4 Symbols and Abbreviated Terms 4.1 Symbols aR slope of the seismic hazard curve Ca site coefficient, a correction factor applied to the acceleration part of a response spectrum Cc corre
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
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 It is essential for accurately assessing the undrained shear strength, denoted as c u, of the soil This factor specifically considers the average undrained shear strength of the top 30 meters of the seabed, ensuring precise evaluations in geotechnical engineering.
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 1000 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 of sand (\$q_{cl}\$) This normalized resistance is averaged over the top 30 meters of the seabed, providing critical insights for assessing geotechnical stability.
S a (T) spectral acceleration associated with a single degree of freedom oscillator period 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 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 mean ELE spectral acceleration associated with a single degree of freedom oscillator period T; obtained from a PSHA
The spectral acceleration for a 1000-year rock outcrop is derived from maps corresponding to a single degree of freedom oscillator with a period T Notably, the maps provided in Annex B are specifically for oscillator periods of 0.2 seconds and 1.0 seconds.
The mean spectral acceleration, denoted as \$S_a,ELE( )\$T\$, is linked to a specific probability of exceedance \$P_e\$ and the period \$T\$ of a single degree of freedom oscillator This value is derived from a probabilistic seismic hazard assessment (PSHA) that corresponds to a target annual probability of failure \$P_f\$ for the same oscillator period \$T\$.
S a,site(T) site spectral acceleration corresponding to a return period of 1000 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 is a crucial factor in time history analysis, reflecting the median code utilization in relation to shear wave velocity This average shear wave velocity is measured over the top 30 meters of the seabed, while the mass density of the soil and the percentage of critical damping are also significant Additionally, the logarithmic standard deviation of uncertainties not included in a seismic hazard curve, along with the vertical effective stress of the soil, plays an essential role in understanding seismic risks.
Abbreviated Terms
L1, L2, L3 exposure level derived in accordance with the standard applicable to the type of offshore structure 1 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 thorough investigation is necessary to assess seismicity in many locations, taking into consideration indicative accelerations and exposure levels, as outlined in section 6.5.
In seismically active regions, it is essential to investigate the characteristics of ground motions and assess the acceptable seismic risk for structures Proper consideration of seismic events is crucial for ensuring the safety and resilience of buildings in these areas.
1 Standards applicable to types of offshore structure, include ISO 19902, ISO 19903, API 2A-WSD, API 2N, ISO 19904 (all parts), ISO 19905 (all parts), and ISO 19906 See the Bibliography.
When designing structures for ground motions caused by earthquakes, it is essential to consider 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 in seismically active regions must be engineered to withstand extreme level earthquakes (ELE) by applying the ultimate limit state (ULS) design principles, while also accounting for abnormal level earthquakes through the accidental limit state (ALS) framework.
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 reserve strength, displacement, and energy dissipation capacity to endure significant inelastic displacement reversals without total loss of integrity, even though some structural damage may occur The seismic ALS design event, known as the abnormal level earthquake (ALE), represents 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 activities 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 seismic event characteristics The simplified procedure (Clause 7) utilizes generic seismic maps from Annex B, while the detailed procedure (Clause 8) necessitates a site-specific seismic hazard study Additionally, the simplified method can be employed for appraisal and concept screening of new offshore developments.
Figure 1 presents a flowchart of the selection process and the steps associated with 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 linearized However, when utilizing seismic isolation or passive energy dissipation devices, non-linear time history procedures must be implemented.
For the design check of structures exposed to seismic base excitations, two permissible analysis methods are available: the response spectrum analysis method and the time history analysis method.
In both methods, base excitations consist of two orthogonal horizontal motions and one vertical motion, with appropriate damping levels aligned with the Expected Limit State (ELE) deformation criteria When applicable, the relevant standards for offshore structure type 2 should be referenced Any increased damping resulting from hydrodynamic effects or soil deformation must be supported by specialized studies The foundation can be represented using equivalent elastic springs, and may also include mass and damping elements, considering the significance of off-diagonal and frequency-dependent factors 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 direction can be calculated separately and then combined using the square 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 maximum values This approach requires careful selection of the sign for each response parameter to ensure the maximization of the combined response.
When employing the time history analysis method, it is essential to utilize at least four sets of time history records to effectively capture the randomness of seismic motions These records must be chosen to represent the predominant earthquake loading events (ELE) Code checks for components are performed at each time step, with the maximum code utilization from each record used to evaluate performance The ELE design is deemed satisfactory if the maximum code utilization is 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.
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, the time history analysis method is advised, along with the use of deck motion response spectra.
