In previous posts, I shared some of onshore LNG terminal and regasification system. How about offshore LNG regasification system? Does it have any differences with onshore regasification system? In this post, I want to share you practical guidance of offshore regasification vessel from DNV (Det Norske Veritas) Classification Notes No. 16.3. DNV Classification Notes No. 16.3 is applicable for all vessels engages in LNG regasification.
Regasification vessel is intended to regasify LNG then delivered the gas to pipeline gas grid, through a Submerged Turret Buoy or through dedicated export manifold. The Classification Notes specifically explains regasification systems and offloading systems for natural gas through submerged turret buoy offshore or through dedicated gas unloading manifolds.
Piping Design
Additional requirement of piping design according to this Classification Notes are:
- Complete stress analysis for each branch of the high pressure piping system downstream of booster pump.
- Stress analysis for piping with temperature below -110 deg C.
- Effects of vibrations imposed on piping systems in regasification plant.
- Expansion bellows should not be used in high pressure piping systems.
Materials, Manufacture, Inspection, and Testing
- Materials shall be suitable for low temperature service for liquefied gas piping and cargo vapor piping. In general, it should be selected for -165 deg C.
- Cargo and process piping shall be joined by butt welding.
- Cargo and process piping shall be hydrostatically tested to 1.5 x design pressure prior to installation.
- Heat exchangers are to be hydrostatically tested to 1.3 x design pressure.
- Testing by using gas instead of liquid may be accepted on special consideration of safety implication.
- Other piping may be hydrostatically tested prior to- or after installation.
- After assembly onboard the complete cargo piping shall be subjected to leak test using air, halides, or other suitable medium.
Safety
Risk Assessment
- Risk assessment should be conducted in early phase of project, and at least covered this topics: collision, fire and explosion, dropped object, and cryogenic leakage
Cryogenic Release
- Deck shall be protected from cryogenic spill at the regasification unit (pumps and vaporizers)
- Alternatively, deck is suitable for cryogenic temperature
- For high pressure liquid pipes downstream booster pumps, spray shield shall be provided for any flanged connection to protect personnel and prevent deck and cargo tanks from being exposed to high pressure LNG.
Fire Safety
- Water spray with application rate of 10 L/m2/min of the horizontally projected area shall be used to protect
- Regasification unit (pumps and vaporizers)
- Metering station
- Export manifold
- Internal surfaces of turret compartment
- Storage tank for flammable fluids
- The regasification plant shall also be covered with dry chemical powder fire-extinguishing systems
- Fire detectors shall be fitted to cover regasification units and export manifold
Gas Detection
- The regasification plant shall be equipped with sufficient number of gas detectors covering:
- Regasification unit
- Ventilation inlet to accomodation and engine room
- Export manifold
- Turret compartment
- Audible and visual alarms to be located on the navigating bridge, in the control position of cargo operation
Process Shut Down (PSD)
- Process Shutdown (PSD) shall be from monitoring system, from manual call points, and from fire detectors in the regasification area.
- Separate transmitter to be provided for control and shutdown function
Emergency Shut Down (ESD)
ESD in the Classification Notes refers to emergency shut down facility for the cargo handling system required by IGC code. Activation of the ESD will:
- stop cargo pumps, spray pumps, and booster pump
- close all valves on the tank dome, in the case of cargo tanks with PSV set at above 0.7 barg
- close the cargo manifold/turret valves
Activation of ESD will activate manual call points.
Blowdown System
- Blowdown system shall be directed to vent system for regasification system containing more than 400 kg of flammable inventory to the vent system
- Activation of blowdown system shall be by:
- Manual call points located in cargo control room, in the regasification unit, and export manifold
- Automatic by fire detectors
Pressure Relief
- If a suction drum in the inlet to the high pressure booster pump is fitted, it shall be provided with a PSV for relevant cases, such as fire exposure or blocked booster pump outlet
- PSV shall be installed in sections of piping system where liquid can be trapped
- PSV should be discharged into cargo tanks or to vent mast
Vent System
- Knock out drum should be installed between PSV and vent mast. KO drum shall be provided with high level alarm
- Capacity of vent mast should be calculated to avoid liquid release to vent mast
- For permanently moored vessel, it may be considered to dispose gas via flare rather than vent