The original NLNG plant site was designed to provide scope for future expansion and the current 4th and 5th train (NLNGPlus) project is the next stage in the development of the NLNG project. The NLNGPlus project will provide further opportunities for exploitation of Nigeria's gas reserves, with resulting economic benefits for the country. In addition, the NLNGPlus project will increase the capacity of the LNG complex to process oil-associated gas, much of which is currently wasted through flaring. The use of associated gas will improve the efficiency of resource use, by providing both LNG and liquefied petroleum gas (LPG) products, as well as providing environmental improvements by reducing flaring by the gas producers in the Niger Delta, which contributes to global warming.

 

The NLNGPlus project will consist of the construction and operation of two additional (air-cooled) LNG trains (processing 1,334 MMscfd of gas) bringing the total number to five process trains with an overall capacity of 2,810 MMscfd gas intake. The new trains are to be integrated with existing facilities wherever practicable so that the site is operated as a co-ordinated 5-train complex.

 

The Base plant (two trains) has a production capacity of 5.8 million tonnes per annum (Mtpa) of LNG and the Expansion project (3rd train) will increase this by 2.9 Mtpa. The NLNGPlus project will further increase the production capacity by 8.1 Mtpa. The Base Project trains were originally designed to process non oil-associated gas (NAG), but additional facilities were installed and modifications made during construction of the 3rd train to enable all the three trains process oil associated gas (AG). NLNGPlus is also designed to process oil-associated gas (AG). By the time all 5 trains will be operational, more than 80% of the feedstock will be associated gas. The NLNGPlus project therefore, like the expanded 3train operation will include sizeable natural gas liquids fractionation units, and the LPG chilling and loading unit which is part of Train-3 scope will be modified to handle more than 1 Mtpa of butane and propane products from NLNGPlus project for export.

 

The NLNGPlus Project is based on the tried and tested configuration of the Propane-Mixed Refrigerant process and also incorporates appropriate technological advancements to ensure the economy and competitiveness of the new design.

 

The NLNGPlus project will include the construction, commissioning, start-up and operation & maintenance of:

The NLNGPlus project is similar to the Base and Expansion Projects. However, the trains are larger and take advantage of important technological developments. Each train is larger than the Base and Expansion Projects. The production capacity is 4 million tonnes/year compared to 2.9 million tonnes/year for the existing three trains. Air cooling will be applied instead of water cooling process used for the existing three trains. Also, NLNGPlus includes extended end-flash for increased LNG production. In general, the common utilities, storage and loading facilities will be shared with the Base and Expansion projects.

 

The maintenance requirements of equipment, electrical and instrumentation and the workload resulting from a regular maintenance and inspection shutdowns will be unchanged from the Base and Expansion projects.

Three gas gathering networks will transport predominantly associated gas to the LNG complex;

 

1 GTS-1 is an onshore system with its main trunkline extending some 149 km to the Northwest of the plant and terminates in a 150 m3 slugcatcher. It is owned and operated by NLNG, being started up as part of the Base Project, and has 5 entry points where custody transfer can be affected.
2 GTS-2 is a proposed second onshore system with a 90 km trunkline also extending to the Northwest but to the south of GTS-1. It terminates with a slugcatcher and will be owned and operated by one or more of the gas supply joint ventures.
3 GTS-3, also known as the Offshore Gas Gathering System (OGGS), is a 320 km offshore line. It is owned and operated by Shell Petroleum Development Company and terminates in a 1250 m3 slugcatcher.

Gas supplies from the three networks are mixed and transported via either of two feedgas supply headers to the LNG trains. One header services Trains 1, 2 and 3, while the other services Trains 4 and 5. Each header operates at a different pressure with the latter being the higher.

 

Each new train comprises an Acid Gas Removal Unit, a Molsieve unit to remove water, a Mercury Removal Unit made of guard bed to trap any trace levels of mercury in the feedgas, a propane pre-cooled mixed refrigerant unit to liquefy the gas and a fractionation unit to produce export grade propane and butane products. The fractionation unit also extracts pentane surplus to the vapour pressure requirements of the condensate product and routes this stream to a dedicated pentane fuel system.

 

The LNG rundown from the two new trains joins a common header routing LNG from all five trains to all 3 tanks.
The trestle of the existing LPG loading jetty will be extended to allow LNG loading. Additional Boil-Off Gas will be routed to the endflash compressors of Trains 4 and 5. The loading system shall be designed to load 10,000 m3/h at both the new and the existing jetty. The new system will be operated such that, only one ship at a time will be loaded but it should be possible that a second ship may be cooled down with a small flow, up to 300 m3/h from the loading line while the first ship is being loaded at full rate.

 

Propane and butane from the fractionation units are routed via the LPG chilling unit to the two existing storage tanks. The LPG chilling unit will be extended to accommodate the additional LPG rundown.

