Mitsubishi Heavy Industries Technical Review Vol. 50 No. 2 (June 2013)12

New LPG Carrier Adopting Highly Reliable Cargo Tank- IMO Tank Type B -HIROSHI TAMURA*1

HIROTOMO OTSUKA*2TOSHINORI ISHIDA*1

SATOSHI MIYAZAKI*3

Today, with the growing Liquefied Petroleum Gas (LPG) consumption in emerging countries
and the growing production of associated gases from natural gas and shale gas, it is expected thatseaborne trade of LPG will increase and its trade routes will be diversified. Mitsubishi HeavyIndustries, Ltd. (MHI), which has continued to develop LPG carriers to meet the needs of the era,
has focused mainly on building an 83,000m3 type LPG carrier as a flagship model to meet large
capacity demands since 2006. On the occasion of latest upgrade aimed at the improvement of fuelefficiency and environmental friendliness, we newly developed a cargo tank to realize customer
benefits of high reliability and low maintenance costs. This tank has already been approved byseveral classification societies and the United State Coast Guard (USCG), and has been applied toan LPG carrier delivered in 2012. We will actively conduct sales activities and continue to improvefurther to meet customer needs.

|1. Introduction
MHI has been building LPG carriers for a half century since 1962. Since building theNichiyuh Maru in 1989, we have built VLGCs (very large gas carriers) for various customers; the78,000m3 series has been the de-facto standard and now the 83,000m3 series has been included in
the lineup since 2006. We have now innovated the hull form, structural designs and layouts to meet
recent trends of strict environmental regulations and to enhance the performance competitiveness.
In our newest innovation, we have adopted IMO Type B tanks to provide customers with furtherbenefits.
This paper describes the features of this tank system, such as structural differences from theconventional tank system, operational advantages, etc.

|2. LPG carrier tank types
In LPG carriers, the cargo tanks which are independent from the hull structure are arranged
in cargo holds and supported by tank supporting systems. Table 1 shows a comparison of tanktypes for LPG carriers. Small to medium-sized LPG carriers with 20,000m3 or smaller capacities
commonly have cylindrical Type C tanks or combinations thereof and transport LPG underpressurized conditions at ambient or lower temperatures. As this Type C tank system results in asignificantly thick tank skin for its large capacity, however, large LPG carriers with 30,000m3 orlarger capacities commonly apply independent prismatic tanks and carry LPG under atmosphericpressure at lower temperatures. The Type A tank, which is commonly used in the independent
prismatic type, is designed based on a conventional structural design standard. In order to avoidhazard, the Type A tank requires a secondary containment structure (secondary barrier) around thetank that is sufficient to maintain the entire tank volume on the assumption of a major cargoleakage. The surrounding structure of the entire hold space is designed to serve as a secondarybarrier. Large LPG carriers in service, including those built by other manufacturers, all use theType A tank system. Meanwhile the Type B tank is designed based on fail-safe design concepts.
*1Engineering Manager, Nagasaki Ship & Ocean Engineering Department, Shipbuilding & Ocean Development*2

Manager, Nagasaki Ship & Ocean Engineering Department, Shipbuilding & Ocean Development*3Nagasaki Ship & Ocean Engineering Department, Shipbuilding & Ocean Development

Mitsubishi Heavy Industries Technical Review Vol. 50 No. 2 (June 2013)13

Through advanced and detailed analyses, it has been proven that the Type B tanks have sufficientfatigue strength to prevent crack penetration during the design life, even if fatigue defects occur,and that the leakage should be limited during the prescribed duration, even if leakage due topenetration is detected. Hence, it is only necessary to have a partial secondary barrier to hold only
the envisaged maximum leakage, which should be limited. The typical Type B tank is used in
MOSS-type LNG carriers, where each tank has a drip tray as its partial secondary barrier.
We have been building both large LPG carriers and MOSS-type LNG carriers for years, andwe have accumulated technological expertise in designing and building independent prismatictanks through Type A LPG carriers, as well as in designing and analyzing Type B tank systems
through MOSS-type LNG carriers. Combining this expertise, we have newly developed theindependent prismatic Type B tank.
Table 1

Tank typeComparison of tank types

Independentcylindrical

Independent prismatic(Reference)
MOSS-type(Independent spherical)

Pressurized at ambient
temperature or lower
temperature

Fully refrigerated at atmosphericpressure

Fully refrigerated at
atmospheric pressure

Structure

IMO tank type
Secondary barrier
Notes

Type CNo requirements
For small vessels lessthan approx. 20,000m3
capacityType A
Complete

