Maersk cargo handling

At some installations replacement Purging of the atmosphere is performed during loading by use of the vapour return line.Purging during loading shall only be performed if thetemperature

     Chapter and Title

A Preparation, Inerting, Purging

B Loading - Various Procedures

C Transportation, Tank Atmosphere, Inhibitor

A P Møller Technical Organisation

Copenhagen Denmark

Copies of this manual or part thereof shall not be made for useoutside the vessel without written consent being obtained fromthe Nautical Department in the Technical Organisation,Copenhagen

This manual shall not effect the legal relationship or liability ofA.P Møller with or to any third party and neither shall suchthird party be entitled to rely upon it

A.P Møller shall have no liability for technical or editorial errors

or omissions in this manual nor for any damage, including butnot limited to direct, punitive, incidental or consequentialdamages resulting from or arising out of its use by A.P

Møller’s directors, officers, employees, agents,representatives, owners or others.

1.1.Objective

The objective of these guidelines is to provide the officers onboard with a guide to conduct cargo -handling safely andefficiently

The objective is also to give advice and information to recentlypromoted officers on procedures proven to be safe andefficient by experienced senior officers and to avoid usingprocedures already proven unsafe and inefficient

1.2.Foreword

These guidelines prescribe how the Owners expect officersinvolved in cargo handling - through organisation andestablished procedures - to utilise all available information andguidelines as comprehensively as possible, so that the cargooperation is conducted safely and efficiently

It is essential that these guidelines are read in conjunction withthe manufacturer's instructions and recommendations for thehandling of equipment involved, and guidelines orrecommendations issued by authorities, I.M.O., NationalAuthorities and Classification Societies

1.3.Document Control and Update

"Guidelines for Cargo handling in Maersk Gas Carriers" is acon-trolled document, and any changes in the contents of thismanual shall be logged in RECORD OF CHANGES

The Master and the Chief Engineer shall have a copy of

"GUIDELINES FOR CARGO HANDLING IN MAERSK GASCARRIERS" available as a personal copy, while the 3rd copyshall be located at the Master’s discretion

1.4.Familiarisation

The Master shall ensure that both he and his officers arefamiliar with "Guidelines for Cargo handling in Maersk GasCarriers"

Before any officer is involved with and responsible for anycargo operation, these guidelines shall be read by that officer

The guidelines are based on experience collected from all ourgas carriers and other sources They are not to be consideredcomplete, as from time to time new products are beingintroduced in our ships Additionally procedures may change

in order to save time or to make cargo operations more safeand efficient

This manual is kept updated with ships assistance by reporting

to Nautical Department about new procedures with as manydetails as possible; thus, new sections can be added to themanual when the need arises

1.6 Reporting

Prior to any tank cleaning or any special cargo operation theMaster shall hold a meeting with the officers involved in thetank cleaning or cargo operation, after which the agreed plancontaining all details of the procedure and estimated timerequired is telexed to Nautical Department

Before the operation is initiated the Nautical Department shall

be informed of the plan thus making it possible to suggest analternative procedure if necessary If instructions to performtank cleaning or other special cargo operation are receivedafter office hours, communication with the Nautical Departmentshall be by telephone Telephone communication shall beconfirmed by telexed from the Nautical Department at earliestopportunity

During tank cleaning or other special cargo operations, dailysituation reports shall be telexed to the Nautical Department.All correspondence concerning cargohandling shall beforwarded with a copy to Gas Tanker Department

A-2 Inerting

On gas carriers certain requirements to the atmosphere in thecargo tanks have to be met The requirements may refer to theoxygen content, dew point, traces from previous cargo, carbondioxide, carbon monoxide, sulphur, etc

In order to meet some of the requirements, the cargo tanksmay be inerted using the vessel's inert gas plant

The inert gas on board is produced by burning off Diesel orGas oil in the Inert gas Generator and thereby reducing theOxygen content The produced inert gas is subsequentlywashed, cooled and dried, further on vessels able to produceNitrogen, the CO2 and the CO produced during theincineration is removed by filtering

With inert gas it is possible to reduce the oxygen content andthe dew point in the cargo tanks On vessels equipped withCO2-strippers, it is also possible to avoid excessive amounts

of carbon dioxide and carbon monoxide, which is normallypresent in the inert gas produced

It may however be necessary to continue the inerting withnitrogen supplied by the shore installation, if the inert gas plant

is unable to produce sufficiently clean inert gas or nitrogen.Regular measurements shall be taken during the inerting,using Servomex and Teledyne oxygen analysers From thesemeasurements, graphs shall be drawn The graphs will clearlyindicate the progress of the inerting, thereby enabling earlyintervention, should the inerting not proceed as planned

Furthermore, these graphs will indicate, the time at which theinerting can expect to be completed

In order to maintain the best possible stratification the cargotank pressure shall be kept to a minimum and all tanks shall

be inerted in parallel

1.2 Nitrogen

If the inert gas plant is producing nitrogen, the nitrogen isintroduced into the cargo tanks via the vapour and upper spraylines The nitrogen shall be heated in the nitrogen heaterbefore being introduced into the cargo tanks

1.3 Preheating Pipes

In order to maintain the stratification as long as possibletanks are opened (not possible on the J-class) Thecompressors may be inerted during the heating of the pipes 1.4 Inerting Cargo Tanks in Series

Inerting cargo tanks in series has proved unsuccessfuland should therefore not be used unless under specialcircumstances, in which case, the Gas Tanker Department willinstruct the vessel

1.5 Disconnecting Inert gas Pipeline

After inerting is completed, the inert gas deck pipelineshall be disconnected from the supply line from the engineroom, as this connection must only be established, when theplant is in operation and inert gas is supplied to deck Thetimes for connecting and for disconnecting shall be entered inthe deck log book

2 After Discharging

2.1 Inertgas

After the cargo tanks have been heated to the requiredtemperature, inerting is commenced As the vapour from mostcargoes is heavier than inert gas, the inert gas is introducedinto the top of the cargo tanks through the vapour line and the

upper spray line The gas is vented off through the liquid line,which shall be connected to the vent stacks.

