Geohazards

1. Sample of data typically used to assess the geohazard failure potential.

All of our creations are subject to the laws of nature—Mother Nature hates things
she did not create.

Events that subject a pipeline to injurious loads/stresses due to land movements and/or geotechnical events of various kinds, are termed ‘geohazards’ in this text. Geohazards may cause sudden and catastrophic movements of large masses of earth or they may be slow-acting forces that induce stresses on the pipeline over a long period of time. They can cause immediate failures or add considerable stresses to the pipeline, limiting its ability to resist other failure mechanisms.

Potentially damaging geohazard events are caused by onshore and offshore phenomena of seismic fault movements and soil liquefaction, aseismic faulting, soil shrink-swell, expansive soil movement, subsidence, erosion, landslide, scour, washout, frost heave, ice berg scour, ice/snow loadings, hail, water/debris impingements, sand dune movements, meteorites, lightning, and others. These terms sometimes describe overlapping phenomena or are different terms for the same phenomenon (for example, erosion and washout) but a full listing ensures that none are overlooked. Many weather-related phenomena can trigger a damaging geohazard event. Freezes and flooding are examples. Events such as falling trees (due to windstorm, ice, etc) can be included either as geohazards or as impacts, covered in third party damage potential (as a modeling convenience as discussed in ).

Water/land movements examined in a risk assessment should include all potential for pipeline damage or failure, onshore or offshore, due to triggering events such as tsunami, hurricane, flood, windstorm, rainfall, moisture and temperature changes and others. Again, terminology that includes overlapping events helps ensure complete coverage of initiating mechanisms.

The geohazard threat is usually very location specific. Many miles of pipeline are located in regions where the potential for damaging land/water movements is nonexistent. On the other hand, land movements are the primary cause of failures, outweighing all other failure modes, for sections of other pipelines.

Geohazards logically fall into a failure cause category often called ‘external forces’. However, that categorization would have to capture exposures ranging from vehicle impact to excavator contact to landslide and many others, resulting is a non-transparent risk model. Geohazards normally warrants consideration as an independent threat. However, several overlapping elements can be involved and can make categorization of the cause of failure as third-party damage vs. geohazards difficult. For instance, a failure scenario involving man-made structures moving along the seabottom during a storm, has elements of both third party and geohazard. Scenarios of structures overturning during wind and ice storms similarly have both aspects. The assessor should choose a modeling structure that is preferable to his users.

Geohazards may be further categorized to make modeling more efficient. Sub classes may include hydraulic or hydrotech for exposures related to water, especially moving waters, and geotech for phenomena not involving water to any significant extent. (Also see PRMM.)

Failure Probability: Exposure, Mitigation, Resistance

Recall that all exposures are evaluated in the absence of mitigation. 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 frequency. It is only after adding mitigation—notably depth of cover—that the threat appears as small as most intuitively believe it is.

As with all threats, 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.

Note also that risk reduction ‘credit’ for things like extra strong pipe to withstand instability events is recognized in the resistance assessment and should not be a consideration in exposure estimation.

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 sometimes more correct to pair specific exposures with pertinent mitigations. This is essential where differing exposure-specific mitigations are employed and/or where mitigations have varying effectiveness depending on the type of exposure. For example, depth of cover plays varying roles in geohazard phenomena of landslide, flood, buoyancy control, and seismic liquefaction and cannot be assigned the same level of mitigation benefit to each.

Spans and Loss of Support

Geohazard phenomena may generate stresses directly or, alternatively, they may change the support conditions, thereby indirectly changing stresses. Therefore, depending on the effect, their role as exposures or resistance reducers or both should be considered in the PoF assessment.

For instance, many of the geohazard events may not directly threaten integrity but rather will indirectly endanger a pipeline via creation of a span. Examples include subsidence, erosion, scour, and even some landslide scenarios. Span modeling is discussed as a nuance of the PoF triad in .

Component Types

As with many threats, differences in component material properties will complicate, for some systems, the modeling of susceptibility to damage from some land movements. For instance, the many types of materials often found in a distribution system pipeline means that assessment must often accommodate flexible and inflexible pipe, a variety of mechanical couplers, plastics and metals, and other considerations. Each component, in consideration of its performance under various loading scenarios, may have varying damage potentials and resistance capabilities.

For instance, for the same reasons that they are less threatened by spans, larger diameter pipelines made from more flexible materials and joining processes that create a more continuous structure, such as welded steel and fused PE pipelines, have historically performed better in seismic events and when exposed to soil movements from frost action and subsurface temperature changes.

Facilities with taller structures such as tanks, will need to include potential for toppling during some geohazard events such as earthquakes, floods, and landslides.

Offshore components typically have additional considerations for scour, lateral forces when spanning, and buoyant. See discussions of cracking and vortex shedding.

Exposures

In measuring or estimating exposure to geohazards, it is important to first list all potentially damaging mechanisms that could occur at the subject location. Then, numerical exposure values should be assigned to each. Pre-dismissal of exposures 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.

Geohazards are normally first considered in the design phase where mitigation or resistance is increased to reduce failure potential, as needed. In the risk assessment, each exposure should have a future frequency of occurrence assigned, by imagining that no mitigation nor resistance is available to prevent failure—the imagined ‘unprotected tin can’ scenario.

Some common geohazard threats to pipeline integrity are discussed here and in PRMM. As previously noted, some nuances of continuous exposure and changes in resistance due to loss-of-support will often need to be considered in defining exposure events and resulting damage and failure potentials.

