LNG Operational Practices

August 7, 2017 | Author: Ahmad Imran | Category: Liquefied Natural Gas, Chemical Engineering, Mechanical Engineering, Gases, Nature
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Contents Preface




Section 1: Post-Refit Operations


1 Primary and Secondary Insulation Space 1.1 1.2

1.3 1.4 :



Inerting/Drying Membrane Ships Drying of Cargo Tank Hold Spaces (Moss-Rosenberg) 1.2.1 Drying and Inerting of Tank Insulation and Annular Space Drying of Cargo Tanks Inerting of Cargo Tanks 1.4.1 Overview 1.4.2 Flammability of Methane, Oxygen and Nitrogen Mixtures 1.4.3 Plant Comparison IG/N 2 1.4.4 Inerting Procedure 1.4.5 Pressurisation Post-Refit Ballast Passage

2 Gassing-up Cargo Tanks 2.1 2.2 2.3

Operational Overview Operational Description Other Useful Points

13 14 15


Operational Overview Basic Cooldown Procedure (Membrane Vessel) Basic Cooldown Procedure (Moss-Rosenberg Vessel)

17 17 18

Section 2: In-Service Operations


4 Loading Operation 4.1 4.2 4.3 4.4

Pre-Loading Procedure Loading Procedure Topping-off Post Loading Procedure

5 Loaded Passage 5.1 5.2 5.3

Overview Normal Boil-off Gas Burning 5.2.1 Operation Forced Boil-off Gas Burning 5.3.1 Operation

7 7 8 8 9 10 11 12 12


3 Initial Cooldown of Cargo Tanks 3.1 3.2 3.3

3 6

23 '

23 24 28 29

30 30 30 30 32 32

LNG Operational Practice

5.4 5.5 5.6 5.7

Loaded Passage Essential Safety Equipment Loaded Passage Administration/Records Inner-Huil Inspection (IHI) - Cold Spotting Loaded Voyage Discharge Preparations 5.7.1 Two days prior to arrival at the Disport 5.7.2 Day of arrival at the Discharge Terminal 5.7.3 Day prior to arrival at the Disport

33 33 34 35 35 36 .36

6 Discharge Operation 6.1 6.2 6.3

6.4 6.5 6.6


Overview. Pre-DischaYge Procedure Discharge Procedure 6.3.1 Older Class of Vessel (GT or TGZ) - Using a Main Cargo Pump 6.3.2 Newer Vessels (GTT Membrane or Moss-Rosenberg) - Using a Stripping/Spray Pump Additional Notes Regarding Cryogenic Cargo Pump Operation Completion of Discharge Procedure Post Discharge Procedure

38 38 40 40 42 44 46 46

7 Ballast Passage - In Service 7.1 7.2



Overview Ballast Voyage Loading Preparations 7.2.1 Two Days before Arrival at the Load Port 7.2.2 Day before Arrival at the Load Port 7.2.3 Day of Arrival at the Load Port Cold State of the Vessel on Arrival at the Load Port

47 48 48 48 49 50

Section 3: Pre-Refit Operations


8 Cargo Tank Warm-up 8.1 8.2 8.3

Tank Stripping / Line Drainage Warm-up Operation Useful Points

9 Inerting of Cargo Tanks 9.1 9.2 9.3

Inerting Overview Inerting Operation Useful Points

10 Aeration 10.1 Aerating Overview 10.2 Aerating Operation 10.3 Useful Points

53 :


53 &4 55

56 56 56 57

57 57 58 58







Introduction On occasions, the text will refer to various Officers by rank. To clarify how cargorelated responsibilities are usually managed between the Officers, here is'.a typical manning structure for the Cargo Control Room (CCR): Chief Officer: Overall responsibility for the cargo, reporting directly to the Master and Chief Engineer as appropriate. Cargo Engineer: Responsibility assigned to the Cargo Engineer is dependant on the operating company. Typically the following would be the case, split-operational responsibility for the cargo with the Chief Officer, but with a direct reporting line to the Master and the Chief Engineer with regard to aspects of maintenance. OiC (Officer In Charge): Chief Officer and/or the Cargo Engineer. OOW (Officer of the Watch): In charge of watchkeeping responsibilities, that is, moorings, security, safety and providing assistance with cargo/ballast duties as directed. If the time frame for cargo operations and/or compliance with Hours of Rest regulations . requires handover of operational responsibility between C/O and Cargo/Eng, then that process should be formal and recorded appropriately in the Deck Operations Log. In order to conduct cargo operations safely and efficiently, synergetic teamwork from Jhe CCR is an essentia? requirement. Operator's own Safety Stem (SMS) must define the fSponsibility clearly and without ambiguity.

Emergency Procedures and Communications This section provides guidance in the event of an abnormal condition during cargo loading or discharge operations. An abnormal condition is anything that could compromise the vessel's ability to carry out a smooth, incident-free cargo operation. An abnormal condition need not be cargo-related. For example, it might be in the Engine Room (E/R) or involve deck machinery, such as a mooring winch failure.

Emergency Procedures Many emergency procedures are covered regularly by the shipboard Safety Management Drills which are applicable to in-port operations. Some examples are: • • • • •

Ship/Shore Fire Blackout Internal Loss of Cargo Compressor House Gas Detection Pollution.

All such procedures should be documented in the relevant Information Books and referred to regularly to ensure all shipboard personnel are fully aware of the appropriate action to be taken. In compliance with the ISM Code clauses 7 and 8 - 8.3, these drills must be regularly rehearsed by the ship's personnel according to an approved calendar to ensure that everybody is familiar with all essential response procedures. If considered necessary, suspend operations to stabilise an abnormal situation. If there is any doubt, a/ways take the safest option. From experience, the incidents listed here have the potential to cause serious disruption to the vessel's continued trading pattern:

LNG Operational Practice

Cargo Pump Failure: If you suspect that the cargo pump load current (amps) is abnormal or the discharge pressure is fluctuating excessively and the condition cannot be stabilised by conventional means, stop the pump immediately. The OIC (officer in charge) will inform all appropriate interested parties - the Master, Chief Engineer, Buyer's Representative, Terminal Control and (in most cases) the Superintendent in HO.* After such an action, do not restart the pump without permission from the appropriate party. Primary Barrier (Membrane) Failure: If you suspect that the primary membrane integrity has been compromised, inform all interested parties - the Master, Chief Engineer, Buyer's Representative, Terminal Control and (in most cases) the Superintendent in HO. If liquid cargo is suspected to have entered the primary insulation space, stop all cargo discharge from the tank. After such an action, do not restart the pump without permission from the appropriate party.

Communications In the event of a serious abnormal condition during cargo operations, the OIC should follow the guidelines laid down by the vessel's operator. Typically, these are: On the vessel (Ship's Staff) - Inform the Master, Chief Engineer, Chief Officer and Cargo Engineer. On the vessel (Non-Ship's Staff) - Inform the Buyer's Representative and/or the Loading Master Off the vessel - Inform Terminal Control and the vessel's Superintendent in HO. Brief all interested parties correctly and promptly about the nature of the abnormal condition. Keep your explanations concise and include any possible effects for the


current cargo operation. Cover the safety implications where appropriate.

Physical Properties, Composition and Characteristics of LNG Natural gas is a mixture of hydrocarbons that, when liquefied, forms a clear colourless and odourless liquid. This LNG is usually transported and stored at a temperature very close to its boiling point at atmospheric pressure (approximately -160°C). The actual LNG composition of each loading terminal will vary depending on its source and on the liquefaction process, but the main constituent will always be methane. Other constituents will be small percentages of heavier hydrocarbons, e.g. ethane, propane, butane, pentane, and possibly a small percentage of nitrogen. For most engineering calculations (e.g. piping pressure losses) it can be assumed that the physical properties of pure methane represent those of LNG. However, for custody transfer purposes, when accurate calculation of the gross heating value and density is required, the specific properties based on actual component analysis must be used. During a normal sea voyage, heat is transferred to the LNG cargo through the cargo tank insulation, causing vapourisation of part of the cargo, i.e. boil-off. The composition of the LNG is changed by this boil-off because the lighter components, having lower boiling points at atmospheric pressure, vapourise first. Therefore, the discharged LNG has a lower nitrogen content than the LNG as loaded, and a slightly higher percentage of ethane, propane and butane, due to methane and nitrogen boiling-off in preference to the heavier gases.


