Deck Cadet Booklet for LNG Operations

TABLE OF CONTENTS PAGE 1. LNG Carrier ---------------------------------------------------------------------------------3 1.1 Definition of LNG 1.2 Composition of LNG 1.3 Characteristics of LNG 1.4 LNG Chain 2. Design Standard and Ship Type -----------------------------------------------------10 2.1 General 2.2 Ship Type 3. Cargo Operation --------------------------------------------------------------------------13 3.1 Cargo Control Room 3.2 Drying and Aeration 3.3 Inerting 3.3.1 Inert Gas and Dry Air Generator 3.4 Gas Filling of Cargo Tanks 3.5 Cooling Down of Cargo Tanks 3.6 Loading 3.7 Loaded Passage, Boil-off Gas Burning 3.8 Unloading 3.9 Ballast Passage 3.10 Tanks Warming Up 3.11 One Tank Operation 3.12 Nitrogen Distribution 3.13 Emergency Operation 3.1.3.1 Emergency Cargo Pump Installation 3.1.3.2 Water Leakage to Barrier, Inner Hull Failure 3.1.3.3 Gas Leakage Detection 3.1.3.4 Liquid Leakage 1. LNG Carrier Liquefied Natural Gas (LNG) is becoming increasingly popular as an alternative to petroleum for power generation because its reserves are as plentiful as those of oil, and it is also attractive as a clean energy source. With LNG carrier services in effect acting as maritime pipelines, the consistent adherence to sailing schedule is of great importance to utility companies, which are responsible for the supply of electric power and gas. As the economies of South Korea, Taiwan and various Southeast Asian nations grow rapidly, so does their energy consumption and demand for LNG. NYK’s LNG fleet will therefore expand in parallel with this growing demand as the company diverges into cross trades. This expansion of operations,　together with the need to keep the carriers in top condition over very long contract periods, means that vessel maintenance and management will become increasingly important. Also, since LNG is dangerous cargo, safe navigation is of paramount importance. It is our responsibility to transport goods swiftly, safely and accurately. Loading, handling and unloading ocean freight are, of course, basic to LNG chain operation and we place safety, accuracy and reliability as top priorities when handling LNG projects. LNG carrier crewmembers who are to be engaged in these operations are expected to learn necessary techniques in order to carry out their jobs smoothly. 1.1 Definition of LNG When natural gas is chilled to approx. -160ºC under the atmospheric pressure, it condenses into liquid about one-six hundredth (1/600) of gas in volume. This liquid is called LNG (Liquefied Natural Gas). The weight of this colorless, transparent liquid is about one-half(1/2) of water with the same volume . 1.2 Composition of LNG Similarly to natural gas, LNG consists of several hydrocarbons of which methane is the main component. Other hydrocarbons making up this compound liquid are ethane, propane, butane, and pentane, plus nitrogen which is often found in natural gas is also dissolved LNG. However, other useless components in natural gas such as H2O, H2S, CO2, heavy hydrocarbons, etc., are removed in the liquefying process. Composition of each hydrocarbon contained in LNG dictates the actual density or specific gravity of LNG. As a difference between LNG and natural gas, it can be mentioned that the components of LNG change while in storage in a tank. This change is caused by the evaporation of light components such as methane and nitrogen, which takes place earlier than that of heavier hydrocarbons. Thus, the concentration of heavier hydrocarbons increase while in a prolonged storage. *Molecular formula : Methane CH4 Molecular weight 16 For Example: Composition of LNG Alaska Brunei Malaysia Australia Indonesia Qatar Composition %) Methane CH4 99.4 89.9 91.2 88.2 90.5 89.7 Ethane C2H6 0.2 5.0 4.4 7.8 6.1 6.8 Propane C3H8 0.1 3.3 2.9 3.0 2.5 2.3 Butane C4H10, C5H12 0.1 1.7 1.3 0.9 0.9 1.1 Other 0.2 0.1 0.2 0.1 0.0 0.1 Calorific Value (MJ/Nm3) 40.12 45.08 44.23 44.94 44.12 44.38 1.3 Characteristics of LNG The followings are enumerated as main properties of LNG. Characteristics in storage and transportation: Cryogenic temperature of about -160 ºC. LNG will require use of suitable materials for cryogenic temperature, consideration toward expansion and contraction due to the change in temperatures, structural design with due regard to thermal stress, effective heat-insulation system, precaution against damage caused by low temperature, etc. Volumetric reduction to about one-six hundredth (1/600) of gas at the normal temperature due to liquefaction. This is advantageous to storage and transportation. Tank pressure will rise due to the boil-off (evaporation). A liquid in the state of boiling point. When equilibrium between gas and liquid is destroyed by rise of temperature or fall of pressure, the liquid will immediately start boiling. Density is about half (0.5) that of water. Inflammable, but combustion range of its vapor is narrow. If 5～14 volumetric percent LNG(Pure Methane) is present in air, it forms an explosive mixed gas. In order to prevent such a formation, considerations are given to avoiding LNG coming into contact with air by for example, keeping the tank pressure slightly higher than the atmospheric. Upon leaking into air, it rapidly