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Proceedings of the Offshore Structural Reliability Conference 2014OSRC

September 16-18, 2014, Houston, Texas, USA

Paper 6.2

EARTHQUAKE RELIABILITY OF ONSHORE STRUCTURES AND COMPARISON TO OFFSHORE STRUCTURES

Andrew Whittaker University at Buffalo

Buffalo, NY, USA

Frank Puskar Energo Engineering Houston, TX, USA

ABSTRACT Earthquakes can pose a significant risk to onshore

structures including buildings, bridges, industrial plants and nuclear energy plants. The onshore engineering community has developed numerous practices and standards over the years to provide guidance to structural engineers involved in the analysis, design and upgrade of these facilities. Examples include the family of FEMA-sponsored NEHRP (National Earthquake Hazards Reduction Program) documents, ASCE Standard 7 for buildings, ASCE Standards 4 and 43 for safety-related nuclear structures, NFPA 57 for liquefied natural gas fuel systems, and Eurocode 8 for structures. These documents provide analysis and design procedures, and prescriptive details. ASCE Standards 7 and 43 are risk informed with target maximum probabilities of unacceptable performance for earthquake shaking associated with specific return periods. These target reliabilities have a historical development based on a combination of theoretical approaches, practical application and real world experience in small and large earthquakes. The offshore structural engineering community has undertaken a similar and somewhat parallel development of earthquake design standards, contained mostly in API RP 2A and more recently in ISO 19901-2 and API RP 2EQ. However, in recent years both onshore and offshore earthquake design standards, and more importantly target reliabilities and associated methods, have been under increasing scrutiny and revision as the practices are implemented worldwide with differing outcomes–sometimes driving the structure design to significantly higher costs (e.g., nuclear facilities) or concerns about high risks taken (e.g., the use of lower target reliabilities for medium consequence offshore structures). This paper discusses and compares the reliability targets and methods used for mission-critical onshore and offshore structures, including

similarities and differences. It also discusses the future challenges faced by each of these engineering communities.

EARTHQUAKE RISKS

Earthquakes present high life-safety risks and are one of the most dangerous natural hazards in terms of life loss. As the saying goes “Earthquakes do not kill people – buildings do.” Few lives are lost due to the earthquake ground motions, however, buildings can collapse in part or whole during earthquakes resulting in injury and life loss as well as damage to or loss of building contents. It is often impractical (or too expensive) to repair earthquake-damaged buildings. Importantly, if damage is significant, a building might have to be repaired to a standard consistent with a modern code, which may require the addition of supplemental seismic framing systems that could negatively impact the building’s function.

Codes and standards containing specific earthquake-related guidance have helped engineers better design structures, ranging from homes to skyscrapers, single-span to cable-stayed bridges and warehouses to nuclear power plants. It is clear that the use of these codes and standards has improved the seismic performance of buildings because the vast majority of damaged structures in earthquakes were constructed with little or no consideration of earthquake shaking. Regions and countries with outdated (or no) building codes or with a large inventory of archaic structures, especially unreinforced masonry buildings, can suffer significantly in earthquakes as demonstrated by the 2010 M7.0 earthquake in Port-au Prince, Haiti that resulted in over 100,000 deaths.

Earthquake codes and standards in the United States have steadily improved since the 1930s. Significant changes have often followed damaging earthquakes such as the 1971 San Fernando earthquake, which prompted the widespread use of

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ductile detailing in reinforced concrete buildings, and the 1989 Loma Prieta and 1994 Northridge earthquakes, which provided the impetus to develop performance-based earthquake engineering for buildings. EARTHQUAKE CODE DEVELOPMENT

The 1906 M7.8 San Francisco earthquake produced significant damage in the city, with almost 80 percent of the city destroyed as a result of the earthquake shaking as well as the major fires that followed the earthquake (Figure 1). Although building codes existed at that time, none required a consideration of earthquakes. Professional scientific and engineering organizations such as the Seismological Society of America and the Structural Engineers Association of California (SEAOC), both formed shortly after the 1906 earthquake, were early advocates for the development of seismic building code provisions. However, even given those efforts and the destruction of San Francisco, it was almost 20 years before there were changes to building codes in California to address earthquakes.

