key technologies of mitsubishi lng carriers

8/2/2019 Key Technologies of Mitsubishi LNG Carriers

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Mitsubish i Heavy Indust r ies , Ltd .Technical Review Vol.38 No.2 (J un . 2001)

47

Key Technologies of Mitsubishi LNG Carriers

- Present and Future -

Kazuaki Yuasa*1 Katsuya Uwatoko*1

Junshiro Ishimaru*2

*1 Shipbui lding & Ocean Developmen t , Headqua rter s*2 Nagasa ki Shipyar d & Machinery Works

CO2emission cont rol is recognized as t he essent ial issue rela ted t o th e greenhouse effect, especially since dema nd

for energy increasing. Use of Liquefied Natural Gas (LNG) must be expanded because it produces less CO2 emission

th an an y oth er fossil fuel. Worldwide, approxima tely 90 million tons of LNG ar e tr ans ported by sea in 1999. Mitsubishi

Heavy Indu str ies , Ltd. (MHI) has developed LNG Car riers (LNGCs) s ince the ear ly 1970s, and h as been intr oducing

many techn ica l innova t ions , such as an insu la t ion sys tem wi th lower bo i l -o f f - ra te (BOR) , and a soph is t ica ted

au tomat ion system , in order t o enha nce their r e l iabili ty and economy. MHI is building i ts la t est LNGCs with both

ma jor con ta inment sys tem s , namely , th e spher ica l tan k type and th e membra ne tan k type , and MHI i s the No. 1shipbuilder of LNGCs in th e World. Given t he safe and r el iable opera tion of LNGCs over th e past th ir ty years , t he

economy of LNG t ra nspor ta t ion ha s become a top ic o f inc reas ing a t ten t ion fo r th e LNG su pp ly cha in . MHI wi l l

con t inue to lead the way by bu i ld ing fu tu re LNGCs tha t incorpora te new techn ica l advancements such as s ize

en la rgement , s tanda rd iza t ion and a l te rna t ive p ropu ls ion p lan ts .

1. In t roduct ion1. In t roduct ion1. In t roduct ion1. In t roduct ion1. In t roduct ion

The world's first Comm ercial seaborn e tr ade of Liq-

uefied Na tu ra l Gas (LNG) began in 1964, shipping LNG

from Alger i a to th e U. K. S ince then the quan t i ty has

been increas ing year by year and , approximate ly 90

million t ons of LNG, worldwide, were tr an sported by sea

borne t ra de in 1999. Approximately 52 m il lion tons of

LNG were t ranspor ted to Japan, account ing for about

60%.

The Thi rd Conference of Pa r t i es to th e Uni t ed Na -

t i o n s F r a m e w o r k C on v e n t i o n o n C li m a t e C h a n g e

(COP 3) in December 1997 establ ish ed a new environ-m en t a l t a r ge t f or C O 2 emiss ions . To meet t h i s t a r ge t

na t u r a l ga s ha s i nc r ea s i ng l y a t t r a c t ed cons i der a b l e

a t t en t i on .

The main component of nat ura l gas i s methan e. I t i s

conden sed t o about 1/600 of th e volum e by cooling to

below th e -160 OC , boiling point, to produce LNG.

LNG has t hree ma jor chara c ter is t i cs as a seaborne

tr ade car go: (1) the -16 0OC, super-low temperature, (2)

a low specific gravity (0.43 to 0.50), and (3) flammabil-

i ty. Up to now, var ious cargo cont a in men t sys t em s

designed to han dle these cha racter ist ics ha ve been pu t

in to prac t i ca l use a nd a bout 110 LNG car r i e r s a r e in

service in t he world.

In t his pa per t he h istory of techn ological development

of MHI, appl icat ion to ships, technical them es for t he

futur e, etc. regarding technological t ren ds an d th e fu-

tu re view of LNG carr iers ar e explained. The whole view

of MHI's lates t LNG carr ier is shown in Fig. 1Fig. 1Fig. 1Fig. 1Fig. 1.

