Tailings and Mine Waste Conference

Risk-informed methodology for prioritizing responses associated with mine wasted Fasking, Todd; Mohseni, Omid; Hansen, Tor 2015-10

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Proceedings Tailings and Mine Waste 2015 Vancouver, BC, October 26 to 28, 2015 Risk-informed methodology for prioritizing responses associated with mine wasted Todd Fasking Omid Mohseni Tor Hansen Barr Engineering Co., Minneapolis, USA ABSTRACT Most mining incorporates processes that upgrade ore to increase value. As part of this processing, some type of waste is typically generated. Mine operation and waste management can present both physical and chemical risks that sometimes negatively affect stakeholders, social license, and/or a company’s financial health. This paper reviews some approaches to prioritize mitigation efforts presented by physical and chemical risks primarily associated with mine wastes.  When looking at exposure risk associated with legacy sites, the prioritization process can account for a number of screening considerations, including chemicals present on site, receptor types, exposure pathways, stakeholder concerns, and mitigation net present value (NPV). Another prioritization method focuses on the physical risk presented by dams and impoundments. A risk-informed decision-making methodology can help companies prioritize assessments and remedial work by examining failure modes and the magnitudes of the failure outcomes. Our paper will present some case studies showing application of these methodologies. 1 INTRODUCTION The act of mining inevitably results in generating some sort of waste. The most obvious waste types are associated with ore beneficiation, which typically results in generation and disposal of tailings. Other types of wastes originate from operations-related support activities; typical examples include releases associated with fuelling, metal cleaning, painting, and sewage treatment. Other sources of waste to consider are those not associated with the primary process but that from a primary waste (for example, dust emanating from stockpiles or stored tailings; drainage that carries substances from stockpiles or tailings off site; and discharges from treatment systems whose effectiveness is limited). In addition,  risk remains from residual operational features such as old underground mine works or mine pits and associated infrastructure (especially from asbestos or PCBs in abandoned structures).  All of these waste types present risks to human health and the environment. The primary risk associated with tailings storage is physical in nature; failure of a tailings storage structure can result in the catastrophic release of a large volume of mobile material that can result in loss of life and incur billions of dollars in remedial costs (Bowker, 2015). Tools are available to help companies and regulators understand the potential reach of these impacts, and methodologies to minimize operational risk (MAC, 2011) and incorporate the lifetime risk associated with these structures (Oboni, 2014b) have been proposed. The main mitigation opportunities are placing the structure strategically and constructing and operating it in a way that limits potential failure and release (Chambers, 2011).  Non-tailings wastes can also present physical risks. For example, the recent Gold King water release from underground works in Colorado resulted in an initial localized inundation threat. However, the primary concerns about this type of waste are typically associated with some type of chemical risk to human health or the environment. Determining the risk levels of these threats can be difficult because of the uncertainty associated with the physical environment and the potential nature of the release. In addition, ongoing interpretation of toxicological research, coupled with related regulatory rulings, have resulted in frequent downward adjustments in Proceedings Tailings and Mine Waste 2015 Vancouver, BC, October 26 to 28, 2015 acceptable levels of and criteria for constituents, causing  continual revision to mitigative plans and actions. Regardless of the type of mining waste, generation and storage present potential issues that operators and owners will need to consider and address during and the active life of the mine. There are many reasons to proactively address these potential impacts; however, with limited resources, owners and operators must be thoughtful and diligent in prioritizing their efforts. 2 RISK CONSIDERATIONS WHEN PRIORITIZING RESPONSES All mine operators and owners, independent of size, bear liability related to waste generation. While smaller operators can focus on fewer facilities and therefore fewer risks, they have a correspondingly small resource pool to draw from and can benefit from prioritizing risk management activities. Larger companies typically have more resources available to address risks, but accordingly have more sites and facilities to deal with. Both types of entities need a methodology for quickly and effectively mitigating risk. Operators need to apply methods to minimize risk at ongoing operations (Boswell, 2011), but both they and owners can benefit from a process that prioritizes risk management during acquisitions and divestitures. Such a process can also help companies that continue to accrue responsibility for sites by giving them a fully developed and coherent system for managing continually changing portfolios. Acquiring assets and being named as a responsible party can yield prioritization information if a system that captures data generated during onboarding is integrated into the risk management process.  