evaporates and forms while vapor cloud by the condensation of moisture. Other physical and chemical characteristics: Colorless and odorless liquid. Large latent heat of evaporation. High volatility. Low viscosity. High dielectric power and extremely poor electric conductivity. It can easily be charged even by static electricity. No causticity and no toxicity. Almost no solubility in water. Small surface tension. LNG Chain The relation between from the gas fields to a consumer are linked like the chain. It is impossible to stable supply if it will be lacking in the one of them. Gas Field Liquefaction Plant Loading Terminal Transfer by LNG Carrier Unloading Terminal City gas fabrication plant City Gas Fabrication Plant or Electrical Power Plant 2. Design Standards and Ship Type 2.1 General The overall layout of a Gas carrier is similar to that of the conventional oil tanker from which it evolved. The cargo containment and its incorporation into the hull of the gas carrier, however, is very different due to the need to carry its cargo under pressure, or refrigerated or under a combination of pressure and refrigeration. Gas carriers designed for pressurized cargoes can usually be identified by cylindrical or spherical tanks which may project through the deck. Similarly the LNG carrier with spherical tanks protruding above the main deck can be easily recognized by its distinctive profile and much larger size. Gas carriers designed to carry their cargo at atmospheric pressure in prismatic tanks are not easily distinguishable from oil tankers except by their freeboard which is significantly greater. This greater buoyancy results from cargoes of a much lower density than most oils and the requirement to have totally segregated tanks for ballast. To examine the design of these ships in greater detail, it is convenient to consult the Gas Codes and the rules of the major ship classification societies which give guidance on the requirements of the Gas Code. 2.2 Ship type Some of the factors to be taken into consideration which affect the design of gas ships are, for example: Types of cargo to be carried. Condition of carriage (i.e. fully pressurized, semi-refrigerated, fully refrigerated). Type of trade, which in turn determines the degree of cargo handling flexibility required by the ship. Terminal facilities available when loading or discharging the vessel. Perhaps more than any other single ship type the gas tanker encompasses many different design philosophies. 3. Cargo Operations Cargo Control Room Cargo control room on LNG tankers usually is incorporated in superstructure or situated on the deck above compressor room. In cargo control room there are all control, communication and safety equipment. All operations for the loading of cargo are controlled and monitored from the cargo control room. The loading of LNG cargo and simultaneous de-ballasting are carried out in a sequence. During the loading operations, communications must be maintained between the ship’s CCR and the terminal: telephone and signals for the automatic actuation of the Emergency Shutdown from or to the ship. New LNG ships are equipped with automation integrated system. This systems include: cargo and ballast operations, machinery and electric generation plant operations, some others independent control systems are interfaced with the Cargo or Machinery Systems. Cargo System is capable of control and monitoring of the cargo and ballast auxiliaries and valves. Automatic sequence control logic programs are provided for each cargo and ballast operation. Displays are composed of overviews, operational graphics, monitoring graphics, operational guidance graphics and alarm displays. Emergency shutdown (ESD), cargo tank protection, and machinery trip and safety systems are totally independent from main system. Independent systems are: Loading Computer, Custody Transfer System, Shipboard Management System. User Stations (US) are interfaces to the main system. The realistic graphics operate in Windows operational system , so operators can navigate easily between displays and invoke new displays or other applications directly by clicking a button, or selecting from a list. US are located in: cargo control room, wheelhouse, engine control room. Cargo operations include: aeration, inerting, gas filling, cooling down, loading, boil off gas burning, unloading, warming up, drying, one tank operations, emergency operations. 3.2. Drying and Aeration Prior to entry into the cargo tanks the inert gas must be replaced with air. The Inert Gas and Dry-Air System produces dry air with a dew point of -45°C. The dry-air enters the cargo tanks through the vapor header, to the individual vapor domes. The inert gas and dry-air mixture is exhausted from the bottom of the tanks to the atmosphere at vent mast by the tank filling pipes, the liquid header, and spool piece. During aerating, the pressure in the tanks must be kept low to maximize a piston effect. The operation is complete when all the tanks have a 20% oxygen value and a methane content of less than 0.2% by volume, and a dew point below -40°C. Before entry, test for traces of noxious gases, carbon dioxide less than 0.5% by volume, and carbon monoxide less than 50 ppm, which may have been constituents