Developing Risk Assessment Standards And Specifications .
Developing Risk AssessmentStandards and Specifications for aDistribution System: A Case StudyTBy Jerry Bond, Sara Sankowich, and Christopher J. LuleyThe development of a tree risk assessment specificationdocument provides the opportunity to examine how thecurrent tree risk assessment standards and best management practices can be systematically incorporated intoutility arboriculture. This article presents the results of aFigure 1. The tree risk program was designed to avoid tree-failure-related outages that might occur under “normal” weather conditions. The decay in thissugar maple (Acer saccharum) scaffold branch would increase the likelihoodof failure under such conditions.56 Arborist News www.isa-arbor.com case study that is not only timely but might also proveuseful to other utilities that are considering updating theirtree risk assessment programs.BackgroundUnitil Corporation is a public utility holding company,headquartered in Hampton, New Hampshire, U.S., whichprovides local distribution of electricity and natural gas inthe states of New Hampshire, Massachusetts, and Maine.It serves more than 101,400 electric customers and nearly71,900 natural gas customers, and provides energy brokering and advisory services to large commercial and industrialcustomers in the United States. The electrical distributionsystem stretches 1,051 miles (1,691 km) in New Hampshire and another 560 miles (901 km) in Massachusetts,with landscapes ranging from dense–customer urban tosparse–customer rural.Unitil decided to create a formal risk assessment specification. The goals set for this program were as follows: Adapt the recently published ANSI clause for treerisk assessment (ANSI 2011) and ISA’s Best Management Practices: Tree Risk Assessment (Smiley et al.2011) to a utility-specific tree risk program. Reduce predictable tree-caused electrical outagesand improve Unitil’s annual SAIDI and SAIFImeasures of electrical performance. Avoid failure of defective trees under normal weatherpatterns with wind speeds less than 55 /- mph (89kph) and, to the degree possible, reduce failure ofdefective trees under catastrophic weather conditions. Promote efficient and cost-effective tree pruningand removal. Adequately allocate funds to appropriate risk levelsacross Unitil’s distribution service territory. Provide a documented means of developing consistency in tree risk assessment among assessorsand throughout the risk management program. Specify methods to conduct assessments in the field,and estimate personnel needs and workloads requiredto complete projected risk mitigation work.
Create means to document and assess the qualityof the work performed. Develop material to support funding and workloadrequirements to be made available for internal managers and regulators.Unitil’s risk assessment specification targeted two broadaudiences: 1) a direct audience composed of the utility’sarborists, contractors, managers, etc.; and 2) an indirectaudience composed of the public, regulators, media, andothers. For different reasons, it is important that each audience be able to understand in detail the basis and methodof Unitil’s procedures to reduce tree-caused outages, raisereliability, allocate funding appropriately, and performwork in a cost-effective manner.To reach such diverse audiences effectively, the specification would take form as two separate documents, sharing the same content and structure, but having a stronglydivergent format and approach: A Standards and Specifications document to holdthe program description, along with the referencesand literature citations supporting it; designed tobe formal, detailed, text-based. A Manual for training and field use that is practical,easily accessible, and image-based.The release of the tree risk standard by the AmericanNational Standards Institute, A300 (Part 9 – Tree RiskAssessment) (ANSI 2011), and Best Management Practices:Tree Risk Assessment by the International Society of Arboriculture (Smiley et al. 2011), offers new perspectives fordeveloping risk standards and specifications for utilities(Kempter 2012). The recent Tree Risk Assessment Manualpublished by ISA also provides up-to-date field guidance(Dunster et al. 2013). Two key points, however, merit serious consideration in any effort to incorporate these newmaterials: Most utility tree risk programs were set up beforethe development of the new ANSI Standard (ANSI)and ISA Best Management Practices (BMP), meaning that the programs lack some of the new publications’ most critical concepts. The ANSI A300 (Part 9) and the BMP bookletwere developed primarily for risk assessors workingwith clients. They provide only minimal guidancefor the programmatic development needs of utilities,and will require considerable adaptation.Utilities desiring to make use of the new ANSI clauseand BMP booklet will also have to utilize literaturesources outside these publications. As an example, recentliterature about