LNG Operational Practices

Contents Preface

Introduction

Section 1: Post-Refit Operations 1

1 Primary and Secondary Insulation Space 3 1.1 Inerting/Drying Membrane Ships 3 1.2 Drying of Cargo Tank Hold Spaces (Moss-Rosenberg) 6

1.2.1 Drying and Inerting of Tank Insulation and Annular Space 7

1.3 Drying of Cargo Tanks 7 1.4 Inerting of Cargo Tanks 8 : 1.4.1 Overview 8

1.4.2 Flammability of Methane, Oxygen and Nitrogen Mixtures 9 1.4.3 Plant Comparison IG/N2 10 1.4.4 Inerting Procedure 11 1.4.5 Pressurisation 12

1.5 Post-Refit Ballast Passage 12

2 Gassing-up Cargo Tanks 13 2.1 Operational Overview 13 2.2 Operational Description 14 2.3 Other Useful Points 15

3 Initial Cooldown of Cargo Tanks 17 3.1 Operational Overview 17 3.2 Basic Cooldown Procedure (Membrane Vessel) 17 3.3 Basic Cooldown Procedure (Moss-Rosenberg Vessel) 18

Section 2: In-Service Operations 21 4 Loading Operation 23

4.1 Pre-Loading Procedure 23 4.2 Loading Procedure 24 4.3 Topping-off ' 28 4.4 Post Loading Procedure 29

5 Loaded Passage 30 5.1 Overview 30 5.2 Normal Boil-off Gas Burning 30

5.2.1 Operation 30 5.3 Forced Boil-off Gas Burning 32

5.3.1 Operation 32

iii

vii

LNG Operational Practice

5.4 Loaded Passage Essential Safety Equipment 33 5.5 Loaded Passage Administration/Records 33 5.6 Inner-Huil Inspection (IHI) - Cold Spotting 34 5.7 Loaded Voyage Discharge Preparations 35

5.7.1 Two days prior to arrival at the Disport 35 5.7.2 Day of arrival at the Discharge Terminal 36 5.7.3 Day prior to arrival at the Disport .36

6 Discharge Operation 38 6.1 Overview. 38 6.2 Pre-DischaYge Procedure 38 6.3 Discharge Procedure 40

6.3.1 Older Class of Vessel (GT or TGZ) - Using a Main Cargo Pump 40 6.3.2 Newer Vessels (GTT Membrane or Moss-Rosenberg) - Using a

Stripping/Spray Pump 42 6.4 Additional Notes Regarding Cryogenic Cargo Pump Operation 44 6.5 Completion of Discharge Procedure 46 6.6 Post Discharge Procedure 46

7 Ballast Passage - In Service 47 7.1 Overview 47 7.2 Ballast Voyage Loading Preparations 48

7.2.1 Two Days before Arrival at the Load Port 48 7.2.2 Day before Arrival at the Load Port 48 7.2.3 Day of Arrival at the Load Port 49

7.3 Cold State of the Vessel on Arrival at the Load Port 50

Section 3: Pre-Refit Operations 51 8 Cargo Tank Warm-up 53

8.1 Tank Stripping / Line Drainage 53 8.2 Warm-up Operation :* &4 8.3 Useful Points 55

9 Inerting of Cargo Tanks 56 9.1 Inerting Overview 56 9.2 Inerting Operation 56 9.3 Useful Points 57

10 Aeration 57 10.1 Aerating Overview 57 10.2 Aerating Operation 58 10.3 Useful Points 58

Glossary 59

Index 65

VI

Introduction

Introduction On occasions, the text will refer to various Officers by rank. To clarify how cargo-related 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:


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 -160C). 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.

VIII

Introduction

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 -110C 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 595C. Volumetric expansion at the liquid/vapour phase change is 600-620 times the liquid volume, hence the dangers of trapping residual liquid between valves.


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

Section


1 Primary and Secondary Insulation Space

1.1 Inerting/Drying Membrane Ships

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).

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 -25C.

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

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

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

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.

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.


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

A


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 valve-specific 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 +20C 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 02 , on completion, confirm it as acceptable i.e. better than-25C.

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.


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.

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-

band of 10%. (Within this control dead-band, 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 (Moss-Rosenberg)

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 -25C. But, if the dry air is supplied by the refit yard, the result can be better than -45C.

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. @ Open the inlet valves to the holds. Activate the Consumer Select facility on

the IG control panel, or its equivalent.