Potential impacts to operators and owners can manifest themselves in two main ways:  1. The burden associated with remediating previous known impacts or those from ongoing exposure or releases. This impact can be immediate and have a substantial impact on an owner’s or operator’s financial health. 2. The financial liability associated with mitigation of potential impacts. The occurrence of and the response to such impacts can have an enormous effect on an owner’s or operator’s social license to operate. In a recent report (EYGY, 2014), social license to operate was identified as the third-largest risk facing metal mining; capital dilemmas were the second. The combined risks speak to the need for firms to prioritize the actions they will take to manage mine wastes with limited resources.  The social-license and capital impacts are in addition to the financial impact an owner or operator would incur for an existing liability. Financial impacts represent potential restrictions on the ability of an entity to operate or expand an existing facility or construct new facilities. They can also be felt in the form of cost overruns or project delays if stakeholders opt to withhold social license via various means. An owner’s past practices and responded to historical liabilities can modify the lens through which stakeholders view existing and new projects. Some methods for quantifying the impact of social perception have been proposed (Oboni, 2014a). Recent prominent North American mine failures (Mount Polley and Gold King) have heightened stakeholder awareness. It is paramount for owners and operators to have a strategy for addressing potential impacts associated with current and historical operations. No firm has the unlimited resources to mitigate every identified risk. As sites age, physical structures and site conditions can change over time—for instance, flood frequencies attributed to climate change. At the same time, clean-up thresholds may be reduced or new compounds may be identified as toxic. The result is that risks believed to be previously mitigated may return as a priority in our changing physical and regulatory landscapes. Therefore, a thoughtful strategy for prioritizing waste-related liabilities appears necessary to support successful long-term mining operations. The following discusses some examples of risk-informed prioritization strategies that we have developed, recommended, or seen employed.  Proceedings Tailings and Mine Waste 2015 Vancouver, BC, October 26 to 28, 2015 The distinction between risk-informed and risk-based is an important one. Risk is a universal term referring to the possibility that something negative (such as an injury or loss) will occur. Risk-informed (more specifically, risk-informed decision making [RIDM]) implies that decisions are made considering risk insights along with other information, whereas risk-based implies that decisions are made considering only risks or risk values. 3 BIG-PICTURE CONSIDERATIONS Understanding the setting in which the wastes will be or have already been placed is critical when developing a risk-informed prioritization strategy. The location of the waste relative to human or other receptors affects the magnitude of potential impacts. Not understanding the site and potential geo-hazards presents a risk all by itself. Uncertainty can lead to unexpected or uncontrolled events and associated receptor exposures. A risk-informed prioritization strategy is most effective when considering operational realities such as market conditions, project phasing, corporate or other regulatory reporting requirements, and acquisitions or divestitures. In some instances it may be beneficial to leverage the natural business rhythm to optimize mitigation of an identified risk. In others, a risk may be too low to justify action. Other project circumstances, however, such as removal of a previously inhibiting feature, may provide an opportunity for action be taken on a relatively low risk if it can be performed at a lower cost. 4 TYPES OF POTENTIAL IMPACTS FROM MINING WASTES To develop a useful risk-informed prioritization strategy, waste-related elements potentially contributing to risk must be identified. Once these elements are identified, the magnitude of the associated impact of each should be defined and quantified to the degree possible to best determine a risk value. The level of uncertainty or lack of knowledge or experience associated with these elements should also be determined. Some level of uncertainty is inherent; however, the sensitivity of the outcome to the level of uncertainty should be understood because the amount of uncertainty in some situations can present an unacceptable level of risk. Risks can be mitigated by acquiring more data or a better understanding of the setting. The site setting and associated potential receptors are very important. Common receptors driving risk include local human, flora, and fauna populations, as well as the presence of water bodies and culturally or historically significant artifacts.  