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Gas and LNG StorageThe Future of Modular LNG Tanks

Gas and LNG Storage | The Future of Modular LNG Tanks

1. Introduction

1.1 LNG and LNG to Power Market Overview

The LNG supply market has doubled in the last decade to 301.5 MMPTA [1], and it is anticipated that the next decade will see further growth, particularly in the USA, Canada, East Africa and FLNG, increasing by 46% to 443 MMTPA by 2021 based on projects currently under construction. This expansion is associated with very high and increasing LNG liquefaction costs. For those terminals coming on line by 2021 the estimated CAPEX is over $1,500/tonne. Efforts to lower the unit costs of liquefaction has seen a move away from very large scale, bespoke trains to a modular, multi-train approach, based on smaller, midscale 0.5 to 1.0 MMTPA trains, such as Energy World’s proposed plant in Sengkang, Indonesia.

At the beginning of 2016 regasification capacity, or potential demand, was 757 MMTPA, including just over 10% FSRU capacity, but from 2000 to 2015 utilization has remained between 30% to 40%. With capacity only expected to expand to 810 MMTPA by 2021, utilization would need to increase to 44% to meet the estimated increase in demand. Clearly there has not been a lack of regasification capacity for the LNG supply, but some analysts have predicted an oversupply of LNG [3].

Global trade was 245 MMTPA in 2015. The average yearly growth of LNG demand since 2000 has been 6.6% pa. If this continues, demand would reach 358 MMTPA by 2021, which would represent a utilization of 80% on the planned liquefaction capacity by 2021, without allowing for capacity taken offline. Some recent reports [2] have suggested the so called glut in LNG has not materialized, and the numbers above could lend support to that view.

But the LNG market is changing, oil prices are lower, LNG prices are being driven down, even renegotiated, and buyers are seeking shorter term, more flexible contracts. Despite these challenges there was 890 MMTPA of proposed new liquefaction capacity in January 2016, key regions being US, Canada, East Africa and FLNG. Clearly many of these projects will not proceed as they compete for supply contracts, but this should encourage demand side expansion.

A number of factors will drive demand side expansion including conversion to cleaner cheaper fuels for power generation, to either reduce particulate pollution from coal fired power stations or convert from fuel oil. In addition, those countries that seek to honor their COP21 commitments are likely to see natural gas and liquefied natural gas (LNG) as an essential transition fuel to a lower carbon future.

For those countries with an established gas distribution network, large scale regasification terminals, in excess of 1.0 bcfd are appropriate, whereas archipelagoes such as the Caribbean [4], Indonesia and the Philippines need to consider a hub and spoke solution in which large scale LNG imports (7.0 MMTPA) can be distributed by smaller LNG carriers (30,000m3) directly to the power station.

Some companies are now considering vertical integration in which they provide both supply of LNG as well as demand in terms of LNG to Power. According to Anatol Feygin of Cheniere “that will be the major growth for LNG demand going forward” and is a model it is looking to replicated globally [3]. Lower LNG prices are also making the fuel more attractive. However, for those countries that do not have an established gas distribution network the capital costs of receiving, storing and regasification at each power station can inhibit the development of LNG to power projects.

Some companies are now considering vertical integration in which they provide both supply of LNG as well as demand in terms of LNG to Power. According to Anatol Feygin of Cheniere “that will be the major growth for LNG demand going forward” and is a model it is looking to replicated globally [3]. Lower LNG prices are also making the fuel more attractive. However, for those countries that do not have an established gas distribution network the capital costs of receiving, storing and regasification at each power station can inhibit the development of LNG to power projects.


Offtaker Power Station Capacity(MW) Storage Tank Capacity (m3)

2018 (Conv) 2032 (New) Total By 2018 By 2023 Total

The Bahamas, BEC (NP) 393 320 713 70,000 90,000 160,000

The Bahamas, GBPC 240 0 240 20,000 20,000 40,000

Barbados, BL&P 60 245 305 35,000 45,000 80,000

Belize, BEL 62 40 102 5,000 15,000 20,000

Dominican Republic, All 1,025 1,800 2,825 370,000 460,000 830,000

Guyana, GPL 140 240 380 35,000 45,000 80,000

Haiti, EDH 238 560 798 55,000 225,000 280,000

Jamaica, JPS 621 1,320 1,941 140,000 155,000 295,000

Suriname, EBS 299 640 939 50,000 105,000 155,000

Total 3,078 5,165 8,243 780,000 1,160,000 1,940,000

Table 1 LNG Storage Tank Capacities for Caribbean hub and spoke scenario [4]