R


Once the O2 level is satisfactory (approximately 21%) and the Dew Point is at (or lower than) -45C, 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 -25C 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 -20C.

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 -25C.

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 -45C. 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 -45C and a flow rate of 14,000m3/h.


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

40 50 60

Methane % 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 -25C, 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


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 21% 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) 02: 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 CH4 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 CH4 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.


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

Note that some portable instruments for measuring CH4 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 (02>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/N2 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.

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: # Inert cargo tanks, cargo pipelines and

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

pipelines and associated equipment. > Aerate cargo tanks, hold spaces and

cargo pipelines.

N2 Generating Plant - used to: if Inert/purge the primary/secondary

insulation/annular space (membrane).


Inert gas and pure N2 can kill you. In order to avoid asphyxiation from O2 depletion, take great care to guarantee the safety of all personnel involved with any operation that uses IG. Make sure there has been a Risk Assessment and all personnel involved in the inerting procedure are familiar with the associated hazards.

The inerting procedure follows the completion of the drying process. Line set-up and preparation are very much the same. With most pipeline configurations, preparations for the drying/inerting procedures involve the fitting of cross-over bends into the cargo piping system. Three dedicated bends are normally required. They are: i IG/Dry Air Plant to Liquid Header

(already fitted before Drying) IG/Dry Air Plant to Compressor House (already fitted before Drying) IG/Dry Air Plant to Vapour Header

Note: depending on the vessel's cargo piping configuration and whether or not an onboard IG/Dry Air Plant is fitted, this may vary. To be sure, follow the directions contained in the relevant Cargo Operations Manual.

As with the drying process, the flow loop directs IG from the plant to the liquid header and into the cargo tanks via the tank branch valves and the loading drop lines. Purged air from the tanks is exhausted to the vapour header via the tank dome vapour valves and released to the atmosphere through the vent mast riser.

Before the IG Plant discharge is allowed to connect to the cargo system, use the IG Blower to blow it through with air. This

prevents accumulated debris from entering the cargo system.

Start the IG generator and let it run, but do not connect it to the system until the oxygen content and dewpoint are acceptable (02


Gas Main. Emergency Cargo Pump Columns (Membrane only). Whessoe Columns / Radar Still Pipes.

' Instrumentation (Deck Boxes, impulse lines, safety relief lines).

> Fuel Gas Supply Piping to the E/R.

When the 02 content sampled from a tank outlet reaches 2% (dew point


impossible due to the Gas Dangerous Zones. For example, cosmetic maintenance such as manifolds, cargo tank domes, inside the compressor house, vent risers (flame trap arrangements) and similar equipment. A complete absence of the. hydrocarbon gas hazard makes Risk Assessment for this work far more favourable at this time. .

(b) The team should consider where extra vigilance will be required when cryogenic equipment, which has been exposed to invasive refit maintenance, contracts during first time operations under cold conditions. Consider tank-loading valves, manifold valves, tank gauging systems and gas compressors. Safety awareness under these conditions is invaluable.

(c) although a load port is next, it is normal practice to spin-test the cargo pumps at some point in the loading procedure. Questions to be answered are: i During the procedure, when is the

most opportune time? Are the pumps turning in the right direction?

i; Is the starting current acceptable? Do all measured parameters settle back to normal after starting?

S Has the motor winding resistance to earth and between phases been monitored throughout the ballast passage and are all readings acceptable? Has the cargo pump motor heating facility been disconnected correctly?

(d) On completion of loading, it is a usual Class/CCT requirement for the cargo tank High/High Levels to be tested by actual cargo transfer within the vessel. Consider these issues:

Is the team familiar with this procedure? Has a Risk Assessment been carried out?

$> Have allowable limits for the test been clearly established?

If these limits are exceeded, what procedure should be followed?

J As each independent High/High Level sensor operates, have the implications of the resulting ESD Trip and required response been fully considered?

Has the terminal been forewarned, guaranteeing official permission and co-operation?

# Has a plan been drawn up and agreed?

It is clear that this ballast passage is not only busy from the post-refit maintenance perspective, but it also demands a great deal of preparation, forward planning and elevation of awareness levels, especially among key members of staff.

2 Gassing-up Cargo Tanks (Sometimes Referred to as Purge Drying)

After lay-up or dry-dock, the cargo tanks are filled with inert gas or N2. If the purging has been done with inert gas, the cargo tanks will have to be purged with LNG vapour and cooled when the vessel arrives at the loading terminal. Unlike N2, inert gas contains approximately 15% carbon dioxide (C02), which will freeze at around -56.6C and produces a white powder which can block valves, filters and nozzles.