Figure 1. Widespread destruction from the 1906 San Francisco earthquake initiated some of the earliest

earthquake engineering studies [MCEER 2008]

In 1925 the city of Santa Barbara, California, suffered a M6.8 earthquake that resulted in thirteen deaths, destroyed most of the historic center of the city and the nearby Sheffield earthen dam due to liquefaction. Shortly after, Santa Barbara County adopted a building code that addressed earthquakes by requiring structures to be designed for horizontal forces.

In 1933 a M6.4 earthquake in Long Beach, California resulted in considerable damage and over 100 deaths, resulting in the Riley Act that required all new structures in California to be designed to withstand a horizontal acceleration of 0.02 times the acceleration due to gravity and for local governments to have a building department. The Long Beach earthquake damaged over 75% of the public school buildings in the city, resulting in the first focus on existing buildings, especially those that have high life-safety exposure such as schools

(Figure 2). The Safety of Design and Construction of Public School Buildings Act of 1933 (also known as the Field Act) established in 1933 regulated the construction of school buildings and inspection of existing school buildings including seismic issues. The Garrison Act of 1939 further addressed existing school buildings and mandated upgrades when needed for earthquake resistance. The Field and Garrison Acts are early examples of “risk-oriented” code development for which structures of high importance are designed for higher forces, less damage and thus a higher level of safety in earthquakes.

Figure 2. Before and after pictures of school destroyed in

the 1933 Long Beach earthquake that resulted in early “risk based” codes for high consequence structures [SSC 2009]

The 1971 M6.6 San Fernando, California resulted in severe damage to several hospitals including a Veterans Administration hospital; 44 lives were lost. An outcome of this earthquake was the 1973 Alquist Hospital Safety Act mandating that new hospital structures be designed to higher seismic safety standards than ordinary buildings because of the critical role hospitals play in recovering from damaging earthquakes. The 1994 M6.7 Northridge, California earthquake again resulted in significant damage to hospitals and other existing health care facilities resulting in CA Senate Bill 1953 requiring health care facilities built before 1973 (before the Alquist Act) to be upgraded to more adequately resist earthquakes. The Northridge earthquake also highlighted problems with connections in steel moment frame buildings resulting in changes to building codes and construction techniques.

The 1989 M6.9 Loma Prieta earthquake, with an epicenter approximately 70 miles south of San Francisco, California, resulted in 63 deaths and considerable damage to roadways, bridges and other transportation infrastructure (Figure 3). The earthquake highlighted the vulnerability of structures supported

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on soft soils with severe damage to buildings and roadways founded on filled soils, even though they were located nearly 70 miles from the epicenter. Structures founded on soft soils typically experienced larger seismic ground motions compared to structures located on stiff soils, where rock motions at depth were not intense. The Loma Prieta earthquake highlighted deficiencies in bridge and roadway infrastructure design and construction and resulted in changes to seismic codes and regulations for these critical structures. As discussed later, the Loma Prieta earthquake also initiated a seismic reassessment (recheck) of oil and gas platforms located off the coast of California.

The report Competing Against Time [Housner et al. 1990], written at the request of the Governor of California after the Loma Prieta earthquake, chronicles the development of seismic codes and standards in the United States and details the damage to buildings, bridges and infrastructure caused by the 1989 earthquake.

Figure 3. Damage to roadway infrastructure in the 1989 Loma Prieta Earthquake near San Francisco Resulted in

Changes in Roadway Design and also Initiated Reassessment of California Offshore Platforms [EQE 1989]

It is clear from the above that revisions and updates to seismic codes and standards often follow earthquakes where engineers and scientists learn what went right and what went wrong. There are also many factors involved – different types of building construction (wood, steel, masonry, reinforced concrete), different types of structure missions each with a different risk profile (buildings, bridges, hospitals, nuclear facilities), different foundation conditions (firm versus soft soils) and the different aspects of the seismic event itself (magnitude, duration of shaking, distance from the epicenter). Fortunately in recent decades, there has been in increase in seismic related research and development that is not associated with a specific event. The National Earthquake Hazard Reduction Program (NEHRP) was established in 1977 and funded by the US federal government with a mission to “reduce

the risks of life and property from future earthquakes in the United States through the establishment and maintenance of an effective earthquake hazards reduction program.” NEHRP is the ongoing nationwide coordinated program to reduce risks to life and property in the United States that result from earthquakes and has resulted in significant improvements to building codes. There has also been significant international seismic related research and code development especially in seismically active areas such as Japan, China and parts of Europe.