Landslide

Slope is often an aspect of a damaging land movement. Landslides, rockslides, mudslides, mudflows, creep, and other related events can occur from heavy rain, especially on slopes or hillsides with removed or heavily cut vegetation or where construction or other activities have altered the land. Debris flows—usually involving steep mountain channels and soil liquefaction (‘mountain tsunami’ in Japanese [1016])—are also included here.

A sometimes used categorization of landslides based on soil movements, geometry of the slide, and the types of material involved results in the following five categories: falls, topples, slides, spreads, and flows [777]. See PRMM Figure 5.5 and Table 5.7.

Landslide events can have frequencies ranging from ‘never’ to ‘multiple times per year’ for longer stretches of pipeline. They are logically related to the frequencies of the underlying causal events such as precipitation and seismic events.

Some available public databases provide rankings for landslide potential. As with soils data, these are very coarse—usually missing smaller, but potentially severe scenarios such as embankments and steep creek banks. These datasets are best supplemented with field surveys or local knowledge. Nonetheless, as a preliminary method of assigning initial threat values to long lengths of pipeline quickly, such ranks, converted into event frequencies, can be useful. The conversion from ranks into frequencies should incorporate the protocols underlying the assignment of ranks in the original data. For example, see the discussion of factors used to establish ranks in the US Natural Disaster Study later in this chapter.

Soils (shrink, swell, subsidence, settling)

Earth movements involving localized changes in soil volume can cause shrinkage, swelling, or subsidence. These can be caused by changing temperatures or moisture contents as well as subterranean water movements and other phenomena. These can cause loss of support as well as additional shear forces and bending stresses on a pipeline component.

Changes in soil moisture content and temperature effects, often occurring in seasonal patterns, have been correlated with both water and gas distribution system break rates. These are often related to soil movements which cause changes in stresses on buried components. Where such correlations between break rates and physical phenomena are established, they can be used in risk assessment and break forecasting as well as in comparative risk assessments between regions with differing climates.

Exposure rates to some of these phenomena can therefore be linked to frequencies of triggering events such as temperature changes and rainfall events. In other cases, the potential may be largely unknown. Refer to discussion of exposure estimates using ‘test of time’ evidence.

1. Frost heave/uplift exposure

Aseismic faulting

Aseismic faulting is a phenomenon where soil masses move along fault lines but without seismic actions. Depending on specific circumstances, this may be assessed as an exposure or alternatively as a resistance issue (ie, additional loadings causing weakness).

Seismic

Seismic events can pose a threat to pipelines in several ways. High stress/strain can be associated with seismic events in either aboveground or buried facilities. Many different phenomena are generated by seismic activities, including fault movements, soil liquefaction, ground shaking, generation of landslides and tsunamis, soil settlement, and others. See PRMM for more discussion.

Understanding seismic events helps to determine how they should be characterized in a risk assessment. For buried pipelines, seismic hazards can be classified as being either wave propagation hazards or permanent ground deformation hazards. Strong ground motions can damage aboveground structures. Fault movements sometimes cause severe stresses in buried pipe.

Permanent ground deformation (PGD) damage typically occurs in isolated areas of ground failure with high damage rates while wave propagation damage occurs over much larger areas, but with lower damage rates. Wave propagation hazards are characterized by the transient strain and curvature in the ground due to traveling wave effects. PGD (such as landslide, liquefaction induced lateral spread and seismic settlement) hazards are characterized by the amount, geometry, and spatial extent of the PGD zone. The fault-crossing PGD hazard is characterized by the permanent horizontal and vertical offset at the fault and the pipe-fault intersectional angle.

The principal forms of permanent ground deformation are surface faulting, landsliding, seismic settlement and lateral spreading due to soil liquefaction. One type of PGD is localized abrupt relative displacement such as at the surface expression of a fault, or at the margins of a landslide. The second type of PGD is spatially distributed permanent displacement which could result, for example, from liquefaction-induced lateral spreads, or ground settlement due to soil consolidation. For localized abrupt PGD, pipeline damage mainly occurs around the ground rupture trace. On the other hand, breaks for spatially distributed PGD may occur everywhere within the PGD zone.

The types of faults and the expected amount of fault offset can be empirically correlated with earthquake magnitude. Relationships for predicting the occurrence and types of landslides, and the amount of earth flow movement based on seismic event characteristics are also available. Wave propagation hazards are also empirically related to maximum moments. [777]

Liquefaction caused by seismic movements can fluidize soils to a point at which their ability to support the component is compromised. An unsupported condition can lead to additional and sometimes excessive stresses. A pipeline is also potentially subject to horizontal force due to liquefied soil flow over and around the pipeline as well as uplift or buoyancy forces. Pipeline responses to such loadings may need to be considered as failure potential or at least impairment of resistance (ie, reduction in stress carrying capacity).

1. Liquefaction of Soils

Modern pipeline design considers seismic potential and will often provide useful input for the risk assessment in terms of event recurrence intervals.

Tsunamis

As a special type of flood or external force event, a tsunamis is a high-velocity water wave, often triggered offshore by major abrupt displacement of the seafloor from initiators such as seismic events or landslides. A seiche is a similar event that occurs in a deep lake [70b]. in deep water, these events are of less concern but have the potential to cause rapid scour, erosion, and flowing water impingements when they occur in shallow areas. Aboveground components can be especially vulnerable to lateral forces and debris loadings. This threat can be quantified by considering the potential for offshore seismic events, the shore approach geometry and other location-specific factors. A history of such events may be used to inform the exposure estimate although the potential may exist along almost every large, deep water body. It can be included with other flooding events or assessed as an independent threat to the pipeline. Refer also to previous discussion of quantifying exposures and span-creating events.