The effects of ELE-induced motions on pipelines, conductors, risers, and other safety-critical components shall be considered
2 Standards applicable to types of offshore structure, include ISO 19902, ISO 19903, API 2A-WSD, API 2N, ISO 19904 (all parts),ISO 19905 (all parts), and ISO 19906 See the Bibliography.
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 blend 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 best estimate values for modeling parameters, including material strength, soil strength, and soil stiffness, which may necessitate a reevaluation of the conservatism usually found 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: the static pushover or extreme displacement method, or the non-linear time history analysis method.
The two methods often work together effectively, as the guidelines 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 accomplished through a displacement-controlled structural analysis For accurate assessment of accidental limit states, the non-linear time history analysis method is the most reliable approach.
To effectively capture the randomness of seismic events, a minimum of four time history analyses must be conducted The selected earthquake time history records should reflect the predominant ALE events When utilizing seven or more time history records, it is essential to demonstrate global structure survival in at least half of these analyses Conversely, if fewer than seven records are employed, global survival must be shown in a minimum of four time history 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), accounting for 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
Only the maps in Figure B.2 are applicable in this document, in lieu of those previously used in API 2A-WSD, 21st Edition and earlier
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 are associated with an average return period of 1000 years and are referred to as \$S_{a,map}(0.2)\$ or \$S_{a,map}(1.0)\$.
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 and the design procedure's complexity is influenced by the structure's seismic risk category (SRC) The L2 exposure level is not applicable in seismic regions due to the impracticality of evacuating platforms before seismic events Seismic zones are defined by acceleration levels from Annex B, which guide the selection of the appropriate design procedure based on the structure's exposure level and ground motion severity To determine the SRC, follow these steps: first, identify the site seismic zone using the 1.0 s horizontal spectral acceleration value from Annex B and refer to Table 1 Next, ascertain the structure's exposure level by consulting the relevant standard for offshore structures, with target annual probabilities of failure outlined in Table 2 Finally, determine the SRC based on the exposure level and site seismic zone as indicated in Table 3.
If the lateral seismic design action is less than 5% of the total vertical actions, which include the sum of permanent and variable actions minus buoyancy actions, then structures classified as SRC 4 and SRC 3 can be reclassified to SRC 2.
Seismic Design Requirements
Table 4 gives the seismic design requirements for each SRC; these requirements are also shown in Figure 1.
3 Standards applicable to types of offshore structure, include ISO 19902, ISO 19903, API 2A-WSD, API 2N, ISO 19904 (all parts), ISO 19905 (all parts), and ISO 19906 See the Bibliography.
Table 2—Target Annual Probability of Failure, P f
Table 3—Seismic Risk Category, SRC
Site Seismic Zone Exposure Level
In seismically active regions, it is essential for designers to create robust and ductile structures that can endure extreme displacements beyond standard design values Adhering to architectural and detailing guidelines for ductile design is crucial, except for SRC 1 structures Additionally, it is important to refer to the relevant 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.
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 1000 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 ( ) shall be determined from Equation (1):
(1) where n is the number of distinct soil layers in the top 30 m of effective seabed; d i is the thickness of layer i; v s,i is the shear wave velocity of layer i
4 Standards applicable to types of offshore structure, include ISO 19902, ISO 19903, API 2A-WSD, API 2N, ISO 19904 (all parts), ISO 19905 (all parts), and ISO 19906 See the Bibliography.
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
3 a 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 procedure can be applied with either PSHA results or seismic maps.
Similarly, the average of normalized cone penetration resistance ( ) or soil undrained shear strength ( ) shall be determined according to Equation (1) where v s is replaced by q cl or c u
When designing deep pile foundations, it is essential to evaluate the site class by examining the 30 meters of soil directly beneath the pile's seat of resistance, as this depth varies for lateral and vertical loads The seat of resistance is typically located at the centroidal depth of the P-Y resisting forces for lateral actions and the T-Z forces for vertical actions Additionally, it is important to calculate the coefficients C_a and C_v accordingly.
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, \(S_{a,map}(0.2)\) and \(S_{a,map}(1.0)\).