2.2 Ethylene

If inerting after ethylene, the inert gas shall be introduced intothe bottom of the tanks though the loading line

2.3 Nitrogen Displacement Discharge

If cargo tanks are to be further inerted after NitrogenDisplacement Discharge the inerting shall be performedaccording to Section E 13 in these Guidelines

3 Inerting with Nitrogen from Shore

3.1 Before Loading

The nitrogen shall always be introduced into the cargo tanksvia the vapour and upper spray lines In order to ensure thestratification in the cargo tanks the nitrogen shall be heated toapproximately 80°C at the manifold, either by using shoreheater or vessel's nitrogen heater

Nitrogen at higher temperatures may damage the insulatingmaterial on the pipelines

Unheated nitrogen which may be colder than the atmosphere

in the cargo tanks shall also be introduced into the cargo tanksvia the vapour and upper spray lines

3.2 Preheating Pipes

In order to maintain the stratification for as long as possible,the pipes shall be preheated with heated nitrogen before thevalves to the cargo tanks are opened (not possible on the J-class) The compressors may be inerted during the heating ofthe pipes

If the supplied quantity is insufficient to maintain a rate of 25% of the volume to be inerted, the volume shall be reducedaccordingly by purging a smaller number of cargo tanks at atime.

20-This rate is maintained until maximum 1% oxygen is measured

in the bottom of the tanks As the cargo tanks at this stagecontain mainly pure nitrogen, no stratification can be expectedand the rate is now increased to approximately 50%

In order to control the flow into each cargo tank, the cargotanks may be divided into two systems

The nitrogen used to inert first tank(s) shall not be reused toinert the next tank(s)

When the cargo tanks are divided into two systems, the pipesshall be heated prior to changing the inerting to the secondsystem

3.4 Stratification

If the stratification is disrupted e.g by too high a rate or bychanging temperatures, it can often be re-established bystopping the inerting completely for an hour or two Inerting isthen resumed at low rate and high temperature The soonerturbulence is detected, the easier stratification may be re-established

3.5 Compressors and Piping

If inerting of compressors and piping has not been completedduring heating of pipes, the compressors and piping should beinerted simultaneously with the inerting of the cargo tanks.When inerting the compressors, it is important to turn thesefrequently as otherwise the associated piping will not beinerted

3.6 Change of Compressor Oil

If unable to reduce the content of previous cargo in thecompressors after ventilating the oil sump, the oil should bechanged The oil should not be changed on request fromshippers or receivers unless so specified by the Gas TankerDepartment

A-4 Purging and cooldown – Gas Carriers with Deck Tanks

Preamble

Before loading it is necessary to replace the Inert Gasatmosphere with product gas to enable cooling of the cargoand cargo tanks

At some installations replacement (Purging) of the atmosphere

is performed during loading by use of the vapour return line.Purging during loading shall only be performed if thetemperature difference between the cargo and the cargo tanks

is within the maximum allowed temperature difference (6°C onfully ref gas carriers 15°C on semi-ref gas carriers) otherwisethe steel may be stressed beyond design limits

Where installations are not equipped with vapour return or notable to deliver vapour to the vessel, it is necessary to load apurge parcel and purge the cargo tanks at sea

In order to avoid calling at an additional port for loading apurge parcel, the fully ref gas carriers are equipped with 2deck tanks for loading and/or storing of purge parcels

The larger deck tank of 210 m3 is normally used for storingLPG

The smaller deck tank of 60 m3 is normally used for storingammonia, however, if vessel is not likely to be in the ammoniatrade, the tank may be used for storing additional LPG forcooling purposes

The semi-ref gas carriers are equipped with a deck tank of

210 m3 only

Before purging is commenced a number of conditions have to

be taken into consideration, such as:

- Number of tanks to be loaded with the product inquestion

- Estimated product loss

- Do the deck tanks contain sufficient product to accomplishrequirements

1 Purging with LPG and Chemical Gases

1.1 Purge Parcel

In order that there is sufficient product to perform a fullconditioning of the cargo tanks, as much cargo as possibleshould be transferred to the deck tank(s) during normaloperation in the LPG trade (when changing grades etc.) Ifpossible the large deck tank should always contain Propane.1.1.1 Loading Purge Parcel

If the deck tank(s) does not contain sufficient product toperform a full purging of the cargo tanks, additional productshall be loaded either in the deck tank(s) or in a cargo tank ifalready conditioned

1.2 Purging

1.2.1 Transferring Liquid to the Vaporiser

Deck tank pump should be used to transfer the liquid if thedeck tank pressure is below 7 bar

1.2.3 Flow Direction of Vapour

The cargo tanks shall be purged in parallel as this willminimise the flow through each cargo tank and thus increasethe stability of the stratification The additional product used forpurging in parallel compared to purging in series is marginal.Vapour is directed from the vaporiser to the cargo tanksthrough the liquid filling line

Nitrogen or inert gas is removed from the cargo tanks throughthe vapour lines which shall be connected to the vent stackswith elbows

1.2.4 Pressure

In order to maintain the best possible stratification the cargotank pressure shall be kept to a minimum

1.2.5 Monitoring and Recording

It is essential to monitor the progress closely during the entirepurging operation with regular measurements taken to ensureuniform distribution of vapour in all tanks

From these measurements, graphs shall be drawn Thegraphs will clearly indicate the progress of the purging, therebyenabling early intervention, should the purging not proceed asplanned Furthermore, these graphs will indicate, at which timethe purging can expect to be completed

When purging with VCM, the Gascope must not be used asthe measuring filament of the instrument will be destroyed byVCM vapours

1.2.6 Starting Compressors

Before starting the compressors, it is good practice to continuethe purging for 15 minutes after 100 % reading is reached.1.2.6.1 Blow off

Should the atmosphere in the cargo tanks not be 100 %pure gas, the inert gas/nitrogen may be vented off via thecondensers until the top of the cargo tank is 100 % gas

1.3 Cooling

On completion of purging, the liquid remaining should betransferred to the liquid line It is then used to cool the cargotanks via the upper spray line

1.3.1 Pressure

If the tank pressure falls too quickly during cooling there may

be insufficient pressure to complete the operation Providedthere is sufficient liquid in the cargo tanks and the pressure isnot above 0.25 bar, the cargo pumps should then be startedand liquid circulated through the upper spray line

1.3.2 Temperature

The change in temperature shall always be within therecommended limits This will be complied with by uniformlycooling the tank through the upper spray line.

