Tiêu chuẩn iso 19901 2 2004

3.20 seismic reserve capacity factor ratio of spectral acceleration which causes structural collapse or catastrophic system failure to the ELE spectral acceleration 3.21 site response

Symbols

a R slope of the seismic hazard curve

C a site coefficient, a correction factor applied to the acceleration part of a response spectrum

C c correction factor applied to the spectral acceleration to account for uncertainties not captured in a seismic hazard curve

Copyright International Organization for Standardization

C r seismic reserve capacity factor, see Equation (7)

The C v site coefficient is a correction factor used to adjust the velocity component of a response spectrum The term c u refers to the undrained shear strength of the soil, specifically the average undrained shear strength within the top 30 meters of the seabed.

G max low amplitude shear modulus of the soil g acceleration due to gravity (9,81 m/s 2 )

M magnitude of a given seismic source

N ALE scale factor for conversion of the site 1 000 year acceleration spectrum to the site ALE acceleration spectrum p a atmospheric pressure

P ALE annual probability of exceedance for the ALE event

P ELE annual probability of exceedance for the ELE event

The target annual probability of failure (P_f) is influenced by the cone penetration resistance of sand (q_c) and the normalized cone penetration resistance (q_cl) Additionally, the average normalized cone penetration resistance of the top 30 meters of the seabed (q_cl) plays a crucial role in assessing the stability and safety of marine structures.

S a (T) spectral acceleration associated with a single degree of freedom oscillator period T

S a T mean spectral acceleration associated with a single degree of freedom oscillator period T; obtained from a PSHA

S a,ALE (T) ALE spectral acceleration associated with a single degree of freedom oscillator period T

S T mean ALE spectral acceleration associated with a single degree of freedom oscillator period T; obtained from a PSHA

S a,ELE (T) ELE spectral acceleration associated with a single degree of freedom oscillator period T

S T mean ELE spectral acceleration associated with a single degree of freedom oscillator period T; obtained from a PSHA

S a,map (T) 1 000 year rock outcrop spectral acceleration obtained from maps associated with a single degree of freedom oscillator period T

NOTE The maps included in Annex B are for oscillator periods of 0,2 s and 1,0 s

S T mean spectral acceleration associated with a probability of exceedance P e and a single degree of freedom oscillator period T; obtained from a PSHA

S T mean spectral acceleration associated with a target annual probability of failure P f and a single degree of freedom oscillator period T; obtained from a PSHA

S a,site (T) site spectral acceleration corresponding to a return period of 1 000 years and a single degree of freedom oscillator period T

T natural period of a simple, single degree of freedom oscillator

T dom dominant modal period of the structure

The return period (T) is crucial in time history analysis, while the median code utilization (\$u^\hat{}\$) reflects the efficiency of structural design Shear wave velocity (\$v_s\$) and the average shear wave velocity of the top 30 meters of the seabed are essential for understanding soil behavior Additionally, mass density of soil (\$ρ\$) and the percentage of critical damping (\$η\$) play significant roles in dynamic analysis The logarithmic standard deviation of uncertainties not captured in a seismic hazard curve (\$σ_{LR}\$) and the vertical effective stress of soil (\$σ'_{v0}\$) are also critical parameters in assessing seismic risk.

Abbreviated terms

L1, L2, L3 exposure level derived in accordance with the International Standard applicable to the type of offshore structure 2) MOU mobile offshore unit

PSHA probabilistic seismic hazard analysis

In the structural design of offshore structures located in seismically active regions, it is essential to account for actions and effects resulting from seismic events Seismically active areas are identified based on historical earthquake activity, including both frequency and magnitude While Annex B offers maps that indicate seismic accelerations, a detailed investigation is necessary to determine seismicity for many locations, taking into consideration indicative accelerations and exposure levels, as outlined in section 6.5.

2) International Standards applicable to types of offshore structure, include ISO 19902 and ISO 19903, and when available, ISO 19904 (all parts), ISO 19905 (all parts) and ISO 19906 See the Bibliography

When designing structures in seismically active regions, it is essential to investigate ground motion characteristics and acceptable seismic risks Structures must be engineered to withstand earthquake-induced ground motions, while also considering additional seismic hazards that may arise These hazards should be evaluated through specialized studies to ensure comprehensive safety and resilience.