ONSHORE CODES FOR SEISMIC DESIGN OF STRUCTURES

For a number of decades, the basic onshore building guidance, as represented by provisions in the Uniform Building Code (UBC), was to design a structure for Design Basis Earthquake (DBE) shaking with a 10% probability of exceedance in 50 years, which corresponds to a return period of 475 years. This characterization of the DBE can be traced to ATC Report 3-06 [ATC 1978], which defined earthquake shaking using a response spectrum defined by effective peak acceleration and effective peak velocity-related acceleration, with modification factors for soil type. Various factors were included in the UBC to account for building size and shape, framing type and construction materials, natural period of the building and expected ductility to define an equivalent lateral load for design, where the resultant product had an inferred margin against collapse. The performance level associated with the DBE was life safety, namely, to prevent substantial loss of life in the structure. The inferred margin against collapse was a factor of 1.5 on ground motion intensity [FEMA 2003].

In the late 1990s the basic definition of ground motion was changed in response to concerns that defining ground motion based on a return period of 475 years and targeting life safety as the performance level would not provide a uniform level of protection against collapse, measured in terms of mean annual frequency of collapse, across the United States because the slopes on seismic hazard curves varied significantly across the country [Luco, et al. 2007]. Put differently, an increase in intensity of 1.5 from the 475-year shaking might return 2500-year shaking in the Western United States and a 1000-year shaking in the Eastern United States, producing a collapse probability for 1000-year shaking in the East that was much greater than that in the West. The change involved defining hazard using a Maximum Considered Earthquake (MCE) spectrum with a 2% exceedance in 50 years (return period of 2475 years) and designing for life safety using a DBE spectrum with ordinates 1/1.5 times those of the MCE spectrum, therein preserving the margin against collapse of 1.5.

In the 1990s, and after the 1989 Loma Prieta and 1994 Northridge earthquakes, work began in earnest on performance-based earthquake engineering with funding from the Federal Emergency Management Agency (FEMA) and the National Science Foundation. First generation products and tools for performance-based earthquake engineering were published in 1997 as products of the Applied Technology Council (ATC)

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project ATC-33 in FEMA 273 (guidelines) and FEMA 274 (commentary). These documents were updated and merged in 2000 into an ASCE pre-standard (FEMA 356) that was then issued as an ASCE standard, ASCE 41-06, Seismic Rehabilitation of Existing Buildings, and recently re-issued as ASCE 41-13. These first generation tools identified levels of performance for buildings (i.e., collapse prevention, life safety, immediate occupancy, and fully functional) and provided linear and nonlinear methods of analysis and the associated acceptance criteria. In 2001, FEMA embarked on the ATC-58 project to develop second-generation tools for performance-based earthquake engineering, in part to address the shortcomings identified early on by those involved in the ATC-33 project. The second generation products, tools and procedures were published in 2013 in FEMA P58, and address end-user measures of building performance in earthquakes such as casualties, repair cost and business interruption cost. The P58 guidance includes strategies for selecting and scaling ground motions for performance (loss) assessment, methods of linear and nonlinear analysis, fragility functions for structural and nonstructural components, and a calculation tool for estimating distributions of loss for user-defined ground shaking that can include a code-type response spectrum, an earthquake scenario spectrum (i.e., magnitude and distance pair), and seismic hazard curves (enabling a calculation of distribution of losses over a time period).