2 Purging with ammonia

2.1 Purge Parcel

If the purge parcel is loaded in one of the cargo tanks, then thecargo tank shall always be purged to 100% ammonia prior tostarting loading liquid

2.2 Purging

Ammonia reacts with various components in the inert gasforming saline products It is therefore not allowed to purgewith ammonia in tanks containing inert gas

Due to ammonia's narrow range of flammability and highignition temperature it does not generally constitute a firehazard Therefore it is considered safe to purge ammonia intotanks containing fresh air However, some terminals do requirethat the cargo tanks are inerted to a low oxygen level beforeloading Inerting shall in these circumstances be conductedwith nitrogen

Please also refer to paragraph 2.2.5 below

2.2.1 Transferring liquid to the Vaporiser

As the small deck tank does not have a transfer pump, anycargo stowed in this tank should be as warm as possible.Cargo should then be transferred to the cargo tank via thevaporiser to prevent any fully refrigerated product entering thetank

2.2.2 Vaporiser

Please refer to paragraph 1.2.2 in this chapter

2.2.2.1 Steam Supply

Please refer to paragraph 1.2.2.1 in this chapter

2.2.3 Flow Direction of Vapour

The vapour is directed from the vaporiser into the cargo tanksvia the vapour line and the upper spray line The manual valvebetween the liquid line and the condensate line shall be closed

and prior to initiating purging it shall be ensured that this valve

is closed securely

Air or nitrogen is displaced from the tanks through the liquidfilling line, which shall be connected to the vent stacks withelbows

2.2.4 Monitoring and Recording

Please also refer to paragraph 1.2.5 in this chapter

When purging with ammonia, Barrett-testers are used tomeasure the ammonia concentration These testers use freshwater and do not require the use of any special chemicals.2.2.5 Stress Corrosion Cracks

There is a possibility on the fully ref vessels of stresscorrosion cracks when carrying ammonia is present

According to experience stress corrosion cracking may occur

in cargo tanks containing ammonia, depending on type andtreatment of steel

To minimise the risk of stress corrosion cracking, the followingguidelines issued by SIGTTO shall be followed:

2.2.5.1 Temperature

The carriage temperature shall be kept below -30°C

2.2.5.2 Water content

Water will act as an inhibitor against stress corrosion and it

is recommended by shore installations that the ammoniashould contain a minimum 0.2% by weight of water Ifunable to receive confirmation that the water content is aminimum 0.2%, the Nautical Department shall be contactedprior to commencing purging/loading ammonia

2.2.5.3 Oxygen content

Ammonia does not cause stress corrosion cracking unless it

is contaminated by oxygen When manufactured ammoniahas an extremely low oxygen content, less than 1 ppm byweight

During cool down an oxygen content of 0.5% by volume isgenerally considered safe

However, the oxygen content should be less than 0.025% byvolume prior to introducing liquid to the cargo tanks.

2.2.6 Starting Compressors

As measurements are not always absolutely accurate, it is agood practice to continue the purging for 15 to 30 minutesafter 100 % ammonia concentration in the bottom of the cargotanks has been measured before starting the compressors.2.2.6.1 Blow-off

During the first few days of operation of the reliquefactionplant, the incondensible gases which accumulate in thecondensers, i.e the air remaining in the cargo tanks, shall

be vented off

2.3 Cooling

Please refer to paragraph 1.3 in this chapter

3.1 Calculation of the Time Required for Precooling

The steel mass of the cargo tanks on a 35,000 m³ fully ref gascarrier is estimated to be 2698 metric tons The steel has to becooled from + 25°C to - 40°C The cargo tanks are purged with

60 mt of propane vapour at + 25°C The following quantity ofheat must be substracted:

From the steel mass:

2,698,000 kg x 0.12 x (25 + 40) = 21,044,400 Kcal

0.12 is the specific heat of the steel

From the propane vapour:

60,000 x (195.9 - 178.9) = 1,020,000 Kcal

195.9 and 178.9 are the enthalpy of the propane vapour at +25°C and - 40°C

Total capacity of reliquefaction plant: 1,035,000 Kcal

Time required: about 22 hours

The above calculation do not take into consideration the heatflow through the tanks and should be regarded as a guidanceonly

A5 – Gas Carriers without deck tank

Preamble

The compressors are not able to run if inert gas or air remains

in the cargo tanks, and the cargo tank pressure will build up ifthe cargo tanks are loaded without venting off the inert gas orair either to shore or to vent stacks

Objective

To replace the inert gas or air with product gas in the cargotanks before or during loading in order to enable cooling of thecargo and cargo tanks

Procedure

1 Purging with LPG and Chemical Gases

1.1 Loading with Vapour Return to Shore

At some installations replacement of the atmosphere (Purging)

is performed during loading by use of the vapour return line.1.1.1 Product Temperature

Purging during loading shall only be performed if the differencebetween the cargo temperature and the cargo tanktemperature is within the maximum allowed (15°C) as the steelotherwise may be stressed beyond the construction limits

If the difference between the cargo temperature and the cargotank temperature is outside the maximum allowed limit, and it

is impossible to load via the cargo heater and heat the cargo

to within the maximum allowed limit, the cargo tanks shall bepurged with vapour and subsequently cooled down prior tocommencing loading liquid

1.2 Purging without Vapour Return to Shore

Where installations are not equipped with vapour return it isnecessary to load a purge parcel and purge the cargo tanks atsea

1.2.1 Loading Purge Parcel

The purge parcel shall be loaded into one of the cargo tanksand a number of conditions have to be taken intoconsideration when deciding the quantity of the purge parcel to

be loaded:

- Number of tanks to be loaded with the product inquestion

- Estimated product loss during purging

- Do tanks need to be fully refrigerated before loading

It is recommended to load the purge parcel in cargo tank No 1

as this cargo tank is the smallest and thus easier to preparefor loading the purge parcel