2) For deep pile foundations, the site coefficients C a and C v are listed in Table 8
Table 5—Determination of Site Class
Site Class Soil Profile Name
Average Properties in Top 30 m of Effective Seabed
Sand: Normalized Cone Penetration Resistance a
Clay: Soil Undrained Shear Strength kPa
A/B Hard rock/rock, thickness of soft sediments < 5 m > 750 Not applicable Not applicable
C Very dense hard soil and soft rock 350 < ≤ 750 ≥ 200 ≥ 200
D Stiff to very stiff soil 180 < ≤ 350 80 ≤ < 200 80 ≤ < 200
Any profile, including those otherwise classified as A to E, containing soils having one or more of the following characteristics:
Soils that are susceptible to failure during seismic events include liquefiable soils, highly sensitive clays, and collapsible weakly cemented soils Additionally, oozes with a thickness exceeding 10 meters and soil layers characterized by high gas content or significant ambient excess pore pressure are also at risk.
The effective overburden in situ is 30%, characterized by layers exceeding 2 meters in thickness that exhibit a significant contrast in shear wave velocity (greater than ±30%) and/or undrained shear strength (greater than ±50%) compared to adjacent layers The cone penetration resistance is defined by the equation \( q_{cl} = \left( \frac{q_c}{p_a} \right) \times \left( \frac{p_a}{\sigma'_{v}} \right)^{0.5} \), where \( q_c \) represents the cone penetration resistance, \( p_a \) is the atmospheric pressure at 100 kPa, and \( \sigma'_{v} \) denotes the vertical effective stress Additionally, clay with over 30% calcareous or siliceous material of biogenic origin is noted The site-specific 1000-year horizontal acceleration spectrum should be determined accordingly.
1) A seismic acceleration spectrum shall be prepared for different oscillator periods (T), as shown in Figure 2
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: except that (3)
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):
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.
The site vertical spectral acceleration at a period \( T \) is defined as half of the corresponding horizontal spectral acceleration, without further reduction for water depth effects Additionally, the acceleration spectra derived from the previous steps are based on 5% damping; to adjust for different damping values, a correction factor \( D \) should be applied to scale the ordinates accordingly.
(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, resulting in site-specific design response spectra for a 1000-year period.
Figure 2—Seismic Acceleration Spectrum for 5 % Damping
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 1000 year spectral acceleration [see 7.1 c) and 7.1 d)]:
(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\$, evaluates the structural system's ability to withstand static reserve strength and accommodate significant non-linear deformations, differing between various structure types such as steel and reinforced concrete.
The C r factor indicates the ratio of spectral acceleration that leads to catastrophic structural failure to the ELE spectral acceleration Estimating the C r value before designing the structure is crucial for achieving an economical design that can withstand damage from an ELE while also meeting ALE performance standards Prior detailed assessments of similar structures can justify the C r values For fixed steel structures, specific C r values are provided in Table 10 Alternative C r values may be utilized in design, but they 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 and 2.0 for L3 structures
5 Standards applicable to types of offshore structure, include ISO 19902, ISO 19903, API 2A-WSD, API 2N, ISO 19904 (all parts), ISO 19905 (all parts), and ISO 19906 See the Bibliography.
Table 9—Scale Factors for ALE Spectra
Exposure Level ALE Scale Factor
Table 10— C r Factors for Steel Jacket of Fixed Offshore Platforms
The ductile design recommendations outlined in section 5.3.6.4.3 are implemented, and a non-linear static pushover analysis is conducted in accordance with API RP 2EQ to assess the overall performance of the structure under Accidental Limit Event (ALE) conditions.
Variable up to 2.80, as demonstrated by analysis.
The recommendations for ductile design in 5.3.6.4.3 are followed, but a non-linear static pushover analysis to verify ALE performance is not performed Variable up to 2.00, as demonstrated by analysis.
The structure must have at least three legs and a bracing pattern that includes leg-to-leg diagonals with horizontals or X-braces without horizontals The slenderness ratio (KL/r) for diagonal bracing in vertical frames is capped at 80, and the condition F y D/Et must not exceed 0.069 These same limitations apply to X-bracing in vertical frames, with the length L determined by the loading pattern.
A non-linear analysis to verify the ductility level performance is not performed.
If none of the above characterizations apply 1.10
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 findings from the PSHA For detailed methodologies, refer to sections 8.2 to 8.5.
Probabilistic Seismic Hazard Analysis
A Probabilistic Seismic Hazard Assessment (PSHA) involves estimating ground motions at a site by evaluating the probability of earthquakes of various 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 is influenced by 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 identical 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.
A Probabilistic Seismic Hazard Assessment (PSHA) produces a series of hazard curves for various spectral accelerations linked to a structure's 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 The mean value of these hazard curves is essential for constructing a uniform hazard spectrum that corresponds to a specific exceedance probability, Pₑ It is important to note that all references to hazard curves in section 8.4 pertain to the mean of the hazard curve.