Effects of seismic events on subsea equipment, pipelines and in-field flowlines shall be addressed by special studies

6 Seismic design principles and methodology

Design principles

Clause 6 addresses the design of structures against base excitations, i.e accelerations, velocities and displacements caused by ground motions

Structures located in seismically active areas shall be designed for the ultimate limit state (ULS), abnormal environmental events and the accidental limit state (ALS) using different levels of earthquake

The Ultimate Limit State (ULS) requirements ensure that structures are designed with sufficient strength and stiffness to prevent significant damage during rare earthquake events Specifically, the seismic ULS design focuses on the extreme level earthquake (ELE), aiming for minimal or no damage to the structure While production operations may be halted during such an event, it is essential that the structure undergoes inspection afterward to assess its integrity.

The ALS requirements ensure that a structure's foundation and framework possess adequate strength, displacement, and energy dissipation capacity to endure significant inelastic displacement reversals without total integrity loss, even though some structural damage may occur The seismic ALS design event is characterized by the abnormal level earthquake (ALE), which is a highly intense earthquake with a very low probability of occurrence during the structure's intended lifespan While the ALE can inflict substantial damage, the design must prioritize maintaining overall structural integrity to prevent collapse, thereby safeguarding lives and minimizing environmental harm.

The return periods for both the Expected Loss Event (ELE) and the Annual Loss Event (ALE) are influenced by the level of exposure and the anticipated intensity of seismic occurrences Additionally, the target annual failure probabilities outlined in section 6.4 can be adjusted to align with the objectives established by property owners in collaboration with regulatory bodies, or to comply with existing regional standards.

Seismic design procedures

Two seismic design procedures are available: a simplified method for structures where seismic factors are minimal, and a detailed method for those significantly affected by seismic considerations The choice of procedure is based on the structure's exposure level and the anticipated intensity of seismic events The simplified procedure, outlined in Clause 7, permits the use of generic seismic maps found in Annex B.

The detailed procedure outlined in Clause 8 mandates a site-specific seismic hazard study, while the simplified procedure can be utilized for appraisal and concept screening of new offshore developments Figure 1 illustrates the flowchart depicting the selection process and the associated steps for both procedures.

During the ELE event, structural members and foundation components can experience localized non-linear behavior, such as yielding in steel and tensile cracking in concrete Consequently, ELE design procedures mainly rely on linear elastic structural analysis methods, with non-linear soil-structure interaction effects being linearized However, when utilizing seismic isolation or passive energy dissipation devices, it is essential to implement non-linear time history procedures.

For structures experiencing base excitations due to seismic events, the design check for earthquake load effects (ELE) can be conducted using one of two permitted analysis methods: response spectrum analysis or time history analysis.

In both methods, base excitations consist of two orthogonal horizontal motions and one vertical motion, with appropriate damping levels aligned with the Equivalent Linear Elastic (ELE) deformation criteria When applicable, the relevant International Standard for offshore structures should be referenced Any increased damping resulting from hydrodynamics or soil deformation must be supported by specialized studies The foundation can be modeled using equivalent elastic springs, and may also include mass and damping elements, considering the significance of off-diagonal and frequency dependence It is essential that the foundation's stiffness and damping values are consistent with the ELE level of soil deformations.

In response spectrum analysis, it is essential to account for the correlation between vibration modes when combining responses in three orthogonal directions Responses from each earthquake directional component can be calculated separately and then combined using the root of the sum of the squares method Alternatively, a linear combination can be employed, assuming one component reaches its maximum while the other two are at 40% of their respective maxima In this approach, the sign of each response parameter should be chosen to maximize the overall response combination.

When employing the time history analysis method, a minimum of four sets of time history records is essential to effectively capture the randomness of seismic motions These earthquake records must be chosen to represent the predominant Extreme Limit Events (ELE) Code checks for components are performed at each time step, with the maximum code utilization from each record used to evaluate component performance The ELE design is deemed satisfactory if the maximum code utilizations are below 1.0 for at least half of the records Additionally, if fewer than seven sets of records are utilized, a scale factor of 1.05 should be applied to the records.

Deck equipment must be engineered to endure motions that reflect the transmission of ground movements through the structure, as deck motions can significantly exceed those at the sea floor For accurate assessment of deck motions, particularly relative motions and response spectra, the time history analysis method is highly recommended.

The effects of ELE-induced motions on pipelines, conductors, risers and other safety-critical components shall be considered

9 a SRC 3 structures may be designed using either a simplified or detailed seismic action procedure, see Table 4

Designing structures to resist ALE events without significant non-linear behavior is often uneconomical Consequently, ALE design checks permit non-linear analysis methods, allowing structural elements to exhibit plastic behavior, foundation piles to reach axial capacity, and skirt foundations to slide Ultimately, the design relies on a combination of static reserve strength, ductility, and energy dissipation to effectively counter ALE actions.