1.2.1.1 Purging prior to loading Purge Parcel

Prior to loading the purge parcel it may be necessary topurge the cargo tank into which the purge parcel is to beloaded, depending on the temperature of the purge parcel

If the temperature difference between the purge parcel andthe cargo tanks is outside the maximum allowed limit, thecargo tank must be purged with vapour and subsequentlycooled down prior to commencing loading the purge parcel.1.2.1.2 Loading via Cargo Heater

If the sea water temperature is low and sufficient energy toevaporate the purge parcel during purging may not beextracted from the cargo heater, it may be advantageous toload the purge parcel via the cargo heater and thereby addheat to the purge parcel, which in turn will increase theefficiency of the subsequent purging of the remaining cargotanks

If the cargo tanks require cooling prior to loading, the purgeparcel should not be loaded through the cargo heater,unless the sea water temperature is extremely low

1.3 Purging

The cargo tanks shall be purged in parallel as this willminimise the flow through each cargo tank and thus increasethe stability of the stratification The additional product used forpurging in parallel compared to purging in series is marginal

1.3.1 Transferring Liquid to Cargo heater

The deep well pump is started with the recirculation valve onthe liquid line open

1.3.1.1 Transferring via Condensate Line

The liquid is pumped through the condensate line to thedome on cargo tank No 3 and from here via a flexible hosefrom the stripping line to the liquid drain in the bottom of thecargo heater

The flow is controlled by the manually operated valve close

to the pneumatic valve 1C361 on the stripping line, whichshould only be opened slightly

1.3.1.2 Transferring via Liquid Line

The liquid is pumped via the liquid line through the cargoheater in reverse order compared to normal flow direction.The flow is controlled by the manually operated valve at thecargo heater just before the liquid enters the cargo heater.The valve shall only be cracked open

1.3.2 Vaporising Liquid

It is essential that all liquid is vaporised in order to avoid thecargo heater being filled with liquid, which will obstruct thevaporisation and slow down the entire operation

Further if liquid is allowed to pass the cargo heater and enterthe bottom of the cargo tanks the cargo tanks may bedamaged

1.3.3 Flow Direction of Vapour

Vapour is directed from the heat exchanger to the cargo tanksthrough the liquid lines entering the tanks partly through theliquid line and partly through lower purge line for distributingthe vapour throughout the length of the tank

The pneumatic valves on the condensate line on the domesshall be closed if the liquid is transferred via the condensateline

Nitrogen or inert gas is removed from the cargo tanks throughthe vapour lines which shall be connected to the vent stackswith elbows

1.3.3.1 Cargo Tank No 3 Starboard - Condensate Line

If using the Condensate Line for transferring liquid to thecargo heater, vapour to cargo tank No 3 Starboard can only

be directed through the liquid line, as the condensate line isused to supply liquid from cargo tank No 1 to the cargoheater

1.3.4 Pressure

In order to maintain the best possible stratification the cargotank pressure shall be kept to a minimum

1.3.5 Monitoring and Recording

It is essential to monitor the progress closely during the entirepurging and regular measurements shall be taken during thepurging to ensure uniform distribution of vapour in all tanks.From these measurements, graphs shall be drawn Thegraphs will clearly indicate the progress of the purging, therebyenabling early intervention, should the purging not proceed asplanned Furthermore, these graphs will indicate, the time atwhich the purging can expect to be completed

When purging with VCM, Gascope must not be used as themeasuring filament of the instrument is destroyed by VCMvapours

liquid is transferred via the upper spray line and start-up line

as this will assist in cooling the entire tank more evenly

1.4.1 Pressure

Transferring the liquid via the upper spray line and the start-upline will also assist in increasing the tank pressure, which willmake the compressors more efficient

As the liquid is cooled down, the tank pressure will decrease,and if the tanktop still requires cooling, then the deepwellpumps should again be started and the liquid recirculatedthrough the upper spray line and the start-up line

1.4.2 Temperature

The change in temperature shall always be within the designlimit of the cargo tanks Further the cooling shall be performed

as uniform throughout the cargo tank as possible

2 Purging with ammonia

2.1 Loading with Vapour Return to Shore

The cargo tanks shall always be purged to 100 % ammoniaprior to starting loading liquid If the shore is unable to delivervapour, then liquid is directed very slowly to the cargo heaterwhere all liquid shall be vaporised The vapour is directed tothe cargo tanks via the upper spray line and the air/nitrogen isvented of via the lower spray lines The lower spray lines shall

be connected to the vapour line

2.2 Purging without Vapour Return to Shore

Please refer to paragraph 1.2 and 1.2.1 in this chapter

2.2.1 Purging prior to loading Purge Parcel

The cargo tank into which the purge parcel is loaded shallalways be purged to 100 % ammonia prior to starting loadingliquid If shore is unable to deliver vapour, then liquid isdirected very slowly to the cargo heater where all liquid shall

be vaporised The vapour is directed to the cargo tanks via theupper spray line

2.2.2 Loading via Cargo Heater

Please refer to paragraph 1.2.1.2 in this chapter

2.3 Purging

Ammonia reacts with various components in the inert gasforming saline products It is therefore not allowed to purgewith ammonia in tanks containing inert gas.

Due to ammonia's narrow range of flammability and highignition temperature it does not generally constitute a firehazard and it is thus considered safe to purge ammonia intotanks containing air Some terminals do, however, require thatthe cargo tanks are inerted to a low oxygen level beforeloading Inerting shall in these circumstances be conductedwith nitrogen

2.3.1 Transferring liquid to Cargo Heater

Please refer to paragraph 1.3.1, 1.3.1.1 and 1.3.1.2 in thischapter

2.3.2 Vaporising Liquid

As the vapour is lighter than air and nitrogen, it shall bedirected into the top of the cargo tanks It is extremelyimportant that all liquid is vaporised in the cargo heater andthat no liquid is allowed to enter the cargo tanks to be purged,

as this will destroy the stratification and risk damaging thecargo tanks

2.3.3 Flow Direction of Vapour

The vapour is directed from the cargo heater to the cargo tankdomes via the liquid line On the domes the vapour is directedinto the cargo tanks via the upper spray line The manual valve

on the liquid loading line shall be closed and prior to initiatingpurging it shall be ensured that this valve is securely closed.Air or nitrogen is removed from the tanks through the lowerspray lines The lower spray lines shall be connected to thevapour lines which in turn shall be connected to the ventstacks with elbows The manual valves on the vapour line tothe cargo tanks on the domes shall be closed and prior toinitiating purging it shall be ensured that these valves areclosing tightly

2.3.4 Pressure

Please refer to paragraph 1.3.4 in this chapter

2.3.5 Monitoring and Recording

Please also refer to paragraph 1.3.5 in this chapter.