The most recent code changes (as reflected in IBC 2012 and ASCE 7-10) move from a “uniform hazard” definition of ground motion to a “uniform risk” definition. This risk-targeted approach was initially applied to the nuclear industry via ASCE 43-05 [ASCE 2005] and its DOE predecessor documents. Whereas the uniform hazard definition provides a consistent definition of ground motion exceedance in a specific time interval, it cannot deliver a uniform probability of failure for structures designed for that ground motion. There are significant variations in the probability of collapse because of uncertainty in the collapse capacity or factor of safety against collapse relative to the ground motion for which the structure is designed. The original approach defined a uniform hazard with the assumption that the overall design approach would lead to a low probability of collapse (

Pf = 0.10 [FEMA P-750, 2009]) in

the event of MCE shaking. Rather than rely on this inferred probability of failure, which depended on the slope of the seismic hazard curve at the building location, the mean annual frequency of failure was defined explicitly (1% probability of exceedance in 50 years) and the risk-targeted MCE ground motion was back-calculated with knowledge of the 2475-year hazard, the slope on the hazard curve in the vicinity of this return period, an assumed 10% probability of collapse conditioned on MCE shaking, and an assumed log standard deviation on the collapse fragility curve for a code compliant building. A risk coefficient was applied to the 2475-year MCE spectrum such that the product, a risk-targeted spectrum, used in conjunction with the prescriptive rules and details in ASCE 7 and in the materials standards incorporated by reference in

ASCE 7, would deliver a building with a 1% or less probability of failure in 50 years. (Note that this target is not sought for buildings on sites located close to major active faults.)

Table 1. Seismic Design Approaches in the US Onshore Buildings Industry

Parameter Prior to 1998 1998-2009 2010 - present

Ground motion definition

Uniform hazard

Uniform hazard Uniform risk

Performance level Life safety No collapse No collapse

Design basis earthquake1

10% in 50 years 2/3 of MCE1 2/3 of MCER

1

Maximum earthquake -- 2% in 50 years

(MCE) UHS ×Cr

(MCER)

Collapse margin

1.5 on DBE shaking2

1.5 on DBE shaking3

0.1 for MCER shaking

MAF4 collapse -- -- 0.01 in 50 years

1. DBE = Design basis earthquake; MCE = Maximum considered earthquake; MCER = risk targeted maximum considered earthquake

2. Assumed collapse margin 3. Stated (although not checked) collapse margin 4. MAF = Mean annual frequency

The United States Nuclear Regulatory Commission (NRC)

requires seismic analysis and design for all nuclear power plants in the nation. ASCE 43-05 [ASCE 2005] provides target performance goals for structures, systems and components (SSCs) in nuclear structures. ANSI/ANS 2.26 describes the criteria to be used for selecting a Seismic Design Category (SDC) and a limit state for each SCC. Each SDC has an associated target performance goal, expressed as a mean annual frequency of exceedance of the ANSI/ANS-specified limit state for that SSC. Seismic design categories range from 1 to 5, and limit states from A to D, depending on factors that include the risk of facility operation and the safety-related function of the SSC. A nuclear facility may contain SSCs that span from SDC 1 to SDC 5, with target performance goals ranging from 10-3 to 10-5. The intent of ASCE 43 is to achieve the target performance goals for each SSC using a risk-informed design basis ground motion and specified procedures for calculating seismic demand, capacity and acceptability. Alternately, the analyst can choose to achieve the target performance by demonstrating 1) less than 1% probability of unacceptable performance for the risk-informed design basis earthquake, and 2) less than 10% probability of unacceptable performance for ground motion shaking equal to 150% of the risk-informed design basis earthquake. The US NRC also requires a plant level assessment of the high confidence of low probability of

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failure for earthquake shaking 167% of design basis, which generally requires a seismic probabilistic risk assessment.

From a risk perspective, the ASCE 43 methodology is much more mature than that of ASCE 7, noting that the risk focus of ASCE 7 is on the building frame only, with the assumption that casualties (the product of building collapse) are due only to the failure of framing. ASCE 43 takes a much broader view of risk and considers all of the structures, systems and components in a nuclear facility that contribute to safety. Indeed, in many nuclear facilities, the contribution of structural damage to the mean annual frequency of unacceptable performance (i.e., core damage frequency in a nuclear power plant) is small, with the bulk of the risk associated with non-structural (e.g., mechanical, electrical) components and systems.