2. Cargo conta inment sys tems of LNG carr iers2 . Cargo conta inment sys tems of LNG carr iers2 . Cargo conta inment sys tems of LNG carr iers2 . Cargo conta inment sys tems of LNG carr iers2 . Cargo conta inment sys tems of LNG carr iers

Var ious cargo contain men t system s ha ving var ious

Fig . 1 T h e l a t e s t L NG ca r r i e r b u i l t b y M HIF ig . 1 T h e l a t e s t L NG ca r r i e r b u i l t b y M HIF ig . 1 T h e l a t e s t L NG ca r r i e r b u i l t b y M HIF ig . 1 T h e l a t e s t L NG ca r r i e r b u i l t b y M HIF ig . 1 T h e l a t e s t L NG ca r r i e r b u i l t b y M HIThe wh ole view of MHIs la tes t LNG carr ier is shown.


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configura tions, ma terials an d str uctures su itable to LNG

have been p r oposed and pu t i n t o p r ac t i ca l u se . T he

sphe r ica l i ndependen t t an k t ype and m em br ane t an k

type are adopted ma inly at p resent du e to their economy

and rel iabi l i ty. TTTTTable 1able 1able 1able 1able 1 sh ows a compa r ison of th ose

containment systems.

2 .1 Independent t a nk type2 .1 Independent t a nk type2 .1 Independent t a nk type2 .1 Independent t a nk type2 .1 Independent t a nk type

Wi th the independent t ank type , t he hu l l and t anks

are independent s t ruc tures a nd se l f-suppor t ing t anks

are a rra nged inside the h ull . Therefore, deformation due

to ther mal expan sion a nd contraction is not directly con-

veyed to the h ull . The l iquid car go load in the t an ks a cts

on the self-supporting ta nks , not directly on th e insula-

t ion ma ter ial , and al l loads act on the ta nk su ppor t ing

members.

Therefore su fficient s t rength an d insu lat ion perfor-

m a n ce a r e r e q u i r e d for t h e s u p p o r t i n g s t r u c t u r e .Seconda ry barr iers are r equired to be inst alled from the

viewpoint of hull protection a gains t leak age of LNG in

an accident emergen cy.

At pr esent a bout half of the LNG carriers in th e world

are th e spher i ca l i ndependent t an k type . MHI has im-

proved the spher ical indepen dent t an k type developed

by Moss Rosenber g Verft a .s. (presen tly Moss Mar it ime

a.s.) and twenty-two LNG carriers have been delivered

an d/or a re being built so far .

I n t h e sphe r i ca l i ndependen t t a nk t ype t he w ho le

cargo l iquid load is borne by th e membra ne st r ess of

the ta nk sh el l in a shel l s t ructure tan k, therefore st ress

concentr at ion can be avoided. Also the sp herical ta nk is

insta l led on a cyl indr ical ski r t in t he hold and ha s the

featur e tha t deforma t ion due t o thermal expansion an d

cont raction of the s pherical tan k is reas onably absorbed

by the bending of the ski r t .

Becau se of th e s imple configura t ion a nd s t ruc tu re

of ax ia l symmet r y of the sph er i ca l t ank an d cy lindr i -

ca l sk i r t , h i gh accu r a cy s t r e s s an a l y s i s i s pos s i b l e .

Ther efore th e des ign concept , "No LNG leaks . Even

when cracks a re genera ted , progress of th e cra cks i s

ex t r em e l y s l ow an d l eaka ge of L N G i s s l igh t . " w as

e x p e r i m e n t a l l y a n d a n a l y t i c a l l y c e r t i f i e d a n d t h e

pa r t i a l s econda r y ba r r i e r i s adm i t t ed . T he sphe r i ca l

t an k type i s acknowledged to have the h ighes t sa fety

as t an k type B of the In tern a t iona l Code for the Con-

s t ru c t ion a nd E quipment of Ships Car ry ing Liquef ied

Gases in Bu lk ( IGC Code) .