When purging with ammonia, barrett-testers are used forcontrol of the ammonia concentration Measuring ammonia,barrett-testers are working satisfactorily with fresh water, as 1litre of water can absorb approximately 800 litres of ammoniavapours It is therefore not necessary to use specialchemicals

2.3.6 Starting Compressors

As measurements are not always absolutely accurate, it isgood practice to continue the purging for 15 minutes after 100

% ammonia concentration has been measured in the bottom

of the cargo tanks before starting the compressors

2.3.6.1 Blow-off

Should the atmosphere in the cargo tanks not be completelypure ammonia, the air may be vented off from top ofcondensers until the atmosphere is 100 % ammonia in thecargo tanks

2.4 Cooling

Please refer to paragraph 1.4, 1.4.1 and 1.4.2 in this chapter

3.1 Calculation of the Time Required for Precooling

The steel mass of the cargo tanks on a 15,000 m³ semi-ref.gas carrier is estimated to be 1464 metric tons The steel has

to be cooled from + 25°C to - 40°C The cargo tanks arepurged with 30 mt of propane vapour at + 25°C The followingquantity of heat must be subtracted:

From the steel mass:

1,464,000 kg x 0.12 x (25 + 40) = 11,419,200 Kcal

0.12 is the specific heat of the steel

From the propane vapour:

30,000 x (195.9 - 178.9) = 510,000 Kcal

195.9 and 178.9 are the enthalpy of the propane vapour at +25°C and - 40°C

Capacity of each cargo compressor is 155,000 Kcal

Time required: about 26 hours

The above calculation do not take into consideration the heatflow through the tanks and should be regarded as a guidance only

A6 – Vaccum prevention in cargo tanks

The cargo tanks in the semi-refrigerated vessels areconstructed to sustain a vacuum of 0.3 Bar while the cargotanks in the fully refrigerated vessels can only sustain avacuum of 0.1 Bar

If the pressure in a cargo tank decreases to minus 0.1 Bar inthe semi-refrigerated vessels and to minus 0.05 Bar in the fullyrefrigerated vessels an alarm function will activate a warningbuzzer and light signal Further decompression to a vacuum of0.2 Bar (on the fully refrigerated vessels 0.07 Bar) willautomatically shut-down the cargo pumps and compressors.During normal working conditions the tanks are thus protectedagainst structural damage caused by excessive vacuum

In the event of abnormal circumstances the alarm system andbuilt-in safety measures may be unable to preventdevelopment of a damaging vacuum Thoughtless action mayresult in a violent chemical-physical reaction which will beimpossible to halt before reaching critical limits

1 Ammonia

If water is injected into a tank containing a rich ammoniaatmosphere it will cause an implosion Unless the manhole isfully open, the energy of the implosion will make the tankcollapse

If flexible hoses laid out over the stern of the vessel are usedwhen venting off ammonia vapours, it must be ascertained thatthese hoses are not lowered below the water surface asammonia has great affinity with water and may result in waterbeing sucked up into the hose and into the tanks where moreammonia vapours will be dissolved in the water The tankpressure is thereby reduced dramatically and the tanks maycollapse

2 LPG

When purging with butane vapour into a tank containing fullycooled propane, the butane vapour may condensate due to

the low temperature The condensation may under specialcircumstances continue until a physical balance is achieved at

a vacuum of 0.5 Bar, which pressure is so low that risk ofdamaging the tank exists

Prior to initiating any operation involving change-over betweenproducts, all possible risks shall be evaluated in the stage ofplanning in order to avoid any damage

B – LOADING VARIOUS PROCEDURES

B1 – Loading VCM in Rafnes - S-Class

Preamble

The Contract of Affreightment with Norsk Hydro stipulatesparcels of up to 14,000 mts for the S-Class vessels In order toaccommodate 14,000 mts of VCM the cargo must be cooled toless than 0° C

The cargo tanks are purged during the loading by returning thenitrogen used for inerting the cargo tanks to the shore tankswith only very limited flaring off, which results in a relativelyhigh back pressure on the vapour return

Objective

To ensure that the cargo intake at Rafnes is optimised byinitiating cooling of the cargo as soon as possible

Procedure

1 Inerting with Nitrogen

Prior to commencing loading in a cargo tank, the cargo tankshall be inerted with nitrogen delivered from shore until therequested oxygen level has been reached The inerting withnitrogen shall be performed according to Section A 2 in theseGuidelines except the lining up which shall be as describedbelow

1.2.Lining Up

In order to commence loading and cooling cargo as soon aspossible tanks 2 and 4 are purged in parallel and in series withtanks 1 and 3, thereby making it possible to load tanks 2 and 4while completing the purging of tanks 1 and 3

1.2.1 Shore Tanks Full on Arrival

If the shore tanks are full on arrival the shore installation mayrequire the vessel to start loading as soon as possible, inwhich case cargo tank no 1 should be inerted with nitrogenand loaded first

1.3.Swing Elbows

The ship arrives with the systems connected to one systemexcept elbows 1C910 A and 1C929 A on the condensate line,which shall be out Before arrival valve 1C071 aft of the portbooster pump on the liquid line below the heat exchanger shall

be blanked off between the valve and System I (tanks 1 and 3)

to ensure that any possible leakage will not cause VCM

liquid to mix with nitrogen during inerting of tanks 1 and 3simultaneously with loading tanks 2 and 4

1.4 Manifold Connections

The nitrogen hose is connected on the vapour manifold MainSystem I, the loading arm is connected on the liquid manifoldMain System II and the vapour return on the vapour-manifoldMain System II