1.2 LNG to Power Storage Requirements

For the Caribbean the IADB report [4] forecasts gas demand of 490 MMSCFD or almost 23,000m3 of LNG per day. Assuming a hub and spoke scenario for distribution from the Dominican Republic, Table 1 summarizes the storage capacity by country required in 2018 and then expansion through to 2032 to meet the forecast demand.

For perspective a modern combined cycle gas turbine has a thermal efficiency of between 45% to 55%. Therefore a 100MW power station consumes approximately 800m3 of LNG per day or over 24,000m3 per month. The report outlines conversion and new build forecasts for CCGT, single cycle gas turbines and reciprocating gas fueled power stations.

The results highlight a key issue for development of LNG to power projects. The storage capacities at the end of each spoke are relatively small. If the capacity for the Dominican Republic is ignored the average storage capacity is approximately 50,000m3. If Belize is also ignored and a nominal tank size of 20,000m3 is assumed, each phase of development could be based on multiples of a standard tank size. By 2018 the Caribbean market could require 20 x 20k m3 LNG storage tanks with perhaps another 40 x 20k m3 or 20 x 40k m3 LNG tanks by 2023.

The Caribbean is only one example. Other countries, such as the Philippines and Indonesia have much greater demand for power station conversion and new build. Also, projects in Central America are being considered, but based on receiving 150,000m3 LNG carriers unloading to FSRUs or onshore LNG regasification terminals that are effectively oversized for the power station capacity.

1.3 Background to Modular LNG Tanks

The authors have been involved with LNG tank design and development for almost 20 years. In that time the traditional solution for LNG storage in excess of 10,000m3 has been a stick built 9% Ni steel single or full containment LNG storage tanks. Most LNG projects have targeted throughputs greater than 1000 MMSCFD or 7MMTPA. The storage volumes for this size of regasification or liquefaction plant have exceeded 160,000m3. Indeed as the capacity of LNG carriers has increased up to 266,000m3 (Q-Max) the onshore storage tank size has also increased to ensure filling or discharge can be achieved within 24 hours.


Figure 1 27,500 m3 ethane/ethylene/LNG carrier operated by Evergas ©

Relatively little work has been done to develop cost effective storage tank sizes for the LNG to Power market. Tank sizes greater than 160,000m3, required to receive a standard export LNG carrier, would provide 10 months of storage for a 100MW CCGT. Even for a larger power station it is clear that there is a mismatch between the storage tank and the exporting LNG carrier. Smaller carriers exist, using Type C or membrane technology, but there is a definite requirement for smaller ships to support cost effective LNG to power delivery. Ships in the range of 10,000m3 to 30,000m3 would allow smaller marine facilities and be compatible with the required onshore storage.

Presentations at the Trinidad Oil and Gas Conference in 2014 [5] and Gastech 2015 [6] have highlighted the market opportunity for LNG to power and emphasized that the design and delivery of smaller LNG tanks is essential to reduce overall cost and schedule to ensure that the cost base is reasonable and the market sustainable.

Another market that is expected to see significant expansion is the LNG marine fuels business. Eagle LNG has recently completed its project in Maxville, FL, USA and Conrad Shipyard is building an LNG bunkering barge. The LNG volumes for each ship are suitable for Type C storage containers, but aggregated onshore LNG storage tank volumes in excess of 10,000m3 are necessary.

Figure 2 Economies of scale- tank volume [8] or number of tanks [9]


1.4 The opportunitiesThe small to midscale LNG market, supplying power stations or the marine fuels business, requires a smaller capacity LNG storage tank, in the range of 10,000m3 to 100,000m3. The traditional solution based on 9% Ni steel technology is stick built on site. It is well known that the unit price of LNG stored reduces as the single tank size increases [8]. However economies of scale can also be achieved by production volume.

The modular LNG tank seeks to reduce the unit cost for smaller LNG storage volumes by targeting offsite manufacturing productivity levels. The economies of scale are based not on the volume of a single tank but the number of units produced to achieve the required volume.

