Mitsubishi Heavy Industries Technical Review Vol. 53 No. 2 (June 2016)
11

Structural Design and Construction Method for“Apple-Shaped Liquefied Natural Gas Cargo Tank”
for LNG Carriers
MICHIHISA WATANABE*1 RYUSUKE TAKADA*2

TAKASHI OKAFUJI*3

HIROSHI TSUJII*4

MASAHIRO KASHIWAGI*3 YOICHI KAMITANI*3

Recently, the demand for Liquefied Natural Gas (LNG) as power-generating fuel has rapidly
increased. On the other hand, the production of a new type of natural gas, due to the developmentof technologies for gas extraction from shale formations (referred to as “Shale Gas Revolution”),
has been expanding mainly in the United States. Due to the increasing demand for this natural gas,
the marine transport of LNG is expected to increase significantly. Currently, regarding thetransport of American shale gas, which has attracted enormous attention, there are concerns suchas restrictions on the dimensions of vessels after the expansion of the Panama Canal andlong-distance transport across the Pacific Ocean. Therefore, it is important for LNG carriers toachieve a high level of safety and improve cargo loading space efficiency. Mitsubishi Heavy
Industries, Ltd. (MHI) developed the 155 km3 Sayaendo LNG carrier in 2011, and has delivered
five vessels of the same model so far. In order to satisfy the needs of our customers, MHI has
completed the development of an “Apple-Shaped Tank” that inherits the features of the existing
MOSS spherical tank, which has enjoyed a great reputation for reliability. As of April 2016, MHI
has received orders for eight new LNG carriers with apple-shaped tanks, and began the
manufacturing of aluminum tanks in October 2015.

|1. Introduction
While accumulating experience in constructing MOSS type LNG carriers, in order to satisfy
the needs of our customers such as increased cargo loading capacity, MHI has sought to increase
the capacity of the existing MOSS spherical tank. In the “155 km3 Sayaendo,” MHI has adopted astretch tank in which a cylinder is inserted into the equatorial portion of the tank in order toenhance the transport efficiency of LNG. In response to the grand transformation of the LNG
transport market, MHI has successfully developed the world’s first “Apple-Shaped Tank,” which isan upgrade of our existing model with the equivalent level of safety, in order to increase the cargoloading capacity while taking the restrictions at the Panama Canal into consideration. We will
introduce the technical details below.

|2. Apple-Shaped Tank
The MOSS-type tank has been a popular choice because its sphere shape provides the moststable strength that reduces the plate thickness of the tank’s surface material. Subsequently, theadoption of a stretch tank has become widespread for the purpose of increasing cargo loading
capacity. On the other hand, the newly-developed apple-shaped tank adopts an axisymmetric tankshape, and inherits the same conventional concept of supporting internal pressure via hoop stress. It
is categorized as a MOSS-type tank, with its considerable amount of construction experience andgreat reliability, and is classified as an IMO Type B independent tank. The tank structure consists
*1
*2
*3
*4

Nagasaki Ship & Ocean Engineering Department, Shipbuilding & Ocean Development Division, Commercial Aviation &Transportation Systems
Ship Engineering Department, Mitsubishi Heavy Industries Shipbuilding Co., Ltd.
Strength Research Department, Research & Innovation Center
Chief Staff Manager, Shipbuilding Department, Mitsubishi Heavy Industries Shipbuilding Co., Ltd.

Mitsubishi Heavy Industries Technical Review Vol. 53 No. 2 (June 2016)12

of a combination of three elements, i.e., a spherical shell, torus portion and cylinder portion, which
achieves high flexibility in terms of the shape and capacity.
The new apple-shaped tank was developed for the purposes of meeting two contradicting
demands, i.e., to deal with the restriction on the ship breadth in order to pass the Panama Canal andto increase the transport efficiency through greater capacity. For example, Figure 1 shows
comparisons between a 180 km3 apple-shaped tank with a 155 km3 stretch tank, as well as with a
180 km3 spherical tank. The adoption of the apple-shaped tank has led to a successful increase incargo capacity by more than 15% without making the hull dimensions greater than the LNG
carriers equipped with stretch tanks.

Figure 1

Comparison of tank types and hull dimensions|3. Development of apple-shaped tank
3.1

Structural design of MOSS spherical tank

Figure 2 illustrates the structure of the existing MOSS spherical tank. The tank surface
material, with no inner structural members, is connected together to form a sphere. The center
portion of the tank height is supported by a cylindrical structure called a tank skirt. The bottom of
the tank skirt is secured to the hull structure. In the center of the spherical tank, a pipe tower isarranged vertically, which contains pipes, cargo handling instruments and access equipment toallow people to enter and exit the tank.

