CLASSIFICATION NOTESNo. 61.2

DET NORSKE VERITAS

Veritasveien 1, NO-1322 Høvik, Norway Tel.: +47 67 57 99 00 Fax: +47 67 57 99 11

LNG BOIL-OFF RE-LIQUEFACTION PLANTS ANDGAS COMBUSTION UNITS

MAY 2006

FOREWORD

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Motives

New propulsion systems for the latest generation of LNG Carriers introduce reliquefaction plants and gas combustion units forhandling the boil-off. This is as an alternative to the traditional arrangements where the boil-off is burned as fuel in boilers.

Several LNG carriers now on order will be fitted with re-liquefaction and gas combustion units (incineration). DNV has felt aneed to establish guidance for these installations in the form of a new Classification Note (No 61.2).

In the not too distant future introducing requirements for these installations in the Rules for Classification of Ships will be con-sidered, but only after having gained more experience with such installations. Thus, in the meantime applicable DNV expecta-tions on new ships to re-liquefaction plants and gas combustion units for handling of LNG "boil-off" are outlined in theClassification Note 61.2.

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May 2006

CONTENTS

1. GENERAL................................................................. 41.1 Introduction................................................................41.2 Scope ...........................................................................42. ARRANGEMENT .................................................... 42.1 Re-liquefaction systems .............................................42.2 Gas combustion unit ..................................................53. MATERIALS, MANUFACTURE, INSPECTION

AND TESTING......................................................... 63.1 Materials.....................................................................6

3.2 Piping design and manufacture ............................... 63.3 Certification of components ..................................... 73.4 Strength testing.......................................................... 74. MONITORING AND CONTROL ......................... 74.1 General ....................................................................... 74.2 Extent of monitoring of re-liquefaction plants ....... 74.3 Monitoring of GCU ................................................... 7

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1. General

1.1 IntroductionAn alternative to using LNG boil-off gas as fuel in steam boil-ers, gas turbines or combustion engines is to re-liquefy the va-pour and return the LNG to the cargo tanks.

The IGC Code requires two means for disposal of the boil-offgas and in the case of re-liquefaction is the primary means thesecondary means is typically a Gas Combustion Unit (ThermalOxidizer) where the boil-off gas is burned and the combustiongas cooled and exhausted to atmosphere.

1.2 ScopeThis class note outlines requirements for classification of re-liquefaction installations and gas combustion units.

Since the installations may differ in arrangements and equip-ment the requirements outlined may be subject to adjustmentsto suit the particular installations.

2. Arrangement

2.1 Re-liquefaction systemsFig.2-1 and Fig.2-2 show two examples of re-liquefaction sys-tems.

The system in Fig.2-1 (Moss Maritime / Hamworthy) uses a ni-trogen refrigerant cycle. The nitrogen is compressed in threestages with inter-cooling, then pre-cooled in the cold box andexpanded through a rotary expander from about 57 bar pres-sure to 14 bar.

By this expansion the temperature of the nitrogen falls to –163°C(Joule-Thompson effect).

The cold nitrogen is lead to the cold box where it cools theboil-off gas which has been compressed to about 4.5 bar ofpressure in the low duty compressor.

The condensed boil-off gas is lead to a liquid receiver wherethe LNG is collected and the non-condensable gases, mainlynitrogen, is vented.

The vented gases are either returned to the vapour main or di-rected to the gas combustion unit (GCU) for incineration.

Venting to the atmosphere is normally not permitted since thecontents of methane gas in the vented gas is 20-25% at 4.5 barof pressure in the liquid receiver.

Figure 2-1Re-liquefaction system (Moss Maritime / Hamworthy)

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Classification Notes - No. 61.2 5

May 2006

Figure 2-2Re-liquefaction system (Air Products) process flowsheet

The system in Fig.2-2 (Air Products) is also utilizing a nitrogenrefrigerant cycle.

The main difference is that while the system in Fig.2-1achieves the temperature reduction through expansion of thenitrogen alone, the system in Fig.2-2 utilizes the J-T effectboth by expansion of the nitrogen and by expansion of the boil-off gas.

For this reason the boil-off gas has to be compressed to about40 bar. After cooling in the cold box (load heat exchanger) thegas is expanded into the N2 rejection column at a pressure of20 bar.