2 .2 Membrane t a nk type2 .2 Membrane t a nk t ype2 .2 Membrane t a nk type2 .2 Membrane t a nk t ype2 .2 Membrane t a nk typeIn the membrane tan k type, the insulat ion mater ial

is inst alled on inner h ull and i ts sur face is covered with

a m eta l l ic membrane sh ee t . This conta inment sys t em

aims a t redu cing th e meta l l ic ma ter ia l exposed to low

temperat ures. The membran e keeps liquid-t ightness to

prevent cargo leakage, and ha s no strength against cargo

load. Th e cargo l iquid load a cts directly on th e hu ll via

the insu lat ion ma ter ial , therefore the insu lat ion st r uc-

tur e must h ave not only insu lation performance but also

st ren gth. Referr ing to Table 1, th e featu res of the Ga z

Transpor t system a nd Technigaz system which are th e

mains t r eam membrane t ypes a re expla ined .

A special ma ter ial ha ving extrem ely small coefficient

T a b le 1 Com p a r i son o f L NG ca r g o c on t a in m e n t sy s t e m sT a b le 1 Com p a r i son o f L NG ca r g o c on t a in m e n t sy s t e m sT a b le 1 Com p a r i son o f L NG ca r g o c on t a in m e n t sy s t e m sT a b le 1 Com p a r i son o f L NG ca r g o c on t a in m e n t sy s t e m sT a b le 1 Com p a r i son o f L NG ca r g o c on t a in m e n t sy s t e m s

Tank material

Insulation material

BOR(insulation thickness)

Spherical tank type

Insulation

Tank

Skirt

Partial secondary barrier

Insulation Tank(aluminum alloy)

Aluminum alloy

Thermal brake(stainless steel)

Steel for low temperature

Aluminum alloy or 9% nickel steel

By thermal expansion andcontraction of tank and skirt

Plastic foam

0.15%/d (about 220 mm)

Membrane type

Gaz Transport system

Inner hull

Membrane, Insulation

Primary membrane

Primaryinsulation boxSecondarymembrane

Secondaryinsulation box

(Measures are not required due tovery low coefficient of thermalexpansion of membrane)

Insulation boxes filled with perlite

0.15%/d (about 530 mm)

The same as primary barrier

Technigaz system

Inner hull

Insulationsecondary barrier

Membrane

MembranePlywood

Secondarybarrier

PolyurethanefoamPlywood

Mastic

Stainless steel

By expansion andcontraction of membrane

Plastic foam

Triplex

Tank section

Insulation structure

Measures forthermalexpansion andcontraction

Secondary barrier Drip pan (partia l secondary barrier)

36% nickel steel (Invar)

0.15%/d (about 250 mm)


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of th erm al expan sion called Invar (36% nickel steel) is

used as th e membran e for the Gaz Tran spor t sys t em,

therefore measur es for th ermal expansion an d contr ac-

t i on a r e e s s e n t i a l l y n ot r e q u i r e d . T h e i n s u l a t i o n

st r uctur e of th e insulat ion boxes fi l led up wi th per l i te

are bui l t l ike br icklaying. Other featu re of this system

is tha t t he secondary bar r i e r uses Invar which i s t he

sam e m a t e r i a l a s t h a t for t he p r i m ar y ba r r i er . Th r ee

LNG carr iers of this system are u nder const ru ct ion by

MHI.

The Technigaz sys t em adopt s cor ru gated s t a in l ess

steel as the mem bran e. The sta inless steel is corrugat ed

both longi tudina l ly and t r an sver se ly, and the therma l

expansion an d contr act ion is absorbed by the corruga-

tion. As for t he insu lation stru cture balsa wood was used

in the ini t ial Mark I system, and Mark I I I system was

developed to achieve a lower BOR. In Ma rk III s ystem ,

reinforced plast ic foam was a dopted as th e insulat ion

mater ial and Tr iplex (aluminum foi l sheet reinforcedwith glass cloth ) was used as seconda ry barr ier . MHI

gained exper i ences in bui ld ing a sma l l vesse l wi th

M ar k I sys t em .

T he G az T r anspo r t sy s t em and Techn i gaz sy s t em

were developed by separate companies or iginal ly. At

present th ese companies have been merged into the Gaz

Transpor t and Technigaz (GTT) and a new system in-

corpora t ing the advantages of bo th sys t ems i s be ing

developed. The basic idea is that for the membr an e will

use In var a nd r einforced plastic foam for insu lation an d

th e seconda ry bar rier is simplified as t horoughly as pos-

sible. The st udy ha s been ma de from th e above point of

view. Att ention was given to the n ew system a s a futu re

system.