Type B
PartialType B
PartialFor large vessels

For LNG carriers|3. Development of LPG carrier with Type B tanks
3.1

Features of the developed LPG carrier

The principal particulars and general arrangement of the new LPG carrier are shown in
Table 2 and Figure 1, respectively. The main dimensions and layout are the same as its
predecessor in order to maintain compatibility with shore terminals. The main improvements fromits predecessor are:- Performance: 5% better propulsion performance is obtained by improving the hull form
based on CFD and model tests;- Layout:a simpler structure is achieved by converging fuel tanks between the cargohold and the engine room to reduce narrow compartments in the double-hullstructure, and a space for installing a ballast water treatment system isprovided at the forward bottom in the engine room;- Reliability: highly reliable Type B tanks are adopted.
Table 2Comparison of principal particulars among LPG carriers with 83,000m3 capacity

Length × breadth × depth
Draft (design/summer)
Cargo capacity
Tank type
Secondary barrier
Insulation
Service Speed
Main engine output

Conventional
219.0m × 36.6m × 21.65m
11.15m/11.60m
83,000m3
Independent prismatic
Tank Type A
Complete secondary barrier
Polyurethane foam
17.0kn
NR: 12,330kW
NR: 11,700kW

Newly developed
219.0m × 36.6m × 21.65m
11.10m/11.55m
83,000m3
Independent prismaticTank Type B
Partial secondary barrier
Polyurethane foam
17.0knMR: 13,700kW
MR: 13,000kW

Mitsubishi Heavy Industries Technical Review Vol. 50 No. 2 (June 2013)
14

Figure 1 General arrangement of the newly developed LPG carrier with
83,000m3 capacity

3.2

Structural design of Type B tanks

Verification of the reliability of prismatic tanks, which have to meet strict safety standards, iscrucial and also entails technically difficult challenges. These challenges result from the fact that a
prismatic tank with a stiffened plate structure has a much longer total weld-length than a sphericaltank, the internal structure is complicated and there are many structural discontinuities and that a
precise understanding of tank behavior under extreme wave conditions including the interaction
between the tank and the hull is necessary because prismatic tanks are not welded on the tanksupport systems for thermal-contraction reasons.
The IMO Type B is one category of tank that complies with the safety requirements forliquefied gas carriers (IGC code) established by the International Maritime Organization (IMO).
For this type of tank, fatigue and fracture mode analyses should be precisely conducted for thestress on each section of the structure, which can be accurately determined using an advanced
analysis technique. In addition to the evaluation of fatigue crack initiation life, a safety assessment
based on behavior prediction is required in the event that a crack is initiated and propagated. To
meet the strict requirements, the Direct Loading Analysis Method (MHI-DILAM), which is a
proprietary integrated wave load and strength analysis simulation system, is applied to the design
for a highly-reliable tank structure. Figure 2 is a flowchart of the Type B tank design process.

Figure 2

Design flow chart of Type B cargo containment system

Mitsubishi Heavy Industries Technical Review Vol. 50 No. 2 (June 2013)15

Further to ship motion and wave load analyses, which are essential for direct loading
analysis, a stress and buckling assessment is performed. Here, MHI-DILAM reproduces theextreme conditions that occur roughly once every 25 years in the North Atlantic Ocean regarded as
the world’s most severe marine meteorological condition. The Finite Element Analysis Method(FEM) is used to evaluate the structural response under the extreme conditions (Figure 3). It isnoteworthy that MHI-DILAM sufficiently evaluates the complicated behavior of the tanksupporting structure (which sandwiches resin blocks between the hull and tank) of a prismatic tank.
In the structural design process of an independent prismatic tank, it is essential to know thecomplicated interaction between the tank and the hull. In that process, non-linear effects such as thereaction forces of the supporting system or the frictional forces on the resin blocks, which vary withthe contact conditions, have to be taken into account. Non-linear FEM analysis using MHI-DILAM
enables the faithful simulation of the behavior, and allows the design of highly rational and reliabletank structures.
Fatigue strength assessment with MHI-DILAM uses a two-phased approach, “screeninganalysis” and “hotspot analysis.” The former uses a large-scale full ship FEM model, and the latter
uses a local fine mesh FEM model. This approach verifies many of the structural discontinuities ofa prismatic tank without omission, and correctly identifies the critical locations among structuralmembers. Then, the fatigue life is accurately evaluated by full spectrum analysis (Figure 4).

Figure 3 Simulation of ship motion and structure
response under extreme conditions

Figure 4

Example of hot spot analysis

In the fracture mode analysis, the stress field around a crack tip was calculated based on
detailed analysis results obtained from MHI-DILAM, and the propagation was simulated by
predicting its path and growth rate (Figure 5). In the leak analysis, leakage from a micro crack isestimated from the stress fluctuation around the crack in order to verify that LPG leakage can bedetected and the gas can be stored safely over 15 days as required by the IGC code. These analyses
enable the design of highly-reliable tanks that meet the IGC code requirements in all locations
inside the cargo tanks.

Figure 5

Simulation of fatigue crack propagation

Mitsubishi Heavy Industries Technical Review Vol. 50 No. 2 (June 2013)16

Through the fatigue analysis, etc., the detailed structural design of the new LPG carrier wasimproved based on a tank structure with a proven and trusted experience. For example, it has softer
girder ends, better bracket shapes of the tank supporting structure and a lower stress concentration
at the end connections of the longitudinals than its predecessors.
In addition, we quantitatively evaluated the damage risk of the structural members based on
extensive and detailed reliability assessments through the screening and hotspot analyses describedabove. Based on the evaluation, we provide customers with guidelines describing priority
inspection points. The installation of inspection platforms at such points ensures effective
maintenance and inspection.