The flammability range of methane in air (21% oxygen) is approximately 5.3 to 14% (by volume). To reduce this range, the air is diluted with nitrogen or inert gas until the oxygen content is reduced to 2% prior to loading after dry-dock. In theory, a.n explosion cannot occur if the O2 content of the mixture is below 13%, regardless of the percentage of methane, but for practical safety reasons, purging is continued until the 02 content is below 2%. This is because the flow path of gases through the tanks can make mixing less effective. At 2%, we can be sure that all areas are well away from the flammable range. The boil-off vapour from LNG is lighter than air at vapour temperatures above -110°C or higher depending on LNG composition. Therefore when vapour is vented to the

atmosphere, the vapour will tend to rise above the vent outlet and will be rapidly dispersed. When cold vapour is mixed with ambient air, the vapour-air mixture will appear as a readily visible white cloud due to the condensation of the moisture in the air It is normally safe to assume that the flammable range of vapour-air mixture does not extend significantly beyond the perimeter of the white cloud. The auto ignition temperature of methane, i.e. the lowest temperature to which the gas needs to be heated to cause self-sustained combustion without ignition by a spark or flame, is 595°C. Volumetric expansion at the liquid/vapour phase change is 600-620 times the liquid volume, hence the dangers of trapping residual liquid between valves.

LNG Operational Practice

For clarity, the following style has been used to distinguish between the various types of text in this book: A text block like this will be used for general comment or a description of a step. An indented text block like this will be an instruction. • A bulleted point like this will either be a check-list item or part of some other sort of list. Items in this italic type are extremely important and should have special note taken of them.

Items in this italic type relate to safety and must always be closely reviewed and be given full consideration.

Normal Trading Cycle for a Modern LNG Carrier (membrane type used as an example)

Normal Cargo Operation

Post-Refit Operations


Post-Refit Operations

1 Primary and Secondary Insulation Space

1.1 Inerting/Drying Membrane Ships

ships operating manual should be followed as there may be slight differences in the operating procedures required.

Before putting a cargo tank into its initial service, or after dry-docking, it is necessary to replace the ambient humid air in the insulation space with dry nitrogen (N2).

The following outlines the more complex procedure involved where the use of vacuum pumps is required:

To do this, use the ship's vacuum pumps to evacuate the insulation spaces and refill them with N2. Repeat this procedure until the oxygen content is reduced to less than 2% and the humidity to less than -25°C.

To avoid major damage to the secondary barrier, never evacuate a primary insulation space while the associated secondary space is under pressure and never fill a secondary space whilst the primary space is under a vacuum.

Vessels fitted with the GTT Mklll type of containment system, or similar, are not equipped with vacuum pumps. Instead the by-pass valves on the exhaust from the interbarrier and insulation spaces are opened and a continuous supply of Nitrogen is provided so that the spaces are 'swept' with a free flow of Nitrogen. Once the exhaust from the spaces meets the above criteria, the exhaust by-pass valves are closed and the spaces pressurised. In all cases the detailed instructions in the

Measurement devices which may otherwise be damaged, should be isolated prior to the commencement of the test. Install temporary manometers of known accuracy, each bearing an in-date test certificate, to monitor the pressure in the space concerned. At all times, the barrier spaces must be protected against over-pressure, which might cause membrane failure.

LNG Operational Practice

Diagram 1.1 Vacuum Pumps

Typically, evacuation of the insulation spaces will take approximately 8 hours. Three cycles are usually necessary to reduce the oxygen to less than 2% by volume. Before being refilled with N2, the insulation spaces are evacuated to 200 mbarA. It is important to have a clear understanding of the pressures involved. For example, a primary space evacuated to 200mbarA exerts the same force on the primary


membrane as a positive pressure from within the tank of 800mbarG - that is, 3.5 x the tank relief valve lifting pressure. This procedure for evacuating the insulation spaces is also used to check the integrity of the barriers during the periodic global test and the same stringent precautions to avoid major damage must be in place. To avoid possible damage to the secondary membrane, evacuate the secondary insulation spaces before you evacuate the

Post-Refit Operations

primary insulation spaces. In normal design, the pipe work at the vacuum pump's suction ensures that either: (a) the evacuation of the primary spaces cannot take place without having first evacuated the secondary spaces, or (b) that both primary and secondary insulation spaces are evacuated simultaneously. Two electrically-driven vacuum pumps, installed in the cargo compressor room and usually cooled by fresh water, draw from the appropriate headers and discharge to the vent riser. Changes in temperature or barometric pressure can produce differentials far in excess of 30 mbar in insulation spaces that are shut in. With the cargo system out of service and during inerting, always maintain the secondary insulation space pressure to a level at or below the primary insulation space pressure. Severe damage to the membranes will result if the differentials exceed 30 mbar. After evacuation, the next step (or cycle) is to fill the insulation spaces with N2. Repeat the cycle until the oxygen content in the spaces is less than 2%. Adjust the opening of the primary space supply valves to balance the pressure rise in all the spaces. (This procedure is a valvespecific matter.) During filling, maintain the pressure in the primary space at 100 mbar above the secondary space. When the pressure in the primary spaces reaches 300 mbar A (100 mbar above the pressure in the secondary spaces), crack open the secondary space supply valves on each tank. Again, adjust the opening of these valves to balance the pressure rise in all the spaces.

For this operation, liquid N2 is supplied from shore to the liquid manifold. Where it passes to the stripping/spray header through the appropriate manifold shore-connection liquid valve. Then, it is fed to the LNG vapouriser. The N2 gas produced is passed at a temperature of +20°C to each insulation space. The initial filling process is supplied from a bulk liquid N2 source where the ship's N2 generating plant has insufficient capacity for this purpose. The final filling of the insulation spaces up to 2 mbar is carried out at a reduced rate of flow. Three cycles are usually necessary. After the final filling, check the 02 content in all the spaces. If it is higher than 2%, repeat the inerting operation. Also check the 02 content at the vacuum pump discharge at regular intervals. Although the N2 produced by vapourisation from a liquid source is very dry, the final humidity of the insulation spaces is an important consideration. As with the 0 2 , on completion, confirm it as acceptable i.e. better than-25°C. Do not shut down the LNG vapouriser until it has warmed back to the ambient temperature. The primary and secondary insulation spaces are filled with dry N2 gas. This is automatically maintained by alternate relief and make-up as the atmospheric pressure or the temperature rises and falls. It is necessary to maintain pressures of between 2 mbar and 4 mbar above atmospheric. Because of this, reinstate the vessel's normal supply from the A/2 generating plant and the associated pressure control systems as soon as possible after the procedures listed above have been completed.

LNG Operational Practice

Ensure that all relief valves have been reinstated and any blanks are removed. To prevent membrane damage, vigilance, careful planning and tight operational control are essential at all times. Note these key points that cover the importance of maintaining insulation spaces in the inerted condition: The N2 provides a'dry and inert medium for the following purposes: • To prevent the formation of a flammable mixture in the event of an LNG leak. • To permit easy detection of an LNG leak through a barrier. *© To prevent corrosion within the insulated spaces. • To deter the migration of leaking LNG vapour from primary to secondary spaces. In normal service, the last point which is more associated with the older class of membrane vessels, is achieved by controlling the secondary space pressure at approximately 1-2 mbar higher than the primary space. Now, Gas Transport Technology (GTT) recommends equal pressures of between 2-4mbar in both spaces. N2 produced by the two N2 generators is stored in a pressurised buffer tank, commonly 22 m3 in size.'This supplies the primary and secondary supply or pressurisation headers through make-up regulating valves located in the cargo compressor room. From the headers, branches are led to the primary and secondary insulation spaces of each tank. Excess N2 from the insulation spaces is vented to the appropriate mast riser through the relief regulating valves.

band of 10%. (Within this control deadband, neither supply nor relief is taking place.) Now, GTT recommends a very slight control range overlap, to ensure a continuous minimal N2 flow through the space, thus maintaining dryness and corrosion protection.