3. MHI 's technica l development a nd char acter i s t ics3 . MHI ' s technica l development a nd char acter i s t ics3 . MHI 's technica l development a nd char acter i s t ics3 . MHI ' s technica l development a nd char acter i s t ics3 . MHI ' s technica l development a nd char acter i s t ics

of LNG carrierof LNG carrierof LNG carrierof LNG carrierof LNG carrier

3.1 Technical developmen t3.1 Technical developmen t3.1 Technical developmen t3.1 Technical developmen t3.1 Technical developmen t

MHI bui l t t he wor ld 's f ir s t l a rge r e f r igera t ed type

LPG car r i e r i n 1962. S ince then man y LPG and LNG

carr iers h ave been bui l t by MHI. MHI is proud of ha v-

ing th e r ich experiences in building t he lar gest l iquefiedgas car r iers .

The techn ical int roduct ion of Technigaz mem bran e

system in 1969, Moss spherical ta nk syst em in 1971 and

Gas Transpor t membra ne system in 1973 were sequen-

t i a l ly per formed an d var ious improvement s for those

systems h as been developed.

The opt imum sh ip forms ha d been developed ma inly

based on model t es t s . In 1990s , Comput a t ional F lu id

Dynam ics (CFD) technology was esta bl ished and was

put into pract ical use. At presen t LNG carr iers ha ving

good propulsive perform an ce can be developed in a sh ort

t ime.

In th e s t ru c tura l des ign , s imula t ion t echnique for

sloshing loads wa s esta blished a nd h igh accura cy an aly-

s i s t e chn i ques abou t s t r u ct u r a l s t r eng t h a nd f a t i gue

were developed. MHI 's or iginal m eth od to ana lyze fa-

t i g u e s t r e n g t h i n w a v e , D i s cr e t e A n a l y s i s M e t h o d

(DISAM) , was a l so put in t o prac t i ca l u se . S t ru c tura l

design of high r eliability became possible by these a na ly-

s i s t echniques . MHI has been doing th e bes t e f for t t o

develop improvement techn ology to ma ximize t hei r

characterist ics . In part icular, in the late 1980s MHI h eld

the leadership to develop the large spher ical tan k type

LNG carriers h aving high tr an sportat iona l economy, the

so-ca l led second gen era t ion LNG car r i e r s ha ve been

developed . The fi r s t 125 000 m 3 of 4-t a nk sh ip wasdelivered in 1989 for NWS project an d all series ships

ha ve been operated sm oothly wi thout t rouble.

3.2 Char acter ist ics of del ivered ships3.2 Char acter ist ics of del ivered ships3.2 Char acter ist ics of del ivered ships3.2 Char acter ist ics of del ivered ships3.2 Char acter ist ics of del ivered ships

T he cha r a c t e r i s t ic s of t he l a t e s t L N G ca r r i e r a r e

shown compa ring with th e first genera tion LNG carr ier.

(1) The t a nk capac i ty i s en larged f rom 125 000 m3

t o135000 m 3 t o decrease t r an spor t a t ion cos t . I n th e

futu re t he capaci ty wil l be fur t her enlarged.

(2) In the sph erical tank type ship, a lower BOR becam e

possible due to the development of a t herm al brak e

in th e ta nk s kirt . Alth ough a su per low BOR of 0.10%/

d is possible techn ically, at pres ent , 0.15%/d is judged

to be the m ost economical from the viewpoint of pra c-

tical use. Most lar ge-sized LNG car riers h ave adopt ed

0.15%/d. The Gaz Tran spor t system at t ained a BOR

of 0.15%/d by thicker insu lation in a rea sonable ra nge

and simplification of the therm al insulat ion st ructur e.

(3) A higher service speed an d decreas e of th e ma in en-

gine output a nd fuel cost were at ta ined at th e same

time by the optimu m design of th e hu ll form an d pro-

peller and development of improved heat efficiency

technology.