A good reference case was the production of 25,000 m3 tanks in South Carolina [9]. The estimated productivity improvements, interpolated from the stated productivity for the initial 10 tanks, are shown in Figure 2. It is noted that the first sphere in that project experienced severe component fit up and some welding issues.

Since the basic tank unit can be in the range of 10k m3 to 40k m3, larger total volumes can be achieved with multiple tanks, which can also align with project phasing goals.

The following sections in this paper will provide an update on development of the modular LNG tank concept.

The key modular LNG tank drivers are:

– Standardize tank design by volume based on site specific seismic isolation

– Offsite tank pre-fabrication in parallel with foundation construction

– Dedicated fabrication yard leading to improved productivities and higher quality

– Offsite pre-commissioning of tank

– Reduced manhours executed on site

These drivers target a “plug and play” capability while reducing costs and schedule compared to the stick built traditional solution.









2. Technical Development

Figure 3 Initial Modular Tank Concept [6]

2.1 Initial ConceptThe initial concept [6] was based on either 9% Ni or membrane technology. To reduce the overall weight the modular tank provides single containment capability, thereby eliminating the concrete wall and roof. The tank was erected on a cellular concrete base which provided a robust susbstructure for subsequent transportation by water from the fabrication yard to the project site.

At the project site the tank was supported on bearings, founded on shallow footings or piles. Trenches between the foundations allowed access for the self-propelled modular transporters (SPMTs).

2.2 Current Concept2.2.1 DesignAfter the presentations in 2014 [5] and 2015 [6], specific project opportunities focused further development.

The initial concept considered a maximum volume of 36,000m3, and this was considered to be close to the upper bound of what could, or should, be pre-fabricated and transported, before costs were negatively impacted. However, an opportunity to consider a 40,000m3 single containment design on the US GoM coast provided the basis for the next phase of development. Technical assumptions are presented in Table 2. The updated design in shown in and Figure 5


Figure 4 40k m3 9%Ni Steel Single Containment Modular LNG Tank General Arrangement

Figure 5 40k m3 9%Ni Steel Single Containment Modular LNG Tank Details


Remark Value Remark Value

Design Standards NFPA59A, API625/620 Outer Tank

LNG Storage Tank Type Single Containment Material Steel ASTM A36

Foundation Type Piled supported, elevated Outer tank diameter 40.000 m

Inner Tank Min width annular space 1.250 m

Material 9Ni ASTM A533 Type 1 Dome Roof

Net Capacity 40,000 m3 Material Steel ASTM A36

Gross Capacity 42,696 m3 Spherical Radius 40.000 m

Inner Tank Diameter 37.500 m Insulation Material

Height (ambient) 39.380 m Bottom Cellular Glass

Annular Expanded Perlite

Suspended deck Glass fiber blanket

LNG Product Seismic Design

Temperature -170 oC OBE (pga) 0.037 g

Density (BOG) 440 kg/m3 SSE (pga) 0.074 g

Density (max) 470 kg/m3 Wind

Latent heat of vaporization 511,000 J/kg ASCE 7-05 63 m/s

Design boil off rate (vol) 0.05 %/day Soils US GoM typical

Maximum filling rate 850 m3/hr very soft to firm cohesive 0-30 ft

Max out pumping rate 2,250 m3/hr firm to stiff cohesive 30-100 ft

Pressures slightly over consolidated >100 ft

Maximum design pressure 190 mbar

Minimum design pressure -5 mbar

Table 2 40k m3 Single Containment Tank Design Data

The key technical developments are summarized below.

– The tank is elevated above ground to provide both space for the SPMTs and also air flow to eliminate base slab heating.

– The cellular concrete base slab is replaced with a steel grillage and concrete deck. This reduced weight which is a significant issue for the larger tank volume.

– 9%Ni was chosen over membrane based on owner preference and concerns over permitting delays that might arise since membrane tanks have not yet been approved by FERC. This issue is discussed further in the next section 2.3.2.