SEISMIC PROBABILISTIC RISK ASSESSMENT

In the aftermath of the Fukushima nuclear accident, many nuclear plants in the United States will be re-assessed to judge their ability to withstand earthquake shaking more intense than their original design basis: a procedure termed seismic probabilistic risk assessment (SPRA). SPRA determines the annual frequency of unacceptable performance, such as core melt and release of radiation. NUREG/CR-2300 [USNRC 1983] provides general guidance for performing a SPRA. The guideline identifies two methods for SPRA: 1) Zion, and 2) the Seismic Safety Margin (SSM). The Zion method was developed for the Oyster Creek probabilistic risk assessment and was later improved and applied for estimate seismic risk assessment at the Zion Plant [Pickard, Lowe, and Garrick, Inc., et al. 1981]. The SSM method was developed in an NRC-funded project at the Lawrence Livermore National Laboratory [Smith et al. 1981]. Although the procedures for computation of risk differ, both are based on the total probability theorem, which was also used by Cornell to develop probabilistic seismic hazard analysis [Cornell 1968].

The risk computation in the Zion and SSM methods can be performed using the following equation [Huang et al. 2011a 2011b]:

( )( ) H

UP UPd aG a dadaλλ = ∫ (1)

where λUP represents the annual frequency of the unacceptable performance; ( )UPG a represents the fragility curve, which characterizes the probability of the unacceptable performance given a value of a ground-motion parameter, a; and λH is the seismic hazard curve. Figure 4a presents a sample fragility curve, where the unacceptable performance and ground-motion parameter are core melt and peak ground acceleration (PGA), respectively. The average probability of core melt at a PGA between 0.45 and 0.55 g is about 0.5 (see the solid circle of Figure 4a). Figure 4b presents a sample hazard curve for PGA, where the annual frequency of PGA between 0.45 and 0.55 g is 0.0011 (see the ΔλH of Figure 4b). The product of 0.5 and 0.0011 represents the annual frequency of core melt contributed by the range of PGA between 0.45 and 0.55 g. If this analysis is

repeated for the entire range of PGA of Figure 4b, the summation of the products of ( )UPG a and HλΔ is the annual frequency of core melt, namely, the

λ f of (1).

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1Peak ground acce lera tion , a

0.2

0.4

0.6

0.8

1

Pro

babi

lity

of c

ore

mel

t, GUP(a)

a. Sample fragility curve

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1Peak ground accelera tion, a

0.0001

0.001

0.01

0.1M

ean

annu

al fr

eque

ncy

of

e

xcee

danc

e, λ

Η(a

)

ΔλΗ=0.0011

b. Sample seismic hazard curve

Figure 4. Sample fragility and seismic hazard curves [Huang et al. 2011a]

To develop a cumulative distribution function of unacceptable performance, an analyst must develop a) fragility curves for structural and nonstructural components and systems, and b) accident sequences. The former characterizes the probability of failure of components given either a ground-motion intensity or structural response (if earthquake shaking is the focus of the probabilistic risk assessment). The latter relates the failure of a component to unacceptable performance. Accident sequences, which are often identified by systems engineers, are required for analysis because the failure of a component or system may activate a safety system and the accident might not lead to unacceptable performance. A typical approach to establish critical accident sequences involves 1) identification of initiating events that might result in

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unacceptable performance, and 2) the use of event trees and fault trees, as discussed in Huang et al. [2011a].

Huang et al. describe a modern seismic probabilistic risk assessment methodology that takes advantage of developments in earthquake ground motion, nonlinear dynamic analysis, performance based earthquake engineering, and fragility analysis and functions. In this five-step methodology (Figure 5), Step 1 requires the analyst to develop fragility curves for the structural and nonstructural components of the facility and the accident sequences that could result in unacceptable performance. Step 2 characterizes the earthquake shaking using a seismic hazard curve. Step 3 involves response-history analysis of the facility subjected to the seismic hazard of Step 2 to estimate the deformations in the structural (primary) system and the accelerations, forces, and displacements that serve as demands on the facility’s nonstructural (secondary) components and systems. Damage to the structural and nonstructural components and systems is assessed in Step 4 using the demands computed in Step 3 and fragility curves developed in Step 1. Monte-Carlo-based procedures to generate a large number of response data that are statistically consistent with those of Step 3 and to assess the possible distribution of damage to structural and nonstructural components of the facility for each set of simulations. Step 5 involves the computation of seismic risk (probability of unacceptable performance) using the results of Step 4 and the accident sequences developed in Step 1.