Knowing the characteristics of the waste being disposed is also critical. The chemical composition of the wastes (including pH) not only indicate how toxic and mobile constituents are, but also influence mitigative approaches and determine how far chemicals of concern might migrate. Particle size and water content are two of the geotechnical characteristics that can affect related chemical and physical risks. Finally, disposition of the waste and associated engineered design aspects contribute to the level of project risk–for instance, the presence and construction methods of dams, impoundments, pits, stockpiles, structures, and abandoned mineworks. Aspects of the site setting, potential receptors, waste characteristics, and contaminant fate and transport are commonly assembled into a conceptual site model (CSM). The CSM evolves as more information is gained to better support development of a risk-informed prioritization strategy.  5 METHODOLOGY FOR ADDRESSING AND PRIORITIZING ASSOCIATED RISKS 5.1 Physical  5.1.1 Background As earlier, physical damage is the primary risk presented by potential failure of a tailings storage structure. The resulting catastrophic release can initially result in deaths and damage or destroy Proceedings Tailings and Mine Waste 2015 Vancouver, BC, October 26 to 28, 2015 habitat and structures, and secondarily in long-term environmental disruption. The failure risk presented by tailings storage structures is typical of those associated with dams. To address this type of risk, we look to a dam-type setting. The U. S. Federal Energy Regulatory Commission (FERC) has decided to employ risk-informed decision making (RIDM) as a method of evaluating risk and prioritizing actions associated with regulated water-storage and power-generation structures. The FERC has chosen to implement this methodology due to shortcomings in standard methods. While a risk-based process like a potential failure-mode analysis (PFMA) details how a dam might fail, it does not directly consider the scope of potential consequences. RIDM is developed as a method to help owners define actions that most effectively address identified potential problems. By considering the full range of consequences, it allows for better utilization of resources. This approach can be easily applied to any risk evaluation in which the implications of failure should be considered. It can also provide a means for understanding the impact of uncertainty and variability beyond that provided in traditional analyses. The two figures that follow are from FERC guidelines. Figure 1 shows the curve used to help decide acceptable (tolerable) risk associated with dam failure:  Figure 1. Risk acceptability for dam failure To move an outcome on the above figure to an area with a better risk profile, the RIDM methodology uses an approach represented by Figure 2:  Proceedings Tailings and Mine Waste 2015 Vancouver, BC, October 26 to 28, 2015  Figure 2. Resulting change on risk curve associated with actions Below is an example of how this technique is transferable to applications outside of concrete-dam failure analysis. In the analysis of the San Bruno natural-gas pipeline release and explosion, a quote from the review committee stated: “There is no evidence top management has taken the steps necessary to be well-informed about the key aspects of decisions selected to manage major risks that concern PG&E.”  “Quality (risk) analysis could both facilitate two-way communication between top management and individuals with substantial knowledge about each of the relevant aspects of utility operations and provide a clear understanding of all the information available to make a key risk management decision.”  “Management could then ensure a full range of alternatives were considered in the decision and examine how each measured up in terms of each of PG&E's relevant objectives. They could examine what assumptions and judgments were used in integrating the available information to indicate the pros and cons of the alternatives. A quality analysis would highlight any significant missing information and provide a basis to examine whether it would be worth gathering if possible.” (FERC, 2013).  This is suggestive of how this methodology could be applied to risks outside of those being considered by the FERC. It is easy to imagine how this could be applied as a process to address the physical and chemical risk associated with mine wastes. 5.1.2 Case Study At a client’s request, Barr Engineering Co. developed a recommended risk-informed approach to address all significant potential failure modes for a surface-water impoundment project. The project was primarily driven by erosion concerns at an existing emergency spillway. Concerns included the spillway’s capacity to safely pass the probable maximum flood (PMF), erosion of the unlined emergency spillway during less extreme floods, and flows exiting the emergency spillway that could erode and undermine the adjacent embankment dam and lead to a dam failure. An ongoing preliminary-options study identified seven options for mitigating the risk of erosion and reservoir release. Other potential failure modes had been developed in detail in a previously prepared PFMA report and were brought forward as appropriate. The options evaluated included extending the existing left embankment to natural high ground; allowing the emergency spillway Proceedings Tailings and Mine Waste 2015 Vancouver, BC, October 26 to 28, 2015 to erode minimally and installing a steel sheet pile cut-off at the crest to prevent significant head-cutting into the reservoir; providing erosion protection on the entire emergency spillway as well as localized areas downstream; and constructing a new concrete gated spillway to pass the flow. Figure 3 shows a flowchart modified from a draft FERC tool used to guide the RIDM process applied in this case.  