– Side wall discharge is proposed. This is consistent with NFPA 59 and if in-tank shut off valves are provided the design spill is significantly reduced. The tank elevation also ensures that the pump does not need to be recessed below ground to achieve the minimum NPSH. Typical details were presented at LNG 12 [10] refer to Figure 6. The results of the techno-economic evaluation concluded that side wall pump discharge could reduce costs by up to $6MM for a 2 x 140k m3 storage tanks (1998 prices). But the prize is even greater for the modular LNG tank. Not only is the pump platform significantly reduced in size, refer to Figure 7, but the tanks can be manifolded reducing the total number of pumps. The pumps can also be located outside of the bunded areas with easy access for maintenance.

– For larger total volumes, based on multiple units, the modular LNG tank will require individual bunded areas. This area can be optimized based on the work carried out by Coers (2005) [11].


2.2.2 ExecutionBased on the design described above an execution plan was developed working with Great Basin Industries and Mammoet. The overall scope of work was divided into a number of work packages as summarized in Table 3.

The following notes highlight some important issues regarding the execution plan.

– Fabrication yards do exist along the US GoM coast. The work to date has not undertaken a detailed evaluation of potential sites, but greenfield development is also an option. This approach will increase the initial start-up costs and therefore it has been assumed an existing facility will be utilized.

– Fabrication facilities are not limited to the project country, indeed the modular LNG concept envisages regional fabrication yards that will support LNG storage tank in that area, thereby reducing the shipping times and costs.

– The hydrotest is not carried out at the fabrication yard. It was concluded that owners and or regulators may require proof that the 9%Ni inner tank was not damaged during transportation. Transferring the test to the project site significantly reduces the foundation loads at the fabrication yard.

– The inner and outer tanks are erected as complete prefabricated rings in a stepped sequence starting with the outer tank then the inner tank. A linear layout for multiple tank erection is shown in Figure 8. A heavy lift crane is used for ring installation.

– The roof is prefabricated as one piece and lifted into position. No air lift is envisaged.

– After roof erection the bottom insulation and inner tank bottom plate can be installed, providing weather protection to the insulation.

– It is assumed that the fabrication yard has a bulkhead suitable for load out of 5,000 te, however temporary loading ramps can be used, founded on a piled ground beam where soil conditions are not strong enough.

Figure 6 Proposed side entry pump suction nozzle for a single integrity LNG tank [10]

Figure 7 Comparison of roof platforms with and without side wall discharge (courtesy Cheniere and Coers [11])


Figure 9 Modular LNG Tank Transportation (courtesy of Mammoet)

Figure 8 Modular LNG Tank erection (courtesy of GBI and Mammoet)


– The tank will be moved on to the transportation vessel using SPMTs. Whether the SPMTs remain for the duration of the tow depends on distance. For short tows the SPMTs will travel with the tank, although the tank will be lowered on to temporary supports on the transportation vessel. For long tows (more than several days) two sets or SPMTs are require, one at the fabrication yard and one at the project site.

– Sea fastenings will depend on the specific tow route. For inland water way tows or sheltered water tows initial calculations indicate vessel motions will not require any seafastening for the inner tank. The outer tank will be fastened to the vessel deck. For longer tows or open water tows, temporary sea fastening of the inner tank will be required. Calculations have shown that the inner tank top ring stiffening and or shell thickness could be increased to cater for the inertial loading. Alternatively temporary restraints to the outer tank shell will provide resistance to the inertial loads. These restraints can be removed once the tank is installed at the project site and prior to hydrotesting.

– Enabling works at the project site are relatively modest and cost effective. For load below 5,000 te temporary unloading ramps can be used. This will be founded on a piled ground beam. Temporary onshore mooring onshore points will be required for a traditional Mediterranean spread mooring pattern.

The key benefits of the proposed execution plan are

1. Tank erection is not waiting on construction of the project site tank foundation.

– Regulatory processes normally prevent any construction on site before project permits have been secured.

– Many LNG sites require significant enabling works including, but not limited to, bulk earthworks before foundation construction can commence.

2. Tank fabrication and erection can start once material is procured and delivered to the fabrication yard.

– Many large LNG tanks have seen lead times for 9% Ni steel plate of 12 to 18 months. This is very market dependent but it has mitigated the schedule delay waiting for foundation construction and outer wall construction.

– Material pre-ordering can reduce the lead times, and financial commitments prior to final regulatory approval can further reduce the schedule.

– Tank erection commences with fabrication and erection of the steel grillage and outer tank carbon steel outer tank rings. This material is on much shorter lead times.

