Risk Assessment of Induced Earthquake Hazards in the Northern Netherlands

Risk Assessment of Induced Earthquake Hazards

in the Northern Netherlands

M.G.J.M. PETERS, J.A. KLEINJAN and A. KOSTER Grontmij Nederland BV

Abstract. During the last decade, at least eight earthquakes occurred in the Northern Netherlands with a magnitude of 3 or

higher on the Richter scale. These earthquakes are not tectonic but induced by natural gas production from the Groningen Gas Field. Before that time there were no earthquakes in Groningen with such magnitudes. Research conducted by the Ministry of Economic Affairs (EZ) and the State Supervision of Mines (Sodm) showed that both number and magnitude have increased in recent years. The Ministry has given priority to investigate the induced earthquake risks and effects on the most critical structures, installations and infrastructure, such as industrial plants in the Delfzijl – Eemsmond region and public buildings like hospitals and schools. This paper describes a method to develop a risk analysis in order to check the earthquake resistance of structures with the highest risk profile.

Keywords. Assessment and Management of Natural Hazards, Geotechnical Risk Management and Risk Communication,

Reliability and Risk Analysis of Geotechnical Structures

1. Introduction

1.1 Background of induced earthquakes

Since 1960, the Netherlands have been extracting natural gas from large on-land reservoirs in Groningen. The gas production is still continuing and leads to induced seismic events with small to moderate magnitudes. The highest magnitude since the production is recorded during the Huizinge event on August in 2012 and is measured with a moment magnitude of Mw = 3.6. Since 1992 the Dutch

Ministry of Economic Affairs is investigating to quantify seismic hazard by executing monitoring programs which have actually been intensified.

The gas is produced from the Rotliegend Sandstone as reservoir, which was developed during the Permian, about 250 million years ago. During that period, the Netherlands were located next to the North German Basin, also called the Zechstein Sea and the landscape was covered with sand dunes. These dunes finally formed the sandstone which is nowadays located at a depth of about 3 km, see Figure 1.

Figure 1. 3D-view of the top of the Rotliegend Sandstone

of the Groningen field (source: NAM). The blue colours indicate the lower levels, the red colours indicate the higher levels. The north arrow is situated on top of de figure, the surface area is roughly 60 x 60 km2_.

In Figure 2 a geological cross section is given with the position of the Rotliegend sandstone on top of the Carbon layer. The reservoir is covered with the Zechstein salt layer, which behaves as a rather plastic, impermeable layer. The gas is stored in the pores and fractures of the sand stone layer.

Exploitation of the gas leads to a reduction of the pore pressure in the sandstone. As a result, the sandstone layer is compacting under the dead weight of the soil layers above. This

© 2015 The authors and IOS Press.

This article is published online with Open Access by IOS Press and distributed under the terms of the Creative Commons Attribution Non-Commercial License.

compaction occurs along and between the fractures in the sandstone and leads to seismic vibrations to higher soil layers. Induced earthquakes have specific characteristics compared to tectonic earthquakes. Regarding the seismic signal, we only see one or two peaks in a short time, where tectonic earthquakes show more peaks during a longer period. First a compression wave occurs causing vertical accelerations, following by a shear wave which causes dominant horizontal accelerations. The magnitude is limited (up to Mw = 5.0 on Richter scale). The maximum measured peak ground acceleration (PGA) on surface level until now is about 0.08 to 0.09 g.

Figure 2. Geological cross section Groningen gas field

(source: SodM)

Figure 3. Some characteristic accelerograms from induced

and tectonic earthquakes

1.2 Performed seismic hazard analysis

Based on the research conducted by the State Supervision of Mines (SodM) and presented in January 2014 [1], it can be concluded that the frequency and strength of earthquakes has increased in recent years. Probabilistic Seismic Hazard Analyses (PSHA) have been executed by the Dutch Institute for Meteorology and Seismology (KNMI, [3]) where Gutenberg-Richter relations are made between the magnitude and the probability of annular exceedance, with a maximum Mw = 5.0 at 0.2% probability (= 1/475 years). Based on empirical relations between magnitude and

PGA (Akkar, [3]) and measured results on the field, a PGA-contour map for the Groningen gas field can be generated. A PGA-contour map is implemented in the current draft version of the National Guideline NPR 9998:2015. The Guideline aims to include the importance factors for deriving the PGA-design values, required for seismic assessment and design.

