Draft Section 6, Rev A
API 625, Aug 3, 2007
Draft for API Refrigerated Tank Task Group
SECTION 6
DESIGN CONSIDERATIONS
Draft No.
A
Date:
August 3, 2007
Section 6 Leader: Sheng-Chi Wu
______________________________________________________________________
SECTION 6.1 Tank Sizing and Spacing Requirements
(drafted by Anant Thirunarayanan)
6.1.1 General
Design of the refrigerated tank system requires considerations that shall be given while
evaluating the available site with reference to the following:
a. Sizing of the tank
b. Spacing of the tank
6.1.2 Sizing Considerations
(included later)
6.1.3 Spacing Considerations
Refrigerated tank system shall be within a secondary dike or impoundment area
depending upon the type of the tank system. The spacing shall be provided in such a
way that it is within the thermal radiation protection zone beyond the impoundment area.
The thermal radiation protection zone shall be large enough so that the thermal
radiation flux from a product fire does not exceed the limit specified by corresponding
NFPA standard for people and property.
The siting and layout of the tank system shall be evaluated based on relevant NFPA
guidelines and local regulations having jurisdiction of the plant. The thermal radiation
distances shall be calculated by a suitable analysis. The wind speed, ambient
temperature and relative humidity of the site shall be used in the analysis. The analysis
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shall also address phenomena such as spills, fires, vapor cloud dispersion apart from
thermal radiation in relation to lay out and safe distances from the tank system.
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SECTION 6.2 Maximum Design Liquid Level (drafted by Anant Thirunarayanan)
The tank shell shall be designed to the maximum design liquid level. This level is higher
than the normal maximum operating level of the tank. This will include the sloshing
height for seismic conditions as required in Appendix L of API 620 in cases where the
tanks are located in seismic zones. The net capacity of the tank for refrigerated product
is basically governed by the normal maximum and minimum operating levels.
SECTION 6.3 Performance Criteria (drafted by Jack Blanchard)
Design and erection of tank systems in accordance with this standard shall
satisfy the performance criteria of this section.
6.3.1 Normal Operating
The primary liquid container shall contain the liquid and the primary vapor
container shall contain the vapor under all normal operating loads and conditions.
6.3.2 Abnormal and Emergency Conditions
Performance criteria for primary and secondary liquid and vapor containment are
defined with the individual loading definition below.
6.3.3 Commissioning and decommissioning
The tank system shall be provided with components that allow the criteria in
Section 12 to be met. In addition, the tank system must be capable of being
decommissioned including purging to a gas to air mixture considered safe for
personnel access.
6.3.4 Boil-off
The tank insulation system shall be provided to produce a boil-off rate below the
rate required by the plant design or maximum specified by the owner. The boiloff rate, normally specified in percent per day of maximum normal operating
capacity assuming a pure product, shall be based on climatic conditions as
specified for the project.
Climatic conditions that are normally considered in the design include:
Maximum daily mean temperature (over 24 hour period)
No wind
Solar radiation effects
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6.3.5 Cool Down
The tank system shall include a separate fill line specifically for cool down of the
tank. The system shall have a means for control of the flow to meet the cool
down rates defined in Section 12. For products stored at temperatures below 600 F (-510 C), the cool down line shall incorporate spray nozzles and shall
introduce liquid near the top center of the primary liquid containment tank.
6.3.6 Roll-Over
The tank system shall include a means for measuring changes in density over
the full liquid height as a means of detecting potential roll-over conditions. A
means of mitigating a detected stratification, by recirculation or other means shall
be included. If the tank system can receive product at more than one density, a
top and bottom fill shall also be provided.
6.3.7 Design Metal Temperature
Primary and secondary liquid containment tanks and process lines carrying liquid
or gas:
Generally, design metal temperatures shall be selected no higher than the
pure product boiling temperature at one atmosphere (See Appendix A).
However, design conditions such as introduction of sub-cooled product or
a cool down procedure which eliminates the warm product purge may
require a lower design metal temperature.
Primary vapor containers subjected primarily to ambient temperatures.
Design metal temperatures shall be equal to the lowest one day mean
ambient temperature for the locality involved.
Local areas of the primary vapor container exposed to lower than ambient
conditions:
Areas such as process nozzle thermal distance piece connections to the
vapor container may be subjected to temperatures below ambient
conditions. The design metal temperature for these locations shall take
this local cooling effect into consideration.
