Third-Party Damage

1. Assessing third-party damage potential: sample of data typically used

Background

Much attention has been directed towards preventing third-party damages to pipelines in many industrialized countries. Nonetheless, recent experience shows that this remains a major threat in many places, despite often mandatory protective measures such as one-call systems.

The US pipeline regulator reports that third-party interference is the most common cause of pipeline failures on land, accounting for 20 to 40 percent of failures within most time periods as well as most of the casualties and pollution. [71]

The majority of offshore pipeline accidents are not caused by third-party damages, but this failure mechanism seems to result in more of the deaths, injuries, damages, and pollution [71]. Consequently, this is a critical aspect of the risk picture for offshore facilities also.

See PRMM for a discussion of underlying causes of third-party damage.

Many do not realize the susceptibility of apparently-strong pipeline components to eventual failure from even minor contacts. A simple scratch on the pipeline can, over time, be as serious as a deep gouge, damaging the coating, accelerating corrosion and/or cracking, and leading to eventual failure. A deep-enough scratch can set up a stress concentration area that at some future point could cause failure from fatigue or a combination of fatigue and corrosion-induced cracking.

While a pipeline operator understands the dangers posed by any interference, some contractors and the general public may not. Communication with any and all parties who may need to excavate will increase safety. Hence, the mitigative benefits of public education.

Assessing third-party damage potential

Third-party damage, as the term is used here, refers to any accidental damage done to a component as a result of activities of personnel not directly associated with the pipeline (ie, not as employees or contractors). This failure mechanism is also sometimes called outside force, mechanical damage, or external force, but those descriptions would presumably include damaging earth movements, water impingement, and others. Third-party damage is chosen as the descriptor here to focus the analyses more^[1] on damage caused by people not associated with the pipeline. Potential earth movement damage and impacts not directly related to human action (but often indirectly related) are addressed elsewhere in the assessment. Intentional damages are covered in the sabotage assessment.

Accidental damages done by pipeline personnel—first- and second-party damages, not third parties—could be covered either here or alternatively in the incorrect operations assessment. Including first- and second-party impact potential here, rather in human error event estimation is usually more intuitive. Mitigation measures are often the same for many excavation and vehicle impact exposures, regardless of who is atop the equipment. This is often important for exposures inside facilities/stations where third party activities are improbable but first- and second-party activity levels are generally higher.

Accidental damage includes impacts on unburied components. The argument can be made that aboveground components enjoy the benefit of being visible, thereby avoiding the damages (reducing risk^[2]) caused by not knowing exactly where the pipeline is (as is often the case for buried sections) and usually having less threat from corrosion (unless insulated, for example). The opposite would be true for an above ground component in an environment with higher impact damage threats and less corrosive soil environment (ie, this threat trade-off results in an increased risk).

Pairings of Specific Exposures with Mitigations

Although an often-justifiable short cut in risk modeling is to collect many types of exposures and pair them with a single collection of mitigations, it is more correct to pair specific exposures with pertinent mitigations. Where differing exposure-specific mitigations are employed and/or where mitigations have varying effectiveness depending on the type of exposure, pairings of specific exposures with pertinent mitigations will be essential, as previously noted.

For example, the following exposures are often paired with customized mitigations to better reflect real-world threats:

• Excavation—agriculture; treated differently from other types of excavation, ie, shallower but more frequent exposure events from agricultural activities.
• Excavation—construction; often characterized by deeper, infrequent events
• Impacts—vehicles; impacts entail different forces than excavation damages in many key aspects so a vehicle impact is different from falling objects
• Impacts—falling objects; discrimination among types of objects—trees, buildings, anchors, tools, etc—is often appropriate.

The mitigative benefit of depth of cover, public education, barriers, and others is different for each of these exposure types. Other pairings may be equally appropriate. For instance, drilling and boring type excavations are materially different from many other types of excavation and may warrant independent treatment in the risk assessment. See further discussion under mitigation.

Exposure

In measuring or estimating third party damage exposure, it is important to first list all potential damaging activities and events that could occur at the subject location. Then, numerical frequency-of-occurrence values should be assigned to each event. Pre-dismissal of threats should be avoided—the risk assessment will show, via low PoF values, where threats are insignificant. It will also serve as documentation that all threats are considered. A frequency of zero or nearly zero can be assigned to extremely remote exposures. For instance, the exposure from falling trees where no trees are present, is obviously zero. Recording this ‘zero’ value demonstrates completeness in assessing exposure.

The exposure level will often change over time, but is often relatively unchangeable by the pipeline operator. Relocation is sometimes the only means for the pipeline operator to change this exposure, and even then, relocation may not result in a permanent reduction in exposure.

Recall that all exposures are evaluated in the absence of mitigation. This is important since it adds clarity and completeness to the assessment. For example, the unmitigated exposure from falling trees might be estimated to be on the order of several times per year—perhaps coinciding with severe storm—wind, ice, flood, etc— frequency. It is only after adding mitigation—notably depth of cover—that the threat appears as small as most intuitively believe it is. Failure to separate the exposure from the mitigation may lead to an inappropriate dismissal of a threat, especially if conditions change. For example, if the pipeline is re-located to an above-ground location under large trees the separation of exposure/mitigation will recognize the change in risk.

It is important to maintain a discipline of assessing exposure separately from mitigation and resistance, avoiding any temptation to short-cut the assessment to a perceived outcome that may not adequately reflect true risk. The ‘unprotected beverage can’ analogy puts the proper perspective to the exercise of producing the exposure estimates.