Perform plant system analysis

Characterize seismic hazard

Simulate structural responses

Assess component damage

Compute the risk

Figure 5. SPRA methodology [Huang et al. 2011a]

An important step in the SPRA procedure is the characterization of the seismic hazard and the selection and scaling of earthquake ground motions for response-history analysis. Figure 6 reproduces a seismic hazard curve from Huang et al. In this figure, the hazard curve is split into eight equal intervals of spectral intensity, where the spectral demand

at the lowest intensity is unlikely to cause any damage and spectral demand at the highest intensity is likely to cause unacceptable performance (e.g., collapse in a building, core damage in a nuclear power plant). The spectral intensity at the midpoint in each interval is assumed to represent earthquake shaking in that interval. Ground motions are selected and scaled to each value of midpoint spectral intensity (i.e., ei

in Figure 6). These ground motions are used in Step 3 for response-history analysis.

0 0.5 1 1.5 2 2.5 3Earthquake intensity, e (g)

1E-006

1E-005

0.0001

0.001

0.01

Mea

n an

nual

freq

uenc

y of

exc

eeda

nce

0.21 0.54 0.87 1.2 1.53 1.86 2.19 2.52

Δe1 Δe2 Δe3 Δe4 Δe5 Δe6 Δe7 Δe8

e1

e2e3

e4e5

e6 e7 e8

ΔλΗ,1

ΔλH,2

ΔλH ,3ΔλH,4

ΔλH ,5ΔλH,6ΔλH,7ΔλH,8

Figure 6. Sample seismic hazard curve [Huang et al. 2011a]

The probability of unacceptable performance is calculated at each value of midpoint spectral intensity, ei : PUP(ei ) . The mean annual frequency of unacceptable performance, λUP is then computed by

λUP = PUP(ei ) ⋅ ΔλH ,i

i=1

8

∑ (2)

where ΔλH ,i is the mean annual frequency of shaking in

spectral intensity range i, represented by midpoint spectral intensity ei , and 8 is the number of intervals of spectral intensity.

OFFSHORE STRUCTURE CODES FOR EARTHQUAKES

Offshore platforms have been designed and installed in seismic regions since the late 1950s when the first platforms were installed off the coast of California. These early structures used onshore seismic design practice contained in the Uniform Building Code (UBC) of the time. The first specific guidance for seismic design for offshore platforms appeared in the API RP 2A (RP2A) Recommended Practice for Planning, Designing and Constructing Fixed Offshore Platforms in October 1969 and contained reference to the UBC including the calculation of an equivalent lateral seismic design load using a special structure design factor for offshore platforms.

The RP2A 7th Edition (1976) contained a significant update for earthquakes, replacing the UBC approach with a two-tier design for an Operating Level and Safety Level: consistent with the Operating Basis Earthquake and Safe Shutdown Earthquake used in the nuclear power industry. The

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Operating Level required the platform to be designed for sufficient strength to maintain stresses within yield or buckling for the maximum earthquake expected during the life of the structure. The Safety Level required the platform to have sufficient ductility (ability to deform beyond yield) to prevent collapse during a maximum credible event. The return periods for either the Operating and Safety Level earthquakes were not defined. The Safety Level design was accomplished by showing that the platform would not collapse at a lateral displacement of two times that calculated for the Operating Level. The 8th Edition of RP2A in 1977 changed Operating Level and Safety Level to Strength Level and Ductility Level, respectively. The industry approach for the two level design requirements was to first “design” for the Strength Level and then “check” for the Ductility Level, assigning secondary status to the Ductility Level check.

The 10th Edition of RP2A (1979) contained significant changes including guidance on computer modeling and some of the original guidance for designing framing, braces and joints to improve ductile response. The Ductility Level requirement was revised to the “performance goal” of demonstrating that the structure-foundation system could absorb at least four times the amount of energy absorbed at the Strength Level without collapse, which was accomplished using a plastic or nonlinear static analysis under lateral loadings.