6.3.8 Differential Movements
The design of the tank system shall consider differential movements between
tank components resulting from differential design temperatures and erection vs.
operating temperature. Components that are restrained from free differential
movement shall be designed to incorporate flexibility as the thermal gradient is
run out.
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6.3.9 Foundation Settlement
Design of Liquefied Gas storage tank systems must consider how predicted
settlements (both short term and long term) can affect components such as:
 the bottom insulation system
 the steel or concrete primary liquid container
 the concrete outer wall of Full Containment LNG tanks
 post-tensioning system
 the various tank attachments
 piles or other structural support systems
Depending on the loads, temperature, time and other design parameters,
settlement effects can result in stresses that are either positive or negative for the
structure design.
Tank settlement patterns and resultant tank distortions can be very complex and
unpredictable. Important factors that can affect how a tank reacts to settlement
include heterogeneous soils (both vertically and horizontally), variable as-built
distortions, and sensitivity of structural details to movement.
Predicted settlements shall be determined as part of the site specific
geotechnical study. Soil Improvement, as determined necessary by design of the
tank system may be applied to reduce the predicted settlements.
Settlement can be separated into four specific types: uniform settlement, global
tilt, differential center-to-edge settlement and differential circumferential
settlement. Values provided below are intended for guidance. Variations from
these values are acceptable if accounted for in the design of the tank and
foundation system.
i)
Uniform settlement: The amount of acceptable uniform edge
settlement is dependent upon the connections to the tank.
ii)
Global Tilt: Global tilt (also addressed as planar tilt) is associated
with rigid body rotation of the superstructure caused by non-uniform
soil across the width of the structure. While large tanks may be
able to accommodate significant uniform tilt without damage, other
components usually require lower values of tilt.
Global tilt, measured in inches, of a flat bottom tank shell should be
limited to 5 times the tank diameter divided by the height.
iii)
Differential Center-to-Edge Settlement: Liquefied gas tanks are
constructed with self supporting roofs. Differential settlement
between the center and the edge does not affect the roof. While
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bottom plate can accommodate significant settlement, tank
internals supported by the bottom and the bottom insulation system
inherent in liquefied gas tanks cannot. Significant short or long
term settlement of the bottom can crack or damage the bottom
insulation system which would seriously increase the heat leak of
the structure, potentially causing freezing of the soil under the tank.
Differential settlement between the edge of the tank and the center
should be limited to the radius of the tank divided by 240.
iv)
Differential Circumferential Settlement: Irregular settlement of
soils around the periphery of a tank can cause out-of-roundness
and localized distortions and buckles in a tank. These can affect
the stability or the performance of the tank.
Differential settlement around the periphery of the tank should be
limited to 3/8 inch in an arc radius of 30 feet (1:1000).
In monitoring the settlement, an independent datum reference point located
beyond the influence of local foundations shall be established. Permanent
markers shall be provided to facilitate settlement monitoring around the perimeter
of the foundation at a minimum of 8 locations not more than 30 ft apart. In
addition, for concrete outer wall tanks and for settlement conditions that are
expected to approach the design limits set for the tank, provisions shall be made
to measure dishing settlement. (An inclinometer system can be provided to
accomplish this requirement.)
6.3.10 Protection from freezing of Soil
Soil located below foundations is subject to thermal heat leak through the bottom
insulation system. This can lead to freezing of the soil progressing to frost heave
and potential tank system damage. The tank foundation design shall include a
means to maintain the soil at a temperature above 32oF (0oC).
Foundation heating systems shall be designed to allow replacement of individual
heating elements and to protect against the accumulation of moisture within the
cable conduits.
6.3.11 Concrete Steel Liner
Performance criteria for steel liners, forming a liquid or vapor barrier for a
concrete component, are defined in ACI 376.
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6.3.12 Concrete Reinforcement
Concrete reinforcement requirements for concrete components including
concrete bearing rings placed under the primary liquid container shall meet the
requirements of ACI 376.
6.3.13 Design Considerations for Concrete Tanks
Requirements for the design, erection, inspection, and testing of concrete
primary and/or secondary containment tanks that make up part of the overall tank
system are found in ACI 376. Performance criteria specific to the concrete
components of a tank system are located in ACI 376 Part 4.