In the 2015 version of the NPR 9998, KNMI has developed a map with seismic zones in order to investigate earthquake risks for the industrial areas in North East Groningen (Delfzijl and Eemsmond area), see figure 4. At this moment, Grontmij is analysing the first industrial plants located in this area, on effects and risks from induced earthquakes. For this, a specific guideline is written by Deltares [2] which contains provisional design PGA-values for the focussed investigation sites derived from the executed PSHA by KNMI.

Figure 4. Seismic zones (source: KNMI [3])

Figure 5. Relation between design-PGA and probability

of exceedance for existing structures in Delfzijl area, in this example the design value is determined for consequence class CC2 for an exceedance period of 1300 year (P = 1/1300 =7.7·10-4_{) [2]}

1.3 Aim of the Paper

This paper describes a method to carry out an efficient risk analysis in order to check the earthquake resistance of structures as (industrial) plants, schools and hospitals.

2. Earthquake risk assessment 2.1 Staged analysis process

With regard to the increased amount of earthquakes in the last years, the need for mapping the earthquake resistance of buildings, infrastructure and installations at (industrial) plants has become essential. For these plants, buildings and infrastructure, procedures have been developed by Grontmij and Deltares together to analyse the earthquake resistance on both structural and geotechnical aspects. Initially, the analysis will be focussed on Near Collapse (NC conditions as an ultimate level state).

In order to investigate in an efficient way, not all possible critical parts and installations will be thoroughly analysed at once, but for every plant or building a pre-selection of most important and critical structures is preferred. According to Eurocode 1991-1-7, the risks and effects will be examined from low (coarse) to high (fine), following the next three phases (see figure 6):

1. qualitative risk analysis; 2. quantitative risk analysis;

3. risk evaluation, treatment and

measurements.

The total investigation consists of three phases.

x

In phase 1, a qualitative risk analysis is executed in order to make a selection of the most critical structures and

components.

x

In phase 2, the details of the selected structures will be inventoried and

principles will be defined in order to assess the current state of the structures and to perform soil investigation if necessary. Next, quantitative calculations will be performed (geotechnical, structural, pipelines, etc.). First simple or analytical and, if necessary, more advanced models

as finite element methods (FEM) will be used.

x

In phase 3, for structures and parts which do not meet the requirements, mitigated measurements will be examined by simple considerations in advance and when necessary by designing with more advanced models. Also advice and recommendations will be given for retrofit and additional research to work out.

Figure 6. Staged process to analyse risks and effects of

induced earthquakes on industrial plants, structures and critical infrastructure (source: Grontmij Netherlands BV) 2.2 Phase 1: Qualitative risk analysis

In order to make a prioritization of the most critical structures, a qualitative risk analysis is performed. This consists of the following components:

x

an inventory of the structures / containment systems and environment;

x

identifying the threats;

x

the definition of the various scenarios and their probability;

x

the estimation of the impact (victims, environmental damage);

x

considering whether further quantitative assessment, further research or measures are necessary or not.

Initially, risks under normal operating conditions are determined. The risk is the product of the probability of an undesirable situation and the consequence of the effect that this undesirable situation brings with (risk = probability x impact).

In case of industrial plants with hazardous substances, an undesirable situation is defined as the situation where these products are released from the plant and then presents a danger to persons and/or the environment. This is a so-called loss of containment or loss of control (LOC) situation. To carry out the risk analysis the plant is cut into containment systems which are determined by the probability and the degree of impact in LOC scenarios. These scenarios are normally based on the qualitative risk under usual operating conditions. However, for induced earthquakes as a special circumstance on which plants are exposed to, the risks under these conditions are tightened. Containment systems are within the area of influence of structures (pounding). The integrity of these structures are high or moderately affected by the earthquake, with possible consequences for the plants and other structures or containment systems. The probability of failure of a containment system is influenced. Also, the degree of damage to a containment system and the effects and consequences will be higher. Both aspects lead to increasing risk. By linking the structures to the containment systems under the given special circumstances, a risk identification can be performed with a qualitative (relative) scaling or weight under these circumstances.