Figure 2

MOSS spherical tank structure

The structural design of the existing MOSS spherical tank needs to conform to therequirements specified in the IGC Code (the International Code for the Construction andEquipment of Ships Carrying Liquefied Gases in Bulk) where a strength evaluation is conducted interms of various different loads, such as the inertial force and sloshing load of the cargo due to theship motion and the heat load due to temperature swings inside the tank from the extreme low of-163°C to room temperature, assuming every possible operational condition in practice. The
spherical tank needs no inner structural members as the tank surface material mainly supports theMitsubishi Heavy Industries Technical Review Vol. 53 No. 2 (June 2016)13

load via membrane stress, which means that the surface material alone is sufficient to stably
support the load. In the actual structural design, in addition to ductile failure due to membranestress, fatigue failure, buckling failure and brittle failure are also evaluated. In order to enhancesafety, LNG leakage is also taken into consideration. A crack propagation analysis is conductedassuming that there are initial cracks on the tank surface material, and it is confirmed that there issufficient duration before the surface material would be penetrated. Furthermore, a simulated
calculation for LNG leakage is carried out assuming that the cracks penetrate the tank surface
material. As a result, it was confirmed that the LNG leakage can be handled within the partial
secondary barrier, and that the gas detector was able to detect the cracks in the early stage when
they were still micro-cracks. This design concept is called “Small Leak Protection” and isincorporated into the MOSS tank, as well as some prismatic tanks.

3.2

Extracting verification items in the structural design of apple-shaped tankIn the structural design of the apple-shaped tank, in order to achieve the equivalent level ofreliability as the existing MOSS spherical tank, the same strength evaluation procedure is applied.
Furthermore, it has become necessary to review a new fracture mode due to changes in form and toestablish an evaluation procedure thereof. Figure 3 shows the design flow of the MOSS sphericaltank. We found that it was necessary to examine the buckling strength and sloshing load evaluation,
which were affected by the changes in form.
Structural Design

Stress Assessment

Design Loads

Membrane Stress Analysis by
Elastic Shell FE modelInternal / External overpressure

Surface Stress Analysis by
Elastic Solid FE model

Inertia force due to ship motion

Interaction Force

Thermal Load

Fatigue Assessment
Sloshing Load
Damage factor assessment

Crack penetration analysis

Buckling Assessment
Proving Small Leak Protection
Crack propagation analysis ofPlate Through Crack
Leakage analysis

Figure 3

3.3

Buckling Strength Evaluation by
Theoretical formula

Additional
Verification
items forapple-shapetank

Buckling Strength Evaluation by
Elastic-plastic large deformation
FE analysis

Design flow of MOSS spherical tank and verification items of apple-shaped tank

Buckling strength evaluation

In the buckling assessment of the existing MOSS spherical tank, there are two types ofbuckling phenomena to be evaluated: the buckling due to external pressure and the circumferential
compressive buckling at the medium liquid level (a phenomenon in which the tank structure near
the liquid surface is pulled toward the meridian and deforms toward the inside of the tank due tothe motion of the fluid, leading to the occurrence of circumferential compressive stress). On theother hand, as shown in Figure 4, for the apple-shaped tank, it is necessary to conduct evaluation
not only regarding the buckling caused by the external pressure and the circumferential
compressive buckling at the medium liquid level, in terms of the spherical shell, torus portion andcylinder portion, respectively, but also regarding “the buckling due to internal pressure” whereMitsubishi Heavy Industries Technical Review Vol. 53 No. 2 (June 2016)14

circumferential compressive stress occurs in the torus portion under the internal pressure load dueto the difference in curvature between individual portions. A long cylinder portion may cause shear
buckling in the cylinder portion due to the ship motion under full load (lateral acceleration).1
Regarding each of the expected buckling phenomena, by conducting evaluation inaccordance with a theoretical design formula taking the initial imperfections based on theconstruction tolerance into consideration and the elastic-plastic large deformation FEA (finite
element analysis), it has been confirmed that sufficient buckling strength to support the design
loads was provided. Moreover, regarding the buckling due to internal pressure, which is a buckling
phenomenon specific to the apple-shaped tank, an internal pressure loading test was conducted interms of two types of models; a 1/20-scale model of an actual tank, and a model in which abuckling due to internal pressure is more likely to occur by a lower curvature in the torus portion
than that of an actual tank. The results of the testing of the individual models and an analysis
thereof are shown in Figure 5. In the test for the model with a low curvature in the torus portion
and with a high possibility of buckling, the buckling mode and loads in the test were consistent
with the simulation analysis, which verified the accuracy of the elastic-plastic large deformation
FEA. In the 1/20-scale model of an actual tank, the test and analysis verified that no buckling
occurs under a pressure sufficiently greater than the designed internal pressure.

