Going Big with Floating LNG

Ro Lokken, Chief Offshore Engineer – ExxonMobil, image: LNG 17

In front of one of the most packed audiences of last week’s LNG 17 conference in Houston, Texas, ExxonMobil’s Chief Offshore Engineer, Ro Lokken, moderated a panel discussion on Floating LNG (FLNG).

ExxonMobil announced at the beginning of April their intentions to install a 495-meter FLNG facility off the northwestern coast of Australia at a cost of around USD $12 billion.  They are not the first to make this level of investment however, both Petronas and Shell are currently constructing FLNG facilities at South Korean shipyards.

An FLNG facility is to natural gas what an FPSO is to oil.

Because of the highly complex systems needed to process, separate, liquefy and cryogenically store the gas, these FLNG plants are far more expensive, and in the case of Scarborough, or Shell’s Prelude, they are absolutely enormous.  Once built, they will be the largest floating objects on earth.

Putting this in perspective, if you’ve ever been in the George R. Brown Convention center in Houston, the Prelude FLNG facility will be 10 meters longer than that building, and much taller.

An FLNG facility works by flowing natural gas from subsea wells via manifolds on the sea floor to production risers which enter a turret and swivel system (a central point on the facility about which the vessel pivots), and then on to the topside processing plant where the produced hydrocarbons are treated, and heavier condensates, water, mercury, and the different types of natural gases are separated.  From there, pure methane gas is then liquefied and stored.

Design considerations

At the moment, major EPC firms and energy majors are coming up with and marketing their own unique FLNG designs, but considering the differences between hydrocarbon reservoirs, not all FLNG designs are the same.

Shell’s Prelude FLNG plant, once built, will export 3.6 million tons of LNG per year, plus 400,000 tons of LPG, and 40 to 50 thousand bbls per day of condensate.  For Shell, the LPG and condensate add a significant and crucial source of revenue which ensures the economic viability of the project.

To enable FLNG to be a viable option on dry gas fields, Shell has come up with an FLNG “LEAN” concept which enables a higher rate of processing of LNG with smaller condensate and LPG processing capabilities.

The other consideration is safety.  To produce huge amounts of LNG, you need really effective heat exchangers.  The most efficient heat exchangers use propane as a refrigerant, a gas which is both highly flammable and heavier than air.  Not a huge deal if you’re on land, but on a ship, it’s a significant concern.

Petronas is focused on a 1.5 mtpa design incorporating an inert gas-based refrigeration process, whereas for Shell, a propane-based “Coil Wound Heat Exchanger” refrigeration process is being used to enable it to process LNG at more than twice the rate as Petronas.  It’s all about output, safety considerations, and economics.

Another major design consideration is how to safely offload product.  Off the coast of Borneo, the weather is generally mild, however the same cannot be said off northwestern Australia.  The only option that currently allows LNG ships to transfer their cargo is via an alongside transfer, one that presents both a collision and an explosion risk.

The other option is tandem offloading, where an LNG carrier approaches the stern of the FLNG facility and on-loads product via hose reels to a bow manifold on the ship.  It’s a process used routinely by dynamically positioned shuttle tankers when transferring oil from FPSOs, however, no such dynamically positioned LNG carriers currently exist.

That being said, it’s likely we’ll start to see DP LNG shuttle tanker designs on the market soon.

Suffice to say, there are a lot of considerations to be made when it comes to FLNG, from the field size, field location, CO2 content of the produced gas, the mix of produced hydrocarbons, and the refrigeration process.

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