6.3.14 Seismic Ground Motions
Tanks designed and built to this standard shall be designed for three levels of
seismic motion. The magnitudes of the seismic ground motions are defined in
Section 6.4.2. In addition, the design shall meet the requirements of the
applicable local building codes.
a) Operating Basis Earthquake (OBE): The tank system shall be designed to
continue to operate during and after OBE event. The OBE is also referred
to as Operating Level Earthquake (OLE) in API 620, App L.
b) Safe Shutdown Earthquake (SSE): The tank system shall be designed to
provide for no loss of containment capability of the primary container and it
shall be possible to isolate and maintain the tank system during and after
SSE event. The SSE is also referred to as Contingency Level Earthquake
(CLE) in API 620, App. L.
c) Aftershock Level Earthquake (ALE): The tank system, while subjected to
ALE, shall provide for no loss of containment from the secondary
container while containing the primary container volume at the maximum
operating liquid volume.
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SECTION 6.4 Design Loads and Load Combinations
(drafted by Rama Challa)
6.4.1 Design Loads
The following types of design loads shall be considered in the design of the
liquefied gas containers and foundations. EN 14620-1.2006 Section 7.3 and ACI
376, Chapter 3 provide guidance on the types of the design loads and load
combinations to be used. They include, but not limited to the following:
1.
2.
3.
4.
5.
Dead loads
Product Pressure and Weight
Maximum internal pressure
Construction-Specific Loads
Testing and Commissioning Loads such as test, vacuum and pneumatic
tests
6. Thermally-induced loads experienced during tank purging, cooling and
filling
7. Shrinkage and Creep-Induced Loads
8. Pre-stressed loads for concrete container
9. Live Loads
10. Environmental Loads such as Snow, Wind and ice loading
11. Loads induced by differential settlement
12. Seismic Loads (OBE, SSE & ALE, defined below)
13. Liquid spill condition
14. Loads based on a risk assessment such as fire, blast, external missile etc.
6.4.2 Seismic Loads
Probabilistic seismic hazard studies are required to determine the seismic ground
motions for design of tank-fluid-foundation system. Three levels of the seismic ground
motions shall be considered:
a) Operating Basis Earthquake (OBE): The OBE is defined as the seismic ground
motion having 10% probability of exceedance within 50 year period, i.e. 475 year
recurrence interval.
b) Safe Shutdown Earthquake (SSE): The SSE is defined as the seismic ground
motion having 2% probability of exceedance within 50 year period, i.e. 2475 year
recurrence interval.
c) Aftershock Level Earthquake (ALE): The ALE is defined as half of the SSE.
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6.4.3 Load Combinations
The design loads shall be combined to produce load combinations to be used in
the analysis and design of the liquefied gas containers. Load combinations are
dependent on the material type and governing codes used for the container.
1. Liquefied gas steel containers are designed per the rules of API-620. The
basis of the design is the Allowable Stress Design with a direct
combination of the design loads. The load factor of unity should be used in
the load combinations. The allowable stresses can be increased when the
following transient loads are combined with the normal operating loads:
Test loads:
25% increase
Wind or OBE:
33% increase
For load combinations that involve SSE, fire, blast, loads etc: yield stress
can be used.
2. Liquefied gas concrete containers are designed per the rules of ACI-376.
The basis of the design is ultimate strength design with factored load
combination of the design loads. Chapter 3 of ACI 376 provides guidance
on the load combinations to be used.
SECTION 6.5 Seismic Analysis (drafted by Jack Blanchard and Sheng Wu)
6.5.1 The tank system shall be designed for three levels of seismic ground
motions as defined in Section 6.3.14 and 6.4.2 above. The rules in API 620
Appendix L for LNG tanks shall be applied to all steel tanks designed to this
standard. The rules of ACI 376 shall be applied to all concrete tanks designed to
this standard.
6.5.2 The site-specific horizontal and vertical acceleration response spectra shall be
developed for both OBE & SSE for different damping values of up to 20%.
6.5.3 The ALE earthquake shall be considered only for the seismic design of secondary
containment with full liquid condition, assuming that the primary container is damaged
after the SSE event.