In an ALE analysis, structural and foundation models must account for potential stiffness and strength degradation of components due to cyclic action reversals The analysis should utilize the most accurate estimates of modeling parameters, including material strength, soil strength, and soil stiffness This approach may necessitate a reevaluation of the conservatism usually inherent in the ELE design process.

For structures experiencing base excitations due to seismic events, the ALE design check can be conducted using one of two permitted analysis methods: a) the static pushover or extreme displacement method, or b) the non-linear time history analysis method.

The two methods often work together effectively, as the criteria outlined in section 6.2.2 regarding the composition of base excitations from three orthogonal motion components and damping are also relevant to the ALE design process.

The static pushover analysis method is effective for identifying potential global failure mechanisms and assessing the overall displacement of a structure beyond the elastic limit This can be achieved through displacement-controlled structural analysis For the most precise assessment of accidental limit events (ALE), non-linear time history analysis is recommended, requiring a minimum of four analyses to account for the variability of seismic events Earthquake time history records must be chosen to reflect the most significant ALE occurrences When utilizing seven or more records, global structural survival should be confirmed in at least half of the analyses; if fewer than seven records are employed, survival must be demonstrated in a minimum of four analyses.

Extreme displacement methods are utilized to evaluate the survival of compliant systems, such as tethers on tension leg platforms (TLP) and the portal action of TLP foundations under lateral forces These methods assess the system at maximum allowable displacement (ALE), taking into account the resulting action effects on the structure The TLP hull structure is designed elastically to withstand these actions, while the impact of significant displacements on pipelines, conductors, risers, and other critical safety components must be considered independently.

Spectral acceleration data

Annex B provides generic seismic maps of spectral accelerations for offshore regions worldwide, which should be utilized alongside the simplified seismic action procedure outlined in Clause 7 Each area features two distinct maps presented in Annex B.

 the other for a 1,0 s oscillator period

The acceleration values, measured in g, represent 5% damped spectral accelerations on bedrock outcrop classified as site class A/B in section 7.1 These values, designated as \$S_{a,map}(0.2)\$ and \$S_{a,map}(1.0)\$, have an average return period of 1,000 years.

Results from a site-specific seismic hazard assessment may be used in lieu of the maps in a simplified seismic action procedure

Seismic risk category

The evaluation of seismic action complexity and the design procedure is influenced by the structure's seismic risk category (SRC) This category is determined by analyzing acceleration levels from Annex B, which define seismic zones essential for selecting the appropriate design procedure The choice of procedure is based on the structure's exposure level and the intensity of ground motion To establish the SRC, first identify the site seismic zone by consulting the worldwide seismic maps in Annex B to find the 1.0 s horizontal spectral acceleration, $ S_{a,\text{map}}(1.0) $, and then refer to Table 1 for the corresponding seismic zone.

In seismic zones 0 to 4, it is essential to assess the structure's exposure level by consulting the relevant International Standards applicable to offshore structures The target annual probabilities of failure for each exposure level are outlined in Table 2, which are crucial for the detailed procedure to determine seismic actions Alternative target probabilities may be utilized in the seismic action procedure if endorsed by local regulatory authorities The simplified seismic action procedure has been calibrated to align with the target probabilities specified in Table 2.

Table 2 — Target annual probability of failure, P f

L1 4 × 10 −4 = 1/2 500 L2 1 × 10 −3 = 1/1 000 L3 2,5 × 10 −3 = 1/400 c) Determine the structure's seismic risk category, SRC, based on the exposure level and the site seismic zone the SRC is determined from Table 3

Table 3 — Seismic risk category, SRC

Exposure level Site seismic zone

If the lateral seismic design action is less than 5% of the total vertical action, which includes the sum of permanent and variable actions minus buoyancy actions, then SRC 4 and SRC 3 structures can be reclassified as SRC 2.

Seismic design requirements

Table 4 gives the seismic design requirements for each SRC; these requirements are also shown in Figure 1

In seismically active regions, it is essential for designers to create robust and ductile structures that can endure extreme displacements beyond standard design values Architectural and detailing guidelines for ductile design should be adhered to whenever applicable, with the exception of SRC 1 Additionally, it is advisable to refer to the relevant International Standards for the specific type of offshore structure.

For floating structures, consideration of riser stroke, tether rotation angle, and similar geometric allowances shall be sufficient to address the ALE requirements

Table 4 — Seismic design requirements SRC Seismic action procedure Evaluation of seismic activity Non-linear ALE analysis

2 Simplified ISO maps or regional maps Permitted

Simplified Site-specific, ISO maps or regional maps Recommended

For an SRC 3 structure, a simplified seismic action procedure is generally more conservative than a detailed seismic action procedure It is recommended to use results from a site-specific probabilistic seismic hazard analysis (PSHA) for evaluating seismic activity If PSHA results are not available, regional or ISO seismic maps can be utilized While a detailed seismic action procedure mandates PSHA results, a simplified seismic action procedure can be applied with either PSHA results or seismic maps.