1.2 Drying of Cargo Tank Hold Spaces (MossRosenberg) During dry-docking or inspection, hold spaces contain a certain amount of moist air. These must be dried before any continuation of service, mainly to avoid the formation of corrosive agents within the hold spaces. This formation can occur if the moist air is allowed to combine with sulphur and nitrogen oxides, which may be contained in the inert gas. Dry air is introduced into the hold space through the inert gas filling pipeline. The displaced air is expelled from the top of each hold to the atmosphere. Dry air is supplied by the refit yard or .{if available) by the vessel's own Inert Gas (IG) plant operating in the Dry Air mode. If the vessel has its own IG plant, it can take some 20 hours to reduce the Dew Point to less than -25°C. But, if the dry air is supplied by the refit yard, the result can be better than -45°C. From an operational perspective, the inlet valves to the hold spaces are usually manually operated. The vent valves are situated on each tank top. •

Ensure the IG plant is operating in the Dry Air mode and ticking over while discharging to the atmosphere.

@ Open the vent valves to the holds. Note that on the older class of membrane vessels, the regulating N2 supply and relief valves for insulation space pressure control, can be set-up with an operational Dead-


@ Open the inlet valves to the holds. • Activate the Consumer Select facility on the IG control panel, or its equivalent.

Post-Refit Operations

Once the O2 level is satisfactory (approximately 21%) and the Dew Point is at (or lower than) -45°C, the IG plant discharge valve to the aeration header will automatically open and the discharge valve to atmosphere will close. The Dew Point for each hold is monitored at the respective outlet pipe. When a Dew Point of -25°C or lower is attained, the appropriate filling valve is closed and the corresponding vent valve also closes.

id Inerting of sulation and pace The insulation around the cargo tanks on spherical vessels is part of the leak protection system. The insulation is fitted in such a way that there is a space between the tank material and insulation. This space is known as the Annular Space'. In the event of a leakage of vapour from the tank, the vapour will be transported around the annular space to the gas detector by a constant flow of Nitrogen, enabling the leak to be detected quickly. In the event of a liquid leak the insulation acts as a splash back, directing the liquid into the drip tray located beneath the tank. Prior to introducing cargo vapour or liquid into the cargo system, the insulation and annular spaces around the tanks are purged with Nitrogen. This process removes any moisture that may be present within the insulation, which if left would cause damage through the formation of ice. The Oxygen in the atmosphere is also removed, resulting in an inert atmosphere around the tank. This procedure is undertaken by opening the exhausts valves from the Annular Spaces to atmosphere, and then opening the by-pass valves on the Nitrogen supply to the spaces on each tank. Nitrogen is supplied from the N2 generators as

required. The process is complete when the Oxygen content is below 2% and dewpoint is below -20°C. Once complete, the Nitrogen inlet by-pass valves are closed and Nitrogen supply to the Annular Spaces is regulated by the appropriate control valves.

1.3 Drying of Cargo During a dry-docking or inspection, cargo tanks that have been opened and contain humid air must be dried. Mainly, this is to avoid the formation of ice when they are cooled, but is also to avoid the formation of corrosive agents if the humidity combines with the sulphur and nitrogen oxides (which might be contained in excess in the inert gas). Normal humid air is displaced by dry air. Then, as the next part of the post-refit procedure, the dry air is displaced by inert gas. Dry air is introduced at the bottom of the cargo tanks through the filling pipes. The air is displaced from the top of each tank through the dome arrangement, into the vapour header and up the appropriate vent mast, usually No.1 (for'd). The operation (performed either from shore or at sea), will take approximately 20 hours to reduce the dew point to less than -25°C. During the time that the inert gas plant is in operation for the drying and subsequent inerting of the cargo tanks, the inert gas is also used to dry and inert all other LNG and vapour pipework to below -45°C. Before the introduction of LNG or the associated vapour, any pipework not purged with inert gas must be purged with N2. A modern shipboard IG plant can produce dry air with a dew point of -45°C and a flow rate of 14,000m3/h.

LNG Operational Practice

Area EDFE flammable ! Caution This diagram assumes complete mi which, in practice, may not occur.

Mixtures of air and methane cannot be produced above line BEFC


50 Methane

60 %

Area ABEDH not capable of forming flammable mixture

with air

Diagram 1.2 The relationship between gas/air composition and flammability for all possible mixtures of methane, air and nitrogen

Wet air, which may be contained in the discharge lines from the cargo pumps, float level piping and any associated pipe work in the cargo compressor room, must also be purged with dry air. In order to avoid the formation of corrosive agents, it is necessary to use dry air to lower the cargo tank's dew point to at least -25°C, before purging with inert gas.

1.4.1 Over Commence this procedure immediately after the drying process is complete. Inerting is the process that reduces the oxygen level in the atmosphere of the cargo tanks, pipelines and associated equipment to a level where combustion cannot take place. Removing the flammability hazard

Post-Refit Operations

means that, when gassing-up, the atmosphere in these spaces will never pass through the flammable zone.

Flammability of Methane, Oxygen and Nitrogen Mixtures Using the diagram 1.2 page 8: At all times, the ship must b'e operated to avoid a flammable mixture of methane and air. The relationship between gas/air composition and flammability for all possible mixtures of methane (CH4), air and nitrogen (N2) is shown on the diagram. The vertical axis A-B represents O2 - N2 mixtures with no methane present, ranging from 0% 02 (100% N2) at point A, to 2 1 % 02 (79% N2) at point B. The latter point represents the composition of atmospheric air. The horizontal axis A-C represents CH4_N2 mixtures with no 02 present, ranging from 0% CH4 (100% N2) at point A, to 100% CH4 (0% N2) at point C. Any single point on the diagram within the triangle ABC represents a mixture of all three components, CH4) 02 and N2, each present in specific proportion of the total volume. The proportions of the three components represented by a single point can be read off the diagram. For example, at point D: CH4: 6.0% (read on axis A-C) 0 2 : 12.2% (read on axis A-B) N2: 81.8% (remainder) The diagram highlights three major sectors: 1. The Flammable Zone Area EDR Any mixture whose composition is represented by a point that lies within this area is flammable. 2. Area HDFC. Any mixture whose composition is represented by a point that lies within this area is capable of

forming a flammable mixture when mixed with air, but contains too much CH4 to ignite. 3. Area ABEDH. Any mixture whose composition is represented by a point that lies within this area is not capable of forming a flammable mixture when mixed with air. Assume that point Y on the 02 - N2 axis is joined by a straight line to point Z on the CH4- N2 axis. If an oxygen-nitrogen mixture of composition Y is mixed with a CH4- N2 mixture of composition Z, the composition of the resulting mixture will, at all times, be represented by point X, which will move from Y to Z as increasing quantities of mixture Z are added. In this example point X, representing changing composition, passes through the flammable zone EDR that is, when the CH 4 content of the mixture is between 5.5% at point M, and 9.0% at point N. Applying this chemistry to the process of inerting a cargo tank prior to cooldown, first assume that the tank is initially full of air at point B. N2 is added until the 02 content is reduced to 13% at point G. The addition of CH4 will cause the mixture composition to change along the line GDC. It will be noted that this does not pass through the flammable zone, but rather is tangential to it at point D. If the 02 content is reduced further, before the addition of CH4, to any point between 0% and 13%, (between points A and G), the change in composition with the addition of CH 4 will not pass through the flammable zone. Theoretically, it would only be necessary to add N2 to air when inerting until the 02 content is reduced to 13%. However, the 02 content is typically reduced to 2% during inerting because, in practice, complete mixing of air and N2 may not occur within the tank, and we are always looking to provide a healthy safety margin.