of the inert gas. In addition, take appropriate precautions as given in the Tanker Safety Guide and other relevant publications. Aeration carried out at sea as a continuation of gas freeing will take approximately 20 hours. Take precautions to avoid concentrations of inert gas or nitrogen in confined spaces, which could be hazardous to personnel. Before entering any such areas, test for sufficient oxygen and for traces of noxious gases. Inerting This operation is undertaken to ensure a non-flammable condition with the vapor of the cargo. Inerting of the cargo tanks and piping system are performed before preparation ship for commercial exploitation or before going to dry dock. In figure 3.3-1 is shown inerting operation diagram. Dry air from tank is displaced by inert gas by line OA until oxygen drop to about 4%. Then gassing up operation can start by line AB The heated gas from the cargo tanks is replaced by inert gas by line CD until. Dry air is accomplished by line DO. In this operation it is important to use reasonable margins of safety since the precise shape of flammable zone cannot be known for mixtures. 3.3.1 Inert Gas and Dry Air Generator The dry air and inert gas plant, installed in the engine room, produces dry air or inert gas which is used for the tank and piping treatments prior to and after a dry docking or an inspection period. The operating principle is based on the combustion of a low sulphur content fuel and the cleaning and drying of the exhaust gases. The inert gas plant includes an inert gas generator, a scrubbing tower unit, two combustion-air blowers, a fuel injection unit, dryer unit of refrigeration type, a final dryer unit (adsorption type) and an instrumentation and control system. The connection to the cargo piping system is made through two non-return valves and a spool piece. Inert gas is produced by the combustion of oil with air, followed by further treatments in order to obtain the required qualities and properties. Gas oil is supplied to the combustion chamber by the fuel oil pump and air from the air blowers. Good combustion is essential for the production of a good quality, soot free, low oxygen inert gas. The products of the combustion are mainly carbon dioxide, water and small quantities of oxygen, carbon monoxide, sulphur oxides and hydrogen. The nitrogen content is generally unchanged during the combustion process and the inert gas produced consists mainly of 86% nitrogen and 14% carbon dioxide. Initially, the hot combustion gases produced are cooled indirectly in the combustion chamber by a sea water jacket. Thereafter, cooling of the gases mainly occurs in the scrubber section of the generator where the sulphur oxides are washed out. The sea water for the inert gas generator is supplied by one of the ballast pumps. Before delivery out of the generator, water droplets and trapped moisture are separated from the inert gases by a demister. Further removal of water occurs in the intermediate dryer stage, where the refrigeration unit cools the gas to a temperature of about 5°C. The bulk of the water in the gas condenses and is drained away with the gas leaving this stage by demister. In the final stage, the water is removed by absorption process in a dual vessel desiccant dryer. The desiccant dryer units work on an automatic change over cycle, where the out of line desiccant unit is first reactivated with warm dry air which has gone through the reactivation dryer system. A pressure control valve located at the outlet of the dryer unit maintains a constant pressure throughout the system, thus ensuring a stable flame at the generator. Dew point and oxygen content of the Inert Gas produced are permanently monitored. The oxygen level controls the ratio of the air/fuel mixture supplied to the burner. The oxygen content must be below I % by volume and the dew point of -45°C. Both parameters are displayed locally and remotely. For delivery of inert gas to the cargo system, two combined remote air- operated control valves operated through solenoid valves are fitted on the distribution system, the purge valve and the delivery valve. The inert gas generator can produce dry-air instead of inert gas with the same capacity, however, for the production of dry air: there is no combustion in generator, there is no measure of oxygen content, the oxygen signal is overridden when the mode selector is on dry-air production. After the processes of cooling and drying and, if the dew point is correct, the dry air is supplied to the cargo system through the delivery valve. The combustion air is supplied to the main burner by tw6 ‘roots’ type blowers, each supplying 50% of the total capacity of the generator. The quantity of combustion air to the burner can be manually adjusted by a regulating valve in the excess air discharge line. Fuel (Light Diesel Oil) is supplied at a constant pressure by the gas oil electric pump which has a built-in pressure overflow valve. Before ignition or start up of the unit, and with the pump running, all the fuel is pumped back via this fuel oil overflow valve which also serves to regulate the delivery pressure of the pump. The main burner is ignited by a pilot burner. The main fuel oil burner is of the high pressure atomizing type. The fuel is directed to the burner orifice through tangential slots, which imparts a rotation motion ensuring that the fuel leaves the burner as a thin rotating membrane which is atomized just after the nozzle. 