Soil classification and spectral shape

Having obtained the bedrock spectral accelerations at oscillator periods of 0,2 s and 1,0 s, S a,map (0,2) and

S a,map (1,0), from Annex B, the following steps shall be followed to define the site response spectrum corresponding to a return period of 1 000 years: a) Determine the site class as follows

The site class depends on the seabed soils on which a structure is founded and is a function of the average properties of the top 30 m of the effective seabed (see Table 5)

The average shear wave velocity in the top 30 m of effective seabed (v s ) shall be determined from

= ∑ (1) where n is the number of distinct soil layers in the top 30 m of effective seabed;

13 d i is the thickness of layer i; v s,i is the shear wave velocity of layer i

Similarly, the average of normalized cone penetration resistance (q cl ) or soil undrained shear strength (c u ) shall be determined according to Equation (1) where v s is replaced by q cl or c u

Table 5 — Determination of site class

Average properties in top 30 m of effective seabed

Site class Soil profile name Soil shear wave velocity v s

Sand: normalized cone penetration resistance q cl a

Clay: soil undrained shear strength c u m/s kPa

A/B Hard rock/rock, thickness of soft sediments < 5 m v s > 750 Not applicable Not applicable

C Very dense hard soil and soft rock 350 < v s u 750 q cl W 200 c u W 200

D Stiff to very stiff soil 180 < v s u 350 80 u q cl < 200 80 u c u < 200

E Soft to firm soil 120 < v s u 180 q cl < 80 c u < 80

Profiles classified from A to E may contain soils with specific characteristics that pose risks, such as liquefiable soils, highly sensitive clays, and collapsible weakly cemented soils Additionally, soils with ooze exceeding 10 m in thickness, high gas content, or excess pore pressure greater than 30% of the in situ effective overburden are of concern Layers thicker than 2 m exhibiting sharp contrasts in shear wave velocity (greater than ± 30%) or undrained shear strength (greater than ± 50%) compared to adjacent layers also require attention The equation for the corrected cone penetration resistance is given by $ a q_{cl} = \left( \frac{q_c}{p_a} \right) \times \left( \frac{p_a}{\sigma'_{v0}} \right)^{0.5} $, where $ q_c $ is the cone penetration resistance, $ p_a $ is atmospheric pressure (100 kPa), and $ \sigma'_{v0} $ is the vertical effective stress Furthermore, clay containing more than 30% calcareous or siliceous material of biogenic origin should be evaluated, along with the determination of $ C_a $ and $ C_v $.

To assess shallow foundations, it is essential to calculate the site coefficients, $C_a$ and $C_v$, using the values provided in Table 6 and Table 7 These coefficients are influenced by the site class and the mapped spectral accelerations, $S_{a,\text{map}}(0.2)$ and $S_{a,\text{map}}(1.0)$.

2) For deep pile foundations, the site coefficients C a and C v are listed in Table 8

Table 6 — Values of C a for shallow foundations and 0,2 s period spectral acceleration

F a a a a a a A site-specific geotechnical investigation and dynamic site response analyses shall be performed

Table 7 — Values of C v for shallow foundations and 1,0 s period spectral acceleration

F a a a a a a A site-specific geotechnical investigation and dynamic site response analyses shall be performed

Table 8 — Values of C a and C v for deep pile foundations

F a a a A site-specific geotechnical investigation and dynamic site response analyses shall be performed c) Determine the site 1 000 year horizontal acceleration spectrum as follows

1) A seismic acceleration spectrum shall be prepared for different oscillator periods (T), as shown in

2) For periods, T, less than or equal to 0,2 s, the site spectral acceleration, S a,site (T), shall be taken as:

3) For periods greater than 0,2 s, the site spectral acceleration, S a,site (T), shall be taken as: a,site( ) v a,map(1,0)

4) For periods greater than 4,0 s, the site spectral acceleration may be taken as decaying in proportion to 1/T 2 instead of 1/T as given by Equation (4): a,site( ) 4 v a,map(1,0) 2

T natural period of a simple, single degree of freedom oscillator

S a,site (T) site spectral acceleration corresponding to a return period of 1 000 years and a single degree of freedom oscillator period T

S a,map (0,2) 1 000 year rock outcrop spectral acceleration obtained from maps in Annex B associated with a single degree of freedom oscillator period 0,2 s

S a,map (1,0) 1 000 year rock outcrop spectral acceleration obtained from maps in Annex B associated with a single degree of freedom oscillator period 1,0 s

The vertical spectral acceleration at a period T is defined as half of the corresponding horizontal spectral acceleration, without further reduction for water depth effects The acceleration spectra derived from these guidelines are based on 5% damping To adjust for different damping values, the ordinates can be scaled using a correction factor D, calculated as \$D = \frac{\ln(100)}{\ln(20)}\$.