1.5 Flow Direction

The nitrogen is led through the vapour stations aft of thecompressor house to the top of tanks 2 and 4 in parallel viathe vapour line Initially the air is vented off from the liquid lineand the lower spray line to vent stacks 2 and/or 4 Whennitrogen is detected in the bottom of tanks 2 and 4, vent stacks

2 and/or 4 are closed and the corresponding elbowsconnecting the liquid line to the vent stacks are removed Theair/nitrogen is directed to tanks 1 and 3 via the condensateline

When the required oxygen concentration has been reached intanks 2 and 4, the flow of nitrogen is altered to completenitrogen purging of tanks 1 and 3 directly from the manifold viathe vapour line, and elbows 1C930 on the condensate-line and1C928 A on the Vapour-line are removed

The air from tanks 1 and 3 is vented off via vent stacks 1and/or 3 When the required oxygen concentration has beenreached in tanks 1 and 3, the elbows to the vent stacks areremoved

2 Loading Cargo Tank 2 and 4

As soon as inerting tanks 2 and 4 is completed and elbows1C930 and 1C928 A have been removed the loading iscommenced in tank 2

2.1 Loading via Cargo Heater

If the sea water is colder than the product in the shore tanks,loading via the cargo heater should be used in order to coolthe cargo The cargo heater must be pressure testedaccording to Section E 7 in General Rules for Maersk GasCarriers.

2.2 Removing Nitrogen from Cargo Tanks

Compressor nos 2 and 3 are used for transferringnitrogen/VCM-mixture to tank 4 The compressors are usedone by one as the compressors shall be stopped before themaximum allowed discharge temperature of +90oC is reached.The minimum interval between the start of each compressorshall be in accordance with manufacturer's instructions, i.e 20minutes

2.3 Cooling Cargo

When the atmosphere in tank 2 is pure VCM, the condensate

is returned to the tank and cooling of the tank with twocompressors is commenced

Tanks 2 and 4 are then loaded simultaneously until the vapourpressure in tank 4 is well above the pressure in the shoretanks

2.4 Vapour Return to Shore

When the pressure in tank 4 is well above the pressure in theshore tanks the cooling of tank 2 is stopped and all vapour andcondensate valves to tank 2 are closed Loading is continued

in tank 4

The vapour return to shore is then opened and the pressure intank 4 is reduced until slightly above the pressure in the shoretanks

When the pressure is reduced, the vapour return and allvapour valves on tank 4 are closed, and cooling of tank 2 iscontinued

Above procedure is continued until the vapour in tank 4 is pureVCM, upon which cooling of the tank is initiated Cooling oftanks 2 and 4 is continued throughout the remaining loadingoperation

3 Loading Cargo Tanks 1 and 3

When the atmosphere in cargo tanks 2 and 4 is pure VCM, theblank at valve 1C071 is removed, provided that tanks 1 and 3are inerted to the required oxygen level, and the vapour return

is changed to the vapour-manifold Main System I Before thevapour return is changed the hose must be purged withnitrogen

Loading is then commenced in tank no 3

3.1 Removing Nitrogen from Cargo Tanks

Compressor no 1 is used for transferring mixture to tank 1 The compressors shall be stopped beforethe maximum allowed discharge temperature of +90oC isreached Minimum interval between each start of thecompressor shall be in accordance with manufacturer'sinstructions, i.e 20 minutes

nitrogen/VCM-3.2 Vapour Return to Shore

When the pressure in tank 1 is well above the pressure in theshore tanks the transferring of nitrogen/VCM-mixture from tank

3 is stopped and all vapour and condensate valves to tank 3are closed The loading is continued in tank 3

Vapour from tank 1 is returned to shore via the vapour returnuntil the tank pressure is slightly above the pressure in theshore tanks

When the pressure is reduced, the vapour return and allvapour valves on tank 1 are closed, and transferring ofair/VCM mixture from tank 3 is continued

The above procedure is continued until the vapour in tank 3 ispure VCM, upon which the condensate is returned to tank 3and cooling of tank 3 is commenced Tank 1 is then loadeduntil the vapour pressure is well above the pressure in theshore tanks, upon which cooling of tank 3 is stopped and allcondensate and vapour valves closed

Vapour from tank 1 is returned to shore via the vapour returnuntil the tank pressure is slightly above the pressure in shoretanks

The above procedure is continued until the vapour in tank 1 ispure VCM, upon which cooling of tank 1 is initiatedsimultaneously with the cooling of tank 3.

4 Topping Up

As the cargo is cooled and the capacity thereby increased, thecargo tanks shall be topped up frequently until all tanks areloaded to capacity or until the shore installation stops theloading

B2 – Laoding Warm Propane in semi-ref vessels

Loading of "warm" propane, particularly with a high ethaneconcentration, is an operation which requires the fulloperational capacity of the ship's compressor units and AUXmotor or generators in order to ensure the shortest possibleloading time

In the semi-refrigerated gas carriers, the compressor units aredesigned for cooling of propane containing a maximum of2.5% ethane in the liquid phase without any problems

These gas carriers have been loading several cargoes ofwarm propane containing from 4 to 8% ethane in the liquidphase

Particularly during the initial loading stage such high ethanecontents may create problems as ethane content of forexample 6% in the liquid phase can produce an ethanecontents of 25% in the vapour phase

In order to obtain a satisfactory loading rate, the followinggeneral recommendations are given for the semi-ref vessels:

1 Initially, the loading rate must be low, and the compressors shall

be started up as quickly as possible

2 The temperature in the tanks shall be lowered to below -10°Cand the loading rate adjusted so as to maintain the temperature athighest -10°C

3 The loading rate shall, while closely monitoring the tankpressure, be gradually increased in line with the increasing load onthe compressors

4 It is preferable to maintain a condenser pressure of 20 bar inorder to obtain maximum cooling effect

The condenser pressure can be adjusted by blowing offvapours containing ethane to the tanks through the lower spray lines

in order to keep the ethane trapped in the liquid

5 The loading rate can be adjusted to correspond to thecompressors' capacity by throttling the liquid line valve on the tankdomes so as to maintain tank temperatures below -10°C