3.4 Gas Filling of the Cargo Tanks After lie up or dry dock, the cargo tanks are filled with inert gas or nitrogen. If the purging has been done with inert gas, the cargo tanks have to be purged and cooled down when the vessel arrives at the loading terminal. This is because, unlike nitrogen, inert gas contains 15% carbon dioxide (CO2), which will freeze at around -60°C 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 vapor. This is done to remove any freezable gases such as carbon dioxide, and to complete the drying of the tanks. LNG liquid is supplied from the terminal to the liquid manifold. It is then fed to the LNG vaporizer and the LNG vapor produced is passed at +20°C to the vapor header and into each tank. The LNG vapor is lighter than the inert gas, which allows the inert gases in the cargo tanks to be exhausted up the tank filling line to the liquid header. The inert gas then vents to the atmosphere. This operation can be done without the compressors. The operation is considered complete when the methane content, as measured at the top of the cargo filling pipe, exceeds 80% by volume. The target values for N2 gas and inert gas CO2 is equal or less than 1%. These values should be matched with the LNG terminal requirements. This normally entails approximately two changes of the volume of the atmosphere in the cargo tank. Cooling Down of Cargo Tanks After the cargo tanks has been purge-dried and gassed up, the headers and tanks must be cooled down before loading can commence. The cool down operation follows immediately after the completion of gas filling, using LNG supplied from the terminal. The rate of cool down is limited for the following reasons: to avoid excessive pump tower stress, vapor generation must remain within the capabilities of the compressors to maintain the cargo tanks at normal working pressure, to remain within the capacity of the nitrogen system to maintain the primary and secondary insulation spaces at the required pressures. LNG is supplied from the terminal to the manifold cool down line and from there directly to the spray header which is open to the cargo tanks. Once the cargo tank cool down is nearing completion, the liquid manifold crossovers, liquid header and loading lines are cooled down. Cool down of the cargo tanks, on membrane ships, is considered complete when the mean temperatures of -130°C or lower. Cool down of the spherical cargo tanks is considered complete when the mean temperatures of equator is -125°C or lower. When these temperatures have been reached, and the custody transfer system (CTS) registers the presence of liquid, bulk loading can begin. Vapor generated during the cool down of the tanks is returned to the terminal by compressors (or free flow) and the vapor manifold, as in the normal manner for loading. During cool down, nitrogen flow to the primary and secondary spaces, on membrane ships, and to annular space of spherical tanks will significantly increase. It is essential that the rate of cool down is controlled so that it remains within the limits of the nitrogen system to maintain the primary and secondary insulation space pressures between 2 and 4 mbar. In annular space of spherical tanks pressure is set at 5 mbar. Once cool down is completed and the build up to bulk loading has commenced, the tank membrane will be at, or near to, liquid cargo temperature and it will take some hours to establish fully cooled down temperature gradients through the insulation. Consequently boil-off from the cargo will be higher than normal. Cooling down the cargo tanks, on membrane ships, from +40°C to -130°C, will require a total of about 800 m3 of LNG. Cooling rate is 12°C per hour. Maximum permissible cooling rate for spherical tanks is 8°C per hour for the first 100°C cool-down period and 4°C for the rest of period. In order to protect the tank shell against high thermal stresses recommended temperature difference of tank, equator and skirt must be respected. 3.6 Loading The preoperational procedures must be discussed with the terminal operators. The information exchange between terminal and ship is required and relevant check list should be completed. ESD test must be carried out. (ESDS — emergency shut down system). LNG is taken through liquid line and directed into cargo tanks. Normally when loading cargo, generated vapor is returned to the terminal by means of the compressors or shore compressor. The pressure in the ship’s vapor header is maintained by adjusting the compressor flow. Ship’s tank pressure must be observed. Loading rates should be reduced if difficulties are experienced in maintaining correct tank pressure. On membrane ships the pressurization system of the insulation spaces must be in operation with its automatic pressure controls. The secondary Level Indicating system should be maintained ready for operation. The temperature recording system and alarms for the cargo tank barriers and double hull structure should be in continuous operation. The gas detection system and alarms must be in continuous operation. On