Recall also the discussion of mitigation by others. If additional speed control is initiated on a roadway, that action is better modeled as a reduction in exposure to vehicle impact rather than an addition to mitigation. It is generally more efficient in a risk assessment to establish a protocol whereby mitigative actions taken by the pipeline owner are modeled as mitigation while mitigative actions taken by others are modeled as reduced exposures.

Recall the early discussion of nuances of exposure, mitigation, and resistance estimation. Potential damages to the load-carrying capability of the component are exposures while damages to, for instance, coatings are modeled as reductions in corrosion mitigation. Recall also that an exposure is defined as an event which, in the absence of any mitigation, can reduce the load-carrying capacity. Under this definition, even a minor scratch or gouge is damage since, if a stress concentrator arises, the ability of the component to carry long term fatigue loadings may be reduced. An exposure, therefore, is an event that, when unmitigated, would result in a damage that causes a reduction in load-carrying capacity—immediate or long-term—of the component.

Area of Opportunity

Implicit in a probabilistic risk assessment is the concept of ‘area of opportunity’. Third-party damage potential increases as the area of opportunity for accidental contact increases. The area of opportunity is strongly affected by the level of activity near the pipeline. More activity near a component logically increases the opportunity for a strike. The lowest exposure is associated with scenarios where there is virtually no chance of any digging or other harmful third-party activities near the line.

Population density is therefore typically a consideration in the risk assessment. More people in an area generally means more activity: fence building, gardening, water well construction, ditch digging or clearing, wall building, shed construction, landscaping, pool installations, etc. Many of these activities could disturb a buried pipeline. The disturbance could be so apparently minor as to go unreported by the offending party. As already mentioned, such unreported disturbances as coating damage or a scratch in the pipe wall are often the initiating condition for a pipeline failure sometime in the future.

An area that is being developed or is experiencing a growth phase will often require frequent construction activities. These may include soil investigation borings, foundation construction, installation of buried utilities (telephone, water, sewer, electricity, natural gas), and a host of other potentially damaging activities. Planned or observed development is therefore a good indicator of increased activity levels. Local community land development or planning agencies might provide useful information to forecast such activity.

Excavation damage potential includes drilling/boring and impact driving operations. These are often sensitive to pipeline contacts–it is possible for the equipment operator to hit a facility without being aware of the hit. The drill bits or driver points, designed to go through rock, may experience little change in resistance when going through plastic pipe or cable and can cause much damage to steel pipelines. These are unique forms of excavation with different damage potentials compared to surface excavation. Some mitigation measures may also have differing effectiveness on this type of excavation—for example, marking/locating accuracy requirements, benefits of signs/markers, etc. There may also be other unique exposure aspects. For example, with no visibility from the surface, there will typically be fewer opportunities for a last minute intervention.

The presence of other buried utilities logically leads to more frequent digging activity as these systems are maintained, inspected, and repaired. This increased exposure is perhaps partially offset by a presumption that utility workers are better versed in potential excavation damages than are some other excavators. If considered credible evidence of increased risk, the density of nearby buried utilities can be used as another variable in judging the activity level.

A relatively high activity level normally accompanies a distribution system. Often though, a more experienced group of excavators works near these systems, sometimes to the exclusion of ‘amateurs’. Consider excavators working in densely populated or commercialized urban areas. These excavators are owners of or contractors to other utilities, have more experience working around buried utilities, expect to encounter more buried utilities, are often working under strict procedures and permitting systems, and are more likely to ensure that owners are notified of the activity (usually through a one-call system). Consistent use of a one-call system by local contractors can be an indication of informed excavators. Nonetheless, errors are possible. It is still often advisable to conservatively assume that more activity near a pipeline offers more opportunity for unintentional damage to a pipeline, even when skilled excavators dominate.

From the impact perspective, considerations include nearby rail systems and high volumes of nearby automobile traffic, especially where heavy vehicles such as trucks or trains are prevalent or speeds are high. Aircraft traffic should also be included. Aboveground facilities and even buried components are at risk because a vehicle impact can have tremendous destructive-energy potential.

Offshore facilities, including those under streams, rivers, lakes, oceans, etc, are often exposed to damage potential from anchoring, fishing, dredging activities, and shipwrecks along with dropped objects. New water-crossing pipeline installations by open-cut or directional-drill methods may also pose a threat to existing facilities. Offshore dredging, shoreline fortifications, dock and harbor constructions and perhaps even offshore exploration/production drilling activities may also be a consideration. Debris movement along sea bottoms involving man-made objects, from normal current flows and especially during offshore storms have also damaged components.

Also important to some assessments is the potential for sympathetic reactions—failures in a nearby component creating forces sufficient to damage the subject component. Shared pipeline ROW’s and many above-ground facilities harbor such threats.

Estimating Exposure

Quantifications of exposure can be done very simply or, alternatively, with a high level of associated research and calculation. When direct measurement of exposure rates are unavailable or when plausible exposure levels are not deemed significant enough to warrant the research, simple reasoning exercises can be used to assign values.

Exposure estimates involve predicting future events. Indicators of past activity can inform estimates of future exposures but with varying degrees of relevance, sometimes to the point of being a contrarian indicator. Consider a high historical frequency of vehicle ‘leaving the roadway’ type incidents. An abnormally high frequency will often prompt actions such as reduced speed limits or installation of barriers to reduce the exposure. During construction of a new residential area, activities will be high. But once established, activity levels in the new neighborhood may fall to below-average levels. In both examples, the high past rate portends a decreased future rate.