In the N2 rejection column, cooling of the gas mixture in thepacked bed by liquefied gas brings the methane content in thevented gas down to about 5%.

With this low methane content the gas may be vented to atmos-phere.

2.2 Gas combustion unitDisposal of boil-off gas may be done by incinerating the gas ina gas combustion unit (GCU).

Examples of GCUs for marine use are shown in Fig.2-3 andFig.2-4.

Figure 2-3Gas combustion unit - (Snecma Moteurs)

Boil - OffGas

VentBOG Compressor Skid

Cold Box

CCNR Skid

Flashpot

LNG

Load heatexchanger

N2rejectioncolumn

N2 heatexchanger

Nitrogen(make - up)

CW

CW

Funnel

Gas burnerUV

Block &Bleed Valves

Self checkingFlame detectionsystem

Combined combustion-dilution air fans (3 or +)– Constant flow

Spark igniters (2)

BOG

Full metallic Air cooledcombustion chamber

Mixingdevice

Diaphragm

Control Valve

Air room

Man Hole

Funnel

Gas burnerUV

Block &Bleed Valves

Funnel

Gas burnerUV

Block &Bleed Valves

Self checkingFlame detectionsystem

combustion-dilution air fans (3 or +)–

Air cooledcombustion chamber

Mixingdevice

Diaphragm

Control ValveMan Hole

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Figure 2-4Gas combustion unit - (Hamworthy Combustion)

The unit consists of an outer and an inner steel cylinder. Theinner cylinder is the combustion chamber with gas burners andigniters. Some designs may use pilot burners for ignition whileothers, as in the example, use direct spark ignition.The combustion chamber has no refractory but is cooled by airflowing in the space between the inner and the outer cylinder.

The supply of air for cooling and combustion may be by com-bined fans, or by separate fans for combustion air and coolingand dilution air.

The bulk average exit temperature of the combustion gas mustbe kept well below 500°C in order not to ignite any possibleleakages of methane gas and also to avoid excessive heat radi-ation to surrounding installations. Localised maximum peaktemperatures must not exceed 535°C.

The reduction of gas temperature is achieved by diluting thegas by air before exiting the funnel.

A GCU may be accepted to meet the IGC Code requirementfor both primary and secondary means for disposal of boil-offgas on condition that redundancy is arranged for fans (combus-tion and dilution), burners, igniters, gas flow control valve(s)and control system.

A GCU control system includes both combustion control andburner management.

A redundant GCU control system is a system with dual proc-essors and single I/O or a single processor with an independentmanual/remote control system.

Examples

The design boil-off capacity is normally the maximum boil-off rate.

The amount of boil-off gas consumed by the dual fuel enginesduring harbour operations is defined as the base load (hotelload). This base load may be subtracted from the maximum de-sign boil-off rate to establish the design boil-off capacity of thegas combustion units.

3. Materials, Manufacture, Inspection and Testing

3.1 MaterialsMaterials for the refrigerant piping and cargo vapour pipingare to be suitable for low temperature service. In general mate-rial should be selected for a temperature of –165°C. Materialselection for warmer temperatures in parts of the system maybe considered.Requirements for materials, documentation and testing, seeRules Pt.5 Ch.5 Sec.2 D and E.

Materials for the steel cylinders of the GCU are to be suitablefor the high temperatures.

Material quality is to be documented by Works’ Certificates.

3.2 Piping design and manufacturePiping shall be joined by butt welding. The number of flangesshall be kept to a minimum.

Valves should preferably be of weld-in type.

Requirements for design, welding and testing, see Rules Pt.5Ch.5 Sec.6 C500-C800.

For gas combustion units located in enclosed spaces the gaspiping within the space shall be of double walled type or fullyenclosed in a ventilated duct with arrangements complyingwith Rules Pt.5 Ch.5 Sec.16 B100 and 200.