the end of loading operation loading rates must be reduced as previously agreed with terminal in order to “topping off” tanks. The liquid remaining in headers can be blown into the ships tanks by injecting nitrogen into the loading arms. The maximum allowable filling limits of cargo tanks are given and must be respected without hesitation. Cargo loading can be carried out using a vapor line (we say vapor header), and a liquid line (we say liquid header). Where loading is carried out with a vapor return facilities, liquid is taken on board through the liquid header and directed into the cargo tanks. Vapor generated are returned ashore via the vapor return connection using the cargo compressor of LNG Carrier in order to control the cargo tank pressure. Close watch should be kept on ship’s cargo tank pressure. This is the reason why these operation in done on the closed cycle, not to dispose LNG vapor at atmosphere. *Some gas quantity is sent to main boilers from H/D compressor through L/D heater. 3.7 Loaded Passage, Boil-Off Gas Burning During a sea passage when the cargo tanks contain LNG, the boil-off from the tanks is burned in the ship’s boilers. The cargo tank boil-off gas enters the vapor header. It is then directed to one of the compressors, which deliver the gas to the boil-off or warm-up heater. The heated gas is delivered to the boilers at a temperature of +25°C. The compressors speed and inlet guide vane position is governed by fuel gas demand from the boilers and cargo tank’s pressure. The system is designed to burn all boil-off gas normally produced by a full cargo and to maintain the cargo tank pressure at a predetermined level. If the propulsion plant steam consumption is not sufficient to burn the required amount of boil-off, the tank pressure will increase and eventually the steam dump will open, dumping steam directly to the main condenser. The main dump is designed to dump sufficient steam to allow the boiler to use all the boil- off produced, even when the ship is stopped. The steam dump is designed to open when the normal boil-off value is 5% above the original selected value and when the tank pressure has reached the pre-selected dump operating pressure. The cargo and gas burning piping system is arranged so that excess boil-off can be vented should there be any inadvertent stopping of gas burning in the ship’s boilers. The automatic control valve vents the excess vapor to atmosphere. If the gas header pressure falls to less than 40 mbar above the primary insulation space pressure, an alarm will sound. In the event of automatic or manual shut down of the gas burning system or if the cargo tanks pressure falls to 5 mbar above the insulation space’s pressure), valve will close and the gas burning supply line to the engine room will be purged with nitrogen. 3.8 Unloading When the ship arrives at the LNG receiving terminal and when ship’s and terminal lines are connected preoperational tests can be carried out. Unloading operation begins with one cargo pump and low rate to cool down ships and terminals lines. Cooling down operation lasts about one hour when others pumps can be started and unloading rate increased. Cargo centrifugal pumps should be started against partially open valve in order to minimize starting load. Thereafter the discharge valve should be open gradually until pump load is operating within design parameters. Cargo discharging takes about 15 hours. All pumps run in parallel. The tank pressures tend to fall as cargo is being removed from tanks. Vapors produced by remaining cargo boil off are insufficient to balance the liquid removal rate. To maintain normal tank’s pressure the gas may be provided from terminal via main gas line or can be produced by using the ship forced vaporizer. In second case liquid is taken from the liquid line and diverted through the vaporizer. Towards the end of discharging unloading rate should be reduced (usually by stopping one pump of each tank). On completion of cargo discharge liquid line must be drained and manifold valve closed. Then terminal loading arms can be disconnected from ship’s manifold. From that moment ship takes care of tank’s pressure by burning gas in ship propulsion boilers. Cargo unloading can be carried out using a vapor header and a liquid header, same as loading operation. Unloading operation is carried out with ship’s submerged pump in each tank. In order to control tank pressure, LNG in cargo tanks can be sent to storage tank of shore side, and BOG from shore side, and with return gas blower to ship’s tanks. 