(4) By combination of low BOR and a forcing vaporizer

BOG could be us ed ef fect ively in a l l service speed

ran ge from low load to ma ximum load, so operat iona l

economy could be improved . An envi ronmen ta l ly-

fr i endly sh ip was r ea l i zed by decreas ing the NOx,

SOx, etc.

(5) In considerat ion of long term ma inten an ce ful l at -t e n t i o n w a s p a i d t o s t r u c t u r e d l a y o u t , m a t e r i a l

selection, painting specifications, etc in design stage.

I n p a r t i c u l a r i n t h e s t r u c t u r a l d e s i g n , f a t i g u e

s t r en g t h de s i gn i s c a r r i ed ou t m ak i ng fu l l u se o f

DISAM.

(6) Simplificat ion of opera tion a nd im provemen t of na vi-

g a t i on s a f e t y a r e a t t a i n e d b y u s i n g ex t e n s i v e

automat ion systems.

Fu rth ermore, the LNG carr ier has been developed by

intr oducing new technologies such as au tomat ic ballast

water exchan ge system, measur es for CFC ref r igerants

an d these t echniques h ave par t ly been appl ied to some

building ships.


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BOGBOG BOG

BOGFO

FO FO

Propulsion plant

Plant configuration

Advantages

Disadvantages

Economy

InitialinvestmentFuel effi-

ciency (fuel)

Steam turbine engine

Red

uction

gea

r

Low pressureturbine

Main boiler

High pressure turbine

Fuel efficiency is low

100

100 (BOG + HFO)

100 (87)4 (3)

67 (0)

(Note) Numerals in parentheses are those in the case of BOG exclusive combustion

Dual fuel diesel engine

Auxiliaryboiler

Highpressure

Mainengine Mainengine

Fuel efficiency is betterBOG can be used as fuel

Exclusive BOG burning isimpossible

BOG can not be fired at alow output

105

67 (BOG + HFO)

6610043

Diesel engine withreliquefaction plant

To cargotank

Reliquefactionplant

Fuel efficiency is betterThe cargo part and engine partcan be separated

Heavy fuel oil consumptionis high

Electric power for drivingreliquefaction plant is required

105

65 (HFO)

7799

100

Gas turbine

Exhaust gaseconomizer

Steam turbine

Fuel cost is better comparedwith that of the steam turbineengine

High quality fuel oil is required Dual fuel burning is impossible

104

79 (BOG or Gas Oil)

73100

CO2NOxSOx

Dis-chargegases

Most LNG ships adopt andreliability is high

100% of BOG can be firedduring voyage

Gas combined cycle

Red

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4. Fut ur e v iew of LNG car r iers4 . Fut ur e v iew of LNG carr ier s4 . Fut ur e v iew of LNG car r iers4 . Fut ur e v iew of LNG carr ier s4 . Fut ur e v iew of LNG carr ier s

Recent ly in the in t e rn a t ional meet ings such as th e

Inter na t ional Conference on LNG an d GASTECH, th e

reduct ion of the LNG tra nspor tat ion cost ha s become a

major topic. In par t icular var ious proposals to reduce

the LNG chain cos t a r e be ing d i scussed . In t he back-

ground of the d i scuss ions on th i s subject , sa fe ty an d

reliabili ty have already been demonst rat ed and i t is rec-

ognized tha t economy is the most impor tant i ssue.

Ther efore t he fu tu re v i ew for LNG car r i e r s i s ex-

p la ined pa ying a t t en t ion to economy .

4 .1. Enlar gement4 .1. Enlar gement4 .1. Enlar gement4 .1. Enlar gement4 .1. Enlar gement

The economic mer i ts of enlar ging the LNG car r iers

are th a t i t wi l l r educe the uni t t r an spor t a t ion cos t as

same as other kinds of merchant sh ips. In par t icular in

the LNG project , tra nsport qua ntity is roughly const an t,

therefore th e nu mber of LNG car r i e r s can be r educed

by choosing t he la rger LNG carr iers . Redu ct ion of th enu mber of LNG carriers is connected directly to the re-

duction of capita l and opera tional costs .

