FLOATING STORAGE REGASIFICATION UNIT ( FSRU ) DESIGN INSIGHT
FSRU is used to supply LNG gas for power projects where power stations use LNG to produce electricity. Normally LNG carriers visit these FSRU periodically to provide LNG to these FSRUs. Gas is stored in LNG storage tanks and later liquid fuel is regasified by regasification units. Which will be increasing delivery pressure of gaseous fuel so that it can be transported through subsea piping system to power station.
The FSRU consists of the following major components:
- Double skin construction steel hull structure
Double skin construction is required for FSRU for collision protection and to provide reserve buoyancy in case of any damage to hull.
- Prismatic LNG containment system
These tanks are designed to store LNG at very low temperature ( often around -160 c ) and insulation is specially designed to protect any spill and to reduce brittleness due to excessive low temperature
- LNG loading, unloading and NG export system
System is designed for loading LNG from LNG carrier and unloading to smaller LNG carrier. Natural gas export system is provided to export regasified fuel to the power stations.
- Regasification System
Regasification system is provided on top of hull. In converted FSRUs from LNG carrier these units are often kept at the port or stbd side on the midship however new build FSRUs can be designed so that these units can be kept at the fore body.
- Accommodation Block
Accomodation is provided for personnel working on FSRU and this design should cater for temporary refugee space to protect crew from accidental fire.
- Emergency and essential power system
Emergency power is required for emergency services when black out happens. It should cater to emergency lights, smoke detection system and other emergency systems. However essential power system is required in normal operation condition to run regasification units and other essential loads.
- Power Generation
These days LNG carriers are designed to use boil off LNG so that they don't need other fuels. So in conversion FSRU projects engines are designed to produce power in range of 15 MW to 25 MW. In FSRU mode this power is enough to provide power for regasification units and other loads. So forced boil off can be utilized to fuel the existing engines. However in new build FSRU, duel fuel engines are more economical whereas it can use boiled off LNG as well as MDO or HFO.
- Fuel oil & Boil off fuel gas system
As explained earlier fuel oil and boil off fuel gas system is provided to feed duel fuel engines.
- Permanent mooring system
FSRUs are designed for continuous supply of natural gas to the power stations, so their design life ranges from 15 to 25 years at the location. So their mooring systems are designed for permanent basis.
- LNG Cargo system
- Ballast water system
FSRU ballast tanks are designed to maintain almost constant or low variation in draft changes. So capacity of ballast tanks and ballast system shall match the LNG storage and loading and unloading rate. Low variation in draft is required to maintain operation integrity of LNG export system or LNG arms.
- Inert Gas System
Inert gas system is required to supply inert gas to LNG tanks which are explosion prone area.
- Nitrogen Generating System
Nitrogen used as inert gas in the system
- Hypochlorite Generation Package
- Firefighting system
- Fire and gas detection system
DESIGN BASIS OF FSRU
New build FSRU is based upon a barge shaped hull moored with berthing and mooring dolphins. The FSRU shall be able to receive LNG from LNG Carrier of different sizes,offload LNG to various sizes and send out NG to provide a demand flow of natural gas to the power station.
- New build FSRU based on LNG tanks design with total required storage volume depending on the requirement of power station as well as standard size of LNG carrier which is used for loading the FSRU. Normally LNG carriers are designed in the range of 120000 m3 to 190000 m3. So FSRU shall be designed to cater the full unloading of candidate LNG carrier. Frequency of LNG carrier visit will depend on the natural gas use. So if uses are high, more frequent visit of LNG carrier is required to cater the need.
- Regasification units are designed with redundancy and normally N+1 system is sufficient for operation. so if natural gas requirement is 375 mmscfd, 4 units of 125 mmscfd are sufficient. Number of units are designed in such a way that in case one unit is down, other units can supply required natural gas.
- The proposed arrangement of the vessel shall have a fore peak tank, machinery / workshop space, storage tanks and cofferdam space, diesel oil tanks, aft peak tank, machinery space aft and pump room in fwd.
- Double side tanks and double bottom tanks within the cargo area are to be used as seawater ballast tanks and are to be designed and designated as segregated ballast tanks.
- A single Central Control Room (CCR) shall be located in the Accommodation Block with a Central Equipment Room (CER) located near the CCR. The CCR and the CER shall be of sufficient size to accommodate all control functions of the vessel & topside requirements. (Regasification equipment, loading and offloading equipment, ballast and cargo control stations, designated machinery operations and safety equipment).
- FSRUs are Designed for long life periods normally ranging from 15 to 30 years, So systems shall be designed to avoid dry docking. In water survey provisions shall be provided.
- Mooring system to be fully compatible with the jetty mooring system including provision of associated systems and fairleads
- Accommodation shall be located away from the storage tanks and regasification units.
- The unit shall be designed and equipped for loading & unloading LNG through loading arms or flexible LNG hoses and offloading natural gas via jumper and pipe line system to the power stations
- Redundant cargo fiscal custody control transfer system, based on cargo tank level measuring (radar ullage).
- Vessel operations control,jetty mooring control, docking add control, jetty power,lighting and vessel communications system shall be provided from FSRU
The design draught is based upon maximum allowable cargo tanks filling with a LNG cargo of 0.470 specific gravity, sufficient bunkers & consumables for endurance.
LNG STORAGE AND TANKS REQUIRED
The following and tank are required.
- LNG Tanks
- MDO Storage Tanks
- MGO Storage Tanks
- Potable Water Tanks
- Technical Water Tanks
- Cooling Medium Drain Tank
- Bulk Lube Oil Storage Tanks
- Waste Oil Tank
- Bulk Hydraulic Oil Storage Tanks
- Water Ballast Tanks
REGASIFICATION UNIT
Regasification System
There are various options are available for regasification system that are closed loop and open loop. Following criterias and inputs shall be checked against selected regasification system
- Send Out Gas Pressure
- Send Out Gas temperature
- Maximum send out gas volume
- Minimum send out gas volume
- Number of trains Operating+1 Spare
STEELWORK
For purposes of this specification, the hull is divided into 3 sections: the fore body, cargo block (LNG Storage area) and the after body.