Flooding

Flood waters can impart abnormal forces onto components, including buoyancy effects and debris loadings, loss of support (ie, scour, erosion), and fatigue from moving waters.

This potential threat has been a specific focus with regard to pipeline integrity. In the US, the pipeline regulator, PHMSA has released several Advisory Bulletins on this subject, each of which followed an event that involved severe flooding that affected pipelines in the areas of rising waters. Three of the more notable events (as of this writing) are briefly described below:

• On August 13, 2011, Enterprise Products Operating, LLC discovered a release of 28,350 gallons (675 barrels) of natural gasoline into the Missouri River in Iowa. The rupture, according to the metallurgical report, was the result of fatigue crack growth driven by vibrations in the pipe from vortex shedding.
• On July 1, 2011, ExxonMobil Pipeline Company experienced a pipeline failure near Laurel, Montana, resulting in the release of 63,000 gallons of crude oil into the Yellowstone River. The rupture was caused by debris washing downstream in the river damaging the exposed pipeline.
• On July 15, 2011, NuStar Pipeline Operating Partnership, L.P. reported a 100-barrel anhydrous ammonia spill in the Missouri River in Nebraska. The 6-inch-diameter pipeline was exposed by scouring during extreme flooding.

This advisory bulletin [1017] continues as follows:

As shown in these events, damage to a pipeline may occur as a result of additional stresses imposed on piping components by undermining of the support structure and by impact and/or waterborne forces. Washouts and erosion may result in loss of support for both buried and aerial pipelines. The flow of water against an exposed pipeline may also result in forces sufficient to cause a failure. These forces are increased by the accumulation of debris against the pipeline. Reduction of cover over pipelines in farmland may also result in the pipeline being struck by equipment used in farming or clean-up operations.

Additionally, the integrity or function of valves, regulators, relief sets, and other facilities normally above ground or above water is jeopardized when covered by water. This threat is posed not only by operational factors, but also by the possibility of damage by outside forces, floating debris, current, and craft operating on the water. Boaters involved in rescue operations, emergency support functions, sightseeing, and other activities are generally not aware of the seriousness of an incident that could result from their craft damaging a pipeline facility that is unseen beneath the surface of the water. Depending on the size of the craft and the pipeline facility struck, significant pipeline damage may result.

Though these accidents account for less than one percent of the total number of pipeline accidents, the consequences of a release in water can be much more severe because of the threats to drinking water supplies and potential environmental damage.

A further examination of the advisory [1017], issued by the regulator, provides insight into not only regulatory expectations, but also commonly employed risk mitigation measures at waterway crossings.

To: Owners and Operators of Gas and Hazardous Liquid Pipeline Systems.

Subject: Potential for Damage to Pipeline Facilities Caused by Severe Flooding.

Advisory: Severe flooding can adversely affect the safe operation of a pipeline. Operators need to direct their resources in a manner that will enable them to determine the potential effects of flooding on their pipeline systems. Operators are urged to take the following actions to prevent and mitigate damage to pipeline facilities and ensure public and environmental safety in areas affected by flooding:

1. Evaluate the accessibility of pipeline facilities that may be in jeopardy, such as valve settings, which are needed to isolate water crossings or other sections of a pipeline.
2. Extend regulator vents and relief stacks above the level of anticipated flooding, as appropriate.
3. Coordinate with emergency and spill responders on pipeline location and condition. Provide maps and other relevant information to such responders.
4. Coordinate with other pipeline operators in the flood area and establish emergency response centers to act as a liaison for pipeline problems and solutions.
5. Deploy personnel so that they will be in position to take emergency actions, such as shut down, isolation, or containment.
6. Determine if facilities that are normally above ground (e.g., valves, regulators, relief sets, etc.) have become submerged and are in danger of being struck by vessels or debris and, if possible, mark such facilities with an appropriate buoy and Coast Guard approval.
7. Perform frequent patrols, including appropriate overflights, to evaluate right-of-way conditions at water crossings during flooding and after waters subside. Determine if flooding has exposed or undermined pipelines as a result of new river channels cut by the flooding or by erosion or scouring.
8. Perform surveys to determine the depth of cover over pipelines and the condition of any exposed pipelines, such as those crossing scour holes. Where appropriate, surveys of underwater pipe should include the use of visual inspection by divers or instrumented detection. Information gathered by these surveys should be shared with affected landowners. Agricultural agencies may help to inform farmers of the potential hazard from reduced cover over pipelines.
9. Ensure that line markers are still in place or replaced in a timely manner. Notify contractors, highway departments, and others involved in post-flood restoration activities of the presence of pipelines and the risks posed by reduced cover.

If a pipeline has suffered damage, is shut-in, or is being operated at a reduced pressure as a precautionary measure due to flooding, the operator should advise the appropriate PHMSA regional office or state pipeline safety authority before returning the line to service, increasing its operating pressure, or otherwise changing its operating status.

Flood exposure estimates can arise from several sources, including published flood severity/frequency information, often linked to meteorological events. The subset of flood events that can cause failure to a pipeline component will be a function of the definition of the resistance baseline, as discussed in .