Nonetheless, historic rates are normally good starting points from which to quantify location-specific exposures. Even with questionable direct relevance, at least understanding the range of values that have occurred elsewhere is useful.

Various commonly-available records show past exposure levels. These records may come from public data sources, direct observation by pipeline personnel, patrols by air or ground, incident records, and telephone reports by the public or by other construction companies. The one-call systems (these are discussed in a later section), where they are being used, provide an excellent database for assessing the past level of excavation activity even though this is not often a good indicator of future activity. Roadway and railway owners, as well as government agencies, typically keep records of vehicle incidents. Aircraft and marine vessel incident rates are also commonly available.

Note however, that all such measures of activities are only a lagging indicator; that is, they may show where past activity has occurred but not necessarily be indicative of future activity. Current and past activity are very relevant to estimates of where damages may have already occurred. Perhaps one of the best indicators of the defect introduction rate—for coatings and pipe wall—is the frequency of excavation activity reports. This is considered in the PoF assessment aspects such as coating effectiveness and resistance. For third party damage exposure, used to predict future failure potential, past and current activity levels may be less relevant as indicators of future activity.

Advance notice of pending excavation activity is especially useful for predicting exposures. Regulatory permitting for land development indicating the impending use of the area—development in progress or planned—is a potential source of information on longer term activity levels. Evidence of more immediate activities arise from pre-excavation indications such as survey markings.

Excavation

The quantification of the risk exposure from excavation damage requires an estimate of the number of potential excavations that present a chance for damage. Excavation occurs frequently in the United States. The excavation notification system in some states record hundreds of thousands of calls per month and millions of excavation markings per year, averaging thousands per day in some areas [64].

As noted in PRMM, it is estimated that gas pipelines in the US are accidentally struck at the rate of 5 hits per every 1,000 one-call notifications.

An examination of historical excavation damage accidents supports the hypothesis that a higher population density means more accident potential.

1. Potential Excavator Damages

In 1995 the Gas Research Institute (GRI), now known as the Gas Technology Institute (GTI), conducted a useful study on excavation risk-exposure for the gas industry. Results showed rates of 58 third party strikes per 1,000 miles of all types of pipelines, with transmission pipelines receiving only 5.5 hits per 1,000 miles and distribution lines suffering 71 hits per 1,000 miles [64]. Such data from studies such as this can be used as inputs to exposure estimates. See PRMM and ref [64] for a summary on this.

Ref [9988] cites excavation rate values of 0.076 per km/year for agricultural areas and 0.52 per km-year in commercial and industrial areas. With ‘typical prevention measures’, these rates were thought to lead to hit rates of between 0.004 per km year for undeveloped areas and 0.05 per km-yr for developed areas. This reference also notes that 75% of the excavation impacts are by backhoes which are too small to cause ‘serious damage’ to the larger diameters typical of transmission pipelines. If the risk assessor concludes relevance to the components being evaluated, then this information can be useful in estimating exposure rates, mitigation (for example, see discussions on depth of cover benefit and patrol effectiveness based on excavation evidence), and resistance.

Even in fairly short distances, exposure rates can vary widely. Indicators such as new construction, repeated work on nearby facilities, anchoring and dredging areas offshore, etc can be very location-specific. Higher exposure rates (perhaps on the order of 0.1 to over 100 events/year at certain locations) and lower exposure rates (perhaps less than 0.01 events per mile year) may be associated with common indicators of exposure level. A more complete listing of such indicators is found in PRMM.

Impacts

General categories of impacts include those from vehicles and falling objects, as discussed below.

Vehicles

Type and speed of vehicles are determinants of damage potential. Various traffic impact scenarios are possible for many components. Considerations include moving object congestion, frequency, duration, direction, mass, speed, and distance to facilities. The impact potential is often informed by historical accident frequency, severity and damage caused by cars, trucks, rail cars, offshore vessels, and/or plane incidents.

Vehicle impact potential can be assessed by considering categories of ‘momentum’, where momentum is defined in the classic physics sense of vehicle speed multiplied by vehicle mass (weight). High speed, lightweight vehicles can cause damages comparable to low speed, heavy vehicles. Momentum exposures can be assessed in a quantitative way by estimating the frequency of occurrence around the component being assessed. For example, a high frequency of light aircraft at a small airport might be two or three planes per hour, whereas a high frequency for heavy trucks on a busy highway might be hundreds per hour. For each type of vehicle, the frequency can be combined with the momentum to yield an exposure estimate. Where the potential for more than one type of vehicle impact exists (and mitigations for each are equivalent), the frequencies are additive.

Most roadways in most Western countries will have occasional vehicle excursions but high incident rates at specific locations will not long be tolerated. A section of road that experiences numerous vehicle excursions every week will normally prompt action. Safeguards such as speed control and barriers will be employed by roadway owners and/or owners of exposed facilities to control the rate. This informs estimates of exposure since rates of, for instance, 100 incidents per week at a single road location, would not seem plausible, at least for most industrialized countries.

The type of vehicular traffic, the frequency, and the speed of those vehicles determine the level of exposure. Vehicle movements inside and near aboveground facilities should be especially considered, including

• Aircraft
• Trucks
• Rail traffic
• Marine traffic
• Passenger vehicles
• Maintenance vehicles (lawn mowers, etc.)

The potential damages caused by various vehicle impact scenarios can be challenging to estimate without detailed calculations of many combinations of component characteristics and impact specifics. This is further discussed in resistance, .