During purging/the inert gas in the cargo tanks is replaced with warm LNG vapour. This action removes any freezable gases, such as carbon dioxide, and completes the drying of the tanks. This is known as Purge Drying.


Diagram 1.3 An LNG Vapouriser

1.2 Oimrmromu

LNG liquid is supplied from the terminal to the liquid manifold where it passes to the stripping/spray header through the appropriate shore connection liquid valve or, in some cases, a dedicated cooldown bobbin. Then it is fed to the LNG vapouriser and the LNG vapour produced is passed at approximately +20C to the vapour header and into each tank through the vapour domes. (Note: 20C is a typical process value.)

At the start of the operation, the piping system and LNG vapouriser are vapour locked i.e. the reduced liquid supply for this purpose may be insufficient to prime the

pipework by displacing the vapour, requiring an alternative venting arrangement. The stripping/spray header can be purged into the cargo tanks through the vapour dome through the arrangement of spray valves and associated control valve until liquid reaches the LNG vapouriser. As the LNG vapour is lighter than the inert gas its introduction at the top of the tank creates a clearly defined interface that forces the IG in the cargo tanks to be exhausted by displacement up the tank filling line into the liquid header. The inert gas then vents to the atmosphere, usually through No.1 vent mast. Because the flow of liquid from the terminal is relatively small for this operation, problems experienced may be due to vapour locking at the loading arms. Where a cooldown bobbin is used, there is provision

1/1


for intermittent venting of the loading arm through the appropriate vent mast.

When 5% Vol CH4 (the percentage figure will be specified by the particular port authority) is detected at the No.1 vent mast riser, the exhausting gas is directed ashore via the HD compressor's bypass line, or to the boilers through the gas burning line.

If there is enough backpressure, this operation can be done without the compressors. But if there is not enough backpressure, one or both of the HD compressors in service can be used. However, as the compressors create unnecessary turbulence inside the tanks, it is better not to use them.

The operation is considered complete whe the CH4 content exceeds 88% by volume a measured at the top of the cargo filling pipe.

The target values for the N2 gas and the inert gas CO2 are equal or less than 1%. These values should be matched with the requirements of the LNG terminal.

This normally entails approximately 1.8-2.0 changes of the volume of the atmosphere the cargo tank.

Due to local regulations on venting CH4 ga to the atmosphere, some port authorities may require the entire operation to be

Required Heat Energy at Initial Purging

Tank TK volume (m3) Required NG (m3)

Required LNG (kg)

Required LNG (kg)

Heat Energy (MMBTU)

No.1 21,943 39497.04 32071.6 . 68.4 1648.5

No.2 40,432 72777.06 59095.0 126.0 3037.5

No.3 40,443 72796.9 59111.1 126.0 3038.3

No.4 37,831 68095.62 55293.6 117.9 2842.1

Total 140,648 253,167.00 205,571.0 438.0 10,566.0

Note: 1. LNG Density: 469.0 kg/m3 2. LNG Heating Value: 51.400 MMBTU / ton

carried out with the exhausting gases being returned to the shore facility.

2,3 Other Useful Points Depending on the type of vessel, the pipeline configuration as described in previous procedures usually requires purpose-fabricated bends to be fitted. Two are required: Liquid Header to Vapour Header and Liquid Header to HD Compressor.

Good practice requires this equipment to be purged with cargo vapour: f; Main cargo pump discharge pipes.

LD/HD compressors. Gas Heaters.

When a vapouriser is in use, it is essential to monitor the operating parameters of associated equipment with care. Before introducing LNG, carefully warm the unit. Test all associated trips, alarms and controls beforehand and prove them operable, (refer to the post-refit ballast passage routines). The NG outlet temperature (+207+25C) and the temperature of the heating steam condensate returns are both critical. Take great care when shutting-down the unit, for example, maintain the heating steam until all residual LNG has been dissipated from the unit.


Diagram 1.4 Cargo tank cooldown

Required heat energy at initial purging for a typical 140,000m3 membrane vessel:

The inert gas is purged and replaced by Natural Gas (NG) produced by the LNG vapouriser (+20C outlet temperature) and fed with LNG supplied by the loading terminal. It requires approximately 1.8 complete volume changes to displace the inert gas and attain a CO2 content of


3 Initial Cooldown of Cargo Tanks

Cooldown is the process that brings the containment system to a temperature that will not cause excessive boil-off during loading or unacceptable stresses in the support structures. It follows a procedure that prevents a thermal shock to the primary containment system.