LNG Operational Practice

When a tank full of CH4 gas is to be inerted with N2 prior to aeration, a similar procedure is followed. Assume that N2 is added to the tank containing CH 4 at point C until the CH4 content is reduced to about 14% at point H. As air is added, the mixture composition will change along line HDB; which, as before, is tangential at D to the flammable zone, but does not pass through it. For the same reasons as when inerting from a tank containing air, whe.n inerting a tank full of CH4 it is necessary to go well below the theoretical figure to a CH 4 content of 5% because complete mixing of CH4 and N2 may not, in practice, occur. To summarise procedures for avoiding flammable mixtures in cargo tanks and piping:

Do not confuse the IG/Dry Air generating plant with the N2 generating plant. Both have very different functions. (a) the IG/Dry Air plant has a high capacity, low pressure throughput, from a combustion-based source that is used for inerting large volume spaces with high quality IG. (b) the N2 generating plant is a low capacity, reasonably high-pressure process that is used to supply relatively pure N2 for pressure maintenance purposes, that is, a low flow, low 02 content and without any undesirable by products associated with the combustible source. They provide these onboard services: IG/Dry Air Plant - used to:

;' Before admitting CH4, tanks and piping containing air are to be inerted with N2 until all sampling points indicate 2%Vol or less oxygen content.

# Inert cargo tanks, cargo pipelines and associated equipment.

Before admitting air, tanks and piping containing CH 4 are to be inerted with N2 until all sampling points indicate 5%Vol CH4 or lower.

€> Aerate cargo tanks, hold spaces and cargo pipelines.

Note that some portable instruments for measuring CH 4 content are based on oxidising the sample over a heated platinum wire and measuring the increased temperature from this combustion (catalytic process). This type of analyser will not work with CH4-nitrogen mixtures that do not contain oxygen (0 2 >13% Vol). For this reason, special portable instruments such as the infra-red type have been developed and these are capable of detecting and measuring the hydrocarbon content in inert atmospheres (a purely non-combustible process).

4.3 Plant Comparison IG/N 2 Unlike oil tankers, gas carriers do not use the ship's boilers to create inert gas (IG). Instead, it is produced by a purpose-built IG/Dry Air Generating Plant.


% Dry cargo tanks, hold spaces, cargo pipelines and associated equipment.

N2 Generating Plant - used to: if Inert/purge the primary/secondary insulation/annular space (membrane). Safe carriage of the cargo in compliance with the operator's SMS and associated international regulations. Achievement of maximum out-turn across the manifold for the customer. !

Delivery of the cargo within the customer's quality parameters and/or as per the contractual requirements of an established Sale & Purchase agreement. These may include a maximum delivery temperature (>-158.8DegC) and a window of allowable vapour pressure. If required, SG and N2 content may be determined at source by sampling, as arranged between supplier and receiver. Cargo venting avoidance in-line with the operator's EP Policy. Full use of boil-off as a supplementary fuel source for the vessel's power plant. Maintenance of cargo containment integrity, particularly the inner-hull steel work temperatures.


I nrnviuA Son^yfi Gas Burn-in®

During a sea passage where the cargo tanks contain LNG, the naturally-generated boil-off from the tanks is burned in the


ship's boilers. The operation is started on deck and controlled by the ship's engineers in the CCR and ECR. If the boil-off cannot be used for gas burning purposes, or if the volume is too great for the boilers to handle, then excess vapour as a last resort may be vented into the air through the No.1 vent mast as a last resort.

5.2.1 Op The cargo tank boil-off gas enters the common vapour header through the cargo tank vapour domes. It is then directed to the in-service LD compressor, which delivers the fuel gas to the E/R through a gas heater. The heated gas is delivered to the vessel's power plant at approximately +30°C by a fuel gas control valve. Compressor throughput is controlled by speed of the prime mover and/or inlet guide vanes at the compressor suction. On a modern carrier, the various control requirements are consolidated by DCS (Distributed Control System), which includes the ACC (Automatic Combustion Control). Associated logics use a pressure input from the cargo tank vapour system to make sure the gas delivery to the E/R does not suppress the cargo tank vapour pressure below a pre-determined allowable minimum, or allow it to rise above a predetermined maximum. The system is designed to burn all boil-off gas normally produced by a full cargo maintaining the cargo tank pressure and temperature at the required level. On a standard steam propulsion plant, if fuel consumption is not sufficient to burn the generated amount of boil-off, the tank pressure will increase. This pressure increase can be controlled in one of two ways either provision of a steam dump system, or alternatively, by increasing the speed of the vessel. In most cases, the main steam dump system is designed to dump sufficient steam to let the boilers burn the boil-off gas, even when the ship has stopped. A combination of both methods may be used.

In-Service Operations

Although venting is also a means of automatic vapour pressure control, it is not an option under normal operating conditions and, in the interest of Environmental Protection (EP) compliance and operating efficiency, it should be avoided. This is the standard logic arrangement for boil-off gas pressure control by a Distributed Control System (DCS): To control the flow of gas through the LD compressors, adjust the inlet guide vane position. When gas-burning is initiated, this is directed by the DCS. Select the normal boil-off in the boiler combustion control. Select the maximum/minimum allowed tank pressures. Select the tank pressure at which the main steam dump operates. For normal operation, the normal boil-off value is selected at approx 60%, that is, boil-off provides 60% of the fuel required to produce 90% of full steaming capacity. The minimum/maximum tank pressures are selected at (for example), 1050 and 1090 mbarA (for a standard membrane-type carrier). If the normal boil-off control value has been correctly adjusted, the tank pressures will remain within the selected values. Should the selected normal boil off value be too large, tank pressure will slowly reduce until it reaches the minimum value selected. If the tank pressure value continues to fall below the minimum value selected, the DCS will reduce the normal boil-off value until the tank pressure has increased again above the selected value. If the selected normal boil-off rate is too small, the tank pressure will slowly increase until it reaches the maximum setting selected. If the tank pressure value increases above the maximum selected

setting, the normal boil-off rate will be increased until the tank pressure falls to a level below the selected setting. If the tank pressure continues to increase because the steam consumption is not sufficient to burn all the required boil-off, the steam dump will open. The steam dump is designed to open when the normal boil-off rate is 5% above the original selected value and/or when the tank pressure has reached the pre-selected dump operating pressure. At this setting, an increase of 5% of the normal boil-off corresponds to an increase in tank pressure of approximately 40 mbar above the maximum tank pressure selected. In each case, the Automatic Combustion Control (ACC) compensates for the boiler fuel requirements by varying the amount of HFO delivered -which is variable between 0% HFO (100% gas) and 100% HFO (0% Gas). If anything should stop the gas from being burned in the ship's boilers, the cargo vapour and gas burning piping system is arranged so that excess boil-off can be vented automatically. An automatic control valve, usually sited at the No.1 vent mast, is normally set at approximately 25-30mbar below the tank relief valve (typical "luceat") upper operating level. If the gas-burning system shuts down for any reason, an integral part of the shutdown sequence automatically initiates a thorough purging of all associated lines with N2. Typically the gas burning security valve G will operate (shut) under these circumstances: P Gas to E/R temperature Low/Low. ' Gas detected in boiler combined gas hood. Gas detected in the vent duct. .•'••v Gas duct exhaust system failure. } Gas pressure High/High. Gas pressure Low/Low.


LNG Operational Practice

Manual operation (E/R, CCR, Bridge). ;

: Loss of authorisation from CCR or Bridge. E/R exhaust fan failure. i E/R C0 2 fire-fighting release. • Blackout.

If, during a loaded passage, additional fuel gas from the cargo tanks is required to be burned in the ship's boilers over and above current natural generation, it can be made available by forced vapourisation, using a dedicated Forced Vapouriser. This operation, called Forced Boil-Off, can be used to complement gas burning for up to 100% of the boiler's fuel requirement.

o,;;; Pureed BoH-Df? Gas Burning Before undertaking forced boil-off, consider the economics of gas versus fuel oil burning and the charter agreement, if applicable.