Hamworthy Combustion AMOxsafe® GCUInternational Patent Number PCT / GB 2005 / 004362 pending

BOG

Vent

Double Block& Vent Valves

ControlValve

REGISTER BURNERWITH

DUAL SWIRLERTECHNOLOGY

100%STAINLESS STEEL,

TWIN SHELLCONSTRUCTION

DILUTIONAIR FAN

DILUTIONAIR FAN

AIR JETSFOR

MIXING

COMBUSTIONAIR FAN (2)

Propulsion Primary Secondary Capacity and redundancyDiesel Re-lique-

factionRe-lique-faction

2 x 100% design boil-off gas capacity re-liquefac-tion units. Common cold box may be accepted.

Diesel Re-lique-faction

GCU 1 x 100% design boil-off gas capacity re-liquefac-tion unit.1 x 100% design boil-off gas capacity GCU.

Diesel GCU GCU 1 x 100% design boil-off gas capacity GCU with redundant fans, igniters, burners, gas flow control valves and combustion control systems; or2 x 100% design boil-off gas capacity independent GCUs; or3 x 50% design boil-off gas capacity independent GCUs

Dual fuel diesel electric

GCU GCU 1 x 100% design boil-off gas capacity GCU with redundant fans, igniters, burners, gas flow control valve and combustion control system; or2 x 100% design boil-off gas capacity independent GCUs; or3 x 50% design boil-off gas capacity independent GCUs

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3.3 Certification of components

3.4 Strength testingPiping and heat exchangers shall be hydrostatically tested to1.5 times the design pressure.

Testing by using gas instead of liquid may be accepted on spe-cial considerations of safety implications.

4. Monitoring and Control

4.1 GeneralMonitoring and control systems shall in general comply withthe requirements of Rules Pt.4 Ch.9.

4.2 Extent of monitoring of re-liquefaction plants

4.3 Monitoring of GCU

Component Certification Rule referenceValves DN ≥ 75 mm, DNV

DN < 75 mm, WPt.5 Ch.5 Sec.6 H

Pressure vessels DNV Pt.4 Ch.7Heat exchangers DNVCompressors DNV Pt.4 Ch.5 Sec.4Pumps DNV Pt.5 Ch.5 Sec.6 GGas combustion unit WW = Works (makers) certificate

Item/Parameter Alarm Shut down CommentBOG compressor, gas outlet temp.

H HH

BOG compressor, gas outlet pressure

H HH

BOG compressor, gas inlet pressure

H/L

BOG compressor, gas inlet/outlet temp. diff.

H

BOG compressor, gas inlet temp.

H HH

BOG compressor, vibrations H HHBOG compressor, lube oil press

L LL

BOG compressor, bearings H HH For shaft power above 1 500 kW

BOG pre-cooler level HVapour header pressure H/L LLLNG separator level H/LLNG separator, pressure H/LN2 compressor gas inlet pressure

L

N2 compressor gas inlet temp.

H

N2 compressor 2.stage gas inlet temp

H

N2 compressor 2.stage gas outlet pressure

H

N2 compressor 3.stage gas inlet temp

H

N2 compressor 3.stage gas outlet temp. after cooler

H

N2 compressor/expander, vibrations

H HH

N2 compressor/expander lube oil press

L LL

N2 compressor, bearings H HH For shaft power above 1 500 kW

N2 high press/low press differential

X Outside normal range

Cold box enclosure, gas concentration— methane— oxygen

HL

Cooling water press. LCooling water outlet temp. HN2 compressor room, O2 concentration

L

N2 compressor room, loss of ventilation

X

N2 rejection column, level HN2 rejection column, pressure

H/L

BOG compressor room O2 concentration

L

BOG compressor room HC gas concentration

H X SD may be time delayed

BOG compressor room fire detection

X X Blow down of gas pressure

BOG compressor room, loss of ventilation

X

ESD activated XCargo tanks overfill danger

H HH Activate at levels above existing

Loss of power supply to control and monitoring system

X X

Instrument air, pressure L

Item/Parameter Alarm Shut down CommentFlame failure X XLoss of combustion air supply X XLoss of cooling air/dilution air supply

X X

BOG inlet temperature L LLCombustion gas exit temperature

H HH

Methane gas concentration in gas pipe duct

H HH

Loss of ventilation in gas pipe duct,Alternatively loss of N2 pressure

X X

Fire detection in GCU space X X

Item/Parameter Alarm Shut down Comment