Figure 3. Risk analysis incorporated into RIDM process Key to the evaluation was designing for the probable maximum flood (PMF). Part of the analysis involved determining whether the level of protection associated with the PMF was necessary or whether a lesser flood event (e.g., a 1-in-10,000-year event) was reasonable considering the risks and other contributing factors included in the RIDM process. The estimated PMF flow was much higher and much less likely than an annual exceedance probability of 0.01 percent (i.e., a flood with an annual recurrence interval of 10,000 years). Costs for the different response actions are typically developed during the RIDM process. Here, we estimated that the cost to complete spillway modifications to the PMF capacity based on FERC standards-based requirements was on the order of $10–$20 million. Designing to this level of protection might not be necessary if the RIDM process justified a lesser design flood. The estimated PMF flow was two times higher and much less likely than the 10,000-year event. Designing to the lesser flood event would reduce the cost of dam safety modifications to somewhere in the range of $.5–$10 million, and the savings could be allocated to address other Proceedings Tailings and Mine Waste 2015 Vancouver, BC, October 26 to 28, 2015 potential dam-safety issues at other sites. The 10,000-year flood event has been accepted by the FERC at other facilities with the risk-informed justification that it satisfied tolerable risk guidelines in use by other federal agencies. 5.2 Chemical 5.2.1 Background Non-tailings mine wastes can present physical risks (e.g., reactive phosphorus, inundation from accidental water releases). The primary concern typically associated with these waste types, though, is some type of chemical exposure to either human health or the environment. Heavy metals are a key concern, but these wastes can also contain radionuclides, VOCs, and other toxic compounds. pH (acidic or caustic) represents both a direct risk during release and an increase in risk posed by other constituents via chemical reactions that may occur. The risks associated with these wastes are defined by a number of variables, particularly the location of the facility. Factors including the environment (topography, geology, hydrology, climate), potential receptors (flora, fauna, and proximity to human populations), and the regulatory setting dictate a large portion of the risk, as do the type and quantity of waste: large amounts of inert waste rock present a different degree of risk than a similar amount of stored liquid waste or liquefiable waste that contains relatively more toxic compounds. All of these variables, and others, can affect the level of risk presented by the source or site. With all of these potential risk drivers, developing a methodology to prioritize and address potential risks is a necessity. One approach to prioritizing responses to risks would use a risk-informed ranking system reflecting each property’s potential human and sensitive-environmental-receptor exposure risk. Properties with nearby receptors would be ranked higher than properties considered to be isolated from receptors. Some risk categories that could be used to guide the ranking process include:  Acute—documented conditions that could result in fire, explosion, or other acute public-health impact.  Highly likely human—characterized by a documented (or a high likelihood of future documentation of) exceedance of a public health criterion, proximity to potential public receptors, and a reasonable exposure pathway to the potential receptor.   Highly likely environmental—characterized by documented (or a high likelihood of future documentation of) exceedance of an environmental criterion, proximity to a potential sensitive environmental receptor, and a credible exposure pathway to that receptor.   Potential health or environmental with a receptor—characterized by potential exceedance of public health or environmental criteria, proximity to potential receptors, and a credible but incomplete exposure pathway to the potential receptors.   Potential health or environmental without a receptor—characterized by potential exceedance of public health or environmental criteria, but isolated from potential receptors, and uncertain or likely incomplete exposure pathway to the receptors.   Mitigated—response actions have been completed and known current and future risks to public health and the environment have been addressed; long-term monitoring of remedies may be required.  Resolved—ongoing public health and environmental risks no longer exist or could not be identified.  