Figure 4

List of buckling modes expected in apple-shaped tank

Figure 5

Results of internal pressure loading test (at 1/20 scale) and simulation analysis

In terms of the initial imperfections and plate thickness, which are the major factors for the
buckling strength, it was confirmed that the incidence of the combination of the values used for the
buckling strength evaluation was sufficiently low, by calculating individual variations (probability
distribution) according to the past construction records of spherical tanks. As shown in Figure 6,

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the probability distribution of the buckling strength was calculated using the Monte Carlo
Simulation, where the buckling strength calculation is conducted numerous times by giving valuesof individual factors as random numbers that follow the probability distribution, and assigning
those values to the buckling strength evaluation formula, in order to obtain the failure probability.
As a result, it was confirmed that the failure probability due to the buckling of the apple-shapedtank was expected to be the equivalent of or less than that of the MOSS spherical tank if the initial
imperfections and plate thickness were controlled at the same level as in the existing spherical tank.
Accordingly, we have set new construction tolerances in the torus and cylinder portions of theapple-shaped tank so that the tolerances will be at a similar level as the existing MOSS sphericaltank.
The torus portion has a different curvature for the circumferential and meridian directions.
As the curvature in the circumferential direction varies, the manufacturing difficulty in the bending
process is likely to be higher than the spherical shell. However, through the bending test describedin 4.2 below, we have confirmed that bending accuracy equivalent to the spherical tank can be
secured without issue, and that the newly developed apple-shaped tank realizes a tank structure
which has a buckling strength equivalent to or greater than the existing MOSS spherical tank.

Figure 6 Calculation of probability distribution of buckling strength utilizing Monte
Carlo Simulation

3.4

Strength evaluation for sloshing

Sloshing refers to the movement of liquid having a free surface, causing a large impact
pressure when the liquid inside the tank is undergoing a strong motion and is hitting the tank walls
due to a particular situation of the loading condition, LNG liquid level and sea state. In the existing
MOSS spherical tank, the curved surface makes the liquid shift along the tank walls. Therefore,
compared with prismatic tanks such as membrane tanks, the impact pressure due to sloshing is
negligible. The apple-shaped tank also has a smooth shape that does not concentrate pressure in aparticular area. However, since there has been a gradual change of shape from the spherical tank,
we have verified that it has the same level of excellent sloshing properties as the existing model,
just to be safe.
Upon discussing the sloshing properties here, we have conducted examination in terms ofregular wave excitation and irregular wave excitation. Regular wave excitation is an evaluation
method using a constant excitation amplitude/cycle. In recent years, numerical simulation
technology has developed rapidly, enabling sloshing evaluation based on irregular wave excitation
taking the ship motion into consideration, which is closer to what happens in reality.
Firstly, MHI internally conducted evaluation based on a model experiment using regularwave excitation with a constant excitation amplitude and cycle, as well as computational fluid
dynamics (CFD Analysis), in order to compare the sloshing load between two models: theapple-shaped tank under development and the existing spherical tank. The analysis accuracy wasverified in the model experiment and it was confirmed that the extent of the sloshing load in theapple-shaped tank for the actual project was the same as the spherical tank, and that the increase in
the load was within 10%.

Mitsubishi Heavy Industries Technical Review Vol. 53 No. 2 (June 2016)16*

Secondly, the sloshing load was evaluated by DNV-GL which has a sloshing evaluation
technique under irregular excitation conditions. The evaluation method based on irregular wave
excitation is shown in Figure 7. In this evaluation, it is important to identify the condition where
the sloshing load increases. Therefore, a numerical analysis was conducted through irregular wave
excitation for a duration of about 30 minutes. Then, a screening of conditions where the maximum
load occurs (situations of loading condition, LNG liquid level and sea state) was carried out.
Especially when identifying the sea state, evaluation needs to be conducted for both the sea
state close to the tank resonance period and those in which the inertial force due to the ship motion
is large. The sloshing load was compared between two analysis models of the apple-shaped tankand spherical tank, by conducting a detailed analysis in terms of irregular wave excitation for twohours, utilizing the condition in which the maximum sloshing load identified in the screening
analysis occurred. The results were sufficiently consistent with the results of our study. Evaluationof the experiment/analysis under regular wave excitation and irregular wave excitation concluded
that the sloshing loads that occurred in both cases were the same level (an increase of within 10%).
Based on these evaluation results, in the design of the apple-shaped tank, strength evaluation was
conducted allowing for a 10% increase in the sloshing load, and is even safer than the sphericaltank.
* DNV-GL: International service provider for certification, technical consulting, classification, etc.