pruning and tree failure can be used tojustify action under various consequence-of-failure scenarios that would greatly speed up and simplify fieldwork. The bibliographies contained in two recent publications on pruning (Clark and Matheny 2010) and riskassessment (Matheny and Clark 2009) offer good sourcesfor information on studies relevant to utilities.Figure 2. The tree risk program is guided by two essential documents: a Standardsand Specifications manual, which defines the program and cites supportingliterature and references; and a Training Manual that is designed for field andeducational supportAs the authors of Unitil’s tree risk program materials,we wanted to pay careful attention to the effect of anyrisk assessment program on fieldwork. Unlike risk assessors in many other parts of the arboricultural industry,utility arborists have very limited time at any one treegiven the large number of trees that require attentionalong utilities. It was therefore paramount that the resultof our efforts be commensurate with actual work capacityand practices.Finally, it is important to recognize the serious dataquality issues that exist within many utility risk management programs (Wetteroff 2011). Most utilities believethat their efforts are sufficient, but that belief often lacksactual data (Brown 2009). Moreover, much of the datathat does exist frequently relies on tree failure information from observers lacking appropriate training andexperience.This lack of data is exemplified in the commonlyheard dichotomy between “hazard” and “healthy” trees.Not only is this dichotomy fundamentally invalid, sincehealth and stability are distinct (biological and mechanical) issues, but the concept of “hazard tree” as used byuaugust 2014 57
Developing Risk Assessment Standards and Specifications (continued)many utility arborists is so broad that it tends to foreclosethe very evaluation of failure potential that constitutesthe essential element in the practice of risk assessment(Lonsdale 1999).The tree risk program that developed contained thefollowing sections.OverviewIn order to establish a foundation for this work with alevel of exactness and accessibility for our audience, wedeveloped four major points: Definitions: since this aims to be a true specification, all terms need to be defined as precisely aspossible. Especially important are new terms introduced by the ANSI Standard and ISA’s BMP publication, including the concept of risk itself (Smileyet al. 2011), since prioritizing by the level of risk(instead of the presence of hazard) constitutes acritical change to industry practices. Risk is thecombination of the probability of an event occurringand its potential consequences. Goals and means: an explicit statement of realisticgoals and efficient methods also belongs up front.This is an appropriate location to introduce andconsider the suitability of suggestions made inrecent utility literature: collection of reliable poststorm data, creation of a hazard tree database basedon observations by trained observers, prioritizationof work (e.g., by species, customers served, circuitoutage history), adoption of targeted ground-to-skypruning (e.g., to first recloser), etc. Limitations: it is important to address the limitations of risk assessment because its effectivenessappears to be undermined by recent utility studies(e.g., Primrose et al. 2010; Guggenmoos 2011). Itmay well be prudent for many utilities to demonstrate how an improved tree risk assessment programcould increase reliability on its particular distribution system. Quality control: Integrated Vegetation Management programs should include a quality QA/QCprogram, per ANSI’s A300 (Part 7) (ANSI 2006).Following the Standard, it is critical to include specified items such as creating an annual report summarizing circuits assessed and trees identified, randomsampling circuits for adherence to risk assessmentspecifications and assessing circuits for residual treerisk following contractor work.Scope of WorkThe scope-of-work section covers topics such as tree location and selection criteria, level and type of risk assessment, inspection interval, method of reporting, andmitigation (Smiley et al. 2011). For each location withinthe distribution system, a scope of work is defined basedon the consequences of failure (number of customersserved and/or potential physical damage to utility hardware). A primary emphasis of the Unitil program was toestablish a scope of work that lowered risk tolerancewhere consequences were high, and raised it where consequences were low. This approach was guided by theANSI A300 (Part 9) and ISA BMP as they outline howrisk tolerance affects risk rating, from fieldwork to legaldefense, and we wanted to take that into account for theUnitil specification.The definitions and applications of the following itemswere detailed: Target: people, property, or activities that could beinjured, damaged, or disrupted by a tree failure. Inthe context of the Unitil tree risk