Double bottom, double side, transverse and longitudinal bulkheads of the plane type are to be provided within the cargo area. Double bottoms are also to be provided for the machinery spaces.Double sides are to be such that the fuel tanks are protected.The LNG cargo tanks will be subject to sloshing effects during operations. The FSRU vessel cargo, ballast and other tanks must be designed to support and compensate for the induced loads resulting from sloshing.The construction and arrangement of the vessel shall be developed with due consideration to easy access and future maintenance. The basic structural design shall provide continuity of structural members throughout the length of the hull. Where discontinuities are necessary, the terminations of members shall be designed to provide effective structural connections with a suitable transition in stiffness. The calculated fatigue life at any point in the hull structure shall be equal to or exceed the design life multiplied by the applicable factor of safety, as defined in Basis of Design and in compliance with Class requirements.
HULL DESIGN
The FSRU shall have an all welded steel double hull designed to support LNG cargo tanks.
A specific gravity of 0,5 Kg/dm3 shall be considered for the cargo for longitudinal strength and tank scantling design. No reduction in scantling shall be applied in connection with “corrosion control” allowance permitted by Class.
The cargo area shall be of double decks, double hull, and cofferdams which are located at forward and after part of cargo area and between cargo tanks with double bottom construction for the storage of liquefied natural gas cargoes at cryogenic temperature (-162ºC) and atmospheric pressure, and four (4) pairs of water ballast tanks surrounding the sides and bottom. of cargo tanks compartments.
The cargo tank structure shall be designed to have a double hull arrangement in way of the LNG cargo tank sides and bottom. For Periodic Close-up Survey purpose, permanent access to all the upper parts of the ballast tanks and cargo tanks is to be arranged utilizing permanent staging and passages through structures. The ballast tank configuration shall be optimized with respect to damage stability, vessel trim, motion behavior and equal load distribution of the hull in normal operating conditions.
Ballast tank capacity shall be sufficient to maintain design draft at all loading condition. The hull areas in way of mooring fenders and supply boats landing areas shall be specially considered and adequately reinforced to withstand the operational loads and in accordance with the requirements of Classification Society.
The side of the hull used for mooring shall be analyzed to ensure structural safety for all conceivable mooring conditions of the FSRU. The approach velocity in either case is to be considered as a minimum 25-cm/sec. The resulting reaction stresses in the plating and stiffening of the side-shell support structure at 50% fender deformation as calculated by linear elastic analysis shall not exceed 0.8 times the structural material yield strength.
TEMPORARY SAFE REFUGE
The accommodation block shall be designed and constructed that it or part of it can be used as a Temporary Safe Refuge (TSR) for the maximum POB. The TSR shall be designed to be able to withstand impairment from the Hazards on the Installation for a minimum of one hour and be fully protected on all sides by A60 rated fire walls, bulkheads, floors and ilings. The forward bulkhead of the accommodation block, and also the lifeboats, are to be protected from jet fire or explosion on the process deck or cargo deck by means of a fire/blastwall. Fire and Blast rating above any A60 rating shall be determined during detailed design.
MOORING SYSTEM
Multiple Mooring Support Structure shall be installed on the forward and aft part of the FSRU Vessel to allow for Jetty mooring.
GAS COMBUSTION CHAMBER
A gas combustion chamber supporting structure shall be fabricated and installed on the aft of Main deck area.
REGASIFICATION MODULE SUPPORT STOOLS
The final arrangement is to be decided during the detailed design but shall coincide with frame positions of the hull and tank boundaries.
SEA CHESTS AND BILGE WELLS
High and low sea chests or trunks are to be provided as required. The location of the sea chests and seawater lift pumps shall be determined during Detailed Engineering.The sea chest grids are to be designed to facilitate underwater inspection.
Sea chest blanks and plugs are to be provided for each and every sea chest and pipe penetration of the hull. The water tight cover plates are to be designed and fabricated from materials that will give neutral buoyancy together with securing devices are to be provided for diver installation. All blanks and plugs are to be fit tested in the dry dock prior to launching.
The sea chests are to be provided with vent connections, weed clearing connections and marine growth prevention connections. The zinc anodes shall be replaceable. Internal examination of all sea chests and the repair of the sea inlet valves shall be required during operational conditions of the vessel. As such, watertight aluminum blanking shields shall be provided for all the different sea chests and shipside openings over 100 mm diameter.
The sea chest blanking shields shall be suitable for installation by divers, and shall be flat and fixed to the vessel hull coamings. Therefore coamings shall be provided and welded to the outside of the sea chests.
UNDERWATER OVERBOARD BLANKS
Similar as sea chest, all underwater overboard shall be blanked off with aluminum blanks. The blanks shield to be used must be suitable for installation by divers and shall be flat and fixed to the vessel hull coamings. Therefore coamings shall be provided and welded accordingly to the underwater overboard openings.
Storage provision shall be provided to properly stow the blanks shield when not in use. A dedicated storage rack shall be installed in an appropriate place.
AIR LOCKS
Access between hazardous area on the open weather deck and non-hazardous spaces shall be by means of an airlock. This shall consist of two self-closing, substantially gastight, steel doors without any holding back arrangements, capable of maintaining the overpressure, at least 1.5 m but no more than 2.5 m apart. The airlock space shall be artificially ventilated from a non-hazardous area and maintained at an overpressure to the hazardous area on the weather deck.
