Agenda Item 625-1001
Title: Seismic Ground Motion Consistencies
Date: May 7, 2013
Revision: 0
Handled By:Jack Blanchard
ChicagoBridge and Iron Company
14105 S. Route 59
Plainfield, IL 60544-8984
Telephone: 815-439-6324
Fax: 815-439-6560
Email:
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Purpose: Align API625 6.5.2 with API620 L.4.and L.3.2.6 to allow non-USA API625 tank designs to be more consistent with local regulations and owner risk evaluations.
Source:
Impact: ALL non-LNG Tanks in moderate to high seismic areas: Allow more economic and non-isolated designs which are more in line with local regulations and owner risk evaluations.
General discussion:
API620 Appendix L paragraph L.3.2.6 allows adjustment of the recurrence interval for performance based designs and L.4.1 recognizes that the defined ground motions (for OLE, CLE and ALE) applied to determine design response spectra are dependent on regulations and specific site locations. However, API625 6.5.2,and API 620 L.4.2 require specifically defined seismic ground motions set to specific recurrence interval criteria.
The design magnitude for seismic events should be consistent with regulatory documents such as building codes. When regulations do not address the need for continued operability, the Purchaser should establish the basis for continued operability. API should not take on the responsibility for setting these magnitudes. Therefore the provisions in API620 L and API625 which specify magnitudes are proposed to be changed
Examples of inconsistency between API provisions:
API 620 section L.4 contains provisions for tanks requiring performance level designs. API625 section 6.4.10 requires three levels of seismic performance designand refers to API620 L for seismic design of steel tanks (6.6.1). However, API625 section 6.5.2 is not aligned with the following highlighted references in API620 section L.4 and does not allow for adjustment of the required ground motion.
Current API 620 Section L.4:
L.4.1 Ground Motions
The definition of the ground motions to be used with the OLE, CLE and ALE events may vary depending on regulations for the specific location. Within the U.S. federal regulations 49 CFR 193 and NFPA 59A are the primary regulatory and standard documents for LNG storage tanks. The user is referred to those documents or similar regulatory documents when the tank is located outside the U.S., for ground motion definitions to be used with this appendix.
L.4.2 Operating Level Earthquake (OLE)
Unless otherwise defined by the governing local regulations, the operating level earthquake ground motion shall be defined as the motion due to an event with a 10 % probability of exceedence within a 50-year period (a 475-year recurrence interval).
L.4.3 Contingency Level Earthquake (CLE)
Unless otherwise defined by the governing local regulations, the contingency level earthquake ground motion shall be defined as the motion due to an event with a 2 % probability of exceedence within a 50-year period (a 2475-year recurrence interval) which is the maximum considered earthquake in API 650, Appendix E and ASCE 7.
Design Ground Motions
CLE (SSE): For all refrigerated gas storage tanks including LNG tanks, the defined ground motion for SSE in API 625 6.5.2 is consistent with ASCE7 (MCE reduced by a 2/3 factor and increased by I = 1.5).However, for tanks built in other locations such as western South America, the local regulations may be based on a shorter recurrence period (usually 475 years). A ground motion based on the shorter interval (and increased by an I factor for refrigerated gas storage) may be more appropriate where the frequency of seismic activity exceeds that of the USA. Applying criteria,consistent with local regulations,is more appropriate than setting criteria based on the USA.
OLE (OBE): When API 620 / 625 are applied, there are no regulations worldwide, except NFPA 59A, that require an OBE design. API 620 L.4.2 would therefore always require a 475 year recurrence interval, even when the local regulations set 475 years for CLE. For tanks other than those covered by regulations, the defined ground motion for the OLE should bebased on plant safety and the need for continued operability. This is related to risk of property damagerather than risk to life. Therefore, the basis for OBE ground motion should be determined by the purchaser’s plant risk assessment.
Summary of Proposed Changes:
This agenda item sets the SSE ground motion to that required by the governing local regulations as a minimum. The OBE ground motion is established based on the owner’s risk evaluation or governing regulation. ALE remains unchanged as ½ of SSE.
Changes to API625:
Changes are limited to section 6.5.2 to tie the design magnitude to regulations for SSE and risk assessment for OBE.
Changes to API620 Appendix L:
API620 L,4 was written (prior to API625) to provide direction for application of API620 Appx L to LNG tanks designed to NFPA 59A. Thus some of the provisions do not have flexibility for designs not required to meet NFPA 59A.
The requirements in API620 L.4.2 are more restrictive than L.4.1. Changes are proposed to make OLE (OBE) a purchaser defined value.
The requirements of API620 L.4.3 set specific criteria for the US which may be inconsistent with ASCE7. Therefore, the definition should be changed to reflect the defined MCE / MCERin ASCE 7 Chapter 21. This is consistent with NFPA59A: 2013. These changes reference ASCE 7.
API620 L.4 includes rules for applying I, Q, and damping to OLE and CLE designs which are consistent with NFPA 59A and aligned with ASCE 7 design methods. When other criteria is used to define OLE and CLE, some rules need to be adjusted for the new basis.
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The following text is for reference. It is agenda item 620-1001:
L.4.1 Ground Motions
The definition of the ground motions to be used with the OLE, CLE and ALE events may vary depending on regulations for the specific location. Within the U.S.ASCE 7 is required by most state building codes and federal regulations 49 CFR 193 and NFPA 59A are the primary regulatory and standard documents for LNG storage tanks.The user is referred to those documents or similar regulatory documents when the tank is located outside the U.S., for ground motion definitions to be used with this appendix.
For Appendix Q and R tanks, the seismic design liquid level to be applied shall be as defined in API625.