Scour and erosion

Erosion is a readily recognized threat for shallow or above-grade pipelines close to river banks or other areas subject to higher-velocity flows. Many pipelines are exposed to threats from scour in less apparent situations, such as bridge foundations. A potential integrity threat occurs when cover erodes during flood flows, exposing the pipeline to moving waters and transported debris. The pipeline could become overstressed from lateral forces, buoyancy, or lack of support. Scour potential estimates are available for many waterways, often expressed in terms of maximum scour depths related to storms of certain recurrence intervals.

These scour estimates can directly inform exposure frequency estimates in the risk assessment. For instance, knowing that, say, a 3 foot deep scour potential is associated with a 100 year flood event, provides input for frequencies of loadings such as water/debris impingement and vortex induced vibration (ie, these exposures are expected every 100 years for components having 3 ft of cover or less). Presumably, more frequent storms can also produce scour, but to lesser depths.

Relationships between frequencies of events that cause various scour depths will also inform mitigation effectiveness estimates for depth of cover and other protections in place or contemplated; for example, rock cover, concrete mattresses, etc.

1. Scour at Bridge Piers^[1]

Sand movements

The potential for wind erosion, and dune formation and movement, is another possible source of damage or at least span-creation. Seabottom sand ripples, dunes, and other instabilities are the equivalent phenomena in the offshore environment. Any of these may produce changes to loads, depth of cover, and support conditions and should be included in the risk assessment.

Exposure estimates may be based on design-phase studies, when available. In some cases, instability may be almost continuous, for instance in a high wave energy zone offshore, but only rarely severe enough to endanger the pipe component. See discussion of spans and support conditions under .

Weather

The threats associated with meteorological events should be included, either as damaging phenomena or as triggering events for subsequent damaging phenomena. Events such as a wind storm, tornado, hurricane, lightning, freezing, solar flares or storms, hail, wave action, snow, and ice loadings against unprotected components may be independent damage producers, along with any previously discussed phenomena they may precipitate. Even when the exposure is minimal and/or mitigation will normally eliminate the threat, inclusion into the risk assessment is important.

Electromagnetic pulses (EMP) from lightning or solar storms can damage electronic components. Such damage can lead to ‘failures’ such as service interruption and, in rare cases, perhaps even loss of integrity—leak/rupture. A sometimes complex chain of events needs to be identified and scrutinized to fully understand certain potential scenarios involving failures of electronic components.

Lightning strikes are a common cause of damages to electronic components as well as initiators of wildfire. US government maps are available showing lightning strike density, expressed in the mean annual number of flashes per square kilometer. Maps have been created with rankings from zero 100 for the country, where 100 represents the highest lightning strike density and zero represents the lowest lightning strike density. With assumptions of some fraction of lightning strikes being potentially threatening to a component, such rankings can inform estimates of exposure rates.

A frequency of occurrence for each possible weather event, in the absence of mitigation, is a logical starting point for exposure estimation. National weather agencies typically have databases that can be consulted. For example, points along the US Gulf of Mexico have a hurricane recurrence interval of about 25 years. This suggest a windstorm and flood exposure of 1/25 per year from hurricanes alone. This value can be refined based on hurricane magnitudes and considerations of surge heights, sustained wind speeds, and other location-specific characteristics that lead to varying damage potentials. Then protective measures, such as depth of cover, are assessed as universal or exposure-specific mitigations.

Fires

While often not a direct threat to integrity of a buried pipeline, fires can lead to increased erosion and landslide potential. Above ground components may be threatened by more intense or longer duration fires or when less heat resistance components (for example, gaskets, tubing, seals, plastics, instrumentation, etc) become exposed. Minor leaks may ignite and blocked-in, liquid-full components may be subject to BLEVE ruptures.

Wildfire prediction models based on factors such as topography, fuel, live shrub moisture content, weather, wind, lightning ignition efficiency are used in the US, with mapped results available from government sources. Exposure estimates can emerge from such sources and others, eg, meteorological data.

Other

Additional threatening phenomena are at least peripherally related to geohazards, as noted here.

Excessive external pressure is a potential threat to some offshore components’ integrity, perhaps best included in the assessment as a type of geohazard. Pipelines in deep water are subjected to external forces from the hydrostatic pressure of the water column. Especially when there is reliance on internal pressure to protect the pipe from buckling, this is a source of exposure and/or an element of the resistance estimate.

Onshore scenarios of external pressure are also plausible. In one operator’s experience, hydrogen permeation through steel repair sleeves caused numerous buckles to the pipe beneath. The source of hydrogen was high CP levels and the annular space pressure of around 300 psig was reportedly sufficient to cause the buckling. [1001]

Stability issues are inherent in many geohazards. See discussion of spans and support conditions under .

US Natural Disaster Study

In the US, maps are available showing relative threats to pipelines from some common geohazards. While expressed with a only relative scale, the derivation of the rankings provides a way to generate frequency estimates for many of these phenomena, at least on a coarse—large geographical areas, possibly missing smaller but important features—level. It is useful to examine the methodology of establishing these hazard indices.

Excerpts from this ref [1018] are shown below to assist the risk assessor in determining the usefulness of such relative-scale information into a contemplated assessment. Note that, in the absence of more definitive information, a relative scale itself can be ‘grounded’ with frequency values and thereby used in preliminary exposure estimates (for example, score of 70 = 0.1 events/year, etc).