In order to make a first qualitative analysis for a (industrial) plant, all structures, installations and containments are considered. With use of existing documents, like Safety Reports, process descriptions, qualitative risk analysis (QRA), environmental risks analysis (MRA) etcetera, a complete inventory of relevant stationary containments with dangerous goods is made. The existing QRA is used to determine the largest safety risks, the existing MRA is used to determine the largest environmental risks. Based on the largest threats for safety and environmental risks a first selection of risky structures is made.

The first selected structures are examined further on structural and geotechnical aspects in order to give a better review and assessment of the containment systems and prioritization for the quantitative analysis. This review is based on a visit to the plant, a so-called: walkthrough. During this walkthrough, various earthquake related properties of structures as

containment systems with dangerous contents like tanks, silos, pipelines and vessels are considered and indicated on a recording form. According to this recording form, several structural and geotechnical "security" aspects were drawn up in a assess-division form. An example is given in figure 7.

Figure 7. Example of recording form for structural

aspects according to ASCE guidelines for seismic evaluation and design of petrochemical facilities [4]

The qualitative assessment of structures and containment systems on (industrial) plants is based on the Guidelines for Seismic Evaluation and Design of Petrochemical Facilities [4], API 650, Appendix E (Seismic design of storage tanks) [5] and CalARP (California Accidental Release Prevention) [6].

Initially, the civil/seismic engineer formulates the structural and geotechnical features. For example for tanks, some criteria can be:

x

height, width, wall thickness, tank capacity;

x

slenderness, asymmetry, uniformity;

x

roof and anchorage;

x

foundation type, subsoil;

x

site conditions and presence of bunds;

x

etc.

Based on these features, seismic risks are qualitatively assessed. It is done by using a scale where the height of the risk is based on structural and geometric configurations that can have an adverse effect during earthquake (such as: surface, slenderness, asymmetry, high gravity centre, stiffness differences, transitions and connections, etc.). Assessment

can be made by using scores for each feature related to sensitivity to earthquake effects. The total score will be related qualitatively (or relatively) to the probability of the risk. Here, an example for two aspects (geotechnical and structural) with total score related to earthquake effects for tanks is given in table 1.

Table 1. Example total scores related to probability

Probability Geotechnical risks Tank structural risks A Very small 0 – 1 0 – 3 B Small 2 4 – 6 C Average 3 7 – 9 D Large 4 10 – 12 E Very large > 4 > 12

The consequences are related to the severity to health damage and to environment. An example is given in table 2.

Table 2. Example Severity related to consequence

Severity Damage to health Environmental damage 0 No damage No damage / no effects 1 Small injury, no

treatment necessary.

Small impact /no effect outside boundary 2 Injury with short

absence (< 1 week)

Modest impact / no resting consequences 3 Very serious injury

with long absence (> 1 week)

Local impact

4 1 to 3 victims Large impact / serious environmental damage 5 More than 3 victims Extensive

environmental damage over a large area Interaction with operators and process engineers is essential regarding consequences of damage. For example, a civil engineer might assume that the highest consequences of failed process piping are associated with pipes carrying the most toxic material. In reality, other considerations, such as whether the system will continue to feed material through the line, or whether pressure drop will shut down production of the material, may be more significant factors in prioritization of the hazards.

Finally the qualitative risk assessment is made by merging the probability and consequence for each structure or containment system. An example of the risk matrix is given

in table 3. The structures with the highest risk are given the highest priority for the quantitative analysis in phase 2.

Table 3. Example of qualitative risk matrix

sever ity

probability

A B C D E 0 low low low low low 1 low low low medium mediu

m 2 low low medium medium mediu

m 3 low medium medium high high 4 medium medium high high high 5 medium medium high high high

Other buildings with high consequence level In case when QRA-like risk assessments are not usual or available, for instance for critical public buildings with high consequence levels like hospitals or schools, a similar approach for analyzing earthquake resistance can be followed (Tier 1), starting with a prioritization of the most critical parts. For schools, a first quick scan can be made with a rapid visual screening procedure (RSP), followed by a qualitative consideration with an evaluation checklist for seismic evaluation of existing buildings, according to the ASCE Guidelines or FEMA Guidelines 154 [7] and P-424 [8]. For hospitals, qualitative risk assessment can be performed based on FEMA Guideline 577 [9].