Cooldown is achieved by pumping LNG through the spray header and cooldown grids at the top of each tank. This allows the LNG to vapourise at the sprays and allows cold gas to enter the tank.

A controlled and design-compliant rate is essential to avoid damage to the primary containment and overcome potential problems: \ To avoid excessive stresses being

induced in the pump tower or trellis. To prevent vapour generation exceeding the capabilities of the HD Gas Compressors at any time during the process. To avoid structural deformation to the insulation arrangement being caused by non-uniform contraction.

To maintain the pressures in the insulation spaces to the requirements of the N2 generating plant and associated system. In the initial stages of cooldown, pressures in these spaces can collapse and slow the rate at which we can complete the procedure. This is particularly the case on the older class of membrane vessel.

Procedure (Membrane Vessel)

Unlike rigid cargo tank designs, vertical thermal gradients in the tank walls are not a significant limitation on the rate of cooldown.

LNG is supplied from the terminal to the spray header, which is open to the cargo tank spray rails. Once the cargo tank cooldown is almost complete, the liquid manifold cross-overs, liquid header and loading lines are also cooled.

To avoid splashing the cargo tank bottoms with LNG liquid, control the stripping/spray header pressure, especially in the initial stages.

Cooldown of the cargo tanks is considered to be complete when the mean temperatures (except for the 2 top temperature sensors in each tank) indicate -130C or lower. When these temperatures have been reached and the Cargo Transfer System (CTS) registers the presence of liquid, bulk loading can begin. (GTT has defined that LNG loading is possible when the mean target temperature is lower than -80C). In practice however, GTT recommends that the cooldown operation is continued until -130C has been attained, in-line with most LNG terminal requirements.

Vapour generated during the cooldown of the cargo tanks is returned to the terminal through the HD compressors and discharged to the vapour manifold, as in the normal manner for loading.

During cooldown, N2 flow to the primary and secondary spaces will increase significantly. It is essential that the rate of cooldown is controlled so that it remains within the limits of the N2 system to maintain the primary and secondary insulation space pressures between 2 mbar and 4 mbar.


Once cooldown is completed and the build-up to bulk loading has commenced, the tank membrane will be at, or near to, liquid cargo temperature. But it will take some hours to establish fully cooled temperature gradients through the insulation. Consequently, boil-off from the cargo will be higher than normal at-this stage.

On a typical new 140,000m3 vessel, cooling the cargo tanks from +40C to -130C, over a period of 10 hours, will require a total of about 800 m3 of LNG to be vapourised. Therefore, the cooldown rate in the cargo tank and insulation spaces is dependent on the degree of LNG spraying.

Preparation for Tank Cooldown: Prepare the cofferdam steam or gycol heating system as appropriate.

Prepare the records for the cargo tank, secondary barrier and inner-hull steel temperature readings.

Check that the insulation space A/2 supply system is in automatic operation and has the capability of supplying the additional N2 necessary to compensate for the initial pressure drop experienced as the primary space atmospheres collapse.

If appropriate to the system, before cooling, raise the N2 pressure inside the primary insulation spaces to 8 mbar as extra compensation for the inevitable initial pressure drop.

Ensure the buffer tank is maintained at maximum operating pressure throughout the procedure.

Check that the gas detection system is in normal operation.

Prepare the N2 generating system for maximum output.

Prepare both HD compressors for use.

Cargo Tank Cooldown: After cooling the lines, a liquid line pressure of 2 bar is required on the vessel's spray rail. This is controlled by a shore request from the vessel and adjusted to meet required parameters onboard.

Adjust the spray rail pressure to obtain an average temperature drop of 20C per hour in the first five hours - and 10/15C per hour thereafter.

Start one HD compressor (or both as necessary) to maintain the tank pressures at about 100 mbarG.

Check the N2 pressure inside the insulation spaces. If there is a continuing downward trend that is outstripping the N2 supply system, reduce the rate of cooldown accordingly.

When the average temperature shown by the CTS sensors is -130C, inform the terminal that cooldown is complete and prepare for a gradual ramp-up to bulk loading status.

3.3 Basic Cooldown Procedure (Moss-Rosenberg ;;> ^oe:;

As with the membrane vessels, the cargo tanks are gradually cooled by spraying LNG received from the loading terminal through the spray nozzles located round the centre column of the tank. This operation, which produces cold vapour that has to be returned ashore, must continue until the equatorial region of the tank is at least -115C. Typically, the maximum allowable rate of cooldown is 9C per hour and must never be exceeded.