As of this writing, US databases area available [1018] showing hazard indexes for:

• earthquake (HER = Earthquake Hazard Rank)
• hurricane (HHR = Hurricane Hazard Rank)
• tornado/storm (TSRR = Tornado/Storm Hazard Rank)
• flood/scour (FHR = Flood Hazard Ranking)
• landslide (LHR = Landslide Hazard Ranking)
• other (lightning and snow depth; OHR = Other Hazard Risk)

This index system also includes a summary layer, produced using the composite rank formula:

NPHI = .3(FHR) + .2(EHR) +.2(LSHR) + .l(TSHR) +.l(HHR) +.l(OTHER)

Where:

FHR = flood hazard rank

EHR = earthquake hazard rank

LSHR = landslide hazard rank

TSHR = tornado/storm hazard rank

HHR = hurricane hazard rank

OHR = other natural hazards hazard rank

National Pipeline Risk

Hazard	Variables Included	Methodology	Notes
Hurricane	Historical count	94 year history of hurricanes per coastal county	2
TSRR	Historical count	Number of occurances over 30 years per one degree box	3
Landslide	swelling clays, landslide incidence, susceptibility, subsidence	LSHR = 0.3 (clay) + 0.4 (incidence) + 0.2 (susceptibility) + 0.1 (subsidence)	6
Earthquake	Spectral response acceleration coefficient	Based on single, complex variable ranked 0-100	4
Other	30 year Mean annual lightning strike density; Snow depth with 95% chance of not being exceeded	OHR = .5 (lightning strike) + 5 (snow depth)	5
Flood	Annual flooding frequency, potential scour depth	FHR = 0.5(flooding) + 0.5(scour depth)	1

Table Notes

1. For the Annual flooding frequency layer one-kilometer grid cells were assigned the following values based on the annual chance of flooding:
   Frequent (5O-100%): Flooding = 100
   Rare (O-5%): Flooding = 33
   Occasional (5-50%): Flooding = 67
   No Flooding: Flooding = 0
   These values were then multiplied by the percentage of area they covered for each soil map unit. The percentage values were summed to give the value for each soil map unit. A grid of these values was created and then ranked from 0 to 100. For the Potential scour depth layer one-kilometer grid cells were ranked based on their value (potential scour depth in feet).
   Highest value Scour depth = 100 Lowest Value Scour depth = 0
2. The total number of direct and indirect landfalling hurricanes per coastal county was used from 1990 (assume typo—should probably be “1990”) until 1994. From the county baaed polygon coverage, a point coverage was derived. From this point coverage first a Triangulated Irregular Network (TIN) and then a continuous surface grid was created, in order to more appropriately represent the hazard without the use of political boundaries. These numbers were ranked from zero to 100, where 100 represents the highest number of land-falling hurricanes and zero represents the lowest number of land-falling hurricanes.
3. The centroids of the one-degree cell areas were used to generate a Triangulated Irregular Network (TIN). This resulted in a continuous surface that more naturally depicts the distribution of tornado events. A grid was created at a resolution of one kilometer from the TIN. The values were ranked from zero to 100, where 100 represents the highest number of tornadoes and zero represents the lowest number of tornadoes.
4. The spectral response acceleration coefficient is an indicator of the probability of receiving specific intensities of ground shaking from earthquakes. For the EHR the spectral response acceleration coefficient at a period of 0.3 seconds expressed as a fraction of gravity with a 90% chance of not being exceeded in 50 years is used. The data are prepared by the U.S. Geological Survey (USGS) for the NEHRP Recommended Provisions for the Development of Seismic Regulations for new Buildings.
5. Lightning strike density is expressed in the mean annual number of flashes per square kilometer. Contour lines were digitized from a very small scale map. The areas in between the contour lines were given the mid-value of the class. These values are ranked from zero 100 for the country, where 100 represents the highest lightning strike density and zero represents the lowest lightning strike density. The 95% annual nonexceedence probability was calculated for 239 weather station in the United States. From this point coverage first a Triangulated Irregular Network (TIN) and then a continuous surface grid was created in order to more appropriately represent reality. These numbers were ranked from zero to 100, where 100 represents the highest snow depth and zero represents the lowest snow depth.
6. The LSHR values were ranked from zero to 100, where 100 represents the highest ground failure hazard and zero represents the lowest ground failure hazard.

While relative indices like these are not ready for direct inclusion into a modern risk assessment, the underlying methodology provides insights into the phenomenon and current abilities to forecast them.

Offshore

Offshore pipelines, including those crossing inland waterways such as rivers and lakes, are exposed to many of the same forces as those onshore—landslides, rockfalls, seismic events, etc—plus others unique to the offshore environment. The interaction between the pipeline and the seabed or riverbed will frequently set the stage for external loadings offshore. The following discussion focuses on ocean environments, but will often apply, albeit to a lesser extent, to inland creeks, rivers, large lakes, and sometimes even ponds. See also the discussion of stream scour and flooding.

One of the largest differences between the risk assessments for offshore and onshore environments appears in this issue of stability. This reflects the very dynamic nature of most offshore environments under normal conditions and more so with storm events.

1. Ice gouging, ice keel exposure

Stability Issues

Offshore bottom conditions are constantly changing by normal forces of moving water. This changes the stability conditions for structures resting directly on the seabottom or with shallow cover. Additional instability events associated with storm-related forces, changes in bottom topography, temporary currents, tidal effects, and ice movements are also often relevant to a risk assessment.