The 15th Edition of RP2A (1984) represents the last significant change to the earthquake provisions of RP2A; API and ISO employ this basic approach to seismic design at the time of this writing. Key changes in 1984 included the assignment of a 200-year return period to the Strength Level earthquake and the elimination of the energy absorption approach for the Ductility Level in favor of either 1) the inclusion of specific ductility features into the platform framing, or 2) explicit analysis, with nonlinear response-history analysis preferred. There was no mention of a specific return period for the Ductility Level event but most operators used a return period of between 1,000 and 2,000 years.

By the mid 1980s, the RP2A earthquake guidance had received considerable attention from the offshore industry, with nearly 20 percent of the document devoted to earthquakes, and its procedures representing the state-of-the-art for seismic analysis and design of structures at the time, either onshore or offshore. The mid 1980s was the end of the era of platform construction off the coast of California and there was no impetus or perceived need to update the RP2A seismic guidelines. Because the 1989 and 1994 earthquakes in California triggered research and development of seismic design guidance for on-shore structures, the state-of-the-art baton was passed from the offshore (and nuclear) industry to the onshore buildings industry.

In the late 1990s, ISO began developing a new standard for the seismic design of offshore structures, which eventually evolved into ISO 19901-2 (ISO 2004). The ISO guidance borrowed heavily from RP2A and the work by the onshore industry especially the NEHRP related studies. The two tier design approach was retained although the Strength Level was

renamed Extreme Level and the Ductility Level was renamed Abnormal Level to be consistent with the ISO nomenclature for other types of extreme design conditions such as storms. However, the prior API approach of “design for the Strength Level and check for the Ductility Level” was reversed with the emphasis on establishing a platform design capable of ductile response. This was accomplished by establishing a target annual probability of failure,

Pf (described elsewhere as a

mean annual frequency of failure) for the Abnormal Level event and then, based upon the specific ductility features of the platform, establishing the Extreme Level Earthquake (ELE) spectrum. An Abnormal Level Earthquake spectrum is determined for a platform by seismic hazard analysis (typically) at the site at a mean annual frequency of exceedance based on an exposure Level. The ordinates of the resulting uniform hazard spectrum are then increased to the ALE spectrum by a correction factor, based on the slope of the seismic hazard curve to capture uncertainties in seismic actions and component resistances. The corresponding return period of the ALE is then greater than the inverse of the target annual probability of failure. The ELE spectrum is derived from the ALE spectrum by dividing ordinates by a seismic reserve capacity factor that ‘…represents the available margin of safety for events beyond the ELE.”

This reversal in approach placed emphasis on the ductile detailing of the platform, which is one of the primary findings of the onshore building work in the 1980s and 1990s, namely, that ductile structures perform best in earthquakes. ISO added a risk-based approach for selecting seismic criteria with High Consequence platforms (manned or unmanned) requiring a

Pf

of 1/2500 compared to Medium Consequence (manned or unmanned) platforms with

Pf of 1/1000 and Low Consequence

platforms with Pf of 1/400 as shown in Table 2. In this table,

the exposure levels (formally defined in ISO 19902 [ISO 2007]) can be loosely mapped to a High Consequence platform in a region of moderate to high seismic hazard (L-1), and a Low Consequence unmanned platform in a region of low to moderate seismic hazard (L-3).

Table 2. ISO 19901-2 Earthquake Criteria

Exposure Level Target Annual Probability

of Failure, Pf

L-1 4×10−4 (1/2500) L-2 1×10−3 (1/1000) L-3 2.5×10−3 (1/400)

Following the ISO 19901-2 approach, the return period on

the ALE typically ranges between 3,000 and 3,500 years off the coast of California and between 3,000 and 4,000 years elsewhere in the world. The underlying principles of the risk-targeted approach for onshore buildings and ISO 19901-2 approach for offshore structures are the same, and they are both based on the principles discussed by Cornell [1996].

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In 2014, API followed the approach of ISO 19901-2 and issued the first edition of API RP 2EQ [Younan and Puskar 2010, API 2014]. A key change in API RP 2EQ is the elimination of the Medium Consequence design category since it is difficult to define a manned platform [Puskar et al. 2014].