Falling Objects

Objects dropped or falling from heights above the component being assessed are a potential source of damage. The potential for toppled structures nearby should also be included in the assessment. Falling trees, buildings, walls, utility poles, aircraft, meteors, cranes, tools, pipe racks, etc are often overlooked in a risk assessment. This is an understandable result of discounting such threats via an assumption that a buried component is virtually immune from such damage. While this is normally an appropriate assumption, the risk assessment errs when such threat dismissal occurs without due process. The independent evaluation of exposure and mitigation ensures that, should depth of cover condition change, ie, the component is relocated above grade; or a particular falling object indeed can penetrate to the buried pipeline; are not lost to the assessment.

Many of these exposures can be tied to weather phenomena such as windstorm and ice loadings. Therefore, exposure estimates can be tied to location-specific data on recurrence intervals of such phenomena. For instance, most locations along the Gulf of Mexico have hurricane recurrence intervals of around once every 25 years. This suggests a hurricane-induced wind storm event frequency of 1/25 per year, with perhaps only a fraction of those events actually generating wind-borne debris loadings of sufficient magnitude and direction to potentially cause damage to the component being assessed. These loadings could alternatively be considered in the Geohazard portion of the PoF assessment.

Similarly, aircraft crash rates are well documented and even meteorite strike rates have been approximated.

Offshore, objects can be dropped from some surface activity (construction, fishing, platform operations, mooring close to platforms, cargo shipping, pleasure boating, etc.) and can damage submerged facilities.

The risk of subsea equipment being damaged by dropped objects can be assessed and used to ensure that proper levels of physical protection are provided in the design phase. Drops per lift, based on UK offshore historical data, suggest rates ranging from 10-5 to 10-7. Coupled with lift frequencies ranging from 10^4 to 10^8 per year, results in ranges of historical exposure rates, possibly appropriate for an offshore segment being assessed. These general rates can be made more scenario specific with knowledge of equipment types, loads being lifted, and many other factors.

In offshore scenarios, dropped objects may travel large horizontal distances before reaching sea bottom. This is dependent upon currents and depth and can be included in the probability that a dropped object from a certain location will strike the component being assessed. A buffer distance around fixed sources such as platforms can provide a zone within which components are threatened from that source. Similarly, for moving sources such as vessels and aircraft, an additional probability of proximity can be added to the assessment.

Damage potential is related to energy imparted which in turn is related to object weight, height, and the acceleration of gravity. For subsea installations, the object’s terminal velocity as it travels through the water will determine the energy imparted. This is a function of the object’s weight, shape, water displacement, and resistance to flow, or drag.

Station Activities

Surface facilities will often have different types and frequencies of exposures compared to ROW segments. Attention to the frequency and duration of normal vehicle movements, in-station excavations, facility modifications, and visitor traffic is usually warranted. Controlled access, third-party facilities present, and continuous work inspection are considerations. While damage caused by employees of the pipeline owner/operator is not technically third-party damage, all such station activities may be more efficiently addressed as part of excavations and impacts assessment.

For surface facilities, there is often need for additional emphasis on internal traffic, including loading operations involving trucks, rail, marine vehicles as well as the potential for successive reactions.

Successive reactions

The potential for successive reactions warrants more discussion. A successive, or sympathetic, reaction is the damage caused to one part of a facility by a nearby event (for example, rupture, fire, explosion, etc) on another part of the facility. Accidental rupture and explosion of a vessel containing combustible material can cause heat and/or projectile damage to a other parts of the facility or to neighboring facilities. Debris, projectiles, and impulse loadings from nearby explosions are readily apparent potential causes of damage. Other damage scenarios include neighboring pipelines, even when both the assessed and the neighbor components are buried

Successive or Sympathetic Failures

Segments that are susceptible to such secondary effects will show a higher risk, even if only a very minor increase. Because this event depends on the occurrence of another, the level of exposure for this kind of external force is low. The probability of the initial event is normally low and the successive reaction event, as a fraction of the initial failure probability, should usually be very low.

The damage potential is a function of what is being transported or stored, and the volume and pressure. The potential can be quantified by calculating or estimating the thermal and/or overpressure effects from failure of a neighboring component.

Factors such as barriers, shielding and distance reduce the threat, and estimated¹ exposure or mitigation values should reflect this.

Ideally, the likelihood of failure of the causal event is based on its own complete PoF assessment. This additional assessment might not be possible if the causal event can occur from a neighboring facility that is not under company control. If so, an estimate, perhaps based on generic component information (for example, average natural gas transmission pipeline failure rates), consistent with the PXX level of the assessment, is appropriate.

Example of Successive Reaction Exposure Estimates

Initiating Event Location	Frequency of Failure per year	Fraction Potentially Damaging to Neighboring Equipment	Exposure (events/year)
Pipeline AB, Sta 110 to 145	0.00001	0.1	0.000001
Valve 1 on Pipeline XY	0.00005	0.05	0.0000025
Vessel 1	0.001	0.01	0.00001
Vessel 2	0.05	0.2	0.01
Truck Loading	0.006	0.5	0.003
			0.013	exposure from successive reactions (events/year)
		an event every	76.8	years

Offshore Exposure

Anchoring, fishing equipment impacts, shipwrecks, platform sinkings, debris transport by moving waters, shoreline constructions, dredging are some external forces unique to the offshore environment. Vortex shedding and loadings due to moving waters are aspects of risk captured elsewhere in the assessment (ie, as contributors to cracking failure). As with an onshore assessment, in an offshore third-party damage exposure estimate, the evaluator assesses the probability of potentially damaging activities occurring near the pipeline. A complete list of plausible activities will be necessary for a full assessment. Higher activity logically increases the exposure. Each exposure should be assigned an exposure rate: events per mile-year, for example. Where exposure levels are higher or multiple exposures co-exist, PoF increases.