5.3.1 Operatior The normal gas burning arrangement is maintained and the forcing vapouriser is brought into operation. This uses a single stripping/spray pump in conjunction with the LNG forcing vapouriser. The excess flow from the pump is returned to the same tank

Diagram 2.3 Forcing Vapouriser


In-Service Operations

through the stripping header pressure control valves. The generated vapour then combines with the natural boil-off gas from the vapour header before being drawn into the suction of the LD compressor, and reducing the risk of droplet carry-over. The process is controlled by the DCS. In normal forced vapouriser operation, the controlled return from the pump is always directed back to the same tank where the liquid is being drawn from as an insurance against cargo transfer between tanks.

•'••;>i Loaded Passage Essential Safety

• Catalytic Gas Detection System -Atmospheric sampling of strategic spaces round the accommodation, E/R and compressor house. •i Insulation and Inner-Hull Temperature Sensors # - One of the main line defences against primary containment leakage and inner-hull steel work cold-failure protection. ® Automatic Sequence Controlled N2 Purge System -When a gas burning shut-down is initiated, it renders an inert atmosphere within all gas burning pipe work. A Fully Integrated Alarm System - Sounds in the CCR, E/R and Bridge, as appropriate.

This is a list of essential safety equipment: Air Swept Duct - Conveys fuel gas line, purge lines and N2 lines through the E/R space to the boiler furnace fronts/top. Double-Walled N2-Jacketed Gas Line - Used where the fuel gas lines exits the air-swept duct and connects to the furnace front/top burner gas valves.

Records Maintain these records at all times: ;i Daily Cargo Log. © Daily LD Compressor Log.

Air-Swept Duct Exhaust Fans - Maintains air flow through the air-swept duct. Stand-by fans automatically cut-in on failure of in-use units. Initiated by ampere load-sensing relays.

•s Monitor inner-hull and insulation space temperatures, as appropriate.

Gas Burning Security Valve G - Independent and operated by any of the above listed elements.


Dedicated Fuel Gas Detection System - Samples furnace front/top fuel gas line canopy/hood arrangements and the swept-duct exhaust air for gas leakage. Positive detection incorporated in the gas burning security chain. Main Permanent Gas Detection System - Infra-red, sampling from the N2 rich interbarrier spaces, insulation spaces and hold spaces.

;i Inner Hull Inspection (IHI) Record.

-' Daily trend monitoring of Average Liquid Temperature (for delivery). Daily Fuel Oil Equivalent (usually calculated as part of the Voyage Abstract).

•1 Alarm Test Register (on-going from an established register or PM). tl Weather Report (sea state, barometric pressure, etc, all have an effect regarding cargo conditioning considerations).

LNG Operational Practice

•^6 . h ^ r - r h ^ l ^^pecllon

These are the inspection points: • The position and temperature of cold spots, or the absence of cold spots where previously reported.

At low temperatures, structural steels can suffer brittle fracture. Such failures can be catastrophic because once brittle steel starts to fracture, only a small amount of energy is required to make it spread.

§; On Moss-Rosenberg vessels, condition of the spray-shield and taped joints.

However, in a tough material, considerably more energy is required to turn a small crack into a larger fracture.

© Condition of inner-hull paint work or appropriate tank coating.

Plain carbon steels have a brittle to ductile behaviour transition that generally occurs in the range of -50°C to +30°C. Because of this, the composition of structural steels used in the containment system needs to be carefully chosen and protected from the cold cargo temperatures (-160°C) throughout the service life of the vessel. IHI (or Cold-Spotting) is an important procedure, more so on membrane vessels than the Moss-Rosenberg designs. It is a Classification requirement for the granting of a valid Certificate of Fitness for ships carrying liquefied gasses in bulk that routine cold spot inspections are carried out and that the results are recorded in the LNG survey record book. As a Class requirement, all spaces around the cargo tanks must be inspected once in every six months. To meet this requirement, it is good practice to divide your spaces into zones and inspect a number of zones on each occasion, meaning that all spaces will be inspected during the designated period as defined by Class. These spaces include ballast tanks, cofferdams, whaleback spaces, MossRosenberg hold spaces and duct keels. On completion of loading, and to standardise the entries in associated records, conduct your IHI after the same time has elapsed.

$ Condition/wastage of ballast tank sacrificial anodes.

• The extent of corrosion on both inner and outer hulls, particularly under ballast tank suction strums in-way-of striking plates and behind heating coils. @ Position and amount of sediment in the ballast tanks. 9 Any damage or fractures, with particular attention paid to the external portion of the inner-hull and at the associated turn of bilge areas, especially within the midships section of the vessel. :::: Evidence of hydraulic oil or heating coil leakage (steam or glycol). .;- Condition of scupper pipes. # Condition of fittings for any ballast level detecting systems. Ballast line condition, especially at bends and expansion pieces. # Ballast line/valve integrity checked if appropriate by applying a static head pressure. When the inspection is complete, the report is signed by the Master, Chief Engineer and Cargo Engineer. These are the IHI hazards and safety precautions: 9 Risk Assessment completed and tabled with all those involved present, and then posted at the tank entry location. & Full enclosed space entry procedures implemented in accordance with the operator's SMS and permit to work


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system. Permit posted at the tank entry location. A worksite Toolbox Talk (TBT) given by the Supervisor to all those involved. ;

> Operation discussed at the previous Daily Work Plan Meeting and'the associated requirements for all departments clearly identified in the Daily Work Plan, which is then posted and circulated accordingly. In accordance with standard enclosed space entry procedures, initiate ventilation in good time and maintain it throughout the inspection. Multi-point/level testing for hydrocarbon gas and the presence of enough O2. The contingency safety trolley is fully equipped and standing-by. Inspection route for each team clearly established. All personnel wearing adequate Personal Protective Equipment (PPE). All equipment must be approved, fitness certificates in date as appropriate and electrical equipment Intrinsically Safe (IS) by design. Personal O2 meters worn by all personnel and their correct operation confirmed before entry.


Each person equipped with a torch or helmet mounted headlamp, plus an emergency chemical light-stick.

# The dangers posed by the presence of N2 pockets highlighted and understood by all concerned. ® All participants invited to express concerns before entry, usually conducted at the TBT In the event of a cold-spot forming, the response is controlled and graded according to severity. The following procedures would be normal: i> Note size, location and minimum surface temperature of the affected steel. # Assess the level of stress and loading in the steel structure affected. For this, Head Office should provide back-up. # Take no further action, but continue to monitor until a temperature approaching the brittle to ductile transition range for the steel is reached. $•• If the situation continues to deteriorate, consider the stresses on the vessel and arrange to flood the space concerned. Even on the older TGZ and GT membrane vessels (now in excess of 28 Years old), it would be a very rare and isolated case.

Personal pocket-sized motion sensors with activated locating beacon worn by each team member. One person in each team equipped with a radio, tested and tuned to the correct channel. Temporary in-tank aerial rigged and tested. Communication with the Bridge confirmed and the reporting system fully established. Standby man present at the main deck level to liaise between the in-tank team and the Bridge.

D? s e n a r y Preparation 5,7.1 Two day' at the Disport This is a check-list of tests to be carried out and items to be'verified: ESD system -Tested and proved fully operational.

LNG Operational Practice

Cargo valves - Remote/manual operation tested. - Speed of remote operation checked and confirmed to be within surge avoidance parameters which should be 25-30sec. (Critical with regard to the loading arms and damage-avoidance in the event of an ESD.) Ballast valves - Remote/manual operation tested. - Use line-surge avoidance to check the speed of remote operation. (Important where the ballast lines are made of GRR which is the preferred type of construction on many Moss-Rosenberg vessels.)