Offshore “high-energy” areas, evidenced by conditions such as strong currents, or tides, are common areas of instability. Seabed and riverbed morphology is constantly changing due to naturally occurring conditions (waves, currents, soil types, etc.). Vortex shedding, lateral loadings, scour, and other forces caused by frequent changes in bottom conditions are commonly associated with wave zones and high steady current environments.

At times, the pipeline itself, as an obstruction that has been introduced into the system, contributes to bottom changes. Sand wave migration—size, direction, and rates—can be predicted with an understanding of bottom conditions. Rare occurrence events, often carrying higher energy, may create greater damage potential. This includes hurricanes, severe storms, and rare ice movements.

Bottom instability generates integrity concerns primarily from issues related to support and/or fatigue-loading. A common conservative assumption in risk assessment is that increased instability of bottom conditions leads to increased potential for pipeline over-stressing and failure.

The pipeline can become an unsupported span despite initial installation and efforts to maintain cover. Once uncovered or spanning, it is subjected to additional stresses due to gravity, buoyancy, and wave/current action. Consider scenarios such as a buried line becoming uncovered by scour or erosion of the seabed/streambed perhaps with uplift forces (for example, an emptied liquid pipeline), and subsequently becoming exposed to flowstream forces and impact loadings from floating debris and material being moved along the seabed or riverbed. Such external forces can damage coatings, both concrete and anticorrosion types, and even damage the pipe steel with dents, gouges, buckling, or punctures.

Pipelines exposed to a flowstream may move due to intermittent lateral forces, buoyancy issues, or vortex shedding. Movements of a free-spanning pipeline, resulting in cycling and fatigue loadings may eventually weaken a component to the point of failure. Fatigue and overstressing threats are amplified by larger span lengths, higher water velocities, and larger profiles (diameter).

Rigid pipelines, because of their diminished capacity to withstand certain external stresses, will be threatened under less severe conditions. Mechanical coupling of pipe joints usually adds rigidity and, hence, reduces resistance.

A full evaluation of any potentially damaging offshore phenomena requires an evaluation of many subvariables such as soil type, seismic event types, storm conditions, cover condition, water depth, current speeds and directions, etc, as discussed in PRMM.

As also noted in PRMM, some of the common instability issues and their dominant factors—ie, a ‘function of’—include the following:

• Fault movement damage potential = f{fault type; slip angle; pipeline angle; seismic event}
• Liquefaction damage potential = f{seismic event; soil type; cover condition}
• Slope stability = f{slope angle; soil type; rock falls; initiating event; angle of attack; landslide potential}
• Erosion/scour potential = f{current speed; bottom stability; concrete coating}
• Additional Loadings = f{hydrodynamic forces; debris transport; current speed; water depth}

In new offshore pipeline systems, more threatening areas along the proposed route are normally identified in the design phase studies. The design process is in fact a risk management practice. The risk assessment of a new facility will therefore generally reflect the mitigated threat. The potentially damaging events—the ‘exposures’ in the PoF analyses—should nonetheless be captured in the assessment, regardless of mitigation measures subsequently employed to offset their presence. Even after design-phase mitigation, some risk remains.

A level of reliability is typically chosen in the design phase and can be used to infer the future damage rate—the remnant risk. For instance, a structure could be designed to withstand a 100 year storm or alternatively, a 500 storm flood; a 50 year recurrence interval seismic event or 100 year. There remains the potential, albeit remote, that a more severe event occurs in the structure’s life and produces forces beyond its resistance abilities. That should be reflected in the risk assessment.

For existing systems, seabed and riverbed profile surveys are a useful method to gauge the stability of an area. The effectiveness of the survey technique should be considered as discussed in PRMM.

In summary, offshore pipelines are more threatened in areas where damaging soil movements and/or water movements are more common or more severe. More specifically, this involves scenarios where a high-energy water zone—wave-induced currents, steady currents, scouring—is routinely causing seabed morphology changes; where unsupported pipeline spans are present; where water current action is sufficient to cause oscillations on free-spanning pipelines—fatigue loading potential is high—or impacts from floating or rolling materials; where fault movements, landslides, subsidence, creep, or other earth movements are more probable; and where ice movements are common and potentially damaging.

Risk reduction efforts typically focus on avoidance, correction, or protection techniques. These include reburial as well as various armoring approaches—ie, reinforcing a location using concrete mattresses, grout bags, mechanical supports/anchors, antiscour mats, or rock dumping. Such methods also provide protection against impacts (for example, anchors, shipwrecks, dropped objects, etc) and therefore influence risk from third party activities.

River and Stream Scour

Pipeline crossings of inland waterways are threatened by many of the same phenomena previously discussed. The potential threat from scour has been studied with specific regard to pipeline integrity. In the US, a Dec 2012 PHMSA report [1032] to congress on hazardous liquid pipeline crossings of inland rivers, streams, and other waterways offers some insights into frequencies of cover-depletion events at waterways. This report determined that there are ~2,572 hazardous liquid pipeline crossings of water ways >100ft in width (high water mark to high water mark) out of ~2,841 crossings of inland bodies of water in the US. The authors identified 20 accidents at water crossings between 1991 and Oct 2012, 16 of which involved depletion of cover, either from scour or new river channel creations. These 16 incidents were 0.3% of all reported hazardous liquid pipeline accidents and 0.5% of those accidents exceeding the PHMSA threshold of ‘significant incidents’.