Once sites are prioritized into the larger categories, they can be further subdivided and prioritized using sample criteria such as:  Proceedings Tailings and Mine Waste 2015 Vancouver, BC, October 26 to 28, 2015  Documented (or high likelihood of documentation of) exceedance of regulatory criteria. Sites with exceedances of criteria would be ranked higher than sites with potential exceedances of criteria.  Documented or potential risk to public health  Documented or potential risk to the environment  Off-site impacts  Presence of source material (still contributing to a release)  Regulatory or public request for response action  Property transactions  Resolution of uncertainty  Economics (reasonable annual expenditures, economies of scale) These considerations can be consolidated into a response action matrix or schedule that prioritizes and reflects the relative importance of actions to reduce the risk presented by the inventoried sites. Some of our clients use a matrix similar to a job or task hazard assessment (as illustrated below) with a color guide to help quickly determine the risk rank of the site within a portfolio of sites, or the highest risks posed at a particular site. While these are useful tools, they have limitations in addressing uncertainty and in developing plans to manage risk (Ward, 2014; Cox, 2008). A developed tool would be most effective if it were regularly revisited to redefine prioritization as new information became available. An example application of prioritization on this basis would be removing source material at a site so that risk dropped below human-health exposure levels, changing the rank from high to low. Negotiating a cleanup plan approved by a regulatory body for onsite sources that could eventually result in offsite impact could change a risk from high to medium. Adapted from www.tc.gc.ca/eng/civilaviation/standards/sms-info-oct2005-1367810-2482.htm Figure 4. Sample risk prioritization matrix—safety application 5.2.2 Case Study In this example, the owner had more than 40 sites with known or potential chemical risks to be prioritized, including a number of mining related sites that ranged from closed mines to support facilities to offsite waste-disposal locations. The sites were scattered throughout North America and subject to an array of governing rules and regulations. The prioritization process in this case included an initial review of the sites; establishment of screening criteria, which included chemical risks and other owner-specified parameters; screening and ranking; and identification of major risks and parameters that influenced the ranking. Each site was classified as high, medium, or low Proceedings Tailings and Mine Waste 2015 Vancouver, BC, October 26 to 28, 2015 priority. The most influential risks and parameters within the high-priority sites were then singled out for corrective action in the near term to lower their classification status. The lower-priority sites were still acted on, but did not receive the same level of attention and resources as did the high-priority sites. (This work was performed on a confidential basis in coordination with client counsel, so specific results cannot be shared.) Below is an example of output that could come from chemical risk-based screening of an owner’s multiple facilities. It shows sample high-priority outputs from a screening and prioritization process. Table 1. High-Priority Sites Ranking Property Risk Category Health or Environment Criteria Chemical Source Drivers Corrective Cost 1 Former Landfill Highly likely human Exceeds drinking- water standard Mixed ash and solid waste Public, regulatory, responsible party $5M 2  Historic Processing Area Highly likely environmental Exceeds drinking- water and vapor standard VOC release from metal cleaning Regulatory, potential sale $2M 3  Closed Mine Potential health or environmental with a receptor Exceeds surface-water criteria ARD Public, regulatory $10M  Corrective costs were developed commensurate with the level of project definition and end-usage goal. To make equivalent comparisons, the cost basis should be consistent. Here we used net present value (NPV) to help capture the lifetime cost of the proposed actions. If there is concern about expense per year or short-term cost, the costs can be parsed and compared using that framework as well. NPV is a valuable metric but does have shortcomings when used to address risk, such as the inability to represent sensitivity to inputs (e.g., varying power costs) and the risk of remedy failure (Oboni, 2010). 6 SYNTHESIS The applicability and utility of RIDM seem obvious when the process is applied to waste-related risks. RIDM could be applied directly, as when it is used for developing programs to address tailings dams, or could be modified to address the risk presented by chemical-related exposures. A risk curve similar to Figure 1 would need to be defined based on the tolerances and drivers of the user. The actions taken to alleviate the risk could be viewed in the RIDM context by identifying and implementing actions to move the risk to preferred areas of the curve. Others have suggested prioritization methods to allocate mitigative investments (Oboni, 2012). Below is an example of what the RIDM process might look like if applied to the results of the multiple-site screening process. One approach would be to take the top-ranked risk site (former landfill) and apply the RIDM methodology to arrive at on optimal outcome. The first step is defining the outcome curve. Figure 5 shows an example curve defining unacceptable, tolerable, and acceptable outcomes. Proceedings Tailings and Mine Waste 2015 Vancouver, BC, October 26 to 28, 2015  Figure 5. Modified risk-acceptability curve  One would then define actions that might occur or be implemented that would move the event within the established curve. Examples are shown in Figure 6.   