To avoid thermal stresses on the tank shell and tank support structure, the cooldown procedure must be smooth and uniform.

18


In normal service, the ship will arrive with the equatorial region of the tanks at about (but not warmer than) -119C. Do not commence full rate loading until this figure is attained.

LNG enters the cargo tanks through the spray nozzles and the vapour is returned to shore using the HD compressors as necessary to maintain tank pressures within acceptable limits. On a typical 135,000m3 vessel, the operation will take approximately 24 hours.

On most vessels, when the HD gas compressors are running, service from the LD unit is not required. The E/R fuel gas burning requirement can be supplied by a regulated bleed-off from the discharge of the HD compressors.

Preparation for Tank Cooldown Use the established Time/Temperature graph forms to prepare the records for the cargo tank equatorial temperature monitoring.

Before commencement, set the HD and LD gas compressors for free-flow operation with the appropriate low demand gas heater warming through.

Request shore control to supply LNG at the agreed reduced rate.

The spray lines are cooled first.

Check that the gas detection system is in the normal operating mode.

Cargo Tank Cooldown: On completion of line cooldown, LNG is introduced through the liquid crossover, the spray main and branch lines to the spray nozzles.

Expect a spray rate of 1,000kg/hr per tank.

Monitor the pressure in the cargo tanks throughout the cooldown procedure, particularly in the initial stages when it will rise rapidly. Start the first HD compressor when the pressure reaches 150mbar.

If the tank pressure is allowed to fall to 40mbar below the void space pressure at any time, the HD compressors will automatically shut-down and at the same time the shut-off valves at the domes will close.

If not already in use, commence fuel gas supply to the E/R.

After approximately 2 hours, you may increase the spray rate, either by requesting more flow from shore control or by opening up more spray valves, or by doing both. The time value is usually clearly defined in the cargo manual.

Cargo line cooldown is usually carried out at some point during the tank cooldown procedure.

When: (a) the temperature in the liquid

header at all the cargo tanks has been reduced below -120C,

(b) the equatorial temperature in all cargo tanks has been reduced to below -115C

(c) tank pressures are fully under control

then you may fully open the gate crossover valves in readiness for loading ramp-up.

Jperational Practice

In-Service Operations

Section


4 Loading Operation

Procedure Upon completion of mooring, engage the main engine turning gear and close the master steam stop valve as the gangway is brought onboard and the loading arms are connected.

Note: On steam propulsion plants, old and modern, while the vessel is alongside the loading/discharge berth:

Maintain a slow rotation of the prop shaft at all times while the turbines are hot. You must also raise the engine vacuum and use warming-through steam to maintain a state of readiness.

On older Stal-Laval turbine vessels, the maximum period allowed for a stationary prop shaft under these conditions is only 4 minutes, and on the newer Kawasaki turbine ships, it is only 3 minutes.

If these periods are exceeded, unequal expansion may cause turbine shaft damage, which leads to rotor deformation. Under FWE (Finished With Engines) conditions, that is, between arrival and departure standby, you can arrange slow rotation of the main engine turbines by engaging the electrically-driven Turning Gear.

While on standby, lengthy periods of prop shaft rest are automatically broken by a programme in the Main Engine Bridge Control System called Auto Blasting, which uses steam from the manoeuvring valves to roll the turbines intermittently.

If a system failure occurs during auto-blasting, the prop shaft rpm can increase and cause the vessel to move, relative to the jetty, which is why the

gangway and loading arms are not connected until the Main Engine Turning Gear has been engaged and the turbines appropriately secured.

The normal arrangement at most terminals is two and four loading arms and one vapour return arm.

Connect the CommunicationiESD cable, usually a male/female plug arrangement, fibre-optic or electrical, and provides the various channels for ship/shore communication and ESD interconnection.

Connect the ESD umbilical cable, usually a pneumatic connection

ISGOTT no longer recommends the use of a ship shore bonding cable as the insulating flange at the cargo connection is effective in removing the current and any sparks.

However, some national/local regulations still insist on the connection of a ship/shore bonding cable. If so it should be checked to make sure it is mechanically sound and be connected well clear of the manifold area.

When the loading arms have been connected, they are purged and pressurised with N2 supplied by the terminal to a pressure of 200kPa in each arm. The responsible Ship's Officer checks all associated joints for leakage. (Soapy water is still the preferred means of leak-detection.)