Cargo savealls - Where appropriate, fill with fresh water. In the designated areas, place stainless steel buckets filled with fresh water filled and provide and adequate supply of fresh clean rags and PPE. (Used as a temporary way to stem a minor leak). •& Prepare the Oil Pollution Trolley and its associated equipment. Prepare the deck fire-fighting equipment, rig/arm dry powder monitors and open dry-powder house doors. ® Rig the standard international ship/shore hose connection. CCTV - Check and ensure it is operable on all points.

Manifolds - Check for moisture ingress. Manifold bobbins, filters, gaskets and presentation flanges - Check for mechanical damage. Gas Detection - Check systems (normally infra-red and catalytic) and prove operational on all points. Pressure-test the liquid and vapour manifolds. If using N2, the test pressures should be 5Bar at the liquid manifold and 1 Bar at the vapour manifold. Use soapy water to detect for leaks.

ay of Arrival at the Discharge Terminal Ballast - Adjust for appropriate arrival draught and trim. Deck scuppers - Make sure all closed. Mooring winch savealls -Clean and mop out. - Replace drain plugs. Bunker/hydraulic savealls -Clean and mop out. - Replace drain plugs.


5,7,3 Day prior tc the Disport This is a check-list of tests to be carried out and items to be verified: @ Cargo system - Inspect and ensure the intended offshore manifold is secured and pressurised with N2. ;

Cargo/spray pump motors and associated transmission cables - Check the insulation resistance to earth. Record the results as they may warn of potential electrical failures and a possible break-down of electrical insulation. - Depending on the system, prove the integrity of the electrical safety chain for each pump at the same time As far as is practical, set the cargo lines for the intended discharge procedure. - The Chief Officer and/or the Cargo Engineer are responsible for this. - On the older membrane class, this procedure opens the spray valve on the tank to be used for cooldown, ensures that all other tank spray valves are closed, opens (fully) the loading valve on the same tank, opens the crossover

In-Service Operations

valve between liquid and spray headers and cracks open all discharge valves slightly to remove any possibility of jamming during cooldown. Moorings and associated equipment. - Prepare/check all. -The high freeboard of an LNG carrier and the fine tolerance regarding the ships position relative to the loading arms often places a high demand on this equipment. Cargo boil-off control systems. - Check and prove all systems operative. - Prove the emergency closing facility for the For'd Vent Riser (activated) from the Bridge. Valve opening is usually achieved by a manual adjustment of the controller's Set Value (SV) from the CCR and emergency closure is observed as the Bridge activates the appropriate control. Manifold overside water sprays. - Test the operation to ensure hull protection against LNG leakage. Fire pump, IMO pump, Emergency fire pump and emergency generator - Ensure/confirm that all are available for the loading procedure. Cargo/Ballast Plan - Chief Officer completes the approved plan and obtains the authorising signatures as required. Pilot hoists and/or accommodation ladders - Test as appropriate. Chief Officer to prepare arrival/sailing stability conditions. Chief Officer to prepare arrival/departure paperwork. In accordance with the Company's SMS, the Master chairs the pre-port meeting that provides the interface between the various departments of the SMT At this meeting, terminal requirements, safety, ISPS security/compliance, known system/vessel defects, shore-leave control, intended

reliefs, storing/bunkering requirements and the arrival/departure programme are discussed. Note On the newer Moss-Rosenberg and GTT Membrane vessels, the cargo lines can be cooled before arrival at the disport terminal. This can be done the day before or on the same day, depending on the Expected Time of Arrival (ETA). A similar procedure will usually apply at the load port. Cargo lines and cargo plant are cooled to the lowest possible temperature before arrival at the terminal so that cargo operations can begin as soon as the vessel is moored and all procedures have been completed. As the cargo lines are being cooled, use the actual cooldown liquid to pressure-test the system, that is, as distinct from the N2 pressure test procedure mentioned above. To do this, wait until the manifold spoolpieces, reducers and discharge filters have been fitted - and then increase the system pressure to approximately 5Bar. (This test is not a requirement but it ensures that you have a tight system.) To run a line cooldown procedure, use one spray pump (from No.3 Tank, for example) to pump LNG through the spray header to the liquid manifold pipe work. Vapour displaced from the crossover pipe work passes through the liquid header, the spray bypass, the return valves of No.1, 2 & 4 cargo tanks and then back to No.3 tank, through the filling line. Vapour generated by this process is burnt in the boilers, using the LD compressor and gas heater system. If there is more vapour than is required by the propulsion plant at this stage in the voyage, the excess steam dump is used. When line cooldown is complete, stop the spray pump. If the time between line cooldown completion and berthing is

LNG Operational Practice

extensive, restart the spray pump as required. Return of cooldown liquid to the bottom of the cooldown source tank through the loading line (in this case No.3), can cause a local temperature increase at the appropriate tank bottom sensor. Allow sufficient time for this to stabilise before the post discharge gauging.

6 Discharge Operation






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Most discharge terminals require the manifold valves to be fully shut and put into manual operating mode before the discharge arms are connected or disconnected. Before the operation begins, this condition should be checked and confirmed by a Responsible Officer (usually the Cargo Engineer) to the Terminal Representative on duty at the manifold.

The main engine turning gear is engaged and the master steam stop valve closed prior to the gangway being brought onboard and the loading arm connections made. Most terminals use between two and four discharge arms and one vapour return arm. At most LNG terminals, you are not permitted to immobilise the main engine for maintenance or any other purpose and the main engine must remain in a state of readiness throughout the process. Connect the Communication I BSD cable. This is usually a male/female plug, (fibre-optic or electrical) and provides the required channels for ship/shore communication and ESD interconnection. Connect the ESD umbilical cable, usually a pneumatic connection Connect the shore ground cable at the approved location on the vessel.

To replace the LNG being discharged from the vessel's cargo tanks, return vapour is normally supplied from the terminal or, if the terminal supply is unavailable, by LNG vapourisation on board. Both methods are supplemented by normal boil-off from the liquid in the cargo tanks. These procedures are as detailed as it is possible to make a non-specific class description. Common underlying principles should be evident, especially when compared to the previously-described loading procedures.

When the discharge arms have been connected, they are purged and pressurised with N2 supplied by the terminal. A pressure of 200kPa may be raised in each arm. The Responsible Ship's Officer checks all associated joints for leakage. Use soapy water to check for leaks. As appropriate, use NEW gaskets on all connections.

As with the previously described Loading Procedures, record all key events in the Deck Operations Log (DOL)


Once the discharge arms have been proved leak-free, they are depressurised in a controlled manner. While depressurising,

In-Service Operations

use the portable O2 meter to check the gas. Below 1% is the usual requirement. Dryness of the gas is important and can be part of the standard checks at some terminals. The usual requirement is lower than -50°C. Most LNG terminals have a representative in attendance at the manifold while the discharge arms are being connected. At the same time a Responsible Ship's Officer will witness the connection process and the leakage tests, O^moisture checks and (if required), report the progress of any storing operations to be completed before cargo operations can commence. Back in the CCR or Ship's Office, a predischarge meeting is held to confirm discharge procedures and to exchange relevant information with respect to the vessel's time alongside. At this time, the ship/shore safety check-list and ISPS security check-list are completed. With the vessel upright and even keel, it is usual for the Chief Officer and Cargo Surveyor (or appointed person) to run the official Initial Cargo Custody Transfer data. All parties should confirm their agreement with the calculated quantities. At this point in the procedure, many terminals require EIR fuel gas burning to be terminated and secured. At such terminals, EIR fuel gas burning may not restart until discharge and final gauging have been completed. In the CCR, the ship/shore communication system will be powered up. Test the HotLine and Plant phone and prove that it works. When it has been confirmed that no oil leakage from any of the deck hydraulics has taken place, no matter how slight, bring the manifold spraywater curtain into service. When the vessel and terminal are ready to carry out ESD tests, confirm

whether the ship or the terminal will activate the shut-down as they run through. The OIC normally attends at the manifold to assist and report on the progress of the tests. With permission from the terminal, open the manifold/ESD valves, which are now in the Automatic mode. When the valves are confirmed fully open, reset the ship's ESDS and make sure that the appropriate indicators all show Healthy Once the terminal has completed a similar reset, switch all the ship's associated override facilities to Override Off. Conduct the "Hot ESD" test as agreed with the terminal representative. At this time, it is quite normal for the terminal to ask for the closure of the Manifold/ESD valve to be timed. Now the ship can request permission from the terminal to open the return gas from the shore vapour valve. Set associated lines and valves accordingly. On the older class of vessels, the liquid manifold valves remain closed until the line/discharge arm cooldown procedures are complete and the terminal gives permission. The ESDS, however, is reset and seen to be healthy both terminal and shipside by utilising the appropriate "Manifold Valve not fully open" over-ride facility. On the newer Moss-Rosenberg and GTT vessels, manifold/ESD valves are opened fully at this stage. Enable and ensure all cargo tank level alarms are. operative (Low Level @ 0.5Mtr, Hi Level @ 95% capacity, Tank Fill Level @ 98.5% capacity and HiHi Level @ 99% capacity).

