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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
Gastech 2005
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A company of
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:
?
Gastech 2005
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A company of
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
Gastech 2005
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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
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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
Gastech 2005
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A company of
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
Gastech 2005
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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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A company of
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
Gastech 2005
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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
Gastech 2005
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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
Gastech 2005
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A company of
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
Gastech 2005
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A company of
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
Gastech 2005
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A company of
Moss Reliquefaction system
Cooling water
Nitrogen cycle
57 bara
14.5 bara
-163°C
14 bara,ambient temperature
Gastech 2005
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A company of
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.
Gastech 2005
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A company of
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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A company of
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;
Gastech 2005
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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
Gastech 2005
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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
Gastech 2005
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A company of
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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A company of
Thank you for your attention
Advances in design and layout
of Moss LNG carriers
Gastech 2005