Sample of Exposure Assignments to Offshore Component

Exposure	P95 Exposure Rate (events per mile-year)	Comments
Storm debris movement1	0.02	1/10 x severe storm freq
Anchor drag	0.1	Based on ship traffic
Anchor drop	0.02	Based on ship traffic
Foreign pipeline construction	0.05
Trawling	0.1
Dropped object from vessel	0.05
Ship wreck (sinking)	0.001
Dropped object from platform	0.1

Other Impacts

Detonations, including subsurface detonations from seismograph, mining, or construction, can damage pipeline components and should be included in exposure estimates. See PRMM for more discussion.

Damage from wildlife is not uncommon in some areas. Large animals can damage coatings and instrumentation and sometimes even directly threaten the integrity of pressurized components. Even birds and insects can cause damage that eventually contributes to a failure.

External impacts related to geohazard events with little to no man-made materials involved—landslides, rock falls, sea bottom movements, etc.—are normally considered in the Geohazard assessment.

Mitigation

Given the prevalence of accidental third party damage potential, pipeline operators usually take significant steps to reduce the possibility of damage to their facilities by others. The extent to which mitigation is effective is related to how many damage incidents are avoided. Avoidance of damage in turn depends on how readily the system can be damaged by an event and how often the potentially damaging event occurs.

Continuing the earlier discussion, specific pairings of exposure with mitigation effectiveness is part of a more robust assessment. This recognizes that the same mitigation will often have different effectiveness on different exposure types. Examples include:

• 1 foot depth of cover is generally more protective against agricultural equipment damage than against excavation equipment damage
• Depth of cover may have little mitigative benefit against subsurface boring operations
• Patrol is more effective against exposure scenarios that are slower to manifest, such as cross country pipeline construction and residential developments.

Some assumptions commonly used in assessing mitigation effectiveness for this threat include the following:

• One-call effectiveness is generally an AND gate between sub-variables such as system type, notification requirement, and response. The AND gate is applicable since all sub-variables together represent the effectiveness of the mitigation. If any single aspect is deficient, then the overall effectiveness is suspect.
• The mitigation of patrol is normally an AND gate between patrol type and frequency. Patrol type implies an effectiveness and includes consideration of different types—ground (foot or vehicle), air (speed, altitude, spotter, etc), for example. But regardless of the effectiveness of the each patrol, if not done at sufficient time intervals, overall mitigation effectiveness is suspect.
• External damage protection is typically an OR gate between depth of cover, warning mesh/tape, exterior protection since each measure can act independently to reduce the chance of damage.
• Casing is a mitigation (against external forces) as it is something added to a pipeline system. For risk assessment purposes, slabs, casings, and even concrete coatings are considered to be distinct from the component and therefore best treated as mitigation measures. Under this view, the component is not damaged when only the protection–the casing–is damaged. Some reduction in mitigation may have occurred, but not direct damage to the component.

Component wall thickness or strength, even when ‘excessive’, is not a mitigation. It does not prevent damage. If the wall thickness/strength is greater than what is required for anticipated pressures and external loadings, the ‘extra’ is available to provide additional protection against failure from external damage or corrosion. Mechanical protection that may be available from extra pipe wall material or strength is accounted for in resistance estimates .

Depth of Cover

The depth of cover is the amount of earth, or equivalent protection, over the pipeline that serves to prevent damage to a buried component from third-party activities and impacts. In general, deeper and stronger cover —more resistant to penetration, ie rock or pavement versus sand—provides greater protection. Interestingly, protection does not really begin with the first amount of cover. A small amount of cover, enough to conceal the component but not enough to protect the line from even shallow earth moving equipment can increase risk beyond what a ‘no cover’ scenario would present.

A relationship between cover depth and mitigation effectiveness will be needed. This relationship is most robustly established by obtaining an accurate distribution of types of equipment potentially active in the area and then considering potential reaches and forces from such equipment.

It is often appropriate to employ different relationships for different classes of equipment and practice. For instance, as previously noted, agricultural equipment will often not penetrate the ground to the same extent that many construction excavations will. In this case, more mitigation effectiveness is achieved sooner—at shallower depths—when potential damages from agricultural excavations are assessed.

It may also be appropriate to apply different factors or different relationships between depth (and equivalences) and mitigation benefit for impacts not related to excavations. Two feet of cover prevents damage from falling telephone poles more reliably than from train derailments. The robust solution is to estimate distributions of possible forces from potential impact events and calculate the fraction of those events that are nullified by various mitigation measures. Some studies are available to assist in the determination of an appropriate relationship between depth and mitigation effectiveness. Research from similar pipeline environments can also be useful.

In the absence of more definitive research, an appropriate relationship can be theorized by rationalizing changes in effectiveness when depth of cover changes, considering the types of excavation practice involved. From such rationalizations, equations can be posited and employed in the risk assessment.

A schedule or simple formula can then be posited to assign mitigation effectiveness based on cover. For instance:

12in. of cover = 10% mitigation effectiveness

36in. of cover = 65% mitigation effectiveness

A sample relationship, with exponentially increasing protection as depth increases, is as follows:

Effectiveness = 1 – exp(-[amount of cover in inches] x [factor]

Where the factor chosen reflects the assessment designer’s view of the rate of change in the effectiveness variable as the depth variable changes.