 

Two additional gas turbines generators are added for electricity generation. Process heat will be serviced by a Heat Transfer Fluid (HTF) system which links heat generated from two new waste heat recovery units in the exhaust stack of the gas turbine driver of the propane refrigerant cycle of each new train, to the heat consumers the in-train fractionation Units of the new trains. Waste heat will also be used for heating of the regeneration gas of the dehydration unit.

 

LNG rundown from the five NLNG trains is accumulated in three LNG storage tanks prior to being loaded onto LNG tankers for export. The LNG storage and loading is an existing system comprising three LNG storage tanks, loading pumps, a jetty with a ship loading facilities (Berth 1) and a boil-off gas (BOG) handling facilities. In order to cater for the increased LNG rundown from NLNGPlus project, the facilities are to be extended by addition of a new second LNG berth (Berth 2) and additional BOG handling. The facilities for the second berth are part of the NLNGPlus project scope. The new jetty shall be able to handle ships with sizes ranging from 125,000 to 145,000 m3,

 

The key units of the two new LNG trains are further described in the following paragraphs.

The first step in each LNG train is the Acid Gas Removal Unit, in which acid gases consisting predominantly of CO2 and trace H2S are removed from the natural feed gas.

 

The acid gas is subsequently incinerated in the Acid Gas Incinerator and converted into oxidised products.

In the Dehydration Unit water is removed from the gas. Drying is required to prevent ice and hydrate formation in the downstream Liquefaction Unit, which would cause blockage of lines and equipment. The design of the Unit is based on a configuration of three molecular sieve beds in a parallel configuration.

The purpose of the unit is to remove possible trace quantities of mercury to protect the tubes of the heat exchangers against rapid corrosion of aluminium. The technology applied to remove the mercury from the dried gas is based on a non-regenerable bed of sulphur-impregnated activated carbon or a metal-sulphide based material adsorbent. The life span of the activated carbon is at least 4 years.

The selected liquefaction process is the Propane Mixed Refrigerant (C3MR) process. The gas is cooled down in two steps to approximately -161°C. The LNG is then pumped to storage at a temperature of about -160°C.

 

Two identical, low-NOx gas turbines, per LNG train with a power rating of 71 MW each provide the power for the compressors of the cooling circuits, which are closed loop refrigeration systems. The principle of the cooling systems is basically similar to a normal refrigerator. The compressed cooling agent in the liquefaction units is cooled with air-coolers, which are installed on top of the structure of the LNG trains.

Each of the LNG trains has a Fractionation Unit, in which propane, butane and stabilised condensate are separated from the feed to the Liquefaction Unit. These products are routed to the existing storage facilities.

The LNG is stored in the three existing LNG storage tanks of the full containment type, each with 84,200 m3 gross storage capacity. A full containment tank consists of a concrete outer tank and an inner tank of 9% nickel steel. The boil-off gas is compressed and used as fuel gas for the LNG units.

The marine facilities are designed to accommodate LNG carriers with a cargo load of 122,000 to 145,000 m3. Approximately 127 additional LNG cargoes will be loaded per year (existing: 145 per year). With a loading rate of 10,000 m3 per hour, a ship will be at the jetty for a period of 12-16 hours.

 

The existing LPG jetty will be expanded by extending the trestle to accommodate the LNG loading lines to the jetty head, and bunker fuel loading system. This jetty head will also be expanded to allow installation of the additional LNG loading arm and associated equipment. The existing berth and mooring dolphins require no modifications.

Power for the LNG plant will be generated by a dedicated gas turbine power plant. The power station in the Common Facilities Area currently consists of 6 gas turbines, each with a site rating of approximately 34 MW electrical. The gas turbines will be fired with high-pressure fuel gas from the fuel gas system. Two additional gas turbines, also rated at 34 MW, will be installed to accommodate the additional power demand from the new installations.

The purpose of this disposal system is safely and reliably to collect and dispose of vapour or liquid hydrocarbon containing streams that result from upsets and emergencies. The design is based on segregation of wet, heavy hydrocarbons and light, dry, cold hydrocarbons so that hydrate formation, freezing or condensation does not block any system.

 

The existing flares and liquid burners consist of:

 

1 Four emergency flare stacks (including 1 spare): located in a common derrick structure with a height of approximately 125 m.
2 The liquid burners and the operational flare: located in a separate structure with a height of about 60 m.

 

The liquid disposal burners and the operational flare are designed to burn smokeless. No liquids will be flared during normal operation.

 

An additional and fully segregated flare system will be installed for the NLNGPlus facilities, consisting of three emergency flare stacks (including one spare), located in a new common derrick structure with a height of approximately 150 m.

 

The existing liquid and operational flare systems are capable of handling the extra requirements from the new 4th and 5th Trains LNG facilities.

Drainage and primary wastewater treatment systems are provided to ensure proper discharge of all kinds of effluents from the site. These systems will be integrated with the existing facilities. Several segregated drainage systems will prevent the mixing of different types and qualities of effluents. 
 

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