July, 2025
For a terminal with an annual capacity of 3 million tons of LNG, the gas send-out rate is typically 340tons per hour (t/h). This corresponds to approximately 40cargoes per year, each carrying with 174,000 (m³). The design unloading rate is typically in the range of 12,000 m³/h. (Note : LNG Logistic Study to be addressed at others subject LNG Blog)
LNG Unloading Facility
An LNG unloading berth is a specialized marine infrastructure that enables the safe transfer of LNG from carriers to onshore terminals for storage and regasification. These berths are essential for LNG import terminals, ensuring secure and efficient operations from ship to shore.
LNG carriers use onboard pumps to transfer LNG to onshore storage tanks through four unloading arms. LNG flows via dual 24” pipelines along the jetty. Simultaneously, return vapor from the shore is sent back to the vessel via a dedicated vapor return arm.
Under normal operations, unloading a full LNG carrier at a design rate of 4,430 m³/h using a (3+1 spare) configuration takes about 12 hours, including startup and shutdown times. A spare unloading arm is maintained for regular maintenance.
If the vessel’s onboard Boil-Off Gas (BOG) management system cannot handle the excess BOG, it is processed or disposed of at the receiving terminal. During holding mode, LNG temperature is maintained through a recirculation line. A sampling line is installed on the unloading line.
For BOG, 0.15% wt/day of the ship’s LNG cargo is assumed as a baseline. For longer return gas lines, a return gas blower ensures proper pressure and maintains the designed unloading rate.
LNG Storage Tanks
The tanks are of the full containment type, featuring concrete outer walls and a 9% nickel inner shell. Each tank has a nominal capacity of 160,000 m³, with a working capacity of 140,000 m³. The design pressure is 0.22 kg/cm²G, while the operating range is 0.04–0.19 kg/cm²G. BOG generation is estimated at 0.05% wt/day at maximum fill levels.
Top Loading: Utilizes spray devices for flashing and cooling.
Bottom Loading: Uses dip pipes. Vacuum breakers are included to prevent vacuum formation due to excessive flashing.
Temperature sensors are installed at the tank bottom and shell to monitor cooldown. Gas detectors are placed at critical corners for leak detection.
To prevent stratification and rollover, lighter LNG is loaded at the bottom and heavier LNG at the top. This ensures proper mixing and prevents rollover, which could cause sudden pressure surges due to rapid vaporization.
Rollover Prevention:
- Minimize stratification via smart loading methods.
- Avoid mechanical mixing unless necessary, to prevent increased BOG from heat gain.
- Install density and temperature sensors to monitor stratification trends.
BOG Handling
The BOG rate fluctuates depending on ambient temperature. A single BOG compressor handles flash and displacement gas during unloading. A PSV is installed downstream of the compressor to handle block-out scenarios.
The vapor handling sequence is:
- Vapor return to ship
- Recondenser
- Venting/flaring
Return line temperatures fluctuate cyclically between ambient and cryogenic conditions.
Storage System Layout
Three 160,000 m³ tanks are installed with provisions for a future fourth. The tanks are designed for 0.22 kg/cm²G internal pressure and have a heat-induced BOG rate of 0.05 wt%/day.
The generated BOG is collected via a common BOG header and sent to the compressor, which recondenses it with subcooled LNG. Low-pressure LNG pumps send LNG to the recondensers, serving both cooling and recondenser feed purposes.
LNG Send-Out Pumps
Primary Pumps: Deliver LNG at ~1.2 MPa to recondensers or secondary pumps.
Secondary Pumps: Boost pressure to ~7.55 MPa and send LNG to vaporizers.
A recirculation line returns some LNG to the jetty to maintain cooling. The system can also kick back the entire pump flow to mix LNG during stratification.
Vaporizers
Six seawater-based Open Rack Vaporizers (ORV) operate at 11:1 turn-down ratio. They vaporize LNG at 70–90 kg/cm²G, with an outlet gas temperature of ~10°C.
LNG Recirculation & Metering
All long LNG pipelines are designed with recirculation to maintain cryogenic conditions. Flowmeters are installed on the send-out pipelines, and pigging stations (launcher and receiver) follow the metering station.
Flare and Vent Systems
A dual-header system (LP and HP) connects to respective systems. The flare stack is designed considering wind, radiation, noise, and dispersion risks. Vent stack acts as a backup during flare tip maintenance.
Utilities
Includes:
- Seawater intake for vaporizers
- Liquid nitrogen for purging (with vacuum-jacketed tanks and ambient vaporizers)
- Instrument air systems with -40°C dew point
- Potable/service water systems
- Firefighting water, diesel oil, and backup power systems
Chlorination systems inject NaOCl into seawater to prevent marine biofouling. Discharge temperature is controlled to ensure <4°C differential at 500 m outfall.
Future expansion is considered with tie-in points and reserved space.