Conceptual Relationships Between Depth of Cover and Mitigation Effectiveness

Mitigation credit should also be given for comparable means of protecting the line from mechanical damage including slabs, casings, roadway pavements, etc. Cover for a distribution system often includes pavement materials such as concrete and asphalt as well as sub-base materials such as crushed stone and compacted earth. These are more difficult materials to penetrate and offer more protection for a buried pipeline. Additionally, most municipalities own rights of way and control excavations on public property, especially when penetrating pavements. This control suggests reduced third party damage exposure to a pipeline buried beneath a roadway, sidewalk, etc.

Casing pipe was historically installed to carry anticipated external loads and to protect road and railroad structures from damage if releases occur. A casing pipe is merely a pipe larger in diameter than the carrier pipe whose purpose is to protect the carrier pipe from external loads. Casing pipe can cause difficulties in corrosion control as is discussed later. When the casing carries the external load and protects the section being evaluated from outside forces, it acts as a mitigation.

A robust assessment will determine the benefits of these barriers by quantifying the reduction in PoD that is achieved by the additional protection—how many otherwise damaging events will be interrupted by this protection? This requires estimates of types of equipment and associated forces potentially making contact with the barrier as well as the equipment operator’s response in the case of excavations. When such rigor is unwarranted, a simple schedule can be developed for these barriers by equating the mechanical protection to an amount of mitigation effectiveness. For example, depending on the types of excavation equipment, values such as the following are plausible:

2in. of concrete coating = 50% mitigation

4in. of concrete coating = 80%

Pipe casing = 99.99%

Concrete slab (reinforced) = 90%

4in. asphalt roadway = 85%

It is not only the physical strength of the barrier, but also the implication to the excavation equipment operator of the presence of the barrier. An excavator will normally react differently to a casing pipe than to additional depth of cover. Ideally, he will react to any unexpected encumbrance as an indication that the area is not free of buried structures and should be treated more carefully. This is the idea behind buried warning markers such as highly visible strips of warning tape or mesh with imprinted warnings. Either will logically reduce excavation damage potential and can be valued in terms of its ability to independently protect the component in the location being assessed.

Consider the following sample assignments of mitigation effectiveness:

• Warning tape assessed as 90% effective suggests that nine out of ten excavation scenarios will be halted by this mitigation measure alone (remnant exposure = one hit out of ten excavations).
• Warning mesh assessed as 95% effective suggests that nineteen out of twenty excavation scenarios will be halted by this mitigation measure alone (remnant exposure = one hit out of twenty excavations).

Sea bottom (and lake-, river-, creek-, etc bottom) cover and equivalents (for example, concrete mattress, rip rap rock deposits, concrete coatings, etc) provide mitigation from offshore exposures. Just as with other natural barriers, the water depth can also be treated as a mitigation in the risk assessment.

After assigning a mitigation effectiveness to each protection type independently an OR gate is used to obtain the combined effectiveness. For example, if a component is 60% protected by 30″ of earth cover and also is encased by a steel casing pipe providing an additional 98% protection, then the combined mitigation from these two methods is 1 – (1-60%)*(1-98%) = 99.2%.

Impact Barriers

Since the presence of aboveground components is something that is often difficult to change—their location is usually based on strong economic and/or design considerations—preventive measures must be taken to reduce their vulnerability to any exposures that may accompany the site. Additional types of protection from mechanical damages include barriers against impacts on these unburied components. Exposures from vehicular collision, falling objects, vandalism, and sabotage may be offset by the mitigative benefit of barriers other than burial and those previously discussed.

Barriers and protections, both man made and natural, around aboveground facilities should be identified and assigned mitigation effectiveness in general or for specific exposure types (for example, varying by type and speed of vehicle impact, etc.).

Sample Listing of Protective Measures to be Combined for Mitigation Effectiveness:

• Area surrounded by 6-ft chain-link fence
• Protective railing (4-in. steel pipe or equivalent)
• Trees, wall, earthen berm, or other substantial structure(s) between vehicles and facility
• Ditch (minimum 4-ft depth/width) between roadway and facility
• Waterbodies, min 10 ft +
• Waterbodies, < 10 ft
• Concrete traffic control barriers
• Water filled traffic control barriers
• Offshore protection by articulated concrete mattress

Distance from vehicular traffic on roads, railroads, flight paths, and ship activity are best considered as part of the exposure assessment. As distance increases, the frequency of exposure events logically decreases.

Assignment of mitigation effectiveness can have a basis ranging from simple, SME-based judgments to robust calculations specific to each exposure-mitigation-type scenario. Note the potential for some barrier types to exacerbate an exposure; for example, an earthen berm serving to launch a fast moving vehicle so it becomes an airborne impact threat.

Protection for aboveground facilities.

Security measures that protect against vandalism or other intentional damage may also provide mitigative benefits to accidental damages. Surveillance systems, barriers, lighting, etc. may offer some protection from accidental impacts under some scenarios.

Line locating

A key to avoiding accidental excavation damage is the line-locating process, undertaken by an operator when notified of pending excavations by others. It typically involves a notification system, line locating equipment and procedures, marking practices, supervision during activates, and others. As a multi-faceted process, it is challenging to assess, from a risk assessment mitigation effectiveness perspective. A robust risk assessment examines the various aspects of typical programs and potential criteria for use in assessing overall effectiveness.