Induced Vibration

Vortex shedding, whether by wind or water, can generate sufficient forces under certain circumstances, to move a pipeline segment. This movement can become rapid and relatively large, causing fatigue loadings in the pipe material. Fluid density, speed, cross sectional area in flow stream, frictional drag across the object and other factors influence the onset and magnitude of movements. The exposure generated by this phenomenon is most often captured as cracking. See .

1. Wave-induced pipe movements

To illustrate both a portion of an offshore geohazard assessment as well as the migration into a modern risk assessment approach, an offshore risk assessment example originally presented in PRMM is re-visited here.

An offshore pipeline makes landfall in a sandy bay. The line was originally installed by trenching. While wave action is normally slight, tidal action has gradually uncovered portions of the line and left other portions with minimal cover. With no weight covering, calculations show that flotation due to negative buoyancy is possible if more than about 20ft of pipe is uncovered. This shore approach is visually inspected at low-tide conditions at least weekly. Measurements are taken and observations are formally recorded. The line was reburied using water jetting 8 years ago.

Using rudimentary wave-induced-vibration and fatigue calculations, along with average storm frequencies, the evaluator estimates unmitigated crack growth on this shore approach to be 4 mpy. With the strong inspection program and a history of corrective actions being taken, the effectiveness of cover—eliminates pipe movement when cover is not depleted—is judged to be fully effective except for short periods during storms and between remediation and is assigned a value of 95% effectiveness. This results in a P90 damage rate of 0.2 mpy to be used in subsequent TTF estimates.

Quantifying geohazard exposures

Where geohazards have been rated based on recurrence interval—for example, 100 year flood, seismic event with 10% probability of exceedance in 50 years, etc—those ratings can directly inform an exposure estimate. The land movement potentials from various phenomena can be added so that multiple threats in one location are captured.

Event frequencies of 0.1 to 10 per year or higher may be appropriate for areas where damaging geohazard events are common. Regular fault movements, landslides, subsidence, creep, active earthquake faults, or frost heave are commonly recurring phenomena in some areas.

Event frequencies of 0.001 to 0.1 per year may be appropriate when damaging geohazard events are possible, but when no damage in the subject area has been recorded. A P90+ exposure estimate based on length-time (mile-years, km-years) as described in section may be appropriate to capture the notion of pipelines that have withstood the test of time.

When evidence of geohazard events is rarely if ever seen and movement potential is conceptually approaching nonexistent, then rates of less than 0.001 per year may be appropriate.

In keeping with an “uncertainty = increased risk” bias (see discussion of PXX, ), having no knowledge of earth movement potential should register as high risk, pending the acquisition of information that suggests otherwise.

Mitigation

The potential damages resulting from any events involve considerations for mitigation and resistance. Pipeline components are typically designed with protection from a wide variety of geohazards. Depth of cover will be a typical, and usually very effective, mitigation measure for geohazards along most portions of a pipeline.

However, using characteristics such as depth of cover to screen for vulnerabilities will usually result in dismissing threats from certain phenomena such as fire, as well as certain weather events previously noted. Such screening weakens the risk assessment and should be avoided. Each threat is best measured as an exposure to a theoretical, unprotected component.

Once unmitigated exposures are identified and quantified, mitigations are similarly identified and assessed. In areas where multiple damaging events are possible, the assessment should reflect the combined threats, considering the mitigation benefit from each measure as applied to each exposure. Mitigations, as reactions to a perceived threat, typically may include any of the following: (some taken from ref [1033])

• Inspection / survey
• Stabilization (cover condition, anchors, piles, articulated mattresses, various support types, mix of mitigation, changing exposure, etc.)
• Ground Improvements
• Drainage to control water access by interception ditches, French drain, ditch plugs, etc
• Erosion control vegetation
• Soil densification (for example, by surface loadings, dewatering, or vibrations)
• Slope re-grading, to reduce soil movement potential
• Toe berms, to increase resistance to soil movement
• Retaining walls, to halt movements
• Surface diversion berms, to prevent erosion
• Channel reinforcement by armouring with rock, sandbags, vegetation, etc.
• Channel movement control
• Re-establish depth of cover
• Pipe isolation
• Deep burial to avoid shallow slope movements and frost heave, for example a directional drill
• Synthetic geotextile pipe wrap, manufactured backfill, or straw backfill to reduce friction loadings from ground movements
• Avoidance
• Pipeline re-route
• Above ground pipe components
• Ditch modifications
• Wider ditch to reduce friction and allow movements
• Bedding and padding to prevent contact with rocks/boulders
• Excavation to relieve strain loadings.

Regular monitoring

When the pipeline and/or the potentially threatening phenomenon is visible or otherwise detectable in advance, monitoring can provide intervention opportunities. Regular, appropriately scheduled surveys that yield verifiable information on pipeline location, depth of cover, land movement, rainfall, moisture content, strain levels, water depth/current velocities for offshore pipelines, and other early-warning characteristics should be included in the risk assessment.

Earthquake monitoring systems alert of seismic activity and magnitude often only moments prior to the time of occurrence. This is nonetheless very useful information because areas that are likely to be damaged can be immediately investigated.

Where movements of icebergs, ice keels, and ice islands are a threat, well-defined programs of monitoring and recording ice movement events can be effective in reducing pipeline risk.

Timeliness of detection will be important. Frequency of surveying should be based on historical issues such as flooding, seabed and bank stability, wave and current action, ice storms, and risk factors specific to the pipeline section. The assessment can consider the basis for survey frequency—ideally, a written report with backup documentation justifying the frequency—to determine if adequate attention has been given to the issue of timeliness.