2.3 Phase 2: Quantitative Analysis

If it’s not possible to practise simple measurements to the selected containment systems like shutting down or turning off particular parts of them, a quantitative analysis is performed in the second phase.

Both geotechnical and structural analysis will be performed, and, depending on the situation or structure, also tubo-mechanical (pipelines on industrial plants) or architectural (building objects on schools or hospitals) will be considered.

The second investigation phase consists of the following components:

x

additional inventory of the selected structure and validation of available information about the current state;

x

soil profile characterization by means of available soil investigation results and if necessary execution of additional investigation;

x

quantitative examination of earthquake risks by means of structural and geotechnical seismic analyses.

Figure 8. Geological cross section upper layers with

characteristic CPT’s (source: Grontmij - TNO)

Based on above mentioned data, an overall qualitative assessment of the current state of the selected structures or parts can be given. The current situation will also be considered in order to be able to obtain a reference of the structural and geotechnical state without the earthquake situation.

In order to check the requirements with respect to the ultimate level state (ULS), the selected objects will be subjected to a quantitative assessment, based on the design-PGA values from the executed PSHA.

There are several approaches that can be used for earthquake analysis according to geotechnical and structural effects. The geotechnical analysis contains the schematization of the subsoil including seismic properties, the design ground motion for the site response analysis, the check of earthquake effects to foundations and the considering of deformations and liquefaction effects. The structural analysis contains the schematization including ductility and dynamic properties of the structure and the site response analysis for the earthquake effects with respect to strength capacity of the structural parts, overall stability and connections between structures.

Seismic loading

In order to quantify earthquake effects, there are different levels of modelling the seismic loading. Distinguish can be made between using a design response spectrum or using a time history analysis. The first one describes the response of the structure of various time periods (or frequencies) which for the Groningen area is specified based on the design-PGA value. The response spectrum exists of two parts, the first part considers the short periods from with spectral acceleration Ss

equals PGA at T = 0 s to a maximum of 3.0 times S, between T = 0.1 à 0.0251 s to 0.25 s and the second part considers a decrease of spectra acceleration for larger periods (see figure 9).

A more advanced approach based on time history analysis can be performed by non-linear dynamic analyses with advanced calculation models like PLAXIS or Axis VM. As input at least three representative acceleration signals are required. For the industrial plants in the Delfzijl-Eemsmond area, four recorded representative signals are defined in the guidance for earthquake analyses [2], which have to be modified to the local soil profile and scaled to design values using total stress response models like SHAKE or EERA.

Geotechnical Analysis

In the geotechnical consideration, two main effects are considered: failure of the subsoil and failure of the foundation. Because of the characteristic non-homogeneous soil profile in Groningen which exists of varying clay, peat and sand layers, and the high groundwater level, safety against liquefaction has to be considered. Liquefaction may cause soil strength reduction or bearing failure or even uplift of existing structures. Earthquakes can increase the pore pressure which in turn reduce the effective soil stresses and bearing capacity. Soil liquefaction only occurs in saturated loose sandy or silty soils. The safety factor against liquefaction can be calculated by Idriss &

1

in horizontal respectively vertical response spectra

Boulanger [10], whose model is modified for the Groningen situation according to [2].

Figure 9. Example of design response spectra (green and

blue: horizontal resp. vertical spectra according to PSHA results for CC2-excisting structures Delfzijl area [2] and red: horizontal spectrum according to Eurocode 8)

Figure 10. Example of sensitivity analysis with

calculation safety against liquefaction related to PGA for three CPT’s

Figure 11. Example of sensitivity analysis with

calculation compaction related to PGA for three CPT’s Another subsoil effect is the vertical settlement due to compaction of the loose sand- and silt layers. This check is necessary

for shallow foundations or connections of structures with different kind of foundations (piled foundations vs. shallow foundations) in soils subjected to significant excess pore pressures (factor of safety against liquefaction < 2) and can be calculated with Yoshimine et al. [2, 10].