= (5) where η is the percent of critical damping

Uniform hazard spectra derived from probabilistic seismic hazard analysis (PSHA) can be refined through a comprehensive dynamic site-response analysis to produce site-specific design response spectra for a 1,000-year period.

Seismic action procedure

The design seismic acceleration spectra to be applied to the structure shall be determined as follows

For each oscillator period T, the ALE horizontal and vertical spectral accelerations are obtained from the corresponding values of the site 1 000 year spectral acceleration [see 7.1 c) and 7.1 d)]:

S T =N ×S T (6) where the scale factor N ALE is dependent on the structure exposure level and shall be obtained from Table 9

The ELE horizontal and vertical spectral accelerations at oscillator period T may be obtained from:

The seismic reserve capacity factor, denoted as \$C_r\$, is crucial for structural systems, reflecting their static reserve strength and ability to endure significant non-linear deformations, such as those found in steel versus reinforced concrete structures This factor represents the ratio of spectral acceleration that leads to catastrophic failure to the earthquake level event (ELE) spectral acceleration To ensure an economical design that minimizes damage from an ELE while meeting the acceptable loss event (ALE) performance criteria, \$C_r\$ should be estimated before the design phase Justifications for \$C_r\$ values can be derived from detailed assessments of similar structures, with specific values for fixed steel structures outlined in ISO 19902 Alternative \$C_r\$ values, if available and relevant to the offshore structure type, may be utilized in design but must be validated through an ALE analysis.

To avoid return periods for the ELE that are too short, C r values shall not exceed 2,8 for L1 structures; 2,4 for L2 structures; and 2,0 for L3 structures

Table 9 — Scale factors for ALE spectra

Exposure level ALE scale factor

Site-specific seismic hazard assessment

The design acceleration spectrum is the primary seismic input parameter for the seismic design and analysis of offshore structures Typically, this spectrum is derived from a probabilistic seismic hazard analysis (PSHA) and may be adjusted based on local soil conditions Additionally, deterministic seismic hazard analysis can be utilized to enhance the results obtained from PSHA.

Probabilistic seismic hazard analysis

A Probabilistic Seismic Hazard Assessment (PSHA) estimates ground motions at a site by evaluating the probability of earthquakes of varying magnitudes from all potential sources, such as faults or areas It incorporates the randomness in the attenuation of seismic waves as they travel to the site By summing the individual probabilities from different sources, the total annual probability of exceedance for a specific level of peak ground acceleration (PGA) or spectral acceleration is determined This relationship is depicted in a "hazard curve," which illustrates the probability of exceedance against ground motion or the response of a single degree of freedom oscillator Since spectral response changes with the natural period of the oscillator, a series of hazard curves for different periods is generated.

The results of a Probabilistic Seismic Hazard Assessment (PSHA) are utilized to create a uniform hazard spectrum, where each point on the spectrum represents an equivalent annual probability of exceedance The connection between the return period of this uniform hazard spectrum and the target annual probability of exceedance (P e ) can be expressed as follows:

T return = 1/P e (8) where T return is the return period in years

A Probabilistic Seismic Hazard Assessment (PSHA) relies on a probability-based methodology, making it crucial to account for uncertainties in key input parameters These parameters include the maximum magnitude associated with a specific source, the relationship of magnitude recurrence, the attenuation equation, and the geographical boundaries that delineate the source zone's location.

The results of a Probabilistic Seismic Hazard Assessment (PSHA) consist of multiple hazard curves, each representing spectral acceleration for different structural natural periods, such as T₁, T₂, …, Tₙ Due to uncertainties in the input parameters of the PSHA, each hazard curve is accompanied by an uncertainty band To create a uniform hazard spectrum for a specified exceedance probability $ P_e $, the mean value of each hazard curve should be utilized It is important to note that all references to hazard curves in section 8.4 pertain to the mean of these curves.

Deterministic seismic hazard analysis

Deterministic estimates of ground motion extremes at a specific site are derived by analyzing a single seismic event characterized by a defined magnitude and distance from the location To conduct a thorough deterministic analysis, essential information must be gathered.

 definition of an earthquake source (e.g a known fault) and its location relative to the site;

 definition of a design earthquake magnitude that the source is capable of producing;

 a relationship which describes the attenuation of ground motion with distance

A site may be located near multiple active faults, each with a defined maximum magnitude This maximum magnitude is determined by the length of the fault and historical data regarding previous earthquakes associated with that specific fault.