6 Note the fact that the heavy load on the compressors willgenerate intense heat in the electric-motor room

7 Owners must be consulted if problems are encountered

B3 – Cargo containing water

The various forms in which water is to be found in LPGcargoes are described in the ICS Tanker Safety Guide(Liquefied gas), Section 1.4.1 In the same publication use ofmethanol for de-icing of pumps and filters is dealt with

The ships are equipped with an LPG freeze valve, makeSeiscor During loading of cargoes above 0°C, it can beattached to the manifold and when operated according to themanual indicate the cargo's water concentration in ppm Thefreeze valve shall be used when loading LPG suspected ofcontaining water Thus, if water contents exceed the agreedamount, it will be possible to present a letter of protest to thecargo suppliers in time

As the correct functioning of the freeze valve depends on gasflowing through it and into the atmosphere, it must not be usedwhen loading cargoes with low TLV, such as e.g VCM

For the purpose of ensuring that any icing is discovered beforedischarge is started, when cargo temperatures become lessthan +1°C, it is recommended that the pumps be run for about

15 minutes every day, and also when cargo loaded at atemperature above 0°C is being cooled down below 0°C incargo tanks

Before cargo cooling is initiated consideration should be given

to use the tank overpressure to drain water from the cargothrough the pump sump and stripping line Time permitting,water should be allowed to settle below the cargo for 1 - 2days before draining is carried out

When carrying products, which have been loaded at atemperature below 0°C, it is recommended that the pumps beturned daily in order to ascertain whether they are blocked byice, so that the necessary permission for adding methanol can

be obtained, if de-icing should be required

In the event that pumps and filters are becoming blocked byice, the Technical Organisation must be advised in detail which

of the tanks and filters are affected

If the pumps are found blocked by ice, attempts should bemade to loosen them by inducing warm vapour or liquidthrough the liquid filling line or stripping line This methodrequires meticulous control of the liquid level in the tank towhich gas is being pumped.

Our gas ships are equipped with a tank for methanol, and withpumps and connecting pipe system for adding methanol forthe de-icing of pumps and filters

Charterers seldomly allow adding of methanol to cargo, as thisproduct deteriorates the cargo and may cause damage toplant in which the cargo is being processed

Consequently methanol must never be added to cargo until charterers' permission has been obtained through the Technical Organisation.

For voyages during which adding of methanol has beenapproved and is being made, the quantity added must beentered in the LPG/C REPORT under Remarks

If necessary, a requisition must be forwarded to thePurchasing Department as soon as possible after use ofmethanol in order to replenish the quantity on board

B4 – Loading and Sampling Proplyene Oxide

Initially the shore line and the vessel's liquid line shall be filledwith propylene oxide Valves on the dome/tank shall be keptclosed until an analysis has been made of the propylene oxidecontained in the lines

If this analysis is not satisfactory the propylene oxide shall bereturned to the shore installation by means of nitrogenpressure from shore If the analysis is satisfactory the loading

is commenced and 1 metre of propylene oxide is loaded intothe tank(s) The cargo is circulated for 1 hour after loading,after which, samples of the tank contents are taken

If these samples are satisfactory loading continues untilcompleted

After completion of loading the cargo is again circulated for 1hour after which samples are taken again

As the boiling point of propylene oxide is approximately 34°C

an overpressure has to be created in the tanks by means ofnitrogen supplied from the shore installation The tanks aregenerally pressurised to about 1 bar overpressure

After departure from the loading port during the first few daysthe overpressure will decrease to approximately 0.5 bar as thenitrogen is absorbed into the liquid

Some cargo receivers will only accept the ship arriving with alimited overpressure in the cargo tanks, normally about 0.1bar The overpressure must therefore be reduced during thevoyage

While reducing the overpressure, it must be ensured thatnitrogen is not added to the tanks from the ship's nitrogenbattery through the reduction valve

After the pressure has stabilised reduction of it can be started

It is recommended to halve the overpressure and then closethe tanks until next morning The overpressure will then haveincreased to almost the same level as before blowing off Theoverpressure is halved again This procedure is continued untilthe required overpressure has been reached

The process of reducing the overpressure should be adjusted

to terminate close to the end of the voyage Thus it is ensuredthat potential leaks from the cargo tanks do not require theaddition of nitrogen from the battery to maintain the tankoverpressure

B5 – Number of cooled cargoes

The semi-refrigerated vessels are equipped with a total of 3cargo cooling units enabling them to carry up to two cooledcargoes simultaneously Reference is made to InternationalGas Carrier Code (IGC code) rule 7.2.2.1 , excerpt of which is:Quote

"Where two or more refrigerated cargoes which may react chemically in a dangerous manner are carried simultaneously, special consideration should be given to the refrigeration systems to avoid the possibility of mixing cargoes For the carriage of such cargoes, separate refrigeration systems, each complete with a stand-by unit as specified in 7.2.1, should be provided for each cargo."

however, is purely a commercial problem, as total segregationcannot be expected.

NB: Connecting the standby cooling unit to a tank or pair

of tanks by means of hoses is a violation of the rules.

C TRANSPORTATION, TANK ATMOSPHERE, INBITIOR

C1 – Purifying Of Atmosphere

When initiating cooling of cargoes loaded in a nitrogenatmosphere, nitrogen will most often be present in too high aconcentration for cooling to be possible without removing thenitrogen

At some locations the cargo receivers furthermore require thatthe ship arrives with the cargo in a pure tank atmosphere, i e.with 100 % vapour without nitrogen

Removing of the nitrogen can be carried out by differentmethods:

By use of refrigeration plant.

