(a) Advantages of Using Steam Turbine Propulsion for LNG Carriers

Steam turbine propulsion offers the following advantages for vessels carrying LNG cargo:

1. Utilisation of boil-off gas (BOG): LNG naturally evaporates during the voyage, producing boil-off gas. This gas can be used directly as boiler fuel, helping to control cargo tank pressure and avoiding wastage of the gas.
2. No need for a boil-off gas re-liquefaction plant: Since the natural boil-off gas can be consumed in the boilers, there is no need for energy-intensive and complex re-compression or re-liquefaction arrangements.
3. Fuel flexibility: Steam boilers can operate on natural gas, heavy fuel oil (HFO), marine gas oil (MGO), or a combination of these fuels, providing good operational flexibility.
4. Increased cargo space / reduced fuel storage requirement: As boil-off gas from the cargo can be used as fuel, the vessel does not need to carry excessive quantities of conventional fuel oil, allowing more space to be available for cargo.
5. High reliability and low maintenance: Steam turbines have fewer moving and no heavy reciprocating parts. This results in less wear and tear, reduced frictional losses, lower lubricating oil consumption, and less frequent maintenance.
6. Smooth and quiet operation: Steam turbines provide continuous rotary motion, resulting in low noise and vibration, reduced hull vibration and fatigue, and improved crew comfort.
7. Cleaner combustion: LNG burns relatively cleanly, producing very low sulphur emissions and fewer deposits compared with conventional heavy fuel oil.
8. Simple gas combustion arrangement: Unlike internal-combustion gas engines, steam boilers do not require precise high-pressure gas admission timing and are not affected by problems such as engine knocking.
9. Lower gas pressure: Gas can be supplied to the boilers at relatively low pressure, reducing the hazards associated with high-pressure gas fuel systems.
10. Good redundancy: LNG steam plants are commonly arranged with more than one boiler. If one boiler is shut down for maintenance or becomes unavailable, the vessel can continue operating with the remaining boiler(s).

(b)(i) Safety Precautions for Gas Pipeline Supplying the Boiler and Burning Gas in the Boiler

• Gas pipelines must not pass through accommodation spaces, service spaces, or control stations, unless fully compliant with regulations.
• Fuel piping to be designed to comply with SB – 1/6 of steel vessel rules.
• Maximum pressure in the fuel gas supply line to not exceed 10 bar.
• All pipelines to be welded; flanged connections only permitted at equipment connections.
• Gas-tight compartments containing fuel piping should have direct access to the open deck.
  • If not possible, access via gas-safe spaces must be through self-closing gas-tight doors.
• Compartments to be fitted with mechanical exhaust ventilation.
• Gas detection systems to be fitted in the compartment and boiler room.
• Incorporate block and bleed valve arrangement in pipelines to comply with purging requirements.
• Entire pipeline supplying methane gas to machinery spaces to be double-walled (annular type) and purged with nitrogen before and after gas-burning operations.
• Nitrogen gas pressure in annular space to be maintained; leakage alarms to be activated if methane detected.
• Boiler room fitted with methane gas sensors with alarm and venting arrangements.
• Boiler room to be continuously ventilated with methane monitoring in air.
• Boiler room separated from machinery space by air-lock antechamber with self-closing doors.

(b)(ii) Means of Getting Rid of Excess Gases During Loading or Discharge

• Cooldown process is carried out to prevent excessive boil-off during loading/discharge.
• Cooldown achieved by supplying liquid methane to spray headers via a distribution grid, directed to various tank levels as required.
• Boil-off vapour is passed through a high-duty compressor back to shore via the vapour return line.
• When liquid is detected at the tank bottom, cooldown is considered complete.
• Primary insulation and secondary barrier temperatures maintained between –80°C to –100°C.
• Tank pressure is controlled using compressors and by varying liquid flow to spray headers.
• Before starting loading, the shore flow for cooldown is gradually reduced.
• After cooldown, loading starts slowly and increases gradually to full rate.
• Tank pressures are monitored; maximum loading rate is governed by compressor capacity to return vapour to shore.