CRYOGENIC ABOVE GROUND STORAGE TANKS: FULL CONTAINMENT AND MEMBRANE COMPARISON OF TECHNOLOGIES
Jrme Thierault Caroline Egels
Technical Division BOUYGUES Travaux Publics 1 Avenue Eugne Freyssinet
78061 Saint-Quentin-en-Yvelines - France
Over the last couple of years the LNG industry has shown a continuous trend for:
Being more cost effective
Improving land area utilization
Being able to provide larger and larger tanks to accommodate the progressive increase in the gross capacity of ocean going methane carriers
Reducing construction schedules
Reducing carbon footprint.
The aim and purpose of the proposed paper is to compare the different technologies of above ground storage tanks with regards to:
Safety and integrity of the tank (including seismic event)
Construction cost and schedule
Note: This comparison will reflect latest development in storage tank technologies. Special attention will be paid to the comparison of carbon footprints from a life cycle perspective, knowing that it appears that membrane tanks provide a significant reduction compared with conventional self standing technologies.
Bouygues is in perfect position to compare available technologies of cryogenic storage tanks, thanks to:
Its leading position among civil EPC Contractors, Its large experience of both membrane and full containment technologies, Its involvement in normative working groups dealing with cryogenic storage tanks, Its constant commitment to reduce the impact of construction on the environment.
This paper dealing with cryogenic above ground storage tank is aimed at providing technologies comparison, with special focus on carbon footprint.
In the first part of this note:
The most commonly specified tank technologies will be introduced, Their basic design concept will be presented.
A second part will provide high level comparison of their performances and discuss their design limitations.
Finally, carbon footprints and how it compares between both technologies will be discussed.
3.0 MOST COMMONLY SPECIFIED TANK TECHNOLOGIES FOR LNG CONTAINMENT
3.1 Full Containment Tank
For this containment technology:
The primary container is a thick 9% Nickel welded steel tank. The secondary container is a pre-stressed concrete tank equipped with a thermal corner protection. The space between primary and secondary container is filled with thermal insulation. The primary and secondary containers each possess separate hydrostatic stability and are thus
referred to as self-standing.
Keys: 1. Primary container (9% Ni steel) 5. Insulated suspended deck (aluminum & fibreglass) 2. Bottom insulation (load bearing rigid cellular glass) 6. Hemispherical dome roof (reinforced concrete) 3. Slab (reinforced concrete) 7. Sidewalls (pre-stressed concrete) 4. Slab heating system 8. Wall insulation (loose fill perlite / 1m thick) Notes: - The inner face of items 3, 6 & 7 (red bolded line) is covered by a carbon steel liner aimed at ensuring gas
tightness. - The thermal corner protection extending 5m above the bottom slab protects the wall-to-base joint. - The annular space 8 is connected to the vapour space on the tank and is thus filled with methane gas.
Figure 1: Full Containment Design Concept
3.2 Membrane Tank
For this containment technology:
The primary container is a thin stainless steel corrugated membrane. The secondary container is a pre-stressed concrete tank equipped with a thermal corner protection. The space between primary and secondary container is filled with thermal insulation. This concept is based on the separation of structural and tightness functions.
The primary container ensures liquid and gas tightness, The secondary container provides the hydrostatic stability, The load bearing insulation system transfers hydrostatic loads to the secondary container and
limits the heat entrances to meet the specified boil-off rate criteria.
Keys: 1. SS corrugated membrane (1.2mm thick) 5. Slab heating system 2. Sidewalls (pre-stressed concrete) 6. Insulated suspended deck (aluminum &
fibreglass) 3. Bottom insulation (load bearing PU / 40cm thick) 7. Hemispherical dome roof (reinforced concrete) 4. Slab (reinforced concrete) 8. Wall insulation (load bearing PU / 40cm thick) Notes: - The inner face of item 7 (red bolded line) is covered by a carbon steel liner aimed together with the primary
container (item 1) at ensuring gas tightness. - The thermal corner protection extending 5m above the bottom slab protects the wall-to-base joint. - The insulation space between the membrane and the concrete vessel is isolated from the vapour space of
the tank. A nitrogen breathing system operates on the space to monitor the methane concentration and keep the pressure within normal operating bounds. The nitrogen system can be used to purge the insulation space in the unlikely event of a leak.
Figure 2: Membrane Tank Design Concept
Other tank technologies exist, but none of them compares to the above.
Most well-know technologies are:
Single and double containment tanks (because of their reduced safety in operation such tanks are limited to remote located areas, where very limited population or facilities are present and/or where modest storage capacity is to be provided),
Double concrete tanks (felt difficult to implement because of the lack of references and codes recognizing this technology).
4.0 PERFORMANCE COMPARISON
4.1 Intrinsic Key Performances
Figure 3 below provides a multi-parameters summary of how both technologies compare in term of intrinsic performances:
Robustness / Reliability
Safety in operation
Thermal efficiency (BOR)
Compliance with legislative context
Efficiency in severe seismic context
Full Containment GST Membrane
Figure 3: Comparison of tank intrinsic key performances
Both technologies are considered proven, safe and reliable, with few differentiating strengths and weaknesses.
4.2 Implementation Performances
Figure 4 below provides a multi-parameters summary of how both technologies compare from an implementation constraints standpoint:
Figure 4: Comparison of tank implementation performances
Membrane tank exhibits a number of implementation advantages as against full containment. As a matter of fact, where large tanks are required, membrane can be considered 15% less expensive and 3 months shorter to implement, for the same storage capacity.
However, particular attention needs to be paid to the specifics of the project under consideration that may dictate the storage tank technology. Key parameters of influence are:
Local circumstances, local codes and/or corporate practices, Lack and/or cost of skilled mechanical workforce, Tank size, Earthquake magnitude.