program, this genericdefinition was explicitly restricted to all elementsof the physical system used to distribute electricity. Consequences: a function of the value of the targetand the amount of injury, damage, or disruption(harm) that could be caused by the impact of thefailure. Since risk is a combination of a target impactand its consequences, management of consequencesis well-suited as the means to increase reliability asmeasured by SAIDI and SAIFI. Inspection population: a subset of the trees nearconductors that will form the object of a particularrisk assessment. This subset is defined in the systemadopted for Unitil by the number of customers servedor importance of equipment threatened, with theTable 1. A critical component of the tree risk program was the development of a detailed scope of work. In this table, one part of thescope of work, definition of the “inspection population,” is identified based on the damage potential (consequences of failure) and isdefined here using potential number of customers interrupted if an event were to occur.Customer category/Inspection populationDamage potentialHighAll danger treesModerateHazard trees onlyLowOnly hazard trees whose bole lies at least partially within the distance establishedfor the line-clearance zone58 Arborist News www.isa-arbor.com
option of consulting other factors, such as maintenance history or circuit/segment significance.Inspection methods and intervals also needed tobe clarified, particularly since they vary in theUnitil system.Detailed standards and specifications were developedfor all other aspects of the required scope of work for theUnitil program. As an example of how this actually cameto be in the final documentation, here is the phrasing weadopted for inspection interval:Standard: Risk assessments should occur on a regular,recurring basis when justified by the level of risk ortarget value (Smiley et al. 2011), and it is the responsibility of the controlling authority to schedule repeatassessments (ANSI 2011). Assessments should bebased on existing vegetation, expected growth rates,and action thresholds (ANSI 2006).This was followed by a specification table listing howvarious intervals, from 1 to 5 years, would be used—a tablethat could also prove important when dealing with regulators, media, and the general public. It is important tonote that 1) the scope of work will be unique to each utilitydeveloping a risk management program, and 2) no part ofthe A300 (Part 7) or BMP publications identify or outlinesuch critical details within a utility context.Defect ProtocolRisk assessment requires that structural defects on treesthat could impact the conductors not simply be recognized (the definition of “hazard” tree as commonly used byutility arborists) but also judged for likelihood of failure.To facilitate fieldwork, we set clearly defined actionlevels for utility arborists in the field, something that hasnot been established within the utility or general arboricultural industry. Recommendations were presented intable format for structural defects based on “imminent”and “probable” failure likelihoods under normal weatherconditions (i.e., excluding rare events) within the designatedinspection period. These action thresholds were derived asmuch as possible from the scientific and other publishedliterature; where published literature was unavailable, webased the designated action threshold on our extensiveexperience with best practices in the arboricultural industry.Trunk – LeanImminentProbable A tree with a lean and symptoms of partial root plate failure,such as soil cracking around roots. Presence of active cracks in buttress roots or new trunk cracks. The presence of a lean along with other trunk or rootdefects considered to be probable.Picture 1. Probable likelihood of failure. The combination of lean andthe presence of a sapwood decay fungus on the base of the load-bearingside renders failure probable.Figure 3. To facilitate fieldwork and quality control, a set of clearly defined defect actions levels was provided. As shown here, the recommendationswere presented in an illustrated table for each type of structural defect.uaugust 2014 59
Developing Risk Assessment Standards and Specifications (continued)The Specifications document was populated with careful descriptions and references for each defect. The Manual, on the other hand, included a single page dedicatedto each defect with quality images and detailed captions.Specific action levels bring the double advantage ofimproving the recognition and interpretation of defects,on the one hand, and reducing the work necessary on theother. Note the effect of avoiding the unexamined label,“hazard tree,” in the situation represented in Figure 3.Conducting an actual risk assessment succeeded in 1)identifying and understanding the indicators of sapwooddecay fungi (Luley 2012), and 2) distinguishing between“imminent” and “probable.”Risk ModifiersThe assessment of the stability of trees or tree parts basedon