BOIL-OFF UTILIZATION IN LNG CARRIERS

Presentation submitted to BERNHARD SCHULTE SHIP MANAGEMENT (INDIA)

by Engineering cadets

Fouzdar Sharzad Zubin Ghadage Sagar Shankar

Gupta Shiv Shankar Gurjas Singh

Of

VISHWAKARMA MARITIME INSTITUTE, PUNE

INTRODUCTION TO LNG CARRIERS

• LNG – mixer of hydrocarbons, mainly methane.•The first LNG carrier Methane Pioneer sailed on 25 January 1959 and since then no. of vessels developed. By 2011, 359 vessels were in service.

• BSM is currently managing both conventional steam-powered LNGCs and the new-generation Q-flex ships - powered by heavy fuel-oil burning 2-stroke diesels, with cargo boil-off and reliquefaction plant.

BOIL-OFF

•It is mainly due to the following reasons :1. Ingress of heat into cargo tanks due to the difference

between the temperature in the cargo tanks and temperature of the environment.

2. Sloshing of cargo in partially filled tanks.• Most of BOG is generated during transportation of LNG.• Quantity of BOG during a ship’s voyage changes depends on

the changes in ambient temperature, sea temperature, sea roughness and cargo tank’s contents.

The vapours created due to the ambient heat input (while maintaining constant pressure in a storage vessel) are called “Boil-Off”.

Fig: Generation of BOG

MODES OF UTILIZATION There are three basic modes available :

1. Minimum boil-off/maximum oil:-• Tank pressures are kept high to reduce boil off to a min. and the

majority of energy comes from the fuel oil.• Tank temperature rises due to lack of evaporation.

2. Maximum boil-off/minimum oil:-• Tank pressures are kept low and you have a greater boil-off. Large

amount of fuel used.• Cargo will be delivered cold.(preferable)

3. 100% gas:- • A small pump is started in one tank to supply LNG to the forcing

vaporizer.• mode no fuel oil is used.

• First diesel electric LNG carrier is the Gaz de France at 74 000 m3 She is powered by 4 Wartsila 6L50DF engines each developing 5700kW at 514rpm.

•The energy consumption of the Wartsila 50DF engine is given as 7410kJ/kWh (about 48.5% efficiency). This means that each engine will consume 1 013 688MJ of energy in 24 hours.

•. This means that at the lower boil off figure of 0.1%/day, there is only enough gas to meet the requirements for 1 engine.

•To meet the need to run all the engines, some of the cargo is forced to boil off (F-BOG).

•More economical than running the engines on diesel fuel

BOG Utilization Arrangements

Following arrangements are provided for the same :

1. A steam boiler with a common propulsion steam turbine and steam dump system.

2. A re-liquefaction system.3. A dual fuel diesel engine plant for propulsion and power

generation.4. A gas turbine plant for propulsion and power generation.5. A gas combustion unit.6. Other approved BOG utilization units, such as an auxiliary

steam boiler capable of burning boil-off vapours.

RE-LIQUEFACTION The advantages are:• Flexible fuel system•Optimized fuel cost•More profitable freight contracts• Increased amount of LNG delivered to the terminal,(more profitable than using it as fuel)

Cryogenic plate-fin HE

3 stage centrifugal BOG-compressor

coolers

high-pressure nitrogen stream

Pre heater

BOG FOR PROPULSION• Up until recently all LNG vessels were powered by steam turbines.• Main boilers could be adapted fairly easily to burn boil off gas .• Easy changeover between HFO and gas. BUT.... poor efficiency: about 29% for main propulsion. 25% for electrical generation.

HENCE THIS SYSTEM IS OBESCELETE • efficiency of the diesel engine in the dual fuel concept

approaches 50%.• diesel electric propulsion plant efficiency overall is about

43%.• Burckhardt and Hamworthy have cooperated to develop a

complete concept for handling BOG and injecting natural gas into the dual-fuel, two-stroke engines from MAN Diesel (ME-GI) of Germany.

• BOG is pressurised to about 5.5 bar by compressors & then heated to about 30°C.• The gas is then piped to the engines, where it is injected into the intake air before the air enters the cylinders.

PRINCIPLE OF OPERATION

On the inlet stroke gas is admitted through the gas admission valve and mixes with the inlet air.

The mixture will not ignite on compression because the gas has a high self ignition temperature.

As the piston approaches TDC a small amount of diesel (1%) is injected through the pilot nozzle. Ignition of this pilot fuel then ignites the gas air mixture.

Diesel injector

INSTRUMENTATION AND SAFETY SYSTEMS

•Control and Monitoring System

•Safety Shutdown System

•Gas Detection System

•Fire Extinguishing Systems

GAS COMBUSTION UNIT

•In case the boil off gas cannot be used by the engines, and there is no re liquefaction plant, there must be an alternative way of disposing of it.

• To do this a Gas Combustion Unit is used

LNG HAZARDS

Brittle fracture Explosion

Vapour Cloud Cold Burn

OPERATIONAL SAFETIES

Guidelines ProceduresCargo log book Safety Devices Material safety data sheet Personnel Protective Equipments (PPE) Leaking Manifold Pollution Prevention

SAVINGS AND RECENT DEVELOPMENT• For the new LNG carriers being delivered, the cost reduction may typically be $5 million a year.

•New State-of-the-Art Gas Carrier Velikiy Novgorod Launched.(JANUARY 21, 2014)

•Second ship of the series, Pskov, is due to be delivered in September, 2014.

•MHI’s New-Generation LNG Carrier “Sayaendo” Receives “Best Innovation 2013″ Awards

THANK YOU.