LNG Operational Practice

On passage, it is normal practice to use the dedicated blocking circuits to inhibit certain level alarm systems. The vessel may now begin the discharge procedures, starting with the cooldown of the discharge arms.

are in close radio communication if one is absent

6.3.1 Olc Main Carg Make sure that vapour return from the shore is available.

6.3 Discharge Procedure When the vessel is informed by the shore representative that the terminal is ready for cooldown, depending on the age of the vessel, one of these two procedures will applyThis assumes that both Chief Officer and Cargo Engineer are present in the CCR or,

Fully open the loading and spray valves on the nominated cooldown tank (normally No.3 or 4 on a 5-tank vessel). With the terminal's permission, open the liquid manifold bypass valves by the approved amount (normally 1 A-1 turn).

Diagram 2.4 Custody Transfer System


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Inform the EIR that you are about to start the first cargo pump. Make sure that a Responsible Officer (usually the OOW) is standing-by at the tank and in close radio communication with the CCR. With the discharge valve fully closed, start the nominated main cargo pump. When the ampere loading and discharge pressure stabilises, crack the discharge valve (typically 1 turn) to elevate the amps. Remember that pump protection usually includes a time-delayed trip facility on Low Amps and Low Discharge Pressure. When the discharge valve body is completely frosted, gradually increase opening until the ampere loading is steady at the approved first level (this is nominally 40Amps below normal full load). Close the loading valve until the liquid header pressure is between 1.8 to 2.0BarG. As cooldown progresses, a further adjustment may be necessary to maintain this pressure. Use the midships crossover temperatures and the frosting along the liquid manifold bypass lines and chiksan arms to monitor the progress of cooldown. Regulate the manifold bypass valves to maintain an even rate of cooldown on all discharge arms. When the terminal informs the ship that cooldown is complete, fully open the tank loading valve and allow the liquid header pressure to fall below LObarG. If necessary, throttle in the discharge valve on the pump being used for cooldown to make sure that the chiksan arms are not subjected to

hydraulic shock as the liquid manifold valves are opened. In some discharge ports, a manifold/ESD valve Cold Function test is carried out on completion of cooldown to verify the correct operation of the valves when they are in the cold state. With the terminal's permission, open the liquid manifold valves. When manifold valves are confirmed fully open, switch the ESDS 'Manifold Valve Not Fully Open' override facility to the Off position. This means that the ESDS is now fully enabled. Close the liquid manifold bypass valves. Close the loading valve on the cooldown tank. Adjust the pump ampere value back to the first level by adjustment of the discharge valve opening. Close the cross-over between liquid and spray headers. After informing the EIR, start the remaining pumps with the approved time interval between each start and as agreed at the pre-discharge meeting. This is normally ten minutes for the second pump and five minutes thereafter. Each pump is started in the approved manner. Remain aware of the mechanical damage that flow disruption or repeated starts can cause. When all the pumps are running, inform the EIR and adjust the discharge valves to give either the maximum flow rate permitted by the terminal, the maximum allowable continuous running amps for the pumps or a stipulation regarding EIR

LNG Operational Practice

generating capacity, whichever is the prevailing parameter. This procedure is called Ramp-Up. At all times during Ramp-Up, patrolling watchkeepers will keep a watch on deck lines, fittings and the manifold (including the off-shore manifold). They will remaining in close radio contact with the CCR throughout the process. When the pressure in the tanks has fallen to approximately 65mbarG, ask the terminal to start the Return Gas Blower (RGB). If an RGB is not available at some terminals, then return gas may be supplied by FreeFlow only. If tank pressure continues to fall, you must either reduce the discharge rate or use the ship's dedicated vapouriser to generate vapour internally. Depending on the design, do not allow tank pressures fall below a minimum level, which must be clearly established in the operating procedures (on the older membrane class, normally 40mbar). Ballast operations are carried out concurrently with cargo discharge and in accordance with the Chief Officer's Cargo/Ballast Plan. During discharge, hourly rates and individual tank finishing times are calculated and relayed ashore as required by the terminal. Cargo valves are adjusted to maintain the maximum permitted rate and for all tanks to finish closely together. Under normal circumstances, a certain amount of cargo (heel) is retained onboard in each cargo tank after discharge. This figure is calculated according to an agreed formula that considers the Charterer's requirements, the length of the ballast passage and fuel gas requirements.

Reference to previous records will give you the optimum distribution of this quantity between the tanks because, in practice, the cargo heel does not vapourise equally from each tank.



To cool the manifold and discharge arms, use a liquid supply from the cooldown source tank (typically No.3 on a 4 tank vessel) with the stripping/spray pump discharging into the spray header. The spray header is cross-connected to the liquid manifold through cooldown bypass valves which go round the manuallyoperated liquid manifold discharge block valves. For this procedure, open the liquid discharge manifold/ESD valves. Line-up the spray manifold for return to the source tank through the spray return line. Cool the spray header first. Once you have permission from the terminal, start the spray pump with the discharge valve approximately 10% open. Once the spray header has cooled, obtain permission from the terminal to open the spray pump discharge valve a little more, to ease the spray return into the source tank. Liquid is then forced into the discharge manifold and the discharge arms at a controlled rate. Once the manifold and discharge arms have been cooled (-130°C) and the terminal has given their permission, stop the spray pump and close the manifold cooldown bypass valves.

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Diagram 2.5 Stripping/Spray Pumps

The spray line is drained back to the source tank through the tank's spray master and spray nozzle valves, relieving the expanding residual liquid in the spray header.

When the cooidown procedure is complete, some designs of vessel use the spray pump to prime the discharge columns on the first start main cargo pump(s). This avoids a pressure surge in the lines. To

LNG Operational Practice

.— Diagram 2.6 Stripping/Spray Pumps Start Control

accommodate this, the columns can be vented through appropriate vent valves. The vessel is now ready to start discharge and the principles involved are similar to the procedure already described.

Cargo Pump Operation Operator vigilance and the correct operation of the cargo pumps is of prime importance. Good practice recommends that you maintain an hourly log of associated critical parameters, for example: Ampere Loading / Discharge Pressure / Flow Rate. Investigate abnormal events immediately.


In-Service Operations

Diagram 2.7 Cargo Pump Start Control Diagram

For operator guidance, there should be a clear CCR response procedure, just in case a cargo pump parameter(s) becomes unstable and cannot be steadied out by normal means. If there are fluctuations on the motor ammeter or pump discharge pressure gauge, reduce the flow rate until the readings stabilise down to the NonPumpable Suction Height (NPSH). On a new 135,000 M3 Moss-Rosenberg vessel, when

the flow rate is throttled down to approximately 230 M3/Hr, the required NPSH will be about 10cm. This represents the minimum level attained by pumping. Do not restart a cargo pump with a low suction head of Jiquid remaining in the tank. As a general rule, when the liquid reaches 1 mtr or less, try not to stop the pump until the cargo has been fully discharged. If the shore facility is unable to accept the cargo for intermittent periods, keep the pump

LNG Operational Practice

going by re-circulation back to the tanks until discharge can be resumed and completed.