3.9 Ballast Passage On LNG carriers it is usual practice to retain some liquid in tanks after discharge. This liquid is used to maintain tanks in cold condition in order to be ready for next loading. The quantity of retained liquid depends on: size of the ship, type of cargo containment system, and length of voyage. All LNG vessels, with spherical or membrane tanks, are equipped with spray cool down pumps. The frequency of this operation is much more demanding on LNG tankers with spherical cargo containment system in order to have equator temperature at - 125°C what is required before loading operation. A characteristic of the cargo tanks of the membrane type is that as long as some quantity of LNG remains at the bottom of the tanks, the temperature at the top will remain below -50°C. However, if the ballast voyage is too long, the lighter fractions of the liquid will evaporate. Eventually most of the methane disappears and the liquid remaining in the tanks at the end of the voyage is almost all LPG with a high temperature and a very high specific gravity, which precludes pumping. Thus operator should consider heel ageing for coolant when ballast voyage is too long. Due to the properties of the materials and to the design of the membrane cargo containment, cooling down prior to loading is, theoretically, not required for the tanks. However, to reduce the generation of vapor and to prevent any thermal shock on the heavy structures, loading takes place when the tanks are in a cold state’. The remaining liquid level of membrane tanks must never be above 10% of the length of the tank and the quantities can be calculated by considering a boil-off of approximately 45% of the boil-off rate under laden voyage condition and the need to arrive at the loading port with a minimum layer of 10cm of liquid spread over the whole surface of the tank bottom (with the ship even keel). Additional cool down should be carried out at the LNG terminal, when the cargo tank temperature is higher than -130°C. Maintain the cargo tanks at cold during the ballast voyage by periodically spraying the LNG so that the average temperature inside the tanks does not exceed -130°C It is obvious that spraying will generate more boil-off than without tanks cooling down. The quantity of LNG to be retained on board will have to be calculated with enough margins to avoid the situation at mid-voyage where the residual is too heavy for the pump to operate. If conservation of bunkers is requested it is essential to ensure as much boil-off gas as possible to supply boiler fuel demand, thus keeping fuel oil consumption to a minimum. 3.10 Tanks Warming Up Tank warm up is part of the gas freeing operations carried out prior to a dry docking or when preparing tanks for inspection purposes. The tanks are warmed up by heated LNG vapor. The vapor is re-circulated with the compressors and heated with the cargo heaters to 70°C. In a first step, hot vapor is introduced through the filling lines to the bottom of the tanks to facilitate the evaporation of any liquid remaining in the tanks. In a second step, when the temperatures have a tendency to stabilize, hot vapor is introduced through the vapor piping at the top of the tanks. Excess vapor generated during the warm up operation is vented to atmosphere when at sea, or burning in the boiler. The warm up operation continues until the temperature at the coldest point of the secondary barrier of each tank reaches 5°C. The warm up operation requires a period of time dependent on both the amount and the composition of liquid remaining in the tanks and the temperature of the tanks and insulation spaces. Generally, the warm up will require about 48 hours after vaporizing the remaining liquid. Initially, the tank temperatures will rise slowly as evaporation of the LNG Rolling and pitching of the vessel will assist evaporation. Gas burning should continue as long as tank pressures start to fall. 3.11 One Tank Operation It may be necessary for in-tank repairs to be carried out with the vessel in service. Then tank need to be warmed up, inerted, aerated, entered and work undertaken on the tank internals, change cargo pump, investigate and cure problems with tank gauging systems etc. The warm up, inerting and aeration can be carried out with the remaining cold tanks providing boil-off gas for burning in the boilers. Aeration should be continued throughout the repair period to prevent ingress of humid air to the cargo tank. Tank venting is carried out by means of the gas header line. At the discharge port, the tank to be discharged to the lowest measurable level. Normal gas burning is continued during this operation using vapor from all four tanks. In the first instance, normal boil-off gas procedures are followed until this operation has stabilized, then the operation for warming up one tank using a compressor can be carried out. Normal gas burning is continued during this operation using vapor from others tanks. Inert gas is supplied to the tank by the Inert Gas plant. Dry air is introduced at the bottom of the tank through the filling piping. The air is displaced from the vapor dome into the gas header by the fitted spool piece and is discharged from vent mast. 3.12 Nitrogen Distribution Membrane Systems The primary and secondary insulation spaces are filled with dry nitrogen gas which is automatically maintained by alternate relief and make-up valves as the atmospheric pressure or the temperature rises and falls, under a pressure of between 2 mbar and 4 mbar above atmospheric. The nitrogen provides a dry and inert medium for the following purposes: to prevent 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. Both primary and secondary insulation spaces of each tank are provided with a pair of pressure relief valves which open at a pressure, sensed in each space, of 10 mbar above atmospheric. Nitrogen produced by generators and stored in a pressurized buffer tank, is supplied to