observed defects must often take other factors intoaccount in order to reach a judgment about likelihood offailure—this is one of the most significant results of riskassessment research over recent decades. In this projectwe call such factors modifiers and use them to raise orlower defect classifications.Factors other than tree defects that affect a risk assessment can be grouped into four categories: Load: a generic term describing the effect of various forces acting on a structure. Whether a treewill fail depends, in the final analysis, on the loadit experiences, because load varies to a much greaterdegree and over a much shorter time period thanthe severity of any defect upon which it might act. Health: a measure of the tree’s ability to marshalgenetically determined defenses to compensate forstrength loss due to defects, as well as to respondto damage or load. The most convenient methodof judging a tree’s health is to examine its crowntraits (Bond 2012), for which we supplied a smallanalytic table showing critical thresholds. Site: main concerns for us were soil, biologicalagents, and disturbance history (human as well asnatural). Site disturbance may be the most significant modifier in a utility’s distribution system,since human activities, ranging from urban development to utility pruning, unavoidably alter direction and magnitude of forces on remaining trees. Species: the failure profiles of local species plays astrong role in risk assessment, and some utilities havealready incorporated them into their risk program(e.g., Brown and Dominguez 2008). We constructeda table of species, in an appendix, that constitutetwo percent or more of the inspection populationin Unitil’s distribution area, indicating the characteristics of each species that affect risk assessment(“failure profile”).To keep fieldwork manageable, we only presentedmaterial that a utility arborist could actually use quicklyand effectively. In the Manual, we accomplished this byusing unambiguous photographs and captions to get ourpoint across. Here is a typical caption from the section onhealth, as reflected in crown characteristics:Picture 2. The upper crown of this white ash displaysvitality and opacity levels of only 20%–40% of normal, indicating a critical state of health that increasesthe likelihood of failure over the next inspection period.Figure 4. Risk modifiers (e.g., site, exposure, species, tree health) that affectthe likelihood of failure or target impact were developed to allow the riskassessor to account for these important factors in the field. Here, this leaningblack locust (Robinia pseudoacacia) shows a crown with traits of critical healthand lacking damping lateral branches—traits that would bear on the evaluationof risk by raising the failure potential.60 Arborist News www.isa-arbor.com Note again that the ANSI Standard and ISA’s BMPappropriately do not specify how these modifiers shouldbe included in a risk assessment, just that they should beincluded, making the use of recent published literaturecritical to the successful incorporation of these modifiersinto a utility risk assessment program.
Organization SpecificsEvery utility will want to conclude its specifications with arange of organization-specific concerns about operationalaspects. We addressed topics that would be importantwithin the Unitil context, such as training, scheduling, dataacquisition, specification review, and revision.ConclusionThe main thrust of this work was a revised approach toutility risk assessment in a distribution system. Fundamental concepts were adapted from the A300 (Part 7)and the BMP: risk, scope of work, levels of assessment,and the centrality of consequences. Circuit-specific consequences drive selection and location of trees to beinspected, method and intensity of tree risk assessment,and tolerance for risk. It is important to recognize thatrisk assessment is not identical to risk management. Atree risk specification makes up only one part of the creation and application of policies, procedures, and practices used to identify, evaluate, mitigate, monitor, andcommunicate tree risk (Smiley et al. 2011).Within Unitil, this revised risk assessment program iscurrently being implemented—starting with trainingand establishing basic field application methods. Fromthere, a practical review begins where arborist coworkersmeet in the field and look at actual utility situations, anddiscuss risk assessment outcomes and the actions to betaken. As implementation progresses, arborists will conduct peer review on a semi-annual basis, and assist theSystem Arborist with an annual review and assessment ofthe overall program and the risk levels identified.As trees in the urban forest are not static, neither areutility practices, regulatory environment, and municipality involvement. Just as importantly, research and development are always continuing and growing as well. Forthis reason, Unitil will endeavor t
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