantitively.
The most commonly used method of quantitatively assessing risk on gassing sites is that described in the CIRIA Report . This uses a combination of forward modelling (to evaluate the gas concentration at the receptor) and fault tree analysis to provide a numerical estimate of the risk (i.e. a probability that an adverse effect will occur in any year or other specified period).
The method used in this assessment varied from the method described in CIRIA report  and includes backward modelling (to evaluate the gas flow required for ventilation failure) and the statistical approach of considering the PDFs for gas flow based on the latest guidance  and Atkins’ extensive experience of undertaking gas risk assessments.
The fault tree analysis aimed to determine the probability of an undesirable event (i.e. explosion from methane) occurring based on estimates for the likelihood or frequency of independent, but necessary, conditions (contributory events) arising. For the purposes of this assessment, a number of contributory events to a potential explosion were considered such as probability of ventilation failure, frequency of ignition and probability of occupation.
Figure 4 - Indoor (sub-slab) and outdoor (borehole) continuous gas monitoring results
Figure 5 - Indoor monitoring record: pressure drops against ‘worst-case zone’.
Probability of ventilation failure and methane explosion event
The probability of failure to ventilate the building adequately was derived using a statistical approach made possible by the collection of continuous gas flow and methane concentration data collected from the sub-slab monitoring location at the receptor. Under this approach, a set of PDFs (such as exponential, normal, log normal, gamma, Weibull) were fitted to the methane flow rate (i.e. product of gas flow rate and methane concentration, which is equivalent to the continuous GSV in Figure 4).
Comparing a set of five PDFs with a histogram and empirical density function of the observed data (as shown in Figure 6), the gamma distribution (with shape parameter 0.6 and rate parameter 1400) was identified as the best fit PDF.
Based on the best‑fitting (gamma) PDF, the probability of exceeding the critical flow rate of methane (0.1 l/hr) was calculated as 1.1 x 10-63 and the annual probability of a methane explosion event that would result in harm to human health was calculated as 2.7 x 10-63 (1 in 3.6 x 1062 years). According to the CIRIA guidance , this level of risk is of no concern.
Figure 6 - Indoor (sub-slab) methane GSV histogram and probability density functions.
Best assessment of residual gas risk
The combination of using statistical and fault tree analysis approach and use of the innovative continuous gas monitoring technology conclusively demonstrated that there was not an unacceptable risk of methane explosion within the relay‑room building. Therefore, Atkins was able to recommend that the existing monitoring activities and other control measures including a gas alarm system could safely be discontinued at the site.
Cost effectiveness (including social and environmental benefits)
Throughout the project, Atkins worked closely with National Grid Property to achieve a cost effective and technically robust solution to reduce the operational costs, disruption and administrative complexities involved in maintaining the gas alarm and monitoring system. Some specific benefits the project achieved were:
The innovative technology in the Ambisense GasfluX™ (i.e. telemetry and real time monitoring) was utilised successfully to collect continuous gas and flow data within a short time frame for use in the DQRA;
The outcome of the assessment demonstrated no requirement to undertake further costly remediation (estimated at up to £70,000);
There will be a saving on the operational and maintenance costs of a highly specialised gas alarm and emergency plan, estimated at £50,000 over next ten years;
Increased confidence that site users and critical national infrastructure is not being put at risk; and
Carbon footprint reduced as the need for regular site visits to maintain the gas alarm or to undertake remediation works is removed.
Application of best practice and compliance with legislation, codes and guidance
All works have been undertaken with care and diligence in relation to current best practice and legislation as follows:
Supplementary gas monitoring was carried out from both the sub-slab location and gas from wells constructed according to the current best practice given in BS8576:2013 and with full compliance with critical health and safety requirements. To date there have been no reportable incidents or disruption of the current site operation;
Best practice was applied in the continuous gas monitoring as recommended in the latest CL:AIRE guidance   both to collect sufficient data and to make sure that the critical pressure drop was captured;
The continuous gas monitoring data collected using the latest available technology were verified against conventional monitoring data, which showed good agreement; and
The risk assessment was undertaken using the statistical approach recommended in the latest guidance and compared the results of assessments undertaken in accordance with the CIRIA report .
Effective stakeholder engagement
Atkins liaised with the relevant stakeholders to arrange access for the required site investigation and monitoring, which was undertaken safely on the live substation site. The purpose of the work was clearly communicated in a non-technical way to the staff operating the site. Further, the technical approach had been peer reviewed by the one of the clients technical consultants.
The scope of work and the final outcome of the further assessment meant site operations could continue safely without the need to continue to operate specific gas control measures and protocols.
Robust, sustainable and defensible solution
As discussed above, to discount the residual risk from ground gas at the site, conventional approaches to monitoring were not sufficient due to the site constraints including the conservatism of conventional gas risk assessments. Consequently, an innovative monitoring scheme involving a novel statistical approach and fault tree analysis was adopted working closely with key stakeholders which provided robust scientific evidence to justify that no further gas monitoring or gas alarm was required. The adoption of this approach resulted in a sustainable and cost-effective outcome and released the site for current active operation.