the pressurization headers through make-up regulating valves. From the headers, branches are led to the insulation spaces of each tank. Excess nitrogen is vented through exhaust pressure control valves to vent mast from the primary and secondary insulation spaces. Two nitrogen generators, installed in the engine room, produce gaseous nitrogen which is used. for the pressurization of the barrier insulation spaces, as seal gas for the HD and LD compressors, fire extinguishing in the vent mast and, for purging the fuel gas system and various parts of the cargo piping. The two high capacity units are able to produce 2x90 m3h of nitrogen. The operating principle is based on the hollow fiber membranes through which compressed air flows and is separated into oxygen and nitrogen. The oxygen is vented to the atmosphere and the nitrogen stored in a about 30m3 buffer tank ready for use. Each unit consists: screw compressor, cooled from fresh water cooling system, single stage air/water separator, three air filters arranged in series, electric heater, before passing into the membrane units, an oxygen analyzer, after the membrane, monitors the oxygen content, and if out of range, above 4%, redirects the flow to the atmosphere. On the older generation of LNG vessels nitrogen is stored as liquid in double shell tank and vacuum and perlite insulated. This nitrogen storage usually include one or two storage tanks with vaporizer and heater. Vaporized and heated nitrogen is distributed to above mentioned consumers. LNG tanker of 125 000 m3 capacity, has N2 storage tank of 70m3. Tank pressure is 3 bars, liquid temperature is -196°C. Tank is double wall container with insulation in between. Insulation is additionally improved with vacuum. Nitrogen evaporation coefficient is 0.3% per day. Nitrogen storage control box contains: pressure control valve, level and pressure gauging. When pressure in tank drops bellow service pressure regulating valve open. Liquid passes through vaporizer and increase pressure in tank. Before delivery to consumers nitrogen passes through heater. Nitrogen leaving the heater has temperature of 15°C. 3.13 Emergency Operation 3.13.1 Emergency Cargo Pump Installation In the event that both main pumps have failed in a cargo tank on membrane ships the emergency cargo pump is used. The pump is lowered into the emergency cargo pump column for that tank. Cables and a connection to the local junction box are used to power the pump. The pump, when lowered to its final position, opens the foot valve in the column and the LNG can be pumped out. The pump discharges into the column and to the liquid line. When all equipment, pump, cables, electrical connection box and accessories are in position near the tank in which the pump is to be installed, the derrick need to be prepared to lift the pump and start the pump installation. The cargo tank will inevitably contain LNG, therefore the column into which the emergency pump is being lowered must be evacuated. This is achieved by injecting nitrogen into the column. In the case of a full cargo tank, a pressure of between 20 and 30 mbar is required. The nitrogen forces the liquid out through the foot valve located at the bottom of the column. In the case of cargo pumps failing on a spherical tanks, the unloading can be carried out by pressurizing the tank and forcing the liquid into one or more others tanks. Spray pump can be used to create the pressure that is required for the unloading. 3.13.2 Water Leakage to Barrier, Inner Hull Failure Ballast water leakage from the wing tanks to the insulation spaces can occur through fractures in the inner hull plating. If the leakage remains undetected and water accumulates in these spaces ice will be formed. Ice accumulation can cause deformation and possible rupture, of the insulation. The resultant cold conduction paths forming in the insulation will cause cold spots to form on the inner hull. The pressure differential caused by the head of water building up in the insulation space may be sufficient to deform or even collapse the membrane into the cargo tank. To reduce the risk of damage from leakage, each cargo insulation space has been provided with water detection units. At the bottom of cofferdams there is a bilge well for each tank insulating space. Each of these wells is fitted with water detection units. Each detector is of the conductivity cell type, which causes a change in resistance due to the presence of humidity from the ingress of sea water and activates an alarm. The bilge well serves as the inlet for the nitrogen supply pipe to the insulation space. This supply pipe also acts as a manual sounding pipe to the bilge well. Each bilge well is connected to draining pipe system with a pneumatic pump situated in the forward and aft pipe duct for discharging the water to deck level and then overboard by means of a flexible hose. After the maximum possible water has been discharged from this insulation space, appreciable moisture will remain in the insulation and over the bottom area. Increasing the flow of nitrogen through the space can assist drying out the insulation. This should be continued until the moisture level is below that detected by the water detection system before any cargo is carried in