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Gastech 2005

Advances in design and layout

of Moss LNG carriers

By

Øivin Iversen

&

Roy-Inge Sørensen

Moss Maritime a.s.

Gastech 2005

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A company of

The Moss LNG Carrier Development

DEVELOPMENT AREAS

CARRIER CAPACITY

HULL & PROPELLER EFFICIENCY

ALTERNATIVE PROPULSION

IMPROVE OPERATIONAL PROCEDURES

RELIQUEFACTION OF BOIL OFF

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Moss LNG Carrier sizing

MOSS 147

MOSS 216

MOSS 250

Today’s standard:41m in diameter

Large:47m in diameter

Development of larger spheres

Maximum diameter:

?

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19.0 19.5 20.0 20.5 21.0 21.5 22.0

Speed (knots)

137K

147K

235K

Hull and propeller efficency

• New twin screw hull

• Improvements as a resultof the most extensive model test ever!

147K vs 137K: 7% CARGO INCREASE10% POWER REDUCTION

Bra

ke P

ow

er

Pb

[kW

]

235K: 70% CARGO INCREASEONLY 30% POWER INCREASE

Moss147k Twin screw

4th gen. Moss 147k single screw

3rd gen. 137k single screw

Moss 235k Twin screw

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Alternative propulsion

35

30

40

25

50

45

Medium speeddiesel engine

20

Capacity (MW)501 10

55

Thermal efficiencies %

Gas turbine

Combined cyclegas turbine

Steam turbine

Low speed diesel engine

5

LNG carrier Diesel: Today’s merchant standardSteam: only on LNG carriers & naval vessels

Steam

NBOGas

FBOGas or

Fuel

NBOGasor

Fuel

Diesel

100%

60%

50%

Energy need for propulsion

Alternatives for LNG carriers:• Slow speed diesels with Reliquefaction• Dual-fuel diesel electric propulsion• Gas turbines

The diesel has superior efficency!

PROPULSION TYPE

FUEL

CONSUMPTION

RELATIVE

CONSUMPTION

[g/kWh] [%]

CONV. STEAM PLANT 290 100

COMBINED CYCLE GT 200 69

DIESEL ELECTRIC 200 69

MEDIUM SPEED DIESEL 180 62

SLOW SPEED DIESEL 170 59

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Diesel propulsion reliability

How is the reliability of diesel propulsion vs. steam propulsion?

Steam 99.89%

Single Diesel 99.82%

Twin diesel: one of two engines running 99.99%

Twin diesel: both engines running 99.66%

Modern slow speed diesel engines as reliable as the steam alternativeTwin diesel arrangement will give continous available power

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Ship vibrations

Slow speed diesels:Introduction of additional excitation sources

Detailed investigation of vibration responseof cargo tank structure and global hull structureon a 235k Moss LNG carrier design

• Natural frequency of the hull girder• Lower tank – skirt connection to foundation deck• Cargo tank shell structure• Pipe tower tank structure• Tank cover structure• Top of deckhouse

Gastech 2005

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Ship vibrations

Conclusions from investigation:

• Vibration responce from SS Diesels lower or equal to propeller response

• Highest vibration level from local resonance of structure

• No vibration issues for Moss LNGcarrier with well-adapted propellers

0

1

2

3

4

5

6

7

8

foundation sphere tower cover deck house

mm/s

2ndv

4thv

7thH

4thX

4thPr

0

2

4

6

8

10

12

14

16

18

20

22

foundation sphere tower cover deck house

mm/s

2ndv

4thv

7thH

4thX

4thPr

Maximum vibration level - ballasted

Maximum vibration level - loaded

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Cool down procedure

A study has been conducted to investigate if it would be feasible to start loading cargo tanks earlier than accepted in previous procedures.