Deterministic ground motion estimates do not correspond to a specific return period, like 1,000 years, although the earthquake event utilized may have an associated return period The return period for the largest event on a particular fault can range from several hundred to several thousand years, influenced by the fault's activity rate.

A deterministic seismic hazard analysis may be performed to complement the PSHA results

This article outlines the process of defining earthquake source seismicity and geometry, followed by the establishment of attenuation curves for spectral accelerations across specified periods $ T_1 $ to $ T_N $ It emphasizes the development of seismic hazard curves for spectral accelerations at each period, aligned with a chosen target annual probability of exceedance, leading to the calculation of mean uniform hazard spectral accelerations $ S_{T_a1} $ to $ S_{T_aN} $ Finally, it details the construction of a uniform hazard spectrum representing mean spectral accelerations at the selected target annual probability of exceedance.

2 area source T i single degree of freedom oscillator periods

3 cumulative annual frequency of magnitude M S a (T i ) spectral acceleration associated with a single degree of freedom oscillator period T i

4 attenuation uncertainty d distance from source

M magnitude P annual probability of exceedance

P e target level of annual probability of exceedance S a, e P ( )T i mean spectral acceleration for oscillator period T i at selected target annual probability of exceedance

Figure 3 — Probabilistic seismic hazard analysis procedure

Seismic action procedure

This procedure relies on the findings of a Probabilistic Seismic Hazard Assessment (PSHA) as detailed in section 8.2 and illustrated in Figure 3 The site-specific seismic hazard curve must reflect the annual exceedance probability of spectral acceleration for the dominant modal period of the structure, denoted as $ S_{T_a}^{(dom)} $, with examples shown in Figure 3(c) If specific information about the dominant modal period is unavailable, the seismic hazard curve can alternatively be calculated for a spectral acceleration period of 1.0 seconds.

The ALE spectral accelerations are derived from the site-specific hazard curve and the target annual probability of failure, $ P_f $, as detailed in Table 2 The process for defining ALE and ELE events is outlined in Figure 4, which includes the following steps: first, plot the site-specific hazard curve for $ T = T_{dom} $ on a log-log scale, illustrating the probability distribution of the parameter $ S_{T_a}(dom) $ as shown in Figure 4a Next, select the target annual probability of failure, $ P_f $, based on the exposure level indicated in the analysis.

To determine the site-specific spectral acceleration at $ P_f $, $ S_{a,f} $, refer to Table 2 and Figure 4 a) Next, calculate the slope of the seismic hazard curve, $ a_R $, near $ P_f $ by drawing a tangent line at that point The slope $ a_R $ is defined as the ratio of spectral accelerations at two probability values, $ P_1 $ and $ P_2 $, that are one order of magnitude apart, with $ P_1 $ ideally close to $ P_f $ Finally, consult Table 10 to find the correction factor $ C_c $ associated with $ a_R $, which accounts for uncertainties not represented in the seismic hazard curve.

Correction factor, C c 1,20 1,15 1,12 1,10 1,10 e) Determine the ALE spectral acceleration by applying the correction factor C c to S a, f P (T dom ), the site- specific spectral acceleration at the required P f and the structural dominant period T dom :

The annual probability of exceedance for the ALE event (\$P_{ALE}\$) is directly obtained from the seismic hazard curve, as illustrated in Figure 4 b) The ALE return period is calculated using Equation (8), with \$P_{ALE}\$ being less than \$P_f\$ to account for uncertainties in action and resistance evaluations not reflected in the seismic hazard curve, represented by the correction factor \$C_c\$ For specific structure types with known reserve strength and ductility characteristics, the ELE spectral acceleration is subsequently determined.

The seismic reserve capacity factor, denoted as \$C_r\$, is crucial for structural systems, reflecting their static reserve strength and ability to endure significant non-linear deformations, such as those found in steel versus reinforced concrete structures This factor is defined as the ratio of the spectral acceleration that leads to catastrophic failure to the earthquake level event (ELE) spectral acceleration To ensure an economical design that minimizes damage from ELE while meeting acceptable loss event (ALE) performance standards, \$C_r\$ should be estimated before the design phase Values of \$C_r\$ can be supported by thorough assessments of similar structures, with specific values for fixed steel structures outlined in ISO 19902 Alternative \$C_r\$ values may be utilized, provided they are validated through an ALE analysis, as referenced in section A.8.4.