Besides a reduction of vertical bearing capacity due to reduction of effective stresses (and increase of negative friction), piled foundations have to be checked on horizontal bearing capacity when subjected to site response. Piles have to be checked on strength and moment capacity. This can be done analytical by using the lateral force method for simple cases with an uniform geometry. In case of thick liquefied sand layers, also local buckling resistance has to be checked.

Figure 12. Example of calculated moments and shear

forces in pile foundations Structural analysis

Initially, the more simple approach is considered using the design response spectra for both horizontal and vertical effects. According to Eurocode 8, for more or less uniform structures a lateral force approach can be used as mentioned above. In case of more complex geometry, advanced methods are required, like modal response spectrum analysis or time dependant FEM. In a modal response spectrum analysis, first the normative natural frequencies in the structure with associated mode shapes are calculated for every node, followed by a definition of the related acceleration based on the given response spectrum. Then, the forces are transformed in each node based on the calculated mass and modal shape. Finally a

quasi-static analysis is made of the total structure which can be subjected to a stress check in the ultimate limit state.

Figure 13. Example of structure modelled with a modal

response analysis with Axis-VM

Figure 14. Example of structure modelled in a time

dependant dynamic simulation with PLAXIS 2.4 Phase 3: Recommendations and measurements

For the PSHA, it is recommended to define the PGA-contour map on PGA-values at lower base level (for instance on 30 m depth with Vs = 300 to 400 m/s instead of the current surface values), which the geotechnical engineer can use as input in his total stress or effective stress soil response analysis based on the local soil and site conditions. Also the maximum magnitude and its relation to PGA values need to be reviewed and sharpened, using actual monitoring results and international monitoring data at lower magnitudes.

In general, the analytical approach forms an upper limit of seismic behaviour in de subsoil, the foundation and the superstructure. In case the analytical approach of the structure leads to exceeding the required safety (and unacceptable risks), an optimization can be obtained by using advanced calculation models based on time dependant dynamic simulation with finite element models. In some cases,

analytical approaches are not reliable and may lead to unrealistic results. The use of advanced models will be advised strongly then. Also interaction between soil and structure can be modelled with advanced models. It is not possible to determine in advance whether interaction can be considered as favourable or unfavourable. Although, for the majority of common structures, effects of interaction tend to be beneficial, since the overall damping of a flexibly-supported structure will reduce bending moments and shear forces in the various members of the structure. Finally, advanced models are probably the only tools to be used in design of mitigated measurements, or could function as a prediction/postdiction model as a tool in managing monitoring systems. This will be considered in the third phase of the earthquake analysis.

3. Conditions and Restrictions

The results of the earthquake analyses will be discussed with the owners and managers of the investigated structures. The assumed risk level of the analyzed structures is based on the prescribed safety levels in terms of importance factors according to the guideline [2] while the values are still preliminary. While writing this paper, the National Guidance for Assessment of buildings in case of erection, reconstruction and disapproval, basic rules for seismic actions; induced earthquakes (NPR) is developing. This document might prescribe other design values leading to deviant results.

4. Conclusions

In order to investigate the earthquake resistance of critical structures as (industrial) plants, schools and hospitals, subjected to induced seismic effects in the Northern Netherlands, an efficient approach is essential. Because of the large amount of different existing structural parts, objects and containment systems, a selection of the most critical structures is necessary to start the analysis. A phased approach is developed starting with a qualitative risk assessment to

prioritize the structures and objects for further geotechnical and structural analyses and to design measurements. Close cooperation between the engineer, the client and the editors of the guideline for earthquake analysis in the Delfzijl and Eemsmond area is recommended during the analysis and validation of the assumptions and results.

Acknowledgements

The authors would like to acknowledge the Grontmij earthquake team for sharing the multidiscipline knowledge. We would also like to thank prof. Ilki and asst.prof. Cagayan of Istanbul Technical University (ITU) during our visit in Turkey. Also thanks to ing. B. Berger, drs. J. de Rooij, ir. C. Akogul Msc. and ir. O. Walta for their comments.

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