– Based on an established fabrication yard, tank erection can commence well ahead of a stick built tank at the project site.

3. Significant, labor intensive activities are transferred from the project site to a dedicated fabrication yard.

– Project site, stick built tanks are often remote from large resource centers, reducing productivity and or increasing labor costs.

– Specialist welders are required for the inner 9% Ni tank which incurs a premium for remote sites. Further, in tight labor markets, the transient labor force may be difficult to secure, whereas an established fabrication yard can provide a more reliable resource.

4. Improved productivities and quality – An established fabrication yard focused on tank

fabrication can invest in training and equipment to increase productivities and reduce costs.

– Prefabrication of tank parts can be done in covered areas, further increasing productivities and workmanship quality.

Tank prefabrication Tank Transportation Project Site

Fabrication yard enabling works Supply of all heavy lift equipment Enabling works for receiving tank

Tank fabrication line foundations Supply of all marine equipment Construction of tank foundation

Material procurement Load out at fabrication yard Hydrotesting

Steel grillage fabrication Tow to project site Perlite insulation

Tank ring prefabrication Offload at project site Tank hook up

Tank erection Set down at project site on plinths Bund construction

Tank roof prefabrication Demobilization Final pre-commissioning of tank

Roof Erection Ready for cooldown

Pre-commissioning

Preparation for transportation

Table 3 Execution Work Packages for 9% Ni steel single containment modular LNG tank


Figure 12 Incheon LNG Terminal founded in elastomeric bearings

Figure 11 Peru LNG Tank 130,000m3 with 256 Triple PendulumTM bearing (courtesy of EPS)

Figure 10 23m diameter tank under tow (courtesy of Smith Group)


2.3 Further Development2.3.1 Standard Tank Design by VolumeThe work carried out on the 40k m3 modular LNG tank confirmed technical feasibility and schedule advantages over a stick built solution. It also highlighted the importance of fabrication yard set up costs. When these are spread over many tanks they are not significant, as for any pre-engineered, manufactured product. To ensure that competitive pricing is achieved from the start it was recognized that offsite pre-fabrication should not be delivered on a bespoke design basis for each project. The modular LNG tank concept would be enhanced if standard designs could be offered for any site, anywhere in the world.

A standard tank design would permit the fabricator to further improve its fabrication and erection methods. Key site specific drivers for modular LNG tank design are:

1. Soil conditions and foundation design2. Seismic conditions and inertial loads on tank and

foundation3. Other environmental loading conditions (such as wind

and snow loading)4. Temperatures and effect on insulation design5. Tow route, duration and storm conditions

The soil conditions will always be site specific and provided settlement criteria are satisfied then there is no direct impact on the modular tank design, except for seismic response.

Other environmental loading conditions are not significant drivers of tank shell and roof quantities and conservative assumptions could be made to eliminate this variation.

Preserving a standardize design is always a compromise. Perlite insulation could be maintain a constant thickness and heat leak variations addressed by changes in the roof and base insulation thicknesses. This would impact the overall height of the tank and is not necessarily the most efficient solution. Further work will be required to understand the sensitivity to this issue, but if insulation properties cannot be easily adjusted for a given thickness then conservative insulation thicknesses could be appropriate.

Tank response during the tow has been investigated. It is clear that any extreme motions that would impact the basic tank design can be addressed with temporary sea fastenings and strengthening to the outside of the tank which can be ultimately removed and reused.

The key driver on tank shell design and quantities is seismic loading. This is the most significant lateral load on the tank and in areas of moderate to high seismicity, will govern the tank geometry and shell weight. Some tank designs have adopted seismic isolation to reduce the inertial loading and shell quantities, refer to Figure 11 and Figure 12. According to Earthquake Protection Systems Inc. (EPS) [12] an 85% reduction in seismic loading was achieved, which reduced the overall cost of the tank construction.

Despite the cost savings on the Peru LNG tanks, seismic isolation is not the default approach for dealing with moderate to high seismic loads. Lowering the tank aspect ratio (H:R), using inner tank straps to prevent uplift and advanced nonlinear dynamic soil structure interaction (DSSI) can be used to lower the inertial load effects on the tanks. Seismic isolation automatically elevates the tank and introduces a second foundation or base slab. This increases schedule and cost, to which the isolator cost is also added.