Tank side

Skirt side

Ref. element attop of groove Temp.

sensorlocation

Ref. elementat tank side ofgroove

Ref. elementat skirt sideof groove

• Reduced requirement for the heel

• Reduced or non spraying during the ballast leg

• Reduced cool down time when commissioning the carrier

• Earlier loading start

The advantages are;

Geometry of the tank / skirt model

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Cool down procedureMain parameters for the study;

The stress range margin is significant, indicating that the present minimum temperature requirement of –110°C in the equator profile can be increased.

The study paves the way for further work in order to predict temperature limits and quantify the impact on operations and costs for a full round trip.

Conclusion;

Maximum appearing von Mise stress in groove area

Transient temperature profile at sensors

• four-tank LNG carrier of 137 000 m3

• tank diameter of 40.44 m

• loading at a sensor temp. of –80°C

• other restrictions for cool down as per today

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Moss Reliquefaction system

BOG and high dutycompressors

3 stage nitrogen compressors/expanders

Cold box Separator

Novel features for the system are;

•partial liquefaction and separation of non-condensable ⇒reduced power consumption

•N2-compressor/expander unit

mounted on a common gear

⇒compact system design⇒prefabricated skid modules

The Moss Reliquefaction System is based on a closed nitrogen expansion cycle extracting heat from the boil-off gas (BOG)

N2 reservoir

Orderbook:

8 systems at Korean shipyards

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Moss Reliquefaction system

1.06 bara-120°C

Boil-off gas cycle

From vapor header

To cargo tanks

4.5 bara

-159°C

Non-condensables

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Moss Reliquefaction system

Cooling water

Nitrogen cycle

57 bara

14.5 bara

-163°C

14 bara,ambient temperature

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Moss Reliquefaction systemTechnical merits:

• the system only uses proven components from air-separation, peak-shaving plants world-wide and the onshore LNG production plant at Snurrevarden, Norway.

• the total quantity of Btu’s loaded can now be delivered.

• reduced nitrogen content during the voyage, facilitating tank pressure control and reducing power consumption.

• the system is prefabricated in skid modules for easy installation and hook-up on board.

• no increase in cargo machinery space is necessary.

• the system has automatic capacity control.

• the system can be stopped when the cargo pumps are in operation reducing the need for extra generator capacity.

• the rule requirement for redundancy is easily fulfilled.

• no extra personnel are required for operation and maintenance.

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Moss Reliquefaction system

Economical merits:

• increased cargo quantity delivered.

• reduced heel required on ballast voyage

• large savings in total fuel consumption

• improved propulsion redundancy

• more qualified crew available for handling of the propulsion plant

• more flexibility in contract negotiations for owners and shipyards

Gastech 2005

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Combined high duty heat exchanger

Figure 7-10 Figure 7-11

Different operation procedures for the HD heater and the LNG vaporizer allow combination of the heat exchangers into one single unit

•simpler overall installation

•reduced pipe length, number of

valves and fittings

•reduced cost for equipment and

installation: -150 000 USD/ship

Benefits;

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Simplified BOG systemA new arrangement based on a combined heater / vaporizer with the BOG compressor located upstream of the heat exchanger is proposed

Benefits;

•reduced BOG compressor and motor size

•easier compressor control

•easy downstream throttling control

•the mist separator can be omitted

•simpler piping arrangements

•reduced investment cost: -450 000 USD/ship

•improved operational reliability

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Transportation cost assessment

Moss 137k

steam

Moss 137k

diesel Moss 147k

Diesel

Moss 216k

DieselMoss 250k

Diesel

-30,0 %

-25,0 %

-20,0 %

-15,0 %

-10,0 %

-5,0 %

0,0 %

% c

ost

ch

an

ge

21 knotsservice speed

BOG value set to 3.5 USD/MMBtu

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Summary

• Even larger spheres can be designed

• Maximum size carriers not reached

• Alternative propulsion now introduced

• 8 Moss reliquefaction systems ordered

• Cool down procedure improvements

• Slow speed diesels give no vibration issues

• Diesels give significant cost reductions

Gastech 2005

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Thank you for your attention

Advances in design and layout

of Moss LNG carriers

Gastech 2005