Often called ‘One Call’ systems, excavation notification systems have become commonplace and their use often mandated by law. They have varying amounts of mitigative effectiveness, as evidenced by many operators’ experiences.

The pipeline company’s response to a report of third-party excavation activity is the next critical step in this mitigation measure. Notifications without proper response in a timely manner negate the effects of reporting. Response includes the efficiency and accuracy of the locating equipment and procedures employed as well as the clarity of the markings. Finally, the communications between all parties and the amount of oversight during nearby excavations are important contributors to the effectiveness of these programs.

See PRMM for further details on all aspects of line locating programs.

The assigning of error prevention rates—or mitigation effectiveness, ‘success rate of mitigation’—to the process of line locating is important for risk assessment. Since the accuracy of maps/records is only one facet of the entire locate process, error rates associated with the other aspects must also be included in the assessment. Using a scenario-based analysis tool such as event trees or LOPA can be useful in assessing mitigation effectiveness.

Integrating all of the above considerations into an effectiveness estimate is challenging. The risk assessment requires this estimate of the overall program at each location assessed, recognizing that this effectiveness may vary along a pipeline, from season to season, and over time in any area (for example, change of management resulting in change in focus). Human error potential can be high in these multi-faceted programs.

Company SME’s have typically assigned maximum effectiveness values in the range of 20% to 80%, for one-call/locate programs, based on their experiences with specific pipeline segments. For perspective, the higher end of this range assumes that 8 out of 10 otherwise damaging events are avoided solely (assuming no depth cover, no signs, etc) through the one-call and line locating program while the lower end assumes only 2 out of 10 events are avoided, even with a very good program. Actual effectiveness values are then assigned based on differences from the idealized, perfect program.

Signs, Markers, and Right-of-way condition

Establishing a clear, well-marked ROW is an interesting mitigation practice. On one hand, the more recognizable and inspectable a ROW is, the less the likelihood of accidental interference. This is also helpful in leak detection. However, a manicured path through otherwise difficult-to-traverse terrain, may also invite unwanted activity. Therefore, the risk assessment should fairly evaluate the benefits, if any, to mitigation offered by a clear ROW alone versus a potential increase in exposure.

Various types of signs and markers, including curb markers in paved areas, and painting of fence posts, are used to mark ROW’s. Most will agree that signs and markers do provide some mitigation benefit. However, pipeline accident photos showing burning excavation equipment immediately adjacent to a warning sign, show that the protective benefit is clearly not complete, at least in some locations.

It is usually impractical to mark all locations of a distribution system. Many components are under pavement or on congested private property. Nonetheless, in some areas, markers are used and believed to reduce third-party intrusions.

Subtleties of marker position, frequency, size, colors, lettering fonts, languages, etc are logically related to effectiveness. However, against the backdrop of initial human reaction, desensitization, and other behavioral issues, such considerations are difficult to quantify.

Where mitigation benefit is believed to increase with increased identifiability as a ROW, the evaluator can establish a schedule of mitigation effectiveness associated with various levels of marking/clearing; for example, “…two or more markers visible from all points on the ROW’ provides 10% mitigation”.

In an offshore environment, this mitigation may only be effective at shore approaches or shallow water where marking is more practical and third-party activity levels are higher. At such locations, marking of offshore pipeline routes provides a measure of protection against unintentional damage by third parties. Buoys, floating markers, and shoreline signs are typical means of indicating a pipeline presence. On fixed-surface facilities such as platforms, signs are often used. When a jetty is used to protect a shore approach, markers can be placed. The use of lights, colors, and lettering enhances marker effectiveness.

Company SME’s have typically assigned maximum effectiveness values in the range of 2% to 20%, based on their experiences with specific pipeline segments. For perspective, the higher end of this range—a rating of ‘excellent’—assumes that 2 out of 10 otherwise damaging events are avoided solely through the markers (assuming no depth cover, no public awareness, etc) while the lower end of the range, again with a rating of ‘excellent’, assumes only 2 out of 100 events are avoided. Actual effectiveness values are then assigned based on differences from the idealized, perfect program.

Patrol

Patrol is an important part of pipeline protection and consequence minimization (leak detection, primarily). There is a myriad of patrol types, effectiveness, and frequencies, making the assessment more complex. For instance, air patrol includes the obvious variable of frequency, but also the less obvious considerations of speed, altitude, use of spotter to assist the pilot, use of unpiloted aircraft (for example, drones) and others. See PRMM for a background discussion.

The assessment may also wish to give credits for patrols during activities such as close interval surveys (see Chapter 6) or even daily commutes by employees. For instance, formal patrols might not be part of a distribution system owner’s normal operations. However, informal observations in the course of day-to-day activities are common and could be included in this evaluation, especially when such observations are made more formal. Much of an effective system patrol for a distribution system will have to occur at ground level. Company personnel regularly driving or walking the pipeline route can be effective in detecting and halting potentially damaging third-party activities. Routine drive-bys, however, would need to be carefully evaluated for their effectiveness before credit as a mitigation measure is awarded. Training or other emphasis on the drive-by inspections could be done to heighten sensitivity among employees and contractors.

It is not unusual for operators to conduct formal patrols at frequencies much greater than regulatory requirements. In some instances, daily patrols are perhaps justified and provide a measurably greater safety margin. Frequencies greater than once per day (once per 8-hour shift, for instance) could even be justified by a risk-based cost-benefit analysis.