For the modular LNG tank these costs are already included and the elevated tank is part of the overall concept to allow for installation using SPMTs. In fact the modular LNG tank is very well suited to adopting seismic isolation because all components are included in the existing design for other reasons.

Initial calculations confirm that tuning the elastomeric bearing will lower the seismic loads to those of the base design. The base design could be chosen utilizing the 33% over stress permitted under the Operating Basis Earthquake (OBE). For areas of high seismicity, friction pendulum bearings of the type provided by EPS may be required. The solution for any specific site requires a detailed analysis of the tank foundation system incorporating isolators. It is important that the foundation system (shallow or deep) is incorporated into the model, because significant reduction in loads can arise due to non-linear response in the soil resulting in longer period response and higher levels of damping.


Seismic isolation results in longer period response which is accompanied by an increase in tank transient displacements. This will impact the design of incoming pipework but experience has shown that differential movements can be accommodated in the piping design. If displacements are considered too high then viscous dampers can be added to the isolation system to reduce peak displacements.

Isolation of vertical ground motions is not as common, and has not been proposed for LNG tanks to date. Vertical accelerations will increase the effective weight of the LNG and therefore the hoop stresses. In areas of high seismicity, such as the west coast of the US, peak spectral accelerations approaching 1g can occur, but careful DSSI can mitigate these effects.

Long period ground motions cannot be isolated and these give rise to sloshing effects on the liquid surface. The codes are clear on the requirements for freeboard under both OBE and SSE conditions. As seismic intensity increases, the freeboard height for a given tank aspect ratio increases. To preserve a standard tank design, baffles could be installed on the underside of the roof to disrupt the sloshing wave, but this is a novel approach which might not be acceptable to owners or regulators. Alternatively, it is accepted that the tank height must be increased to address this issue. However it would require only a minor height adjustment to the standard tank design.

Further work is required to understand the variations and impact that vertical and horizontal seismic accelerations have on the modular tank design, but initial results are encouraging and a standardized tank design is possible, which should translate into further reductions in cost and schedule.

2.3.2 Membrane Modular LNG TankMembrane tanks are not new, indeed more than 100 onshore membrane tanks have been built since 1972, and over 85% of all LNG carriers utilize the membrane technology solution. Two membrane tanks are currently under construction for Energy World Corporation at Sengkang, Sulawesi, Indonesia and Pagbilao, Philippines. In addition there have been recent developments in international codes to recognize and incorporate design provisions for membrane tanks. Nevertheless, the dominant tank technology for LNG storage remains 9% Ni steel. A description of the membrane technology and comparison with above ground 9% Ni storage tanks is presented by Ezzarhouni etal (2016) [7].

Whilst this comparison was for a full integrity or full containment design there are many attributes of the system that are compatible with the objectives of the modular LNG tank and would enhance the overall concept, further lowering the costs and reducing the schedule.

Figure 13 Effect of seismic isolation on acceleration and displacements [13]

Figure 14 Top view of the bottom floor showing membrane system (courtesy GTT)


These benefits are summarized below and quantified in Section 3:

– GTT has developed a highly modular membrane system based on pre-engineered, manufactured components. This is well aligned with the objectives of a standardized tank design.

– There is only one structural tank and it is located on the outside. The inner 9% Ni and outer A36 shells are replaced with a 1.2mm stainless steel liner and A537 Class 2 outer shell. Total steel weight and costs reduce significantly.

– Stainless steel and A537 Class 2 have much shorter procurement lead times and will continue to exhibit much lower price volatility.

– The total volume of wall insulation, based on PUF filled plywood boxes, is less. Hence, for the same overall external tank diameter and volume the corresponding tank height is reduced, further reducing the shell quantities.

– The tank transportation weight is lighter than the 9% Ni steel option, despite having all insulation installed prior to load out.

– Membrane tanks do not require hydrotesting. Leak tightness is demonstrated through the ammonia leak test. Foundation proof loading is of questionable value even for 9% Ni LNG tanks and is not required for membrane LNG tanks which use polyurethane foam (PUF) bottom insulation.

– No hydrotest means that the tank can leave the fabrication complete with all insulation installed and fully pre-commissioned. After installation at the project site the ammonia leak test could be rerun to satisfy the owner and regulator that no damage was sustained during the sea tow.