When the refrigeration plant is in operation, the nitrogen can

be removed through the valve on top of the condenser Thevalve is opened when the pressure in the condenser is risingabove the normal operating pressure and partially throttledwhen the pressure decreases to normal operating pressure.The valve is adjusted until the pressure is maintained close tothe normal operating pressure The temperature in thecompressor must however never exceed the allowed values, i

e 60°C for butadiene or 90°C for VCM In order to minimisethe loss of cargo, the gas concentration in the cargo tanksmust be checked during the operation, using the Gascopemodel LV When 100 % concentration has been reached, blowoff from the condenser is stopped

If VCM is carried, the Gascope must not be used as themeasuring filament will be destroyed by the VCM vapours.Riken interferometer may be used, but only to a concentration

of 30 %, which makes this instrument less suitable for thispurpose

If the nitrogen content is too high for the compressors tooperate even with the valve on the condenser fully opened,some of the nitrogen may initially be returned to the tankthrough the by-pass valve

Blow off from the tanks

If the tanks are only partially loaded, another method can be toshift the cargo from one tank to another until approximately 98

% full

The safety relief valves are fitted with the highest set-point.The vapour line between the tanks concerned is kept open inorder to equalise the pressure If more tanks are loaded withsame cargo, pumping from the partly emptied tank iscontinued to these tanks until the tanks are 98 % filled (97.5 %filled in the J-class), while the vapour line is kept openbetween the tanks concerned

Then the vapour line valves are closed, and the cargo isreturned to the tank containing a mixture of product-vapourand nitrogen in the vapour phase until the pressure increases

to appear 0.2 bar below the set point Then the vapour linefrom this tank is opened to the vent mast and the pressure isreduced to a level corresponding with the temperature of thecargo The transfer of cargo is continued until the tank is appr.98% full

Reduction of the pressure should be carried out gradually inorder to minimise the loss of cargo, which will occur due toturbulence in the tank during blow off

Purifying of the tank atmosphere by this method is notcomplete The remaining nitrogen is removed during operation

of the refrigeration plant by blowing off from the top of thecondensers

C2 – Butadiene

Preamble

Butadiene and Crude C4 containing Butadiene are veryreactive chemicals which during transport may react with itselfand form polymers (dimers) and/or react with any oxygen inthe cargo system and form highly explosive peroxides Refer

to form "Notice Butadiene"

Objective

To minimise the risk of any unwanted chemical reaction whentransporting Butadiene and Crude C4 containing Butadiene

Procedure

1 Inerting Cargo Tanks with Nitrogen

In order to avoid the formation of peroxides, which besidesbeing highly explosive also acts as a catalyst forpolymerisation, the cargo tanks, piping and refrigerationsystem shall be inerted with nitrogen until the oxygen in thecargo system is reduced to the level required by the shippers.Regardless of the requirements by the shippers Butadieneshall not be loaded into a tank containing more than 0.2%oxygen

is added to each tank

Before leaving the loading port a certificate shall be delivered

on board stating the following:

- The name and quantity of the inhibitor added;

- The date inhibitor was added and the normally expectedduration of its effectiveness;

- The action to be taken should the length of the voyage exceedthe effective lifetime of the inhibitor;

- Any temperature limitations affecting the inhibitor

If a certificate is not received prior to departure, the Gas tankerDepartment shall be contacted immediately

1.2 Transportation Temperature

As polymerisation increases with heat, it is important that thecargo is cooled until fully refrigerated as soon as possible, andmaintained fully refrigerated during the entire voyageregardless of the discharge temperature

1.3 Draining Water

Depending on the specification Butadiene may contain up to0.5 per cent water, which besides the risk of freezing thepumps also absorbs the inhibitor, with subsequent risk ofpolymerisation

When the cargo has settled, the cargo tanks are checked forfree water by opening the puddle heating/stripping line If no oronly a little water is present the tanks are left until thetemperature in the sump is close to +1 C Any water contained

in the cargo will emerge during cooling and the cargo tanksshall again be checked for water when the temperature in thesump is close to +1 C

Small amounts of water are drained overboard by use of anejector During the draining the soundings shall be closelymonitored to avoid any excessive loss of cargo

If a large amount of water (more than 100 litres per tank) iseither suspected or detected, the Gas tanker Department andthe Nautical Department shall be contacted before anydraining is commenced

1.4 Circulating the Cargo during Voyage

The inhibitor used in Butadiene and Crude C4 containingButadiene is normally Tertiary-Butyl-Catechol (TBC)

If too much inhibitor has been added in the loading port, it may

be difficult to remove the surplus inhibitor during thesubsequent tank cleaning

Circulation of the cargo for a few hours each day during theladen voyage reduces the risk of surplus inhibitor sticking tothe tank surface Circulation shall be through the upper sprayline and filling line

Before circulation is performed the precautions mentioned inSection B3 of these Guidelines shall be taken

1.5 Circulating the Cargo during Discharge

During discharge some of the cargo shall be recirculatedthrough the start-up line When the sounding is at about 1.5metres some of the cargo shall also be circulated through thelower spray lines If performing Nitrogen DisplacementDischarge according to Section E13 in these Guidelines, nocirculation of cargo shall be performed during discharge

1.5.1 Product Washing in the H-class

During discharge product washing according to Section E12 inthese Guidelines shall be performed

Product washing is not to be performed if the vessel isdischarging by use of Nitrogen Displacement Dischargeaccording to Section E13 in these Guidelines, in which case acomplete methanol wash shall be performed when tankcleaning

2 Cooling of Butadiene

As the inhibitor does not boil off and the vapour therefore isuninhibited, the risk of polymerisation or formation of peroxides

in the refrigeration system is present

2.1 Normal Cooling Operation

During normal cooling operation the flow of uninhibited liquidthrough the refrigeration system is constant and the risk ofpolymerisation or formation of peroxides is therefore minimal

2.2 Compressor Discharge Temperature

The Chemical reaction increases when Butadiene is heatedand pressurised and it is therefore important that thecompressor discharge temperature and the suction pressure

do not exceed the values prescribed by the manufacturer, i.e.60°C and 2.0 Bar Gauge The Temperature Selector Switch

shall be set accordingly before cooling of Butadiene or CrudeC4 containing Butadiene is initiated.

2.3 Disconnection of the Refrigeration Plant

If the refrigeration plant or a part thereof is taken out ofoperation and blanked off from the cargo system for aprolonged period, the system in question must be drained andcleaned in a systematic manner in order to positively removeall Butadiene

3.0 Gas Freeing after Discharge

After discharge of Butadiene or Crude C4 containingButadiene all pipes, compressors, condensers, intermediatecoolers, auxiliary manifolds and drain system used for loadingand discharging must be drained and cleaned in a systematicmanner in order to positively remove all Butadiene thuspreventing the formation of peroxides

Tank Cleaning is performed in accordance with Section E1 inthese Guidelines