3.3

Secondary barrier

A Type A tank system has a complete secondary barrier covering the entire cargo hold, but aType B tank system has a partial secondary barrier that is sufficient to contain only the envisaged
cargo leakage. This newly developed LPG carrier has a partial secondary barrier covering the entire
inner bottom plate and lower part of the hopper/stool plate to fulfill its functions under trim andheel conditions. The extent of the secondary barrier is determined based on quantitative evaluationconsidering liquid vaporization by heat ingress and the liquid storage capacity (15 days) with safetymargin calculated as the ratio between the allowable leak rate and the design leak rate from thefracture mode analysis. Where the hold bottom is trapezoidal in shape at the forward and aft end of
the cargo hold, the secondary barrier is designed assuming that the leakage will accumulate there
under heel condition. The secondary barrier of this newly developed tank is made of low
temperature steel in accordance with the IGC code, and the extent of the secondary barrier is muchsmaller than a conventional complete secondary barrier. Figure 6 is an outline view of the partial
secondary barrier.

Conventional (Type A)

Figure 63.4

Newly developed (Type B)

Comparison of secondary barriers

Thermal insulation of tanks

The thermal insulation system of the Type B tank is the same as those found in conventional
Type A tanks, but some modifications have been made to its forming-in-place method. In the caseof a Type B cargo containment system, it is necessary to provide a function to lead the leakage tothe secondary barrier and prevent leakage from spraying (spray shield) so that the hull structureoutside of the secondary barrier is not exposed to low temperatures. In the forming-in-placemethod, the insulation firmly bonds with the tank skin due to its self-adhesion mechanism. Hence,
this method requires a mechanism to lead the leakage to the secondary barrier after cracks occur.
This newly developed tank has leakage channels called “leak paths” on the surface of the tank as
the mechanism. The performance of the leak paths and validation of the spray shields were tested in
the presence of the classification societies. As a result of the test, it was confirmed that theinsulation system satisfies the required leak rate and functions properly as a spray shield.

3.5

Approval from classification societies

During the design concept phase (2009), “Approval in Principle (AIP)” was obtained fromthree organizations: the American Bureau of Shipping (ABS), Lloyd's Register of Shipping (LR)and Nippon Kaiji Kyokai (NK). During the basic design phase, “General Approval” was obtained
from the above three organizations in 2010, and “Concept Approval” was obtained from USCG in2012.

Mitsubishi Heavy Industries Technical Review Vol. 50 No. 2 (June 2013)17|4. Benefits of LPG carriers with Type B tanks
Type B tank provides the following main benefits for LPG carriers:
Highly reliable tank: Detailed analyses have proven sufficient safety performance and therisk of fatigue damage is very low.
(2) Easy and effective maintenance: As the inspection guidelines showing the specific area to bechecked enable effective and consistent maintenance after entering service, it is possible tosustain structural reliability during the ship lifetime.
(3) Low maintenance cost: A conventional cargo hold is entirely made of low temperature steel,but the cargo hold for a Type B tank is made of ordinary steel excluding the bottom area,
which works as the partial secondary barrier. As a result, maintenance or repair is easy if thehull is damaged from contact with pier fenders or tug boats, or if the top side tanks aredamaged by corrosion.
As it is considered that future trade routes will be changed from the conventional tradepattern between the Middle East and Asia to diversified transport routes due to the growing LPG
consumption in emerging countries and the growing production of associated gases from naturalgas and shale gas fields, high reliability in cargo systems and easy maintenance become significant
benefits in selecting and arranging repair yards and materials.
While the hold space is normally filled with inert gas, dry air can be used as a substitute forType B tank systems with certain equipment. This also leads easy inspections, low CO2 emissions
and low fuel costs for producing the inert gas during a voyage.
(1)|5. Conclusion
We have developed an LPG carrier with a Type B tank system based on our technicalexpertise on independent prismatic tanks accumulated through the construction of many LPG
carriers over a half century, as well as design and analysis technology on Type B tanks acquired
through the development of MOSS-type LNG carriers. This newly developed tank will certainly
meet customer needs for a high level of safety and high economic efficiency in diversifying LPG
trading styles. The tank has already been installed in an LPG carrier delivered in 2012, and further
sales activities are under way. We will continue to supply high-quality LPG carriers and pursue
technological development to meet various customer needs, including high environmental
performance and easy maintenance.

References
1. Koichi, S. et.al, MHI-DILAM (Direct Loading Analysis Method)- An Advanced Structural Analysis
Method for Ships and Offshore Structures -, Mitsubishi Heavy Industries Technical Review Vol.47
No.3(2010) pp.65-72