ONSHORE AND OFFSHORE STRUCTURE EARTHQUAKE DESIGN RELIABILITY COMPARISONS

The seismic performance goals for onshore building and nuclear power plant structures, and offshore platforms are difficult to compare directly because analysis and design approaches differ, performance is not directly assessed, and what constitute unacceptable performance varies considerably across the industries. However, summary information is presented below to enable a comparison and foster discussions on risk and reliability in the offshore industry.

Onshore nuclear facilities

The US Department of Energy and the US Nuclear Regulatory Commission regulate the analysis, design and construction of nuclear facilities in the United States. The most stringent requirements are imposed on nuclear power plants. Nuclear power plants are designed to resist extreme loadings as part of the design basis. For nuclear power plants: • Target performance goal of first onset of significant

inelastic component deformation at a mean annual frequency of 1×10−5

• Design basis earthquake derived from a seismic hazard analysis for a mean annual frequency of 1×10−4 (the uniform hazard spectrum is multiplied by a design factor, greater than 1, to derive the design basis earthquake spectrum)

• High confidence of low probability of system failure (around 1%) for shaking 167% of design basis (noting that nuclear plants have active and passive safety features to deal with accidents resulting from earthquakes)

• Mean annual frequency of core damage or large release of radiation for new build plants is not specified but can be assumed to be 1×10−6 or less.

Onshore buildings

ASCE Standard 7 provides a design basis for onshore building construction in the United States. For buildings: • Target performance goal of 10% or less probability of

collapse in maximum considered earthquake shaking • Maximum earthquake spectrum derived from seismic

hazard analysis for a mean annual frequency of 4×10−4 and modified using a risk coefficient that depends on the slope of the seismic hazard curve

• Design basis earthquake spectrum developed by dividing the risk-adjusted maximum considered earthquake spectrum by 1.5

• Mean annual frequency of collapse (modern, code compliant building) is 2×10−4 (or 1% probability in 50 years).

Offshore platforms

For High Consequence (L-1) platforms designed per ISO 19901-2 (1994): • Mean annual frequency of failure is 4×10−4 . • Abnormal Level Earthquake spectrum derived from

seismic hazard analysis for a mean annual frequency of 4×10−4 and modified using a correction that depends on the slope of the seismic hazard curve and other uncertainties not treated directly in design

• Extreme Level Earthquake spectrum obtained by dividing the ALE spectral ordinates by a seismic reserve capacity factor (ISO 19902 [ISO 2007]), which ranges between 1.1 (non-redundant, non-ductile platform) and 2.8 (ductile, redundant platform with global ALE performance checked by nonlinear analysis).

THE FUTURE The seismic design philosophies for the onshore buildings

and nuclear industries and the offshore industry are now either risk-informed or risk based. Of the three industries, the nuclear industry is the most advanced.

Seismic probabilistic risk assessment provides a sound technical basis for the development of performance levels in codes of design practice. The procedure of Huang et al. [2011a, 2011b], which was developed for the nuclear industry, could be adapted for the offshore industry, noting that safety systems are installed on platforms to mitigate specific accidents. Event trees and fault trees for earthquake-induced accident sequences would have to be developed for each platform, new or existing, to perform the calculations and make explicit calculations of the mean annual frequency of unacceptable performance.

An important question, perhaps not yet adequately answered in the offshore and onshore buildings industries is, “What constitutes risk?” In the nuclear industry, risk is defined in terms of core damage or radiation release. In the buildings industry, risk is defined in terms of collapse and no attention is paid (explicitly) to either controlling damage or limiting financial loss, noting that more than 80% of the replacement cost of a building is generally associated with nonstructural components and systems, many of which are not engineered. In the offshore industry, risk is defined in terms of platform collapse and it is unclear whether unacceptable performance is tied to loss of life (irrelevant for unmanned platforms), financial loss or damage to the environment, or a combination of all three. The use of seismic probabilistic risk assessment, advanced methods of structural analysis and fragility curves, and the development of outcome based performance metrics (e.g., loss of life, repair cost, financial loss due to inoperability) would enable the offshore industry to further advance its design practice, evaluate the risk of older platforms, and make decisions regarding standards for manned and unmanned platforms, the need for and value of installing accident-mitigating systems, and so on.

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