The annual probability of exceedance for the Extreme Life Event (ELE), denoted as $ P_{ELE} $, can be obtained from the seismic hazard curve The return period for ELE is calculated using Equation (8) based on this probability Once the return periods for both the Average Life Event (ALE) and ELE are established, the corresponding spectral accelerations for various natural periods can be derived from the Probabilistic Hazard Seismic Analysis (PHSA) results, specifically $ S_{a,ALE}(T) $ and $ S_{a,ELE}(T) $ Additionally, any modifications to the ALE and ELE acceleration spectra due to local geology and soil conditions should be evaluated through a site response analysis.

For floating structures like Tension Leg Platforms (TLPs), where the drag coefficient (\$C_r\$) is uncertain, it is crucial to adopt a design process focused on preventing catastrophic failures in the Accidental Limit Event (ALE) The primary concern is to address extreme displacements and shock waves to effectively design the mooring system Additionally, the hull structure should be designed elastically to withstand the anticipated forces.

To ensure the economic viability of a design, minimum ELE return periods are specified in Table 11 based on exposure levels If the calculated ELE return period from the procedure in this subclause is less than the corresponding period in Table 11, the return period from Table 11 must be applied for S a,ELE (T).

Table 11 — Minimum ELE return periods Exposure level Minimum ELE return periods

L3 50 L2 100 L1 200 a) Derivation of the slope a R of the seismic hazard curve for T = T dom

Figure 4 — Typical seismic hazard curve

21 b) Derivation of spectral accelerations and probabilities for ALE and ELE events Key

Local site response analyses

In the detailed seismic action procedure (8.4), the ALE and ELE design spectral accelerations S a,ALE ( ) T and

Seismic hazard assessments for ALE and ELE events utilize uniform hazard curves with consistent return periods, as outlined in section 8.4 The analyses in sections 8.2 and 8.3 provide ground motion data suitable for various site conditions, including moderately stiff, stiff, or bedrock sites However, offshore locations often feature a soft soil layer above stiffer materials, necessitating adjustments to the ALE and ELE spectral accelerations to reflect local soil conditions To achieve site-specific spectral accelerations for design purposes, a dynamic site response analysis employing linear or non-linear soil models may be conducted.

The procedure outlined in section 7.1 offers an alternative method for modifying acceleration spectra instead of conducting a dynamic site response analysis By following this procedure, an amplification spectrum is derived from the ratio of the acceleration spectrum for the local site class to that of a stiff soil or rock site class This amplification spectrum can subsequently be applied to adjust the acceleration spectra obtained from a probabilistic seismic hazard assessment (PSHA) for stiff soil or rock conditions.

ELE performance

The primary goals of ELE design are to minimize structural damage during an ELE event and to provide a sufficient safety margin against significant failures in more severe situations The subsequent performance requirements for ELE must be thoroughly verified.

All primary structural and foundation elements must endure minimal to no damage from the Extreme Loading Event (ELE) While some non-linear behavior, such as yielding in steel or tensile cracking in concrete, is acceptable, it is crucial to prevent brittle degradation, including local buckling in steel and spalling in concrete.

 Secondary structural components, such as conductor guide panels, shall follow the same ELE design rigour as that of primary components

 The internal forces in joints shall stay below the joint strengths, using the calculated (elastic) forces and moments

Foundation checks must be conducted at both the component and system levels At the component level, it is essential to ensure sufficient margins against axial and lateral failures of piles, as well as vertical and sliding failures of other foundation elements At the system level, adequate margins should also be maintained to prevent large-deflection mechanisms that could harm or degrade the structure and its ancillary systems, such as pipelines or conductors.

 There shall not be any loss of functionality in safety systems or in escape and evacuation systems due to the ELE

Masts, derricks, and flare structures must be designed to withstand transmitted motions with minimal damage The design should incorporate restraints to prevent the toppling of topsides equipment and cable trays Piping must accommodate differential displacements caused by support movements, and sliding supports should be properly maintained to function as intended Additionally, the design should reduce the risk of equipment and appurtenances becoming falling hazards during the Emergency Life Extension (ELE).

ALE performance

The purpose of an ALE design check is to prevent global failure modes that could result in severe consequences, including loss of life or significant environmental harm It is essential to verify the following ALE performance requirements.

Structural elements may demonstrate plastic degrading behavior, such as local buckling in steel or spalling in concrete; however, it is crucial to prevent catastrophic failures, including global collapse or the failure of cantilevered sections of the deck.