Continuous monitoring

Devices or techniques used in monitoring programs that will alert an operator of a significant change in stability conditions or other threats provide some risk reduction. Indicator devices might include strain gauges on the pipe wall itself, or survey markers to detect soil movements near to any component, and seabed or current monitors near to offshore components. Follow-up inspection and action is an essential aspect of the mitigation benefit. Mitigation that provides intervention opportunities is most beneficial when the monitoring is extensive enough to reliably detect all damaging or potentially damaging conditions before failure occurs.

See PRMM for an example evaluation of potential for earth movements.

1. Potential for earth movements

As another illustration of an update to a scoring-type risk assessment, consider the following modified example originally appearing in PRMM.

In the section being evaluated, a brine pipeline traverses a relatively unstable slope. There is substantial evidence of slow downslope movements along this route although sudden, severe movements have not been observed. The line is thoroughly surveyed annually, with special attention paid to potential movements. Survey results have reportedly prompted remedial actions several times in the previous 10 years, although record-keeping is incomplete. The evaluator makes a preliminary assessment of the exposure to be 0.5—an event once every other year—evidenced by the need for multiple remedial actions in a 10 year period. The surveying and subsequent remediation appears to be protective of the segment but are not formally documented. Mitigation effectiveness for the combined survey-remediation protocol is estimated to be 50% in its current state. This equates to an estimate of damage once every 4 years, from this apparently effective mitigation but with unknown error rates and continuance assurance. The evaluator advises the operator that this estimate can be increased if steps such as the following are taken:

• Formalize the survey procedures
• Establish the survey frequency on the basis of failure/damage probability
• Formalize the remediation procedures, especially regarding action thresholds and timing.

Resistance

One common reaction to geohazard threats is increased component strength, specifically the ability to resist external loads considering both stress and strain issues. Other measures to add resistance to geohazards will often be phenomena-specific.

Understanding of failure modes is essential to the modeling of resistance. The following discussion, taken from ref [1034], on seismic induced failure modes illustrates this as well as gives insight into many other geohazard phenomena.

Failure modes for buried pipelines subject to seismic loading.

The principal failure modes for corrosion-free continuous pipelines (e.g. steel pipe with welded joints) are rupture due to axial tension, local buckling due to axial compression and flexural failure. If the burial depth is shallow, continuous pipelines in compression can also exhibit beam-buckling behavior. Failure modes for corrosion-free segmented pipelines with bell and spigot type joints are axial pull-out at joints, crushing at the joints and round flexural cracks in pipe segments away from the joints. The principal failure modes for corrosion-free continuous pipeline with burial depth of about three feet or more are tensile rupture and local buckling. Buried pipelines with burial depths less than about 3 feet (i.e., shallow trench installation) may experience beam buckling behavior. Beam buckling has also occurred during post earthquake excavation undertaken to relieve compressive pipe strain.

Intuitively, beam buckling is more likely to occur in pipelines buried in shallow trenches and/or backfilled with loose materials. That is, beam buckling load is an increasing function of the cover depth. Hence, if a pipe is buried at a sufficient depth, it will develop local buckling before beam buckling.

When strained in tension, corrosion free steel pipe with arc welded butt joints is very ductile and capable of mobilizing large strains associated with significant tensile yielding before rupture. On the other hand, older steel pipe with gas-welded joints often cannot accommodate large tensile strain before rupture. In addition, welded slip joints in steel pipe do not perform as well as butt welded joints

Buckling refers to a state of structural instability in which an element loaded in compression experiences a sudden change from a stable to an unstable condition. Local buckling (wrinkling) involves local instability of the pipe wall. After the initiation of local shell wrinkling, all further geometric distortion caused by ground deformation or wave propagation tends to concentrate at the wrinkle. The resulting large curvatures in the pipe wall often then lead to circumferential cracking of the pipe wall and leakage. This is a common failure mode for steel pipe.

For segmented pipelines, particularly those with large diameters and relatively thick walls, observed seismic failure is most often due to distress at the pipe joints. In areas of compressive ground strain, crushing of bell and spigot joints is a fairly common failure mechanism in, for example, concrete pipes.

For small diameter segmented pipes, circumferential flexural failure have been observed in areas of ground curvature.

Axial pull out of segmented pipe such as cast iron or concrete with rubber gasketed joints and bell-spigot is also a common failure mode for seismic events [1034].

A structure’s design documentation will often state the geohazard events that the structure is rated to withstand—for example, “maximum scour from a 100 year flood”; “seabottom instability from 100 year storm”; “landslide from 50 year rainfall event”. These values are useful in the risk assessment since they suggest a point in the load probability distribution, below which the structure’s survival rate should be high. In the absence of unanticipated weaknesses, the structure should be highly resistive to events of lesser magnitude (normally more frequent events are of lesser magnitude) than the stated design intent. Resistance to more severe events (generally more infrequent) will be questionable.

Knowledge of safety factors will be useful in estimating resistance. Technically rigorous structural analyses can be performed where the most robust resistance estimates are required. These will require more combinations of specific loadings onto specific components and comparing resulting calculated stresses against stress carrying capacities.

Full discussion of resistance is found in .

One touch of nature makes the
whole world kin.

William Shakespeare

1. Structures in the flow stream can cause or exacerbate scour. ↑