the affected tank. 3.13.3 Gas Leakage Detection Fatigue fractures in the primary insulation membrane are generally small and will pass either vapor only, or a sufficiently small amount of liquid, which will vaporize as it passes through the fracture. It is possible, however, that a larger failure of the membrane could occur, allowing liquid to pass through and eventually gather at the bottom of the primary insulation space. A small leakage of vapor through the membrane may not be readily obvious. However, indications are likely to be a sudden rise in the percentage of methane vapor in one primary insulation space. Some porosity in the primary barrier weld will allow the presence of methane vapor in the primary insulation space. The amount of this vapor should be kept to a minimum by the nitrogen purging. If a fracture occurs in the primary insulation barrier below the level of the liquid in the tank, the vapor concentration will increase slowly and steadily. If the fracture is above the liquid level, the concentration will exhibit a fluctuating increase. The vapor concentration in each primary insulation space is recorded daily, to detect any small and steady change. A fracture above the liquid level in a cargo tank will allow a direct flow of vapor into the primary insulation space what will cause increase in pressure in primary insulation space. This flow will vary according to the pressure in the tank. A fracture below the liquid level in a cargo tank, resulting in a small amount of liquid vaporizing as it passes through the fracture, will cause a small increase in pressure. This increase is dependent upon the height of liquid above the fracture and the pressure in the tank. No temperature change will be obvious, unless the fracture is in the immediate vicinity of the sensors below the cargo tank. Leakage of methane vapor into the primary insulation space presents no immediate danger to the tank or vessel. As much information as possible concerning the fracture and leak should be obtained and recorded. 3.13.4 Liquid Leakage A major failure in the primary membrane, allowing liquid into the primary insulation space, will be indicated as follows: rapid increase in the methane content of the affected space, rise in pressure in the primary insulation space nitrogen header, accompanied by continuous increased venting to atmosphere, low temperature alarms at all temperature sensors in the insulation below the damaged cargo tank, general lowering of inner hull steel temperatures. If a major failure of the membrane occurs, liquid from the tank will flow into the primary insulation space until the levels in both compartments are equal. When the contents of the cargo tank are discharged, unless the LNG in the primary insulation space can drain sufficiently quickly to the cargo tank, a differential liquid head will build up, tending to collapse the membrane of the tank. Before discharging a tank with major failure in the primary membrane, it is essential that the primary membrane is punched so that liquid can freely flow back into the tank from the primary insulation space. In this way no hydrostatic head occurs in the primary insulation space which could cause damage to the primary membrane support. The punching device is a 30 kg messenger which is dropped down the float gauge tube so as to punch a hole in the primary membrane at the base. The base of the float gauge tube is fitted with a split perforated base to allow the messenger to penetrate through to the membrane. The membrane is fitted with a thin diaphragm and the plywood insulation boxes are thinner than normal to allow the messenger to penetrate fully. Liquefied Natural Gas Carrier (LNG) Cargo Operation Original Date: 01 March 2008 Version No:1 Page Revision Date: Revision No: 0 46 of 46 Fig. 3.7-1 Boil-off to Boilers Fig. 2.2-7 Ship Type Fig. 2.2-6 Ship Type Fig. 2.2-5 Ship Type Fig. 2.2-4 Ship Type Fig. 2.2-3 Ship Type Fig. 2.2-2 Ship Type Fig. 2.1-1 LNG Tank Design Consumers Fig. 1.4-1 LNG Chain Fig. 1.3-1 Characteristics of LNG Fig. 1.1-1 Principle of LNG Fig. 1-1 LNG Carrier RGB Liquid header Vapor header Loading Pump LNG BOG Shore side Storage tank Fig.3.3.1-2 Inert Gas Generator Monitor Fig. 3.1-1 Cargo Control Room (CCR) Fig. 3.12-2 Liquid Nitrogen System Fig. 3.12-1 Nitrogen Generator System ig. 3.10-1 Warming Up Operation Diagram Fig. 3.8-1 Unloading Operation Fig. 3.6-1 Loading Operation Fig. 3.6-2 Loading Operation Fig. 3.5-1 Cooling Down Operation Fig. 3.4-1 Gassing up Operation Fig. 3.3.1-2 Schematic Diagram of Cargo Tanks Inerting Operation BOG LNG Fig.3.3.1-1 Inert Gas Production Plant Fig. 2.2-1 Ship Type Fig. 3.3-1 Inerting Operation Diagram SHORE LNGC Fig. 3.8-2 Unloading Operation SHORE LNGC Cargo tanks Liquefied Natural Gas Carrier (LNG) is a type of ship transported in double-hulled specifically designed to handle the low temperature (-162 Deg Celsius) of LNG. Tanks are insulated to limit the amount of LNG that boils off or evaporates. This boil off gas is mostly used to supplement fuel for the carriers. LNG carriers are about 300m long, and require a minimum water depth of 40 feet when fully loaded. Cargo tanks Liquid header Vapor header *H/D Compressor To shore Loading Pump LNG Shore side Storage tank BOG LNG