Deterministic Seismic Hazard Analysis
Deterministic estimates of ground motion extremes at a specific site are derived by analyzing a single seismic event characterized by its 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
Figure 3—Probabilistic Seismic Hazard Analysis Procedure
A site may be located near multiple active faults, each associated with a defined maximum magnitude This maximum magnitude is determined by the length of the fault and historical data regarding previous earthquakes from that specific source.
Deterministic ground motion estimates do not correspond to a specific return period, like 1000 years, although the earthquake event utilized may have its own 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.
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 seismic hazard curve specific to the site must be established based on the annual exceedance probability of spectral acceleration, which corresponds to the dominant modal period of the structure, as shown in Figure 3 c).
In lieu of more specific information about the dominant modal period of the structure, the seismic hazard curve may be determined for the spectral acceleration at a period of 1.0 s,
The ALE spectral accelerations are derived from the site-specific hazard curve and the target annual probability of failure, \( P_f \), as outlined in Table 2 The process involves several key steps: first, plot the site-specific hazard curve for \( T = T_{dom} \) on a log-log scale to visualize the probability distribution Next, select the target annual probability of failure, \( P_f \), based on the exposure level from Table 2, and identify the corresponding site-specific spectral acceleration Then, calculate the slope of the seismic hazard curve, \( a_R \), near \( P_f \) by drawing a tangent line at that point, defined as the ratio of spectral accelerations at two probability values one order of magnitude apart Following this, consult Table 11 to find the correction factor, \( C_c \), which accounts for uncertainties not captured by the seismic hazard curve Finally, determine the ALE spectral acceleration by applying the correction factor \( C_c \) to the site-specific spectral acceleration at the specified \( P_f \) 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 from this annual probability using Equation (8) It is important to note that P ALE is less than P f to account for uncertainties in action and resistance evaluations that are not reflected in the seismic hazard curve, which is represented by the correction factor C c.
S a,ALE(T dom) = C c×S a,Pf(T dom) f) For certain structure types whose reserve strength and ductility characteristics are known, the ELE spectral acceleration is next determined from:
The seismic reserve capacity factor, denoted as \$C_r\$, is crucial for assessing the structural system's ability to withstand large non-linear deformations and static reserve strength, particularly when comparing different materials like steel and reinforced concrete This factor represents the ratio of the spectral acceleration that leads to catastrophic failure to the ELE spectral acceleration, and it should be estimated before design to ensure an economical structure that can resist damage from the ELE while meeting ALE performance requirements Values of \$C_r\$ can be supported by previous assessments of similar structures, with specific values for fixed steel structures provided in Table 10 Alternative \$C_r\$ values may be utilized, but they must be validated through an ALE analysis The annual probability of exceedance for the ELE event, \$P_{ELE}\$, can be derived from the seismic hazard curve, and the ELE return period is calculated using the annual probability of exceedance After determining the ALE and ELE return periods, spectral accelerations for both can be obtained from PHSA results Additionally, modifications to the ALE and ELE acceleration spectra based on 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 advisable to implement a design process focused on preventing catastrophic failures in the Accidental Limit Event (ALE) The primary concern is often extreme displacements and shock waves, which are critical for designing an effective 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 12 based on exposure levels If the ELE return period derived from the procedure in this subclause is less than the corresponding period in Table 12, the return period from Table 12 must be applied for S a,ELE (T).
Local Site Response Analyses
The design spectral accelerations for the Alternative Level Earthquake (ALE) and the Maximum Considered Earthquake (ELE) are derived from uniform hazard curves with consistent return periods, as outlined in the detailed seismic action procedure (8.4) These return periods are established following the guidelines in section 8.4 While the probabilistic and deterministic seismic hazard analyses in sections 8.2 and 8.3 provide ground motion data for various site conditions, many offshore locations feature a soft soil layer above stiffer materials Therefore, it is essential to adjust the ALE and ELE spectral accelerations to reflect the local soil conditions A dynamic site response analysis, employing either linear or non-linear soil models, can be utilized to refine these spectral accelerations, yielding site-specific values for design purposes.
6 Standards applicable to types of offshore structure, include ISO 19902, ISO 19903, API 2A-WSD, API 2N, ISO 19904 (all parts), ISO 19905 (all parts), and ISO 19906 See the Bibliography.