Patrol Effectiveness

The effectiveness of any patrol frequency can be determined from an analysis of activities to be detected or at least a reasoning exercise simulating such an analysis. Historical data of findings on previous patrols will often follow a typical rare-event frequency distribution. Once the distribution of ‘findings per patrol’ is approximated, the curve will have some predictive capabilities, to the extent that the types of activities remain constant.

An effectiveness corresponding to the actual patrol frequency should consider the types of activities likely to occur and the ability to intervene. An analysis of the “opportunity to intervene” in various common excavation activities is a necessary aspect of the effectiveness.

The most thorough intervention opportunity analyses begins with a list of expected third-party activities compared to a continuum of opportunity to detect. Estimating detection probability requires an understanding of for how long prior to and after the activity occurs, evidence of its presence can still be seen. Since third-party activities can cause damages that do not immediately lead to failure, the ability to inspect when there is evidence of recent activity is important. Effectiveness changes depend on the type of third party activity. It seems reasonable, for instance, to assume that activity involving heavy equipment requires more staging, is of a longer duration, and leaves more lasting evidence. All of these promote the opportunity for detection by patrol. The frequency of the various types of activities will be very location- and time-specific.

Sample probabilities of non-detection for typical patrol frequencies from ref [50] are as follows:

Twice a day 13%

Daily 30%

Every other day 52%

Weekly 80%

Biweekly 90%

Monthly 95%

Semi-annually 99%

Annually 99.6%

Detection by ‘other than patrol personnel’ is 1/3 as likely as by formal, trained personnel patrol, per some estimates.

Intervention opportunity analyses can be the basis of optimizing patrol frequency in addition to assessing the probability of detection for any given frequency. For example, company management may decide that the appropriate patrol frequency should detect, with a 90% confidence level, at least 60% of all threatening events. This might be based on a cost/benefit analysis. Patrol frequencies required to achieve this goal can be estimated from the analysis.

Damage Prevention / Public Education Programs

A damage prevention program encompasses many of the mitigation activities listed here, but it is often more associated with public education programs—ensuring that all potential excavators understand pipelines and how to avoid damage to them. See PRMM for a full discussion of such programs.

Transmission pipeline company SME’s have typically assigned maximum effectiveness values in the range of 5% to 30%, based on their experiences with public education along specific pipeline segments. For perspective, the higher end of this range assumes that 3 out of 10 otherwise damaging events are avoided solely through the damage prevention program (assuming no depth cover, no signs, etc) while the lower end assumes only 5 out of 100 events are avoided. Actual effectiveness values are then assigned based on differences from the idealized, perfect program.

Other Mitigation Measures

Other emerging technologies that will likely play increasing role in third party damage mitigation include satellite observation, ground vibration monitoring, acoustical sensors, buried sensor cables, motion detectors, infrared activated cameras, and others. Some of these, such as sensors, can be included in a risk assessment as a form of patrol, perhaps a continuous monitoring. Others may warrant an independent place as a mitigation measure in the risk assessment model. This is no problem for the risk assessment model proposed here since any type of additional mitigation opportunity is readily combined with all previous measures. Mitigation estimates are additive (OR gate), reflecting the real-world cumulative benefits, as well as diminishing returns, associated with multiple layers of protection.

Resistance

Adding estimates of resistance to exposure and mitigation moves the assessment from PoD to PoF. Recall that PoD measures the potential for any type of damage that threatens near term or long term load-carrying capacity of the component. PoF is directly related to the fraction of damaging events that result in failure during the period for which the assessment applies.

Factors that make a component less susceptible to failure if damaged—more resistive—include material type, wall thickness, component geometry, toughness, and stress level. Possible weaknesses from past damages, including corrosion, as well as manufacturing and construction issues can also play a role.

The pipe wall thickness and material strength/toughness are among the most important considerations in assessing puncture resistance. The geometry, diameter and wall thickness, influence resistance to buckling and bending. Since internal pressure induces longitudinal stress in the pipe, a higher internal pressure can indicate reduced resistance to certain external forces. Other longitudinal stresses such as those caused by lack of uniform support can similarly impact load-carrying capability.

Potential damage to the component depends on characteristics of the striking object and the impact scenario. Force, contact area, angle of attack, velocity, momentum, and rate of loading are among these characteristics. Potential effects include damages to coating (thereby reducing mitigation effectiveness but not resistance), appurtenances, and component walls, possibly leading to rupture immediately or after some other contributing failure mechanism.

To better estimate resistances to possible loadings that could be placed on the pipeline, exposures such as excavation, vehicle impact, fishing, and anchoring can be grouped based on the types of equipment, vehicles, engine power, type of anchors or fishing equipment, and others–ie, group by similar loadings imparted onto the pipeline. Fishing equipment and anchors that dig deep into the sea bottom or which can concentrate stress loadings (high force and sharp protrusions) present greater threats—they can be more challenging to resist. Analyzing the nature of the exposures will allow resistance distinctions to be made involving types of excavators, vehicle impacts, anchored vessels, fishing techniques, and others. Such distinctions, however, may not be warranted for simpler risk assessments that use conservative assumptions.

See for Chapter 10 Resistance Modeling for modeling options for including resistance in the risk estimates.

1. But not exclusively. It may be more efficient to include, for instance falling trees along with falling utility poles in the same part of the risk assessment, as well as dropped tools and toppled equipment caused by first and second parties. ↑

2. However, ref [67] reports that, due mainly to the greater chance of impact and increased exposure to the elements, equipment located above ground has a risk of failure approximately 100 times greater than for facilities underground [67]. This will, of course, be situation specific. ↑<\small>