6.5.4 When the tank foundation is supported on rock-like site (defined as the site class
A & B in IBC or ASCE 7), the fixed base condition is considered. In this case, the
structural damping values shall be used for determining the seismic responses
6.5.5 When the tank foundation is supported on soil site (defined as the site class C to
F in IBC or ASCE 7), the soil-structure interaction seismic analysis (SSI) shall be
applied. In this case, the dynamic soil and pile stiffness and damping parameters shall
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be included in the tank model for SSI analysis. The dynamic soil/pile properties are
evaluated by considering the effects of seismically induced soil strains and forcing
frequencies. The system damping for SSI shall be calculated for determining the
seismic response, and should be limited to15% for OBE, and 20% for SSE.
6.5.6 In order for tank system to remain in continuous operation during and after OBE,
the elastic method of seismic analysis should be performed using the response
modification factors, R equal to 1. Use of the other response modification factors to
reduce the seismic response may be acceptable, provided the operable performance of
the tank system is not affected by an OBE event.
6.5.7 The response modification factors can be used to reduce the seismic response
for the SSE, provided the design of the tank system meets the performance criteria
during and after an SSE event.
6.5.8 Seismic Design Liquid Height
If the tank system includes high level alarms to restrict the maximum normal
operating level to a level lower than the maximum design liquid level, the
maximum normal operating level may be applied to all seismic design including
freeboard determination. The freeboard height should be determined based on
the OBE sloshing height plus 1 ft allowance or the SSE sloshing height,
whichever is larger.
6.5.9 Resistance to base shear – Sliding
The rules in API 620 Appendix L shall be applied to determine sliding resistance.
In high seismic regions a more extensive analysis may be applied, provided it
includes evaluation of the response of the shell, the fluid, and foundation (in the
case of a slab) to the fluctuation of liquid pressures in the tank. When applying
this approach, the horizontal and vertical seismic response should be applied
based on the component combination of 100% and 30%. The case for the 100%
vertical plus 30% horizontal load case shall be evaluated in addition to the 100%
horizontal plus 30% vertical load case defined by API 620, App L.
6.5.10 Evaluation of Damage from an Earthquake
When it has been determined that a seismic even at the tank has exceeded an
OBE level event, the tank should be shutdown and evaluated for permanent
distortion, continued safe operation, and the need for repairs.
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SECTION 6.6 Foundation Design (drafted by Rama Challa)
6.6.1 General
Liquefied Gas Storage containers shall be installed on suitable foundations
designed to transmit all loadings to suitable load bearing soil strata. Types of
foundation support systems consist of Raft or Mat foundations, pile foundations
(steel H-piles, cast in-situ concrete piles or precast prestressed concrete piles)
and elevated foundations supported on drilled shafts or vertical walls.
Foundation support systems are dictated by detailed geotechnical investigation
of the location for siting of the liquefied gas storage containers. The extent and
detail of the soil investigation shall be specified by qualified geotechnical
engineers.
Geotechnical investigations typically consist of field exploration and laboratory
testing to characterize surface and subsurface conditions. Field exploration work
includes test borings, soil sampling for laboratory tests, cone penetrometer tests,
seismocone tests, down hole vane shear tests, water level measurements, pile
load tests etc. Laboratory work includes soil water content, Atterberg limits, unit
weights and particle size distributions, and compression tests. Other
investigations include reviewing the geological and topographic data such as
locations of known faults etc.
The results of the geotechnical investigations shall be presented in the form of
geotechnical design data required for design of tank foundation (soil or pilesupported foundation). The data shall include the following:
a.
b.
c.
d.
e.
f.
g.
h.
i.
j.
k.
l.
Unit weight of soil
Lateral earth pressure (active, passive and at-rest)
Undrained shear strength
Blow counts
Dynamic soil shear modulus (shear wave velocity)
Ultimate and allowable soil bearing pressure
Modulus of subgrade reaction
Ultimate and allowable pile capacities (compression, tension and
lateral capacity)
Negative skin friction of pile (downdrag)
Dynamic soil and pile stiffness and damping data
Static and dynamic pile group reduction factors
Allowable total and differential settlements
In addition, the geotechnical subsurface investigations shall indicate seismic
hazard data such as the potential of the soil liquefaction; present spectral
response acceleration parameters to be used with Operating Basis Earthquake
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(OBE); Safe Shutdown Earthquake (SSE) and Aftershock Level Earthquake
(ALE) and, if required, soil structure interaction (SSI) effects.
The materials of construction and the foundation type shall be designed to
adequately resist the operating and accident temperature conditions.
Foundation base heating shall be utilized for grade supported mat foundations.