Once the integrity of the loading arms has been proved, they are depressurised in a controlled manner. While depressurising, use a portable O2 meter to check the gas. The usual requirement is below 1%. Dryness of the gas is an important issue and can be part of the standard checks at some terminals. The usual requirement is lower than -50C.

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, commonly referred to as the Initial Gauging.

In the Cargo Control Room (CCR), the ship/shore communication system is powered up. The Hot-Line and Plant phone are tested and proved satisfactory.

Bring the manifold spray-water curtain into service. .

Back in the CCR or Ship's Office, complete the ship/shore safety check-list and ISPS security check-list.

When the vessel and terminal are ready to carry out ESD tests, confirm whether it is the vessel or terminal that will activate the shut-down.

In normal operations, the Officer on Watch (OOW) attends at the manifold to assist with and report on the progress of the tests.

With terminal permission, open the manifold/ESD valves. When the valves are confirmed fully open, reset the ship's ESDS ensuring appropriate indicators all show Healthy. Once the terminal has similarly re-set, switch any associated override facility on the vessel's system to Override Off.

Conduct the Hot ESD test as agreed.

It is normal at this time for the terminal to require Manifold/ESD valve closure to be timed. The IGC Code requires a closure time of less than 30 seconds.

Once ship and shore have confirmed correct ESD operation, both can. re-open the appropriate valves (Manifold/ESD) and reset the ESDS.

At this stage, the ship may request permission from the terminal to send gas ashore. Associated lines and valves are set accordingly. Normally, the terminal allows the vessel to burn boil-off gas throughout

the loading procedure. There are two recognised ways of doing this: 1. the Low Demand (LD) compressor is

shut-down and the Engine Room (E/R) receives the required fuel gas by a controlled bleed system. This is sensitive to combustion control requirements and tapped from the gas-to-shore line.

2. requires continued use of the LD compressor during loading, with the ship's boilers maintaining duel firing throughout. With this system, the LD compressor is removed from the ESD trip logic circuits. (This prevents the LD compressor from being affected by cargo operations.)

Enable and ensure all cargo tank level alarms are operative (recommended as Low Level @ 0.5Mtr, Hi Level @ 95% capacity, Tank Fill Level @ 98.5% capacity and HiHi Level @ 99% capacity).

On passage, you may use the specially-installed blocking circuits to inhibit certain lalarm systems.

The vessel is now ready to begin loading.

42 Lo&dhig Prae&dyre In accordance with the IGC Code, the maximum filling capacity for any cargo tank is 98% by volume. However, in compliance with contractual requirements, backed by Flag State approval and allowing for an average daily boil-off rate of 0.15%, slightly higher loading capacities are in force for LNG vessels.

On average, a membrane vessel will load to 98.5% volume and a Moss-Rosen berg ship will take up to 98.8%. Closure of the tank-filling valve at these levels is usually automatic, but is closely monitored from the CCR and at the tank dome in case manual

24


Diagram 2.1 Ship-Shore Optic Fibre Transmission

intervention is needed. The loading valve must close as the required level is reached.

Critical to this procedure is the level at which the valve starts to close.and the time it takes to close. Therefore, partial-loading valve closure at the topping-off stage is not part of the normal procedure.

Assuming that the vessel has arrived in the Cold Condition, this is the recommended sequence of events:

Ensure manifold/ESD valves, stripping/spray main crossover connections, cargo loading valves and the cargo tank spray rails are

correctly lined-up and set for line cooldown.

To make it easier to empty the ballast tanks, the ship may be trimmed within the terminal's maximum draught requirements during loading.

The structural loading and stability as determined by the loading computer, must always remain within recommended safety limits.

HD compressors must be ready for service.

The secondary float level gauge system must be ready for operation.


Temperature recording systems, alarms for the cargo tank barriers and double hull structure must be in continuous operation.

The gas detection system and alarms must be in continuous operation.

Request the terminal to "start cooldown.

Terminal control will gradually increase the cooldown rate, simultaneously cooling lines ashore and on the vessel.

On deck, the OOW and assistants check the cargo system and report cooldown progress. Frosting provides a very clear indication in this respect, especially at the loading arms, which should be carefully monitored during the procedure.

Any sign of liquid/vapour leakage must be reported back to the CCR immediately.

As a guide, the progress of cooldown is indicated by a sudden reduction in liquid header/crossover temperature, frosting along the liquid header and frosting along the spray header.

