A Screening Methodology for the Identification of Critical Units in Major-Hazard Facilities Under Seismic Loading

The complexity of process industry and the consequences that Na-Tech events could produce in terms of damage to equipment, release of dangerous substances (flammable, toxic, or explosive), and environmental consequences have prompted the scientific community to focus on the development of efficient methodologies for Quantitative Seismic Risk Analysis (QsRA) of process plants. Several analytical and numerical methods have been proposed and validated through representative case studies. Nevertheless, the complexity of this matter makes their applicability difficult, especially when a rapid identification of the critical components of a plant is required, which may induce hazardous material release and thus severe consequences for the environment and the community. Accordingly, in this paper, a screening methodology is proposed for rapid identification of the most critical components of a major-hazard plant under seismic loading. It is based on a closed-form assessment of the probability of damage for all components, derived by using analytical representations of the seismic hazard curve and the fragility functions of the equipment involved. For this purpose, fragility curves currently available in the literature or derived by using low-fidelity models could be used for simplicity, whereas the parameters of the seismic hazard curve are estimated based on the regional seismicity. The representative damage states (DS) for each equipment typology are selected based on specific damage states/loss of containment (DS/LOC) matrices, which are used to individuate the most probable LOC events. The risk is then assessed based on the potential consequences of a LOC event, using a classical consequence analysis, typically adopted in risk analysis of hazardous plants. For this purpose, specific probability classes will be used. Finally, by associating the Probability Class Index (PI) with Consequence Index (CI), a Global Risk Index (GRI) is derived, which provides the severity of the scenario. This allows us to build a ranking of the most hazardous components of a process plant by using a proper risk matrix. The applicability of the method is shown through a representative case study.

Record-to-record variability and code-compatible seismic safety-checking with limited number of records

The Italian code requires spectrum compatibility with mean spectrum for a suite of accelerograms selected for time-history analysis. Although these requirements define minimum acceptability criteria, it is likely that code-based non-linear dynamic analysis is going to be done based on limited number of records. Performance-based safety-checking provides formal basis for addressing the record-to-record variability and the epistemic uncertainties due to limited number of records and in the estimation of the seismic hazard curve. “Cloud Analysis” is a non-linear time-history analysis procedure that employs the structural response to un-scaled ground motion records and can be directly implemented in performance-based safety-checking. This paper interprets the code-based provisions in a performance-based key and applies further restrictions to spectrum-compatible record selection aiming to implement Cloud Analysis. It is shown that, by multiplying a closed-form coefficient, code-based safety ratio could be transformed into simplified performance-based safety ratio. It is shown that, as a proof of concept, if the partial safety factors in the code are set to unity, this coefficient is going to be on average slightly larger than unity. The paper provides the basis for propagating the epistemic uncertainties due to limited sample size and in the seismic hazard curve to the performance-based safety ratio both in a rigorous and simplified manner. If epistemic uncertainties are considered, the average code-based safety checking could end up being unconservative with respect to performance-based procedures when the number of records is small. However, it is shown that performance-based safety checking is possible with no extra structural analyses.