Figure 6. Resulting change on risk curve associated with remedial actions Superimposing the range of options shown in Figure 6 on the risk curve in Figure 5 reveals what effect the options would have relative to risk, as shown on Figure 7.   Proceedings Tailings and Mine Waste 2015 Vancouver, BC, October 26 to 28, 2015  Figure 7. Remedial actions superimposed on the modified risk-acceptability curve The cost associated with actions that would move the event into desired portions of the curve could be developed and assessed to determine a course of action. Further evaluation could be performed by using these costs, including evaluating the highest-priority site or all three options from Table 1 relative to the curve shown in Figure 5. Potential options shown in Figure 6 could be developed for the three high-priority sites. The effectiveness of these actions, assessed as change to the risk versus implementation cost, could be evaluated to determine which site-specific action would provide the most cost-effective improvement. 7 CONCLUSIONS RIDM provides a needed framework for making educated decisions about addressing risk associated with mine wastes, but it is not a simple process to set up due to several factors affecting the outcome:   multiple points of failure  uncertainties  limited resources As pointed out by many others, traditional methodologies such as PFMA and FMEA (failure mode and effects analysis) have weaknesses. RIDM and other prioritization approaches can help operators and owners prioritize application of limited resources to both protect human health and the environment and focus efforts on the most pressing risks. Obviously, this approach benefits stakeholders, but it can also help operators and owners build stakeholder confidence by addressing the protection of human health and the environment while demonstrating a disciplined and systematic methodology for addressing risks identified by owners, operators, or stakeholders.  8 REFERENCES Boswell, J. and Sobkowicz, J. (2011) Duty of Care Applied to Tailings Operations. Proceedings Tailings and Mine Waste 2011. Vancouver, BC, Available at: http://www.infomine.com/publications/docs/Boswell2011.pdf Bowker, L.N. and Chambers, D.M. (2015) The Risk, Public Liability, & Economics of Tailings Storage Facility Failures. California Public Utilities Commission (2011) San Bruno natural gas pipeline release. June 24, 2011. Chambers, D.M. and Higman, B. (2011) Long Term Risks of Tailings Dam Failure. Available at: http://www.csp2.org/files/reports/Long%20Term%20Risks%20of%20Tailings%20Dam%20Failure%20-%20Chambers%20%26%20Higman%20Oct11-2.pdf Proceedings Tailings and Mine Waste 2015 Vancouver, BC, October 26 to 28, 2015 Cox Jr., L.A. (2008) What's Wrong with Risk Matrices? Risk Analysis. Vol 28 No. 2, April 2008, pp. 497–512. EYGY Limited (2014). Business risks facing mining and metals 2014-2015. Available at http://www.ey.com/Publication/vwLUAssets/EY-Business-risks-facing-mining-and-metals-2014%E2%80%932015/$File/EY-Business-risks-facing-mining-and-metals-2014%E2%80%932015.pdf© 2014 EYGM Limited. All Rights Reserved. Federal Energy Regulatory Commission (2013) Risk-informed decision making. Available at https://www.ferc.gov/industries/hydropower/safety/initiatives/risk-informed-decision-making.asp. (Accessed 15 September 2015). Independent Expert Engineering Investigation and Review Panel (2015) Appendix I: B.C. Tailings Dam Failure Frequency and Portfolio Risk, Report on Mount Polley Tailings Storage Facility Breach. The Mining Association of Canada (2011) A Guide to the Management of Tailings Facilities, Second Edition. Available at: http://mining.ca/sites/default/files/documents/GuidetotheManagementofTailingsFacilities2011.pdf Oboni, C. and Oboni, F. (2010) Stop Procrastinating! NPV is dead: Use Risk as a Key Decision Parameter. Canadian Reclamation. Available at: http://www.riskope.com/wp-content/uploads/2013/06/STOP-PROCRASTINATINGNPV-IS-DEAD-USE-RISK-AS-A-KEY-DECISION-PARAMETER.pdf Oboni, F. and Oboni C. (2012) Is it True that PIGs Fly when Evaluating Risks of Tailings Management Systems. 2012 Tailing and Waste Management Conference. Oboni, C. and Oboni, F. (2014) Aspects of Risk Tolerance, Manageable vs. Unmanageable Risks in Relation to Critical Decisions, Perpetuity Projects, Public Opposition. Geohazards 6, Kingston (ON) Canada, June 15-18, 2014 Oboni, F., Oboni, C., and Caldwell, J.A. (2014) Risk Assessment of the Long-Term Performance of Closed Tailings Facilities. 2014 Tailing and Mine Waste Conference. Transport Canada (2005) Safety Management Systems – Information Session – Presentations – Day 1.  Available at http://www.tc.gc.ca/eng/civilaviation/standards/sms-info-oct2005-1367810-2482.htm. (Accessed 15 September 2015). Ward, S. and Chapman, C. (2011) How to manage project opportunity and risk, John Wiley & Sons, pp. 49–51. 

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