In the CCR, the OIC is responsible for monitoring tank pressures, vapour header pressure, liquid header/crossover pressure and temperature. At the same time, the N2 supply to the holds/interbarriers is closely monitored to ensure the correct pressures are maintained.

Usually, the terminal is the first to report that cooldown is complete. The vessel will then request the terminal to standby pending cooldown completion on the vessel. Line cooldown, as monitored in the CCR, is considered complete when the liquid header temperature is approximately 157C. Other indicators include (usually at No.1 Tank), frosting along the loading line, a reduction in noise level at the loading valve, a noticeable drop in the interbarrier space pressure and a significant drop in the tank bottom temperature.

When cooldown on the vessel is complete, inform the terminal and request control to standby pending the start of loading. Tank loading valves are set in the prescribed manner, according to the approved ship specific procedure.

Request terminal control to start loading at the pre-arranged reduced rate.

On deck, the OOW and assistants continue to monitor the integrity of loading arms, cargo lines, associated flanges and instrument connections/impulse lines.

When tank pressures reach the appropriate level, (design dependent) start the first HD compressor, returning gas ashore. But before starting, inform terminal control and the E/R.

On a modern Moss-Rossenberg vessel, this pressure is around 12kPaG. While on the older TGZ or GT vessels, approximately 100mbG would be more appropriate.

Providing that boil-off gas generation and N2 supply to the holds are both under control, request the terminal to increase the loading rate up to maximum over the pre-arranged time scale.

Start the second HD compressor as boil-off generation begins to exceed the capacity of the in service unit. Again, inform terminal control and the E/R before starting.

With both HD compressors running steadily in parallel, tank pressures are lowered to that required for the duration of the bulk-loading period, recommended as 12mbG (12kPaG).

The OIC should remember that in the interest of cargo conditioning considerations while on-passage, achieving

26


Diagram 2.2 HD Compressor

maximum gross heating value out-turn for the receiver, while loading at the lowest pressure appropriate for the containment system, is an important safety factor.

Once the full loading rate is achieved, loading valves can be adjusted to prevent flow preference (unequal filling, which may occur in all designs).

Commence de-ballasting operations as per the Cargo/Ballast Plan requirements. The procedure usually runs in parallel with loading, achieves good drainage of the required ballast tanks and keeps the vessel upright.

In the CCR, an hourly record is maintained of tank levels, Quantity On-board (QOB),

loading rate and manifold pressures. The terminal will normally require QOB and % cargo loaded as recorded by the vessel every hour.

The Cargo Control Room (CCR) officers wil also maintain a close scrutiny on the weather data (wind and sea state) and mooring tensions, provided a mooring tension monitor is part of the ship/shore installation.

Loading operations also provide the opportunity to log-HD compressor parameters. These'are normally recorded every hour.


As bulk loading progresses, boil-off tends to reduce and it may prove necessary to stop one of the two HD compressors.

Adjust loading valve openings to provide the desired stagger between the tanks.

At some point towards the end of the procedure, lower the back-up manual float gauges (typically Whessoe). Record the readings at final gauging and compare them to the remote system (typically Fox IV Trans-sonics or Radar).

T Topping -cm All Responsible Officers - for example, the Chief Officer, Cargo Engineer and the OOW- must report to the CCR in good time.

Give one hour's notice of the first loading rate reduction to terminal control, that is, when the first pump is to stop.

Plan to complete de-ballasting approximately one hour before completion of cargo and before topping-off the first tank. Ensure the vessel is upright and on an even keel.

When the 95% Hi-Level alarm operates, the OOW on duty at the tank dome will test the loading valve (and/or branch valve) operation locally. When this task is complete, the Officer should report positively back to the CCR that the valve concerned operates correctly, automatic' mode has been re-engaged and that the valve is full open.

On the tank designated for receiving the line drains, automatic closure is disabled in case the line drains exceed the appropriate level, even by a minimal amount. The spray valves on each tank are also closed at this time, except on the tank nominated for line drainage where the valve is left fully open.

Give 15 minutes notice to terminal control before the first reduction in loading rate is anticipated.

Then give terminal control a 5 minute warning.

As the first tank shut-down level is approached, request loading rate reduction.

The loading valve must begin to close at the known and pre-determined level.

As each tank successively approaches topping-off, the vessel should provide terminal control with 5 minutes notice for the next loading rate reduction.

As each tank loading valve commences auto closure, the OOW on duty at the tank dome will tell CCR when valve closure has commenced and when valve closure has been completed. Best practice dictates that on completion of closure of each valve, the OOW should disengage the remote activation and, where possible, confirm full closure using the manual gear. In common with sound tanker practice, the CCR will make sure that the level in any closed tank does not continue to creep upwards.