L.4.2 Operating Level Earthquake (OLE)
Unless otherwise defined by the governing local regulations, the ground motion to be applied to meet the OLE performance criteria in L.4 shall be determined as part of a Purchaser defined plant risk assessment considering plant safety and loss of operability. the operating level earthquake ground motion shall be defined as the motion due to an event with a 10 % probability of exceedence within a 50-year period (a 475-year recurrence interval). The OLE design liquid level coincident with the defined ground motion may be defined as less than the Maximum NOL.
L.4.2.10 Sliding Resistance
The calculated sliding force at the base of the tank shall not exceed Vs. The maximum coefficient of friction, , shall be (tan 30o/1.5) where 1.5 is the factor of safety against sliding. The coefficient of friction selected shall consider the materials underlying the tank bottom. Anchorage may not be used to resist sliding. If the sliding force exceeds the allowable, the tank shall be re-configured.
When the OLE ground motion is defined by the Purchaser plant risk assessment, and when a reduced level of safety against operability is consistent with the plant risk assessment, the friction safety factor may be reduced to 1.25. The safety factor may be reduced to 1.0 when the tank bottom is coned up with a minimum 1% slope to increase sliding resistance.
L.4.3 Contingency Level Earthquake (CLE)
The ground motion to be applied to meet the CLE performance criteria in L.4 shall be based on the governing local regulations.
Unless otherwise defined by the governing local regulationsWhen NFPA 59A is required and for all tanks in the US, the contingency level earthquake ground motion shall be defined as the motion due to an event with a 2 % probability of exceedence within a 50-year period (a 2475-year recurrence interval) which is the maximum considered earthquake in API 650, Appendix E and ASCE 7 (MCER) as defined by ASCE 7-10 Chapter 21.
For non-us locations and when NFPA 59A does not apply, the spectra developed from ground motions complying with local regulations shall be adjusted for 5% and 0.5% damping, the appropriate I factor, and soil effects.
L.4.4 Aftershock Level Earthquake (ALE)
This design case shall be applicable only when regulations or project documents specifically require the tank system to be designed or evaluated for aftershocks.
Unless otherwise defined by the governing local regulations, the aftershock level earthquake (ALE) ground motion shall be defined as the motion due to an event with a 2% probability of exceedance within a 50-year period (1 2475 year recurrence interval) which is the maximum considered earthquake in API650, Appendix E and ASCE 7, with the CLE spectral values reduced by 50%.
If the outer tank is not designed as a secondary containment (i.e. it serves as vapor barrier and pressure boundary only and is not constructed of API 620 material suitable for the inner tank) then no design or evaluation for ALE is required by these provisions for the inner or outer tank.
Commentary
LC.4.2 Determination of OBE ground motion from a plant safety & loss of operability risk assessment may consider the following related to the expected life of the structure:
Risk Level / Recurrence Interval / Probability of exceedance in life of the tank30 years life / 50 years life
Low / 100 years / 27% / 40%
Medium / 250 years / 12% / 18%
High / 475 years / 7% / 10%
As an alternate to a site specific response spectra for OBE, the purchaser may specify OBE in terms of a factor times a 475 recurrence interval spectra. For example, 0.5 times the 475 year spectra. Note: The value of 0.5 times the 475 year spectra in terms of recurrence interval and probability of exceedance varies with location.
When the risk assessment targets the limits of elastic tank response and can tolerate limited permanent distortion which does not affect the operability of the of the tank system, an OBE design liquid level consistent with tank operations may be selected. For example, a design liquid level of 80% of the maximum normal operating level may be selected if the tank operates above the 80% level for a small percent of time.
LC.4.2.10 When the risk assessment targets the limits of elastic response, a reduction in the safety factor against sliding is consistent with the assessment target. A minimum SF = 1.25 is maintained due to the accuracy of this calculation. Also, when sliding resistance is increased by providing a coned up bottom, (minimum of 1%)a further reduced friction safety factor is consistent with this approach.
Agenda Item 625-1001: Proposed Changes to API625:
New text is in blue underline.
Deleted text is in red strikethrough.
API 625
6.5.2 Seismic Loads
Probabilistic seismic hazard studies are required to determine the seismic ground motions for design of tank-fluid-foundation systems. The three levels of theSeismic ground motions that shall be appliedconsideredfor the three performance levels defined in 6.4.10 are:
a)Operating basis earthquake (OBE):
Unless otherwise defined by governing local regulations, the ground motion to be applied to meet the OBE performance criteria in 6.4.10 shall be determined as part of a Purchaser defined plant risk assessment considering plant safety and loss of operability.
The OBE is defined as the seismic ground motionhaving 10 % probability of exceedance within 50 year period, i.e. 475 year recurrence interval.
The OBE is also referred to as operating level earthquake (OLE) in API 620, Appendix L
b)Safe shutdown earthquake (SSE):
The ground motion to be applied to meet the SSE performance criteria in 6.4.10 shall be based on the governing local regulations.
When NFPA 59A is required and for all tanks in the USThe SSE is defined as the seismic ground motion to be applied to meet the SSE performance criteria in 6.4.10 shall be defined as the maximum considered earthquake (MCER) having 2 % probability of exceedance within 50 year period, i.e. 2,475 year recurrence interval adjusted by the requirements ofas defined by ASCE 7-10 Chapter 21.
For non-us locations and when NFPA 59A does not apply, the spectra developed from ground motions complying with local regulations shall be adjusted for 5% and 0.5% damping, the appropriate I factor, and soil effects.
The SSE is also referred to as contingency level earthquake (CLE) in API 620, Appendix L.
c)Aftershock level earthquake (ALE)
The ground motion to be applied to meet the ALE performance criteria in 6.4.10 shall be The ALE is defined as half of the SSE.
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