Elevated foundations with adequate air gap between the bottom of the foundation
and grade shall be considered in cases where base heating methods are not
feasible.
6.6.2 Ground Improvement
Ground improvements should be considered in unsuitable areas encountered in
the tank siting. Typical ground improvement methods suggested by geotechnical
engineers consist of the following:
i. Removal of unsuitable soil and replacement with suitable compacted
material,
ii. Preconsolidation surcharging the unsuitable areas and usage of soil or wick
drains to accelerate settlement.
iii. Dynamic compaction of the soil.
iv. Deep soil mixing.
6.6.3 Design Codes
ACI 376 Chapter 8 provides guidelines on foundation design. ACI 318 and ACI
376 guidelines shall be utilized for the structural design of concrete elements
utilized in the foundations. The concrete elements include Raft or Mat
foundations, cast in-situ concrete piles or precast prestressed concrete piles,
drilled shafts and vertical walls. AISC steel construction manual provides
guidelines on design of steel H-Pile Sections.
6.6.4 Factors of safety
In general, the design soil parameters, viz.., bearing capacity, pile compression
and tension capacities etc., are based on allowable stress design (ASD)
approach with an ultimate capacity value divided by a factor of safety. The factor
of safety is based soil type, variability and soil characterization. The factors of
safety presented in Sections 8.3.2 and 8.4.3 of ACI 376 can be used as a guide.
The actual factor of safety should be determined on a case by case basis, based
on the recommendations of geotechnical engineer in charge of geotechnical
investigations.
ACI 318 utilizes an ultimate design strength approach for the design of the
concrete foundation slabs, piers or piles. A design approach comparing ultimate
loads on the soil with the ultimate soil or pile capacities could be also applied.
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6.6.5 Mat Foundations
If the supporting soil is found adequate in respect of load transfer and
settlements then a soil supported mat foundation should be used. Adequate
consideration should be given to the effects of differential settlement, shrinkage,
creep and thermal strains. ACI 376 Table 8.1 recommends factors of safety for
shallow foundations.
Uplift of the foundation shall not be permitted in OBE condition. Uplift of the
foundation shall be permitted for the SSE condition, provided that the soil under
the mat foundation allows for redistribution under the uplift condition. The
Factors of safety against overturning shall be as recommended in Section 8.3.3
of ACI 376. Sliding of the foundation shall not be permitted under the OBE and
SSE events.
6.6.6 Pile foundations
If sufficient bearing capacities and settlements are not available, then the
foundation base slab should be supported by piles. Negative skin friction (down
drag) and pile interaction (group effects) shall be considered when determining
the foundation capacity. The pile design shall consider operating loads, tank
settlements, thermal cycling, drying shrinkage, creep effects, lateral deformations
from wind, OBE and SSE earthquakes.
Adequate factors of safety shall be maintained during normal operational,
OBE/SSE earthquakes and tank upset conditions. Pile foundation factors of
safety are dependent on the results of the soil investigation and the in situ quality
control. Section 8.4.3 of ACI 376 recommends factors of safety considering
these constraints.
6.6.7 Anchorage (drafted by Jack Blanchard)
Anchorage of primary or secondary steel containment tanks shall consider the
following:
 Differential movement between the anchorage and the connection to
the container
 Local stresses at the connection to the container
 Differential strength along the length of the anchor due to thermal
effects
 Connection details where the anchor extends through a containment
boundary (such as the secondary containment bottom of a full or
double containment tank)
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SECTION 6.7 Thermal Corner Protection System (TCP) for Concrete tanks
(drafted by Jack Blanchard)
The design of the wall to slab junction of a concrete container shall consider the
effects of differential thermal expansion due to initial exposure to the liquid
product. Design of the junction shall also consider the application of prestress
forces to insure liquid containment in case of a spill.
A standard solution applies a steel thermal corner protection expansion joint and
a secondary bottom. The TCP may be designed to withstand the full hydrostatic
pressure from a full spill, or may transfer a part of the pressure to the wall
through load bearing insulation. The following shall be considered in the design
of the TCP.
 The location of the top of the TCP as related to the prestress force
diagram.
 Differential thermal movements between the connection to the wall and
secondary bottom including the following conditions: Operating, small
spill, and full spill
 Differential movements due to wall prestress and creep
 Wall rotation due to foundation settlement
 Differential shrinkage between the wall and top of TCP connection
 Erection tolerances between the TCP and the load bearing insulation
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