Table 12—Minimum ELE Return Periods Exposure Level Minimum ELE Return Periods
Figure 4—Typical Seismic Hazard Curve
To modify acceleration spectra as an alternative to dynamic site response analysis, the procedure outlined in section 7.1 can be employed This involves obtaining an amplification spectrum by calculating the ratio of the acceleration spectrum for the local site class to that of a stiff soil or rock site class The resulting amplification spectrum is then utilized to adjust the acceleration spectra derived from a probabilistic seismic hazard assessment (PSHA) for stiff soil or rock sites.
ELE Performance
The primary goals of Extreme Load Event (ELE) design are to minimize structural damage during an ELE and to provide a sufficient safety margin against significant failures in more severe events It is essential to verify the performance requirements associated with ELE to ensure these objectives are met.
All primary structural and foundation elements must endure minimal to no damage from the Extreme Load Event (ELE) While some limited 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, incorporating restraints to prevent the toppling of topsides equipment and cable trays Piping should accommodate differential displacements from support movements, ensuring that sliding supports function as intended Additionally, the design must reduce the risk of equipment and appurtenances becoming falling hazards during the Emergency Life Extension (ELE).
Additional steel jacket platform requirements are given in API 2A-WSD, 22nd Edition.
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 can 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 can show some plastic behavior but must remain within their ultimate strength limits If significant deformations are expected in the joints, they should be designed to maintain ductility and residual strength at those anticipated deformation levels.
— 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 standard applicable to the type of offshore structure 7 apply.
Additional steel jacket platform requirements are given in API 2A-WSD, 22nd Edition.
7 Standards applicable to types of offshore structure, include ISO 19902, ISO 19903, API 2A-WSD, API 2N, ISO 19904 (all parts),ISO 19905 (all parts), and ISO 19906 See the Bibliography.
This annex offers supplementary information and guidance on the clauses found in the main section of ISO 19901 To facilitate easy identification, the same numbering system and heading titles have been maintained, corresponding to the relevant subclauses in the body of ISO 19901.
The background to and the development of the philosophy for this standard are presented in Reference [6] and Reference [40].
When planning and designing offshore structures, it is crucial to account for hazards beyond just seismic motions, as earthquakes can trigger various geological risks Effective site selection studies can help mitigate most of these geologically induced hazards associated with earthquakes.
Soil liquefaction can happen due to repeated cyclic motions in saturated loose cohesionless soils, 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 reduced capacity due to significant degradation of soil strength For more details on how soil liquefaction affects the structural design of offshore platforms, refer to Reference [41].
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 design strategies 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 close to potentially active faults, 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, upon reaching shallow waters, they swell and can break, generating powerful waves that surge inland The primary threat to offshore structures in shallow water comes from the intense inflow and outflow of water, which can exert substantial forces on the structures and lead to severe scour issues.
Mud volcanoes typically occur at existing fault lines, utilizing these zones to transport gas, water, and mud to the sea floor, resulting in cone-shaped surface features While they are not directly triggered by earthquakes, the safest approach to mitigate risks is to position offshore structures away from these areas.
Earthquake-generated shock waves in the water column, caused by sea floor movements, can affect floating structures and their components These shock waves can travel upward, potentially leading to impulsive forces on buoyant or partially buoyant structures, resulting in increased hull pressures and forces on tendons or mooring lines This effect is primarily a concern during severe earthquakes.
Further information on the effect of earthquakes on floating offshore structures can be found in Marshall (1997) [38] and Rijken & Leverette (2007) [39]
A.6 Seismic Design Principles and Methodology
A two-level design check is essential due to the inherent randomness of seismic events and uncertainties in seismic action calculations Relying solely on strength for designing against severe 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 Earthquake Life Safety (ELE) incorporate safety margins to withstand severe events, leveraging both explicit and implicit design equations along with their ability to accommodate significant non-linear deformations To streamline the design process and ensure that the Acceptable Life Safety (ALE) check confirms an adequate design, the ratio of ALE to ELE spectral accelerations is established to maximize the probability of fulfilling both performance objectives This standard's seismic design procedures focus on achieving a balance between ALE and ELE design criteria.
The seismic design of offshore structures is conducted during an ELE evaluation, where the dimensions of structural components are established based on the relevant design equations The ELE design procedure aims to achieve two key objectives: firstly, to ensure that the structure can endure significant seismic events with minimal or no damage, and secondly, to adhere to the applicable design criteria for the specific type of offshore structure.
The design of offshore structures adheres to several key standards, including ISO 19902, ISO 19903, API 2A-WSD, API 2N, ISO 19904 (all parts), ISO 19905 (all parts), and ISO 19906 The ELE design procedure, along with its associated criteria, ensures that the structure is optimized to meet the ALE performance criteria with minimal design 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.