– The design is fundamentally more robust with respect to transportation loadings. Recalling that 85% of all LNG carriers use the technology it is a well proven technology able to accommodate the strains associated with vessel motion. Further, all transportation loads can be designed into the outer tank which can easily accommodate seafastening and temporary strengthening. There is no thin walled inner shell to seafasten.

Additional design benefits of a membrane LNG tank are:

– Thermal cycling of 9% Ni tanks is not recommended because of the inner tank radial movements. However, the membrane tank is not subject to the same constraints as the liner accommodates the thermal strains within the stainless steel corrugations.

– The membrane insulation space is maintained under a nitrogen purge which is continuously monitored. This is considered a more effective method of leak detection than temperature sensors which rely on a spill of LNG rather than vapor.

– The membrane liner permits the use of sumps in the tank bottom thereby increasing the net useable tank volume.

– In summary, the membrane modular LNG tank takes important steps towards the “plug and play” objective.









3. Comparison of 9% Ni Steel and Membrane Tanks

Dimension 9% Ni Modular LNG Tank Membrane Modular LNG Tank

Net LNG storage volume (m3) 40,000

Outer tank diameter (m) 40.000

Inner tank diameter (m) 37.500 38.800

Design Maximum Liquid Level (m) 38.802 36.280

Outer tank height to roof joint (m) 42.280 39.460

Roof rise (m) 5.365 5.365

Overall tank height from ground (m) 50.447 47.627

3.1 QuantitiesTable 4 and Figure 15 summarize the principal dimensions of the 9% Ni and membrane modular LNG tanks.

Table 5 compares the weights, and thereby the quantities, for 40k m3 9% Ni and membrane modular LNG tanks. The following notes explain the key differences.

– The outer tank shell weights are similar weight. The membrane tank is the same diameter, but is shorter because of lower wall and base insulation thicknesses. The membrane tank uses ASTM A537 Class 2 steel compared to A36 for the 9%Ni tank. This is a stronger steel and whilst more expensive per tonne, is more efficient in terms of weight and subsequent welding costs. Bottom shell thickness is 23mm compared to 16mm for the 9% Ni tank.

– The inner tank compares the weight of ASTM A533 Type I 9% Ni steel with 1.2mm A304L stainless steel membrane. Since the membrane is not structural the weight is substantially less, saving 476te on the inner tank weight.

– Roof insulation weights are similar, however the PUF insulation system shows a saving in weight of 336te over the perlite, resilient blanket and foam glass blocks used on the 9% Ni tank.

– The elimination of the inner structural tank and use of PUF insulation has resulted in overall weight savings of 20%. Further the membrane transportation weight is less than the 9% Ni which excludes the perlite.

These results demonstrate that the membrane tank is a lighter design than the 9% Ni steel tank.

3.2 Schedule and Cost A comparison of construction schedules is shown in Table 7. The schedule is based on an EPC contract, with all design data, including soils information available at notice to proceed. The membrane tank is estimated to be ready for transportation at the same time as the 9% Ni but the overall schedule is 2 months quicker because there is no hydrotest and annular insulation to complete at the project site.

Costs are sensitive to local labor conditions and material costs. The costs have, therefore, been normalized and compared to a traditional stick built single containment LNG tank at 100%.

Table 4 Comparison of principal dimensions for 9% Ni and membrane modular LNG tank


Item 9% Ni Modular LNG Tank Membrane Modular LNG Tank

Total (te) Transport (te) Total (te) Transport (te)