Throughout the topping-off procedure, a Nominated Officer in the CCR should record all critical cargo tank levels in the Deck Operations Log (DOL), which will include the point at which the 95% Hi-alarm activated and when the level auto-closure of the loading valve commenced and completed.

Stop the final HD compressor, inform terminal control and the E/R accordingly. If appropriate, commence preparation of the LD compressor for the reinstatement of fuel gas burning.

When the last tank approaches the finish level minus the line drain

28


quantity, ask terminal control to stop loading.

Once terminal control has confirmed all cargo loading stopped, commence line drainage.

With the terminal's permission, you can close the gas-to-shore connection reinstate gas fuel burning with the LD compressor.

On completion of draining, and with the terminal's permission, close the liquid manifold/ESD valves. When closure is confirmed, final gauging is conducted in the presence of the Chief Officer, Loading Master and/or Surveyor.

fk n U ^ ^ i f ^ T I #*%. *" f \ **% f*a -.:-. '-= ! ' . - . v : . " , , - . J c i L - . ; : ; iu

On completion of final gauging, the Loading Master will request the terminal to pressurise the liquid lines and vapour return line to the agreed pressure with N2 (typically 2bar). This procedure pressurises/ depressurises the loading arms to purge them of hydrocarbons. The actual method will vary according to the terminal and vessel type, but whichever way purging is arranged, the principle remains the same. Purging will continue in the approved manner until the CH4 content is reduced to less than 1% by volume (1.2% at some installations).

As the QRCs (Quick Release Connections) are released, you can start the disconnection after the final depressurisation. Follow the standard procedures and safety precautions.

Return back-up float level gauges to their stowed position. Make sure that the in-house reading is correct in each case.

Once the loading and vapour arms have been secured and re-stowed ashore, the vessel should secure and blank the manifold connections for sea conditions.

With the terminal's permission, the fibre-optic/electrical communication IESD link is disconnected and re-stowed ashore.

Shut-down the manifold overside water sprays.

When all shore personnel have cleared the vessel, remove the gangway.

In the CCR cargo tank level, re-engage the alarm blocking circuits.

The Master/Chief Officer will now complete and issue all paperwork as appropriate.

Experience shows that giving the vessel a slight list towards the berth so that she assumes (as near as possible), an upright position after letting-go counters the tension effect of taut mooring wires.

Other vessel-departure procedures are run concurrently with Post-Loading procedures:

Conduct a stowaway search in compliance with ISPS requirements.

Once clear of the jetty, adjust the ballast to bring the vessel upright and trimmed appropriately for the sea passage.

After leaving the-berth, set cargo lines to keep one tank open to the liquid and spray header. This action prevents the build-up of internal pressure through vapourisation of residue liquid. Associated valves are usually manually cracked-back for the first day.


5 Loaded Passage 5.1 Overview During the laden voyage, the movement of the vessel and the ingress of external heat through the tank insulation generates convection currents in the bulk cargo mass. This causes the relatively warm LNG to rise to the surface, producing boil-off gas that becomes available'as a supplementary fuel source for the vessel's power plant. This process can account for 0.15% of the liquid cargo per day.

It is the responsibility of the vessel's operators to achieve these goals: > 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 +30C 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 pre-determined 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.

^n


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 shut-down 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.

r\A


Manual operation (E/R, CCR, Bridge). :; Loss of authorisation from CCR or

Bridge. E/R exhaust fan failure.

i E/R C02 fire-fighting release. Blackout.

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.

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.

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

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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

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.

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.

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.

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.

Records Maintain these records at all times: ;i Daily Cargo Log. Daily LD Compressor Log. ;i Inner Hull Inspection (IHI) Record. s Monitor inner-hull and insulation space

temperatures, as appropriate. -' 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).


^6 .h^ r - rh^ l ^^pecl lon

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.

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

Plain carbon steels have a brittle to ductile behaviour transition that generally occurs in the range of -50C to +30C. 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 (-160C) 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, Moss-Rosenberg 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.

These are the inspection points: The position and temperature of cold

spots, or the absence of cold spots where previously reported.

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

$ Condition/wastage of ballast tank sacrificial anodes.

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

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

O/l

n-Service Operations

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. 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.

;; 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.

D? senary 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.


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.)

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.

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.

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

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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

LNG Operational Practices

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