Cryogenic Gas Industry – Salvus Energy

A significant commercial application of cryogenics is the liquification, transport and storage of cryogenic gas. Liquefied Natural Gas (LNG) is generally 95 percent Methane with a few percent ethane and much lower concentrations of propane and butane. LNG liquefies at – 161⁰C (113⁰ K).

Unlike many applications of cryogenics, the motivation for using LNG is not the provision of lower temperatures but rather the very large volume reduction (greater than a factor of 600) between natural gas at atmospheric pressure and temperature and LNG. It is odorless, colorless, non-toxic and non-corrosive.

A conventional LNG terminal has four functions:

Berthing of LNG tankers and unloading or reloading of cargoes,
Storage of LNG in cryogenic tanks (-160°C),
Re-Gasification of LNG,
Send-out of this gas into the transmission grid

Berthing and unloading

On arrival at the terminal, LNG tankers (length 200 to 350 m) are moored to the unloading berth. Articulated arms are connected to the LNG carrier to unload its cargo and transfer LNG to the terminal storage tanks.

The LNG flows through pipes specially designed to withstand very low temperatures (-160°C). This operation takes at least 12 hours. A volume of boil-off gas is sent back from the terminal storage to the LNG tanker in order to maintain the pressure inside its cargo tanks.

Storage

LNG is stored in cryogenic tanks (designed for low temperatures) capable of withstanding temperatures of -160°C to maintain the gas in liquid form. The outer walls of the storage tanks are made of pre-stressed reinforced concrete. They are insulated to limit evaporation.

Despite the high-quality insulation, a small amount of heat still penetrates the LNG tanks. This causes slight evaporation of the product. The resulting boil-off gas is captured and fed back into the LNG flow using compressor and recondensing systems. This process prevents the occurrence of venting natural gas from the terminal under normal operating conditions.

During maintenance periods, boil off gas can no longer be recovered and is burnt off by the flare stack. It is preferable to burn the methane than to release it into the atmosphere (reduced impact on the greenhouse effect).

Regasification

The LNG is then extracted from the tanks, pressurized and regasified using heat exchangers.

Each tank is equipped with submerged pumps that transfer the LNG to high-pressure pumps. The pressurized LNG (at around 80 times atmospheric pressure) is then turned back into a gaseous state in vaporizers.

Send-out into the national transmission grid

Once returned to its gaseous state, the natural gas undergoes several treatments. Before it is sent out into the national transmission grid (GRTgaz), natural gas, which is odourless, is artificially odourised with a tiny amount of mercaptan , giving gas its characteristic odour familiar to everybody. This is a safety measure since it means that any gas leak can be detected by its odour.

Process Safety Studies

Hazard Identification	Risk Management	HSE Management
HAZOP (Hazard and Operability Study)	Dispersion / Consequence Analysis	Smoke and Gas Ingress Study
Process Hazard Analysis (PHA)	Quantitative Risk Analysis (QRA)	Emergency Systems Survivability Assessment (ESSA)
HAZID (Hazard Identification)	Qualitative Risk Analysis	Escape, Evacuation and Rescue Analysis (EERA)
Hazardous Area Classification	Fire and Gas Detector Location / Mapping Study (3D)	Health, Safety and Environmental Critical Equipment Systems (HSECES)
Emergency Response and Disaster Management Plan (ERDMP) as per PNGRB Rule 2010	Fire and Explosion Risk Assessment (FERA)	HSE Management System (HSEMS)
Layers of Protection Analysis (LOPA)	ALARP Demonstration, Barrier Based Risk Assessment	HSE Impact Assessment (HSEIA)
Safety Integrity Level (SIL) Analysis and Verification	Audits – Fire Safety, Safety, Energy etc.	Reliability Availability Maintainability (RAM)
Bow Tie Analysis (BOTA)	Dropped Objects Study (DOS)	Safety Case
Vent Dispersion	Control Of Major Accident Hazards (COMAH)	Temporary Refuge Impairment Analysis (TRIA)