When discussing enlargement , the theme becomes

the ship-shore inter face wi th exist ing LNG termina ls,

special ly th e receiving term inals. In the stu dy which

ha ve been conducted so far , al th ough regar ding main

dimensions, mooring ar ra ngements , tan k capacit ies, etc.

enlar gement is not serious in bu ilding sh ip, it is consid-

ered th at reconst ruction of th e receiving facil ity ma y be

necessar y depending on the sh ip size from th e view point

of ber th st ren gth an d cargo discha rge rate (1).

4 .2 S tandard iza t ion4 .2 S tandard iza t ion4 .2 S tandard iza t ion4 .2 S tandard iza t ion4 .2 S tandard iza t ion

Becaus e the LNG pr oject r equires a great ini t ial in-

vestmen t , th e faci l it ies ha ve been developed bas ed on

long term contr acts between producers a nd consumers.

Therefore LNG car r iers wer e opt imized as special ex-

c lu s i v e s h i p s fo r t h e p r o j ec t a n d t h e f u n d a m e n t a l

requiremen ts su ch as ship size and sh ip speed etc. , were

decided a ccordin gly.

On th e o ther h and a cont r ac t ma y poss ib ly be con-

cluded bet ween an y of producers an d consu mers of LNG

like gener al seaborn e comm odities. Since some of LNG

is being t raded in the spot ma rket , the t ra de using stan-

dard ships wi l l be expected t o be genera l man ner .

From such a background, the st anda rdizat ion t r ends

are considered f rom a di fferent aspect of size enlar ge-

ment .

The present s t an dard sh ips have been en larged in

genera l f rom 125 000 m 3 to 135 000 m 3. As de scr ibedabove, there ma y exist some LNG termina ls where en-

larged ships a re di f ficul t t o enter . Therefore when t he

s t an dard sh ips a re p l ann ed , it wi l l be most impor tan tto ha ve the f lex ibi l it y in opera t ion . The r e l a t ionsh ip

between sh ip size and ship speed is tha t 18 kn for 145 000

m 3 an d 19 .5 kn for 135 000 m 3 a r e r ough l y t he s am etranspor t quant i ty.

4.3 Pr opulsion plan t4.3 Pr opulsion plan t4.3 Pr opulsion plan t4.3 Pr opulsion plan t4.3 Pr opulsion plan t

On LNG carr iers, BOG is l ighter th an t he at mosphere

an d inflamm able, therefore BOG is permitted t o be used

as fuel in th e engine room. Consequen t ly BOG is ut i -

l ized as a fuel for t he ma in boiler of the steam tu rbine

engine a s sam e as in t he past . However , from the st and

point tha t BOG is considered t o be car go, various plant s

ar e investigat ed aim ing at effective use of BOG economi-

cally.

T a b le 2 Co m p a r i so n o f p r o p u l s ion p l a n t s f or L NG c a r r i e rT a b le 2 Co m p a r i so n o f p r o p u l s ion p l a n t s f or L NG c a r r i e rT a b le 2 Co m p a r i so n o f p r o p u l s ion p l a n t s f or L NG c a r r i e rT a b le 2 Co m p a r i so n o f p r o p u l s ion p l a n t s f or L NG c a r r i e rT a b le 2 Co m p a r i so n o f p r o p u l s ion p l a n t s f or L NG c a r r i e r


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In th is paper th e "steam tur bine engine," " dual fuel

diesel engine," "diesel engine with reliquefaction plan t"

and "gas combined cycle (gas turbine engine + steam

turbine engine)" are compared and evaluated. TTTTTable 2able 2able 2able 2able 2

shows t he plant configura tion, chara cterist ics, economy,

etc. of each plan t. As you can s ee from th e compa rison

ta ble, an y plant can be pu t into pract ical use from the

viewpoint of reliabil i ty a nd safety.

The s t eam t urb ine p lan t ha s been adopted to most

LNG carr iers a nd i t s rel iabi li ty i s high. Both BOG and

hea vy fuel oil ar e combu stible as fuel for th e ma in boiler

an d give the a dvant age of dua l fuel burn ing, etc. In pa r-

t icular emission gas from BOG exclusive combust ion is

th e most clean. However, the steam t urbine plant ha s

th e disadva nt age of inferior fuel efficiency an d high fuel

cost.