6.5 Completion of Discharge Procedure Most terminals require one hour's notice before the first pump.can be stopped. One hour before, inform the EIR that your cargo discharge is about to finish and that the first cargo pump will stop. Normally, the vessel should be upright and on an even keel for the completion of the cargo. The OOW should standby each tank, as the discharge valves are closed, to monitor the operation. The OOW will confirm this to the CCR. As the suction head reduces on the final pump in each tank, adjust the discharge valve to maintain back pressure and ampere loading above the associated tripping levels. Before stopping the pump, throttle-in the main cargo pump discharge valve to 40%. If two main cargo pumps are in use in a tank, when the level reaches an approved predetermined level (typically 0.8Mtr), throttle-in the discharge valve on one pump to 40%, then stop that pump. This action reduces turbulence around the pump suctions as the suction head is being reduced. As the predetermined heel quantity is approached, stop the main cargo pumps in each tank and close the discharge valve. This makes sure that the CCR indicators display healthy operation and full closure.


When all of the pumps have been stopped and the discharge has been confirmed as complete, the terminal will request closure of the liquid manifold/ESD valves. Before controlled closure of the manifold/ESD valves can be accomplished, the ESDS 'Manifold Valve Not Fully Open' override function must be switched to override. This avoids initiating an ESD trip. When manifold/ESD valves are closed, most terminals request the mode of operation to be switched to manual for eventual disconnection of the discharge arms.

6,6 Post Ofeeh&rge Procedure On completion of cargo discharge, and after all cargo pumps have been stopped, open the discharge valve on the tank nominated to receive line drainings. The terminal will then request the officer at the manifold (usually the Cargo Engineer) to open the manifold/ESD bypass valves to drain the discharge arms. On completion of draining, and as requested by the terminal, the bypass valves are closed and the liquid discharge arms are pressurised for purging with N2 supplied from the terminal. In a normal operation, the N2 purge pressure is steadily increased to 2.5barG. Then the supply is stopped and the manifold/ESD bypass valve re-opened to depressurise the discharge arm. This procedure may be repeated as required to purge each liquid arm in turn, i.e. one at a time On the final purge depressurisation, and with a pressure of 1barG still remaining in the chiksan arm, the purge gas is sampled for hydrocarbon content. For most terminals, purging is complete when this is below 1% by volume.

In-Service Operations

After complete depressurisation, disconnect the appropriate discharge arm from the ship's manifold. Normally, the vapour manifold remains open until just before final gauging. Then with permission from the terminal, close the valve. It is good practice to change the operating mode to manual and make sure that there has been a full closure. Purge the vapour arm (as the discharge arms were purged) and disconnect. Once all discharge arms have been disconnected, stop and secure the manifold overside sprays. On the receiving tank for line drains, leave the loading valve partially open so that the final liquid residues can all drain back. If discharge strainers have been used, remove them for inspection. Check for debris. The Cargo Engineer should now prepare to reinstate the LD gas compressor and the supply of fuel gas to the EIR. With the vessel upright and on an even keel, perform the final gauging with all interested parties present, who must all agree with the quantities shown on the final OCT document. Reinstate the fuel gas supply to the EIR. Once the loading and vapour arms have been secured and re-stowed ashore, start securing/blanking the manifold connections for sea conditions. With the permission of the terminal, disconnect the fibre-optic/electrical communication/ESD link and re-stow it ashore.

When all shore personnel have cleared the vessel, remove the gangway. The Master/Chief Officer must be sure that all paperwork has been completed and issued as appropriate. The vessel is then given a slight list towards the berth so that she assumes, as near as possible, an upright position after letting-go. Other vessel departure procedures will have been running concurrently. A stowaway search should be conducted in compliance with ISPS requirements. Once clear of the jetty, adjust ballast to bring the vessel upright and trimmed appropriately for the sea passage. This completes the discharge procedure.

7 Ballast Passage In Service 7.1 Overview There are certain fundamental objectives that the SMT must consider on every ballast voyage. Before itemising these objectives, there is a basic underlying principle to be made clear. Most experienced operators believe that when a gas carrier is'in a fully laden condition, you should avoid serious maintenance (especially of an invasive nature) that could cause a serious disruption. For cargo/deck or related work, the status of the hazard or risk will only increase when a vessel is fully laden.

LNG Operational Practice

Commitment to the customer/receiver means that you should avoid non-essential work in the E/R that has the potential to cause a delay to the schedule.




Loading Preparations 7.2.1 Two Days bef< at the Load Port

Although following this guideline may put ballast passage under pressure, from a safety and commercial point of view, this is a better time to manage such an issue.

This checklist details the tests to perform and the items to verify:

On the ballast passage, these are the main objectives for SMT consideration:

® ESD system: -Tested and proved fully operational.

fr Cold maintenance of the cargo tanks, resulting in the best possible Cold State on arrival at the load port.

• Cargo valves: -Test remote/manual operation. - Check and confirm speed of remote operation to be within surge avoidance parameters, as recommended 25-30sec. (In the event of an ESD, this is important with regard to the loading arms and damage avoidance.)

Maximisation of the use of the LNG boil-off from the retained heel as a supplementary fuel source will help to conserve bunkers. Maintain cargo tank pressures within maker's recommendations - that is, cargo tank pressure on a membrane vessel > 70mbarG or 1083mbarA at all times. This avoids exposing the membrane to unacceptable levels of fatigue stress. @ Completion of any potentially disruptive unscheduled maintenance. Completion of any potentially disruptive planned maintenance (PM). With operator approval, completion of any cosmetic maintenance that is in or adjacent to a gas-dangerous zone. Completion of E/R maintenance on the vessel's power plant that has the potential for blackout, loss-of-way through the water or a delay to the schedule. f • Completion of any alarm or trip tests that could disrupt the schedule.

Ballast valves: -Test remote/manual operation. - Use line surge avoidance to check the speed of remote operation. 1 Manifolds: - Check for moisture ingress. 1' Manifold bobbins, filters, gaskets and presentation flanges: - Check for mechanical damage. s Gas Detection: - Check and prove system operational on all points.

722 Da Load Port This check-list details the tests to perform and the items to verify: # Cargo system: - Inspect the intended offshore manifold. Ensure it is secure and pressurised with N2. # Set Cargo Lines for the intended load procedure. This is the responsibility of


In-Service Operations

the Chief Officer and/or the Cargo Engineer.

programme are all potential issues for the agenda for this meeting.

Moorings - Prepare and check all associated equipment. Note: that the high freeboard of an LNG carrier, and the fine tolerance regarding the ship's position relative to the loading arms, places a high demand.on this equipment.

7.2.3 Day of Arrival at the Load Port

# Cargo boil-off control systems. - Check and prove all these systems operative. These include the HD compressors and associated security/safety circuits.

e Ballast: - Adjust for appropriate arrival draught and trim. Deck scuppers. - Make sure all deck scuppers are closed. ® Mooring winch savealls: -Clean and mop out. - Replace drain plugs.

Manifold overside water sprays. - Test the operation to ensure hull protection against LNG leakage

# Bunker/hydraulic savealls: -Clean and mop out - Replace drain plugs

Fire pump, IMO pump, Emergency fire pump and emergency generator. - Ensure/confirm that all are available for the loading procedure

# Cargo savealls: -Where appropriate, fill with fresh water.

Cargo/Ballast Plan - Chief Officer completes the approved plan and obtains the required authorisation signatures. Pilot hoists and/or accommodation ladders - Test as appropriate. Chief Officer to prepare arrival/sailing stability conditions. Chief Officer to prepare arrival/ departure paperwork. In accordance with the Company's SMS, the Master will chair a pre-port meeting. This is an important interface between the various departments within the SMT Terminal requirements, safety, ISPS security/ compliance, known system/vessel defects, shore-leave control, intended reliefs, storing/ bunkering requirements, arrival/departure

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