Liquefaction assessment based on CSR-hazard curve through empirical procedure

<p>Semi-empirical procedures for evaluating liquefaction potential (e.g. Seed & Idriss, 1971) require the estimation of cyclic resistance ratio (CRR) and cyclic shear stress ratio (CSR). The first can be obtained using empirical relationships based on in situ tests (e.g. CPT, SPT), the latter can be expressed as function of the maximum horizontal acceleration at ground surface (a<sub>max</sub>), total and effective vertical stresses at the depth of interest (σ<sub>v0</sub>, σ’<sub>v0</sub>) and a magnitude-dependent stress reduction coefficient (r<sub>d</sub>) that accounts for the deformability of the soil column (Idriss & Boulanger, 2004). All these methods were developed referring to a moment magnitude (M<sub>w</sub>) equal to 7.5 and therefore require a magnitude scale factor (MSF) to make them suitable for different magnitude values. Usually, MSF and r<sub>d</sub> are computed with reference to the mean or modal value of M<sub>w</sub> taken from a disaggregation analysis, while a<sub>max</sub> is obtained from a seismic hazard curve, including the contribution of various combinations of magnitudes and distances (Kramer & Mayfield, 2005). Thus, there might be inconsistency between the magnitude values used to evaluate either MSF or r<sub>d</sub> and a<sub>max</sub>. To overcome this problem, Idriss (1985) suggests to directly introduce the MSF in the probabilistic hazard analysis of the seismic acceleration. In this contribution, an alternative method is proposed, by properly modifying the acceleration seismic hazard curve conventionally adopted by the code of practice on the basis the disaggregation analysis, so that i) the contribution of the different magnitudes and the associated MSF and r<sub>d</sub>-values are considered, ii) the computational effort is reduced since a CSR-hazard curve is straightforward obtained. This alternative method is used to carry out a simplified liquefaction assessment of a sand deposit located in the municipality of Casamicciola Terme (Naples, Italy), where the results of SPT tests are available from recent seismic microzonation studies. The CSR computed using the proposed procedure is lower than that obtained adopting the classical method suggested by Idriss & Boulanger (2004). This can be explained considering that the suggested method takes into account all the magnitudes that contribute to the definition of the seismic hazard, instead of considering the mean or modal value of the disaggregation analysis. Such an accurate prediction of the seismic demand may represent a basis for more reliable seismic microzonation maps for liquefaction and for a less conservative design of liquefaction risk mitigation measures.</p><p>References</p><p>Idriss, I.M. (1985). Evaluation of seismic risk in engineering practice, Proc. 11th Int. Conf. on Soil Mech. and Found. Engrg, 1, 255-320.</p><p>Idriss, I.M., Boulanger, R. W. (2004). Semi-Empirical Procedures for Evaluating Liquefaction Potential During Earthquakes, Proceedings of the 11th ICSDEE & 3rd ICEGE, (Doolin et al. Eds.), Berkeley, CA, USA, 1, 32-56.</p><p>Kramer, S.L., Mayfield, R.T. (2005) Performance-based Liquefaction Hazard Evaluation, Proceedings of the Geo-Frontiers Congress, January 24-26, Austin, Texas, USA.</p><p>Seed H.B., Idriss M. (1971). Simplified procedure for evaluating soil liquefaction potential, J. Soil Mech. Found. Div., 97, 1249-1273.</p>

Statistical properties of peak ground acceleration and their effect on results of probabilistic seismic hazard analysis

<p>The problem is considered of unrealistic ground motion estimates, which arise when the Cornell–McGuire method is used to estimate the seismic hazard for extremely low annual probabilities of exceedance. This problem stems from using the normal distribution in the modelling of the variability of the logarithm of ground motion parameters. In this study, the statistical properties of the logarithm of peak ground acceleration (PGA) are analysed by using the database of the strong-motion seismograph networks of Japan. The normal distribution and the generalised extreme value distribution (GEVD) models were considered in the analysis, with the preferred model being selected based on statistical criteria. The results indicate that the GEVD was a more appropriate model in eleven out of twelve instances. The estimates of the shape parameter of the GEVD were negative in every instance, indicating the presence of a finite upper bound of PGA. Therefore, the GEVD provides a model that is more realistic for the scatter of the logarithm of PGA, and the application of this model leads to a bounded seismic hazard curve.</p>

Effects of Earthquakes on Flood Hazards: A Case Study From Christchurch, New Zealand

Earthquakes can influence flood hazards by altering the flux, volumes, and distributions of surface and/or subsurface waters and causing physical changes to natural and engineered environments (e.g., elevation, topographic relief, permeability) that affect surface and subsurface hydrologic regimes. This paper analyzes how earthquakes increased flood hazards in Christchurch, New Zealand, using empirical observations and seismological data. Between 4 September 2010 and 4 December 2017, this region hosted one moment magnitude (Mw) 7.1 earthquake, 3 earthquakes with Mw ≥ 6, and 31 earthquakes with local magnitude (ML) ≥ 5. Flooding related to liquefaction-induced groundwater pore-water fluid pressure perturbations and groundwater expulsion occurred in at least six earthquakes. Flooding related to shaking-induced ground deformations (e.g., subsidence) occurred in at least four earthquakes. Flooding related to tectonic deformations of the land surface (fault surface rupture and/or folding) occurred in at least two earthquakes. At least eight earthquakes caused damage to surface (e.g., buildings, bridges, roads) and subsurface (e.g., pipelines) infrastructure in areas of liquefaction and/or flooding. Severe liquefaction and associated groundwater-expulsion flooding in vulnerable sediments occurred at peak ground accelerations as low as 0.15 to 0.18 g (proportion of gravity). Expected return times of liquefaction-induced flooding in vulnerable sediments were estimated to be 100 to 500 years using the Christchurch seismic hazard curve, which is consistent with emerging evidence from paleo-liquefaction studies. Liquefaction-induced subsidence of 100 to 250 mm was estimated for 100-year peak ground acceleration return periods in parts of Christchurch.