4.3 Design limitations
In both cases, the most stringent limiting factor is the size of the hemispherical dome roof.
Numerous studies have led to the conclusion that both tanks exhibit similar boundary limit in that regards, namely:
In benign and mild earthquake areas, an internal diameter of 110m has been found as the limiting factor. This large tank span could be accommodated by the current concrete section thickness, but more limiting is the extent of peripheral demand to constrain the dome outward thrust. In this case
has been found to require substantial/impractical post-tensioning patterns (i.e. 11 no. 37-strand tendons provided at 500mm distance).
In severe earthquake area it has been found practical to set the radius of curvature to 0.8d (in lieu of the typically adopted d) and that inner tank behaviour need be looked at with due care.
5.0 CARBON FOOTPRINTS COMPARISON
5.1 Case Study
The carbon footprints displayed in this paper have been established on the following case study:
One (1) tank of 190,000 m3 storage capacity to be built in Gladstone (Australia). Battery limit lies in nozzle flange on dome top. Externals are thus excluded (can be neglected
because there is no difference in these components, between full containment and membrane tank). Tank is assumed to rest on shallow foundation (in case of deep foundation, the corresponding carbon
footprint is expected to be slightly larger for full containment as against membrane tank).
5.2 System Boundaries
The system boundaries include the complete life time cycle from raw material extraction to LNG tank dismantling (cradle-to-grave):
Figure 5: Tank Life Cycle
However, for the sake of simplification, the contribution of operation, maintenance and dismantling have been neglected. The rationale for this approximation lies in the following reasons:
The aim and purpose is to compare the relative environmental impact between tank technologies, Operation carbon footprint is expected to be similar for both technologies. As a matter of fact
insulation thickness is selected such that both technologies have same boil-off gas rate (0.05% volume per day) and electrical demand (slab heating system),
Maintenance carbon footprint is negligible. Storage tanks (static equivalent) are known to be virtually maintenance free,
Dismantling carbon footprint is expected to be larger for full containment as against membrane tank.
5.3 Calculation Principles
The carbon footprint CF of a LNG tank can be calculated as follows:
CF = CFmanufacturing + CFtransportation + CFconstruction + CFO&M / Dismantling
CFmanufacturing = i Wi x Fi Wi is the weight of the material i Fi is the per unit Global Warning Potential in ton CO2-eq for the production of 1 ton of material i
CFtransportation = (i Droadi x Wroadi x Froadi) + (i Dseai x Wseai x Fseai) Di is the distances travelled by the material i, by road and/or sea Wi is the weight of material i, transported by road and/or sea Fi is the per unit Global Warning Potential in ton CO2-eq of 1 ton.km for each type of transportation CFconstruction = (i Ei x Fi) + (i Di x Fi) + (i Ii x Fi) Ei is the consumption of energy i to perform construction and site staff transport Di is the average distance to be travelled by of personnel to access to site (by road and air) Ii is for equipment i its weight multiplied by mobilization duration Fi is the per unit unit Global Warning Potential in ton CO2-eq corresponding to Ei, Di and Ii.
CFO&M / Dismantling = neglected as explained in paragraph 5.2
5.4 Key parameters of influence
LNG Tanks material take off The following table summarizes the quantities of material involved for both technologies under consideration.
Table 1: 190,000 m3 Tanks Material Take Off
(*): In a full containment the quantity to be laid on the suspended is about half that
of a membrane tank, but this difference in quantity is fully compensated by the existence of the resilient blanket which is fastened to the inner tank shell.
As a matter of fact membrane tank uses far less metals, for similar quantities of rebars and insulation. For the case study under consideration, the difference amounts to -35%.
Per unit global warning potential of each component The following per unit global warning potential of each component have been obtained from suppliers published data, with the exception of concrete which has been computed with SNBPE BETie analytical tool to reflect the specifics of cryogenic tank concrete mix. Orders of magnitudes have been cross checked with published data base by Bouygues in-house team of specialists.
Table 2: Per unit global warning potential of each component
Per unit global warning potential of transport and construction
The following per unit global warning potential have been calculated from ADEME database (French environment and energy management public agency, aiming at protecting the environment and managing energy).
Table 3: Per unit global warning potential of transportation and construction activities
(*): Material such as insulation are too light to take up the maximum authorized
payload of truck and container, thus implying greater unit GWP
Figure 6 below displays the computed aggregated carbon footprints:
Full Containment Membrane
Carbon Footprint in tons CO2-eq
Figure 6: Aggregated Carbon Footprints, with split per contributors
Membrane tank allows a significant reduction of the impact on the environment as against Full Containment tank. This mainly results from the fact that membrane tank uses far less metal and insulation, for similar quantities of concrete as evidenced on figure 7.
Full Containment Membrane
Carbon Footprint in tons CO2-eq
Figure 7: Manufacturing Carbon Footprints split per components nature
The present paper has established that the performances of both technologies are mostly identical.
Main difference lies in implementation constraints:
Membrane tank is generally less expensive and shorter to implement, for the same tank storage capacity,
Membrane tank allows a significant reduction of the impact on the environment (-24% global warning potential for the case study addressed in this paper. Expected to be greater in case of a construction in Europe).
ABSTRACT1.0 BACKGROUND2.0 INTRODUCTION3.0 MOST COMMONLY SPECIFIED TANK TECHNOLOGIES FOR LNG CONTAINMENT3.1 Full Containment Tank3.2 Membrane Tank3.3 Others
4.0 PERFORMANCE COMPARISON4.1 Intrinsic Key Performances4.2 Implementation Performances4.3 Design limitations
5.0 CARBON FOOTPRINTS COMPARISON5.1 Case Study5.2 System Boundaries5.3 Calculation Principles5.4 Key parameters of influence5.5 Results
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