Figure 7

Sloshing evaluation method under irregular excitation|4. Construction method of the apple-shaped tank
4.1

Difference from the construction method of the existing MOSS spherical tankIn the construction of the existing spherical tank, cold press bending is applied to thematerial, aluminum plates to form a shape like a flower petal. The petals are welded together by
MIG welding to form a spherical tank. The petals for the spherical tank were bent with the sameamount of R (radius) in every direction, including the meridian and circumferential directions, toform a sphere-shape. Although some of the petals for the apple-shaped tank have a constant amountof R in the meridian direction, a toroidal shape was adopted where the Bending R in the meridian
direction changes according to that in the circumferential direction. Therefore, in the cold press
bending of the petals, because the apple-shaped toroidal petals have a different Bending R in themeridian and circumferential directions, and the circumferential bending changes in accordancewith the meridian bending, it is necessary to bend the petals while always checking the status of thebending. Accordingly, we examined this complex bending mechanism in a simulated bending test
utilizing a miniature model. Based on the results of the test, we established a bending method byselecting the optimum mold shape and pressure conditions.

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In the assembly process of the petals, after the bending process, up to four petals which haveundergone edge preparation are connected together by a large current automatic MIG welding
device to form an assembly block. Subsequently, multiple assembly blocks form a grand assembly
block of a spherical tank which consists of three portions: the Northern Hemisphere, Equatorial
Zone and Southern Hemisphere. The grand assembly block consisting of the three portions is built
on the spherical tank in the building dock. In the torus portion of the apple-shaped tank for actual
ship construction, in order to cope with a different Bending R in the meridian and circumferential
directions, we will manufacture and utilize a new numerically controlled three-dimensional edge
preparation device, a universal supporting jig for the plate joint positioner of the large current
automatic MIG welding device for block assembly, a binding mount for grand assembly, and rails
for the automatic MIG welding device specific for the Bending R in the torus portion.

4.2

Bending of the toroidal portion

Because the aluminum petals are large in size, for the convenience of the press forming
machine, it is necessary to form them by performing press bending multiple times regardless ofwhether spherical or toroidal. There is, however, little research focused on incremental press
bending and its shape estimation. In this project, we determined the shape of the die for actual ships
after providing some shape options, creating 1/20 scaled-down model dies which correspond to theoptions, and evaluating them by conducting press bending multiple times using aluminum alloy
sheet. In connection with this, the dies were designed according to the springback estimation
formula2 for a surface with an anisotropic shape or the simulation results where a single press
forming is assumed. A 1/20 scaled-down model die is shown in Figure 8.

Figure 8 1/20 scaled-down
model dieFigure 10

Figure 9 Aluminum flower petals formed with a 1/20
scaled-down model die

Measurement result by ARAMIS (Deformation contour in the depth direction)

In the press experiment using the 1/20 scaled-down model die, all the plate dimensions werescaled down to 1/20 of the real thing. The press load is related to the bending stress, and since thebending stress is inversely proportional to the square of the plate thickness, the press load was
1/400 times that of the real thing. The post-press shape was measured using ARAMIS, athree-dimensional deformation measuring device that utilizes a digital image correlation technique.
The post-press 1/20 scaled-down aluminum petals are shown in Figure 9. The petals’ surfaces are
painted white to facilitate measurement by ARAMIS. The measurement results by ARAMIS are
shown in Figure 10. The curvature radius in the vertical direction at respective positions in thelateral direction, that was calculated utilizing the measurement results from Figure 10, is shown in
Figure 11. As a result of the press experiment, we adopted die A, which was the closest to thedrawing shape, prepared a die for the actual ship on the basis of this and carried out a full-scale
prototype test for verification. The resulting prototype is shown in Figure 11. As a result, althoughMitsubishi Heavy Industries Technical Review Vol. 53 No. 2 (June 2016)18

the curvature radius became smaller, the curved shape obtained was almost as expected. Currently,
the press load is being adjusted so that the target shape can be obtained. After having grasped theshape variation of each plate, we are in the process of acquiring data to establish standard press
conditions to obtain consistent quality.

Figure 11

Results of press experiment and full-scale prototype test

|5. Conclusion
In order to respond to the needs of our customers for increased cargo capacity and greatertransport efficiency, MHI has developed the apple-shaped tank as an upgrade of the existing MOSS
spherical tank. We have successfully established the design process for the new tank as a result of avariety of technical assessments in terms of the change in shape. The apple-shaped tank has
buckling strength and excellent sloshing resistance properties equivalent to those of the existing
MOSS spherical tank. Furthermore, it is possible to design the tank shape flexibly. MHI willcontinue to actively contribute to the development of the LNG supply chain both at home andabroad, by applying our new innovation to LNG ship design as a key technology for the ongoingdiversification of LNG transport.

References
1. ASME/BPVC CASE N-284-2: N-284-2 Metal Containment Shell Buckling Design Methods, Class MC
Section III, Division 1
2. Nishioka et al., On Automatic Bending of Plates by the Universal Press with Multiple Piston Heads: 1stReport: Fundamental Study，journal of the society of naval architects of Japan（132），481-501，（1972）