Probabilistic seismic hazard analysis of Nepal

Probabilistic seismic hazard analysis for Nepal has been carried out considering uniform density model. A detailed earthquake catalogue since 1255 A.D, within the rectangular area has been developed and historical earthquakes are plotted in the map of Nepal. Five hundred twenty eight numbers of areal sources are used within the study area to characterize the seismic sources. The completeness of the data has been checked by using Stepp's procedure. Seismicity in four regions of study area has been evaluated by defining 'a' and 'b' parameters of Gutenberg Richter recurrence relationship. Seismic hazard curve of Nepal for soft subsoil condition for 10% probability of exceedence in 50 years period i.e. for return period of 475 years has been plotted.

A Novel Seismic Risk Analysis Method for Structures with Both Random and Convex Set Mixed Variables: Case Study of a RC Bridge

Assessing the demand hazards of structures is requested in the framework of performance-based earthquake engineering. An efficient method for estimating the seismic risk of structures is proposed in this paper. The relationship between multiple limit capacities and corresponding response parameters is denoted by using a generalized multidimensional limit state equation. The limit states of different components are described as random and convex mixed variables. The seismic responses of different components are considered dependent and follow a multidimensional lognormal distribution. The mathematical formula of multidimensional demand hazards of structures is then derived through combining the seismic fragility function and the seismic hazard curve. The proposed method is used to perform the demand hazard analysis and the parameter sensitivity analysis of a multispan continuous concrete girder bridge, selecting column ductility and bearing displacement as the two-dimensional seismic response parameters obtained by Incremental Dynamic Analysis. The results demonstrate that the coefficient of variation and correlation coefficient N, which are involved in the limit state equation, have an impact on the evaluation of the demand hazards.

Seismic Quantitative Risk Assessment of Process Plants Through Monte Carlo Simulations

This paper describes the application of Monte Carlo method for the quantitative seismic risk assessment (QSRA) of process plants. Starting from the seismic hazard curve of the site where the plant is located, the possible chains of accidents are modelled using a sequence of propagation levels in which Level 0 is represented by the components directly damaged by the earthquake whereas the subsequent Levels represent the resulting consequence propagation. In greater detail all units damaged by energy and materials releases from level 0 units are included in level 1 and so forth, so that referring to process units belonging to a generic i-th Level, they are damaged by level (i-1) units and damage units of level (i+1). The sequence of levels represents the damage propagation across the plant through any multiple interacting sequences of accidents. For each unit a damage (DM) - loss of containment (LOC) matrix is generated allowing to estimate the amount of energy and material releases as well as resulting physical effects based on which the scenario at i-th level is generated. The process stops when no further damage propagation is allowed.

A Method of Determining Strengthening Design Ground-Motion Parameters for the Existing Building

At present, seismic strengthening design reference period of the existing building is usually equal to 50 years in China, sometime this is uneconomic and unreasonable. In this paper, determining principle of seismic strengthening design reference period for the existing building with different importance is presented. The seismic strengthening design level of the existing building is put forward. After the shape factor of intensity probability distribution function is used to represent the seismic hazard characteristic of different areas, the seismic hazard curve formula of design acceleration Amax and earthquake influence coefficient αmax are deduced according to the seismic hazard curve of intensity. The seismic strengthening design ground motion parameter for the existing building with different importance is researched in detail by use of hazard curve formula of seismic ground motion parameter based on seismic hazard characteristic zone. At last, the method and the calculation step are explained by a calculation example. The result shows that for the existing building with different design reference period, using same design parameter is unreasonable in different seismic hazard characteristic zone, and the method is more scientific than the code method.