 Stable plastic mechanisms in foundations are allowed, but catastrophic failure modes such as instability and collapse should be avoided

Joints may display limited plastic behavior while remaining within their ultimate strength limits However, if significant deformations in the joints are expected, they must be designed to ensure ductility and maintain residual strength at the anticipated levels of deformation.

 The safety systems and escape and evacuation systems shall remain functional during and after the ALE

 Topsides equipment failures shall not compromise the performance of safety-critical systems Collapse of the living quarters, masts, derricks, flare structures and other significant topsides equipment should be avoided

 Any post-ALE event strength requirements given in the International Standard, when available, applicable to the type of offshore structure 8) apply

This annex offers supplementary information and guidance on the clauses within ISO 19901 To facilitate easy identification, it employs the same numbering system and heading titles as the corresponding subclauses in the main body of ISO 19901.

The background to and the development of the philosophy for this part of ISO 19901 were presented at OMAE 2001 [7]

When planning and designing offshore structures, it is crucial to account for hazards triggered by earthquakes, in addition to seismic motions Proper site selection studies can effectively mitigate most geologically induced hazards associated with earthquakes.

Liquefaction of saturated loose cohesionless soils can occur due to repeated cyclic motions, with the risk decreasing as soil density increases Poorly graded sands are more prone to liquefaction compared to well-graded sands During a strong earthquake, both gravity-based and pile-founded structures in these soils may experience a significant reduction in capacity due to the degradation of soil strength.

Earthquakes can trigger the failure of stable sea floor slopes, leading to sea floor slides Site investigations in potentially unstable areas should prioritize identifying metastable geological features and defining the necessary soil engineering properties for modeling sea floor movements By analyzing soil movement in relation to depth below the sea floor and incorporating coupled soil engineering properties, one can predict the impacts on structural members The most effective way to mitigate this hazard is to position offshore structures away from these regions, although some designs for sea floor slides have been implemented in the Gulf of Mexico.

Seismic activity can lead to fault movement, making it crucial to avoid locating facilities near fault planes that intersect the sea floor If it is necessary to site structures near potentially active features, a geological study should be conducted to estimate the magnitude and time scale of expected movement, which will inform the design of the structure.

Tsunamis are caused by significant earthquakes, undersea fault movements, and large sea floor slides, often triggered by seismic activity In deep water, these waves are long and low, posing minimal risk to structures However, as they approach shallow waters, the wave height increases dramatically, leading to powerful surges that can inundate coastal areas The primary threat to offshore structures in shallow water comes from the intense inflow and outflow of water, which generates strong waves and currents that can exert considerable forces on the structures and lead to severe erosion issues.

Mud volcanoes typically occur along pre-existing faults, utilizing these zones to transport gas, water, and mud to the sea floor, resulting in cone-like surface formations To effectively mitigate the associated hazards, it is advisable to position offshore structures away from these areas.

Earthquake-induced shock waves from sea floor movements can significantly affect floating structures and their components These shock waves can travel upward through the water column, potentially causing impulsive forces on buoyant or partially buoyant structures, leading to increased hull pressures and forces on tendons or mooring lines This phenomenon is primarily a concern during severe earthquakes.

A.6 Seismic design principles and methodology

A two-level design check is essential due to the unpredictable nature of seismic events and the uncertainties involved in calculating seismic actions Relying solely on strength to design for extreme seismic events, without accounting for a structure's ability to dissipate energy and endure significant inelastic displacements, would be economically unfeasible.

Structures designed for the Extreme Limit Event (ELE) incorporate safety margins to withstand severe events, thanks to both explicit and implicit design equations and their ability to undergo significant non-linear deformations To streamline the design process and ensure that the Acceptable Limit Event (ALE) check confirms an adequate design, the ratio of ALE to ELE spectral accelerations is established to maximize the likelihood of achieving both ALE and ELE performance goals The seismic design procedures outlined in ISO 19901 focus on balancing the criteria for ALE and ELE.

The seismic design of offshore structures is conducted during an ELE evaluation, where structural component dimensions are established based on design equations from relevant International Standards The ELE design procedure aims to achieve two key objectives: first, to ensure that the structure can endure severe seismic events with minimal or no damage; and second, to facilitate a design that meets ALE performance criteria with minimal modifications.

The first objective may be seen as an economic goal in that it avoids the need for frequent repairs, while the second objective is a safety goal

Spectral acceleration is often the key parameter in the design of offshore structures, with the ELE design procedure typically defined by seismic design spectra or acceleration time history records.

Tiêu đề	Seismic Design Procedures and Criteria
Trường học	International Organization for Standardization
Chuyên ngành	Petroleum and Natural Gas Industries
Thể loại	tiêu chuẩn
Năm xuất bản	2004
Thành phố	Geneva

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