Outer Tank

Shell 504 504 544 544

Base 74 74 74 74

Roof 107 107 107 107

Inner Tank

Shell 467 467 45 45

Base 66 66 12 12

Insulation

Bottom 502 502 234 234

Wall 368 300 300

Roof 50 50 54 54

Pump Platform 350 350 350 350

Grillage

Concrete 990 990 990 990

Steel 212 212 212 212

Sub-total 3,690 3,323 2,922 2,922

Contingency 554 498 438 438

Total 4,244 3,821 3,360 3,360

Table 5 Comparison of tank weights for 9%Ni and Membrane Modular LNG Tanks

Figure 15 General arrangement for 9% Ni and membrane modular LNG tanks


9% Ni Single Containment 9% Ni Modular LNG Tank Membrane Modular LNG Tank

100% 90% 80%

Table 6 Cost comparison of 9% Ni and Membrane Modular LNG Tank 40k m3

Table 7 Comparison of schedules for 9% Ni and Membrane Modular LNG Tanks

Activity Months from notice to proceed

9% Ni Modular LNG Tank Membrane Modular LNG Tank

Notice to Proceed 0 0

Purchase and fabricate material +5 +4

Grillage construction complete +6 +5

Outer tank erection complete +14 +10

Inner tank erection complete +14 +16

Roof installation complete +15 +11

Insulation complete at fab yard +17 +17

Transport and set tank +18 +18

Hydrotest +19 n/a

Insulation complete at project site +20 n/a

Final pre-commissioning +22 +20

Ready for Cooldown +22 +20


4. Conclusions

The ongoing development work on the modular LNG tank concept has confirmed technical feasibility of both 9% Ni and membrane solutions. The membrane option will offer a more robust design for transportation and also lower costs and shorter schedules.

More importantly, the concept of a cheaper and quicker prefabricated small to medium sized tank with “plug and play” capability, based on a standard design that can be installed for any site, anywhere in the world is achievable.

Single containment is not appropriate for all projects and jurisdictions. Full containment options are too heavy to transport cost effectively, but initial work looking at precast wall panels and wire wound prestressing as used in the water tank industry, combined with the membrane technology should offer cost and schedule savings.

The small to mid-scale LNG and LNG to Power markets require smaller tanks. Cheaper and faster, smaller tanks will greatly assist this developing market.


References

1. IGU (2016), “2016 World Energy Report”, International Gas Union2. Shell (2017) “Shell LNG Outlook 2017”, http://www.shell.com/

energy-and-innovation/natural-gas/liquefied-natural-gas-lng/lng-outlook.html

3. Shiryaevskaya, A., Burkhardt, P., (2017), “Hottest thing in LNG is producing power as record glut looms”, Bloomberg news article 18 January 2017, https://www.bloomberg.com/news/articles/2017-01-18/hottest-thing-in-lng-is-producing-power-as-record-glut-looms

4. Castalia (2015), “Natural Gas in the Caribbean – Feasibility Studies, Revised final report (Vol I and II)”, Report to the Inter-American Development Bank, 30 June 2015.

5. Raine, B., (2014) “Onshore Mid-Scale LNG Terminal Storage and Modularization”, Trinidad Oil and Gas Conference, May 2014

6. Raine, B., Powell, J., (2015), “Onshore Mid-Scale LNG Terminal Storage Modularization”, Gastech 2015, Singapore, 29 October 2015.

7. Ezzarhouni, A., Powell, J., Elliott, S., (2016) “Why a Membrane Full Integrity Tank?” LNG 18, Perth, PO-8, 11-15 April 2016

8. Long, B., (1998) “Bigger and Cheaper LNG Tanks? Overcoming the obstacles confronting freestanding 9% Nickel Steel Tanks up to and beyond 200,000m3”, LNG 12, Perth, 4-7 May 1998, Paper Session 5.6.

9. Veliotis, P.T., (1977) “Solution to the Series Production of Aluminum LNG Spheres”, Society of Naval Architects and Marine Engineers Transactions, Volume 85, 1977, pp 481-504.

10. Antalffy, L. P., Aydogean, S., De la Vega, F. F., Malek, D. W., Martin, S., (1998) “Technical-economic evaluation of pumping systems for LNG storage tanks with side and top entry piping nozzles”, LNG12, Perth, 4-7 May, 1998, Poster Session B.8

11. Coers, D, (2005) “Transshipping LNG – Downscaling Field-Erected Storage Tanks for Lower Profile”, 2005 (Presentation with photos provided by CB&I).

12. Peru LNG, Melchoriate, Peru, “Triple Pendulum bearings protect critical storage tanks”, Earthquake Protection Systems Inc, http://www.earthquakeprotection.com/pdf/Peru_LNG_Dec08.pdf

13. Symans, M. D., “Seismic Protective Systems: Seismic Isolation”, FEMA, Instruction Material Complementing FEMA 451, Design Examples, Seismic Isolation 15-7-1, http://www.ce.memphis.edu/7119/PDFs/FEAM_Notes/Topic15-7-SeismicIsolationNotes.pdf

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