In the dual fuel diesel engine the dual fuel burning

of BOG an d h eavy fuel oil is possible an d fuel efficiency

is bet ter , but the high pressur e inject ion is requir edwhen BOG is intr oduced to th e engine. Also a disadvan -

t age i s t ha t f ue l fo r p i lo t bu r n i ng is neces sa r y and

flexibility is in ferior becaus e BOG exclusive combu stion

is impossible an d moreover in th e diesel engine a lar ge

quan tity of nitrogen oxides is discha rged due t o the h igh

combust ion temperature.

In t he diesel engine with a r eliquefaction plant , since

BOG is rel iquefied and can be di rect ly retur ned to th e

tan ks, the propulsion en gine a nd t he BOG han dl ing can

be perfectly separa ted. The ma in engine is a n ormal die-

sel engine a s used on convent ional m erchant ships, and

th e fuel efficiency is bette r. However, initia l investm ent

for a r eliquefaction plant is n ecessary an d consu mpt ion

of hea vy fuel oil increa ses du e to add itional dr iving elec-

tric power for reliquefaction. Also more nitrogen oxides

and sul fur oxides than those in t he other plants a re dis-

char ged due t o the hea vy oil fired diesel engine (2).

In t he ga s combined cycle, BOG is f ired in th e gas

tur bine and at the sam e t ime a steam turbine is dr iven

by steam generated by the exhaust gas energy from the

gas tu rbine. This plant i s s imi lar t o the land combined

cycle system a nd fuel eff iciency is bet t er th an th at of

th e convent iona l s team t ur bine.Emission gas i s clean,

t he s am e a s t ha t o f t he s t eam t u r b i ne , bu t t he d i s ad -

v a n t a g e s a r e t h a t a h i gh q u a l it y p e t r o le u m f u e l is

required a nd du el fuel combus tion is impossible. In th e

futu re this system may be planed with a n elect r ic pro-

pulsion plant .

5. Conclusion5. Conclusion5. Conclusion5. Conclusion5. Conclusion

MHI as a pioneer of th e l iquefied gas car rier , estab-

lished the techn ology thr ough building th e world's first

l a rge r e f r igera t ed type LP G car r i e r i n 1962. Al so the

key technology of th e spherical tan k type an d the mem -

bran e ta nk t ype LNG carr iers were introduced from 1969

to 1973, therea f t e r var ious t echnologies such a s low

BOR, forcing vapor izer , aut omat ion a nd other systems

have been developed to improve safety, reliabil i ty and

economy and a pplied to ships delivered. At present , MHI

ha s the wor ld 's best bui lding records of LPG carr iers

an d LNG carr iers an d is proposing the lat est l iquefiedgas car r iers m eet ing the customers deman d.

Moreover in recent years on the consensu s th at safety

an d rel iabi li ty in th e seaborne t r ade of LNG ha ve been

proved, furth er improvement of economy is being a big

issue. MHI is developing most economical ships intro-

ducing such technologies as enlargement , s t anda rdiza-

tion, technical development of various pr opulsion plan ts

and proposal for tank improvements t o meet th e futu re

needs of the customers.

Cooperat ion of the pa rties concerned such as the char-

terer, owner, l icenser, classif ication society, company

staf fs h as been indispensable. Special tha nks are given

to the va luable ex terna l suppor t t h e au th or s have had

to date. Future cooperat ion is expected to real ize the

next generat ion LNG carriers ha ving high safety, reli-

abil i ty a nd economy.

Re f e r e n c e sRe f e r e n c e sRe f e r e n c e sRe f e r e n c e sRe f e r e n c e s

(1) I sh ima ru , J . e t a l . , Des igning LNG Carr i e r s for 21s t Cen -

t u r y w i t h E m p h a s i s o n E c o n om i ca l a n d S a f e O p e r a t i o n s ,

Gas tech94 (1994)

(2) Yuasa, K. e t a l . , Economics and New Technology of LNG

Ca r r i e r s , T h e 8 t h A n n u a l M e e t i n g o f J a p a n I n s t i t u t e o f

Ener gy (1999)

key technologies of mitsubishi lng carriers

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