Mobil floating LNG design cuts investment, fab time

Floater allows exploitation of multiple fields

Mobil's offshore LNG plant design can operate and load in the roughest seas, and is suitable to the sequenced exploitation of any number of small remote gas fields. Two floating flare stacks about one mile away can burn off gas in emergencies. [42,647 bytes]
Mobil and BHP both have developed offshore floating vessels to liquefy natural gas in order to take advantage of the growth of gas markets and proliferation of smaller gas fields that cannot be developed with conventional means (see previous two issues for BHP liquefied natural gas).

Mobil LNG released a document earlier stating that natural gas has become the fuel of choice for tighter environmental standards. The document points out that liquefied natural gas (LNG) is experiencing a 6.9% annual global growth rate, compared with 3.1% for all gas forms, and 1.8% for all energy forms.

Floating LNG design enhancement by Mobil, BHP, and others continues to exploit the high cost and time required to install conventional natural gas pipelines to consumer markets and two remaining disadvantages of the newer gas-to-liquid conversion process:

A large portion of the gas stream is lost to power the conversion process.
Several gas separation and catalytic pro cesses are relatively complex.

While gas-to-liquids innovation continues, floating applications with that process may be further in the future than floating LNG systems. One prior disadvantage to the use of LNG processes offshore has been the difficulty of loading during rough weather. New developments in loading arm technology are making it possible to load in all but the roughest sea conditions.

Floating LNG

Liquefaction reduces the gas stream on a BTU/volume basis by 600 times, one of the process' key advantages. However, continued refrigeration through the loading and transport process and the need for a re-gasification facility are the major disadvantages. The location of an LNG facility offshore could tip the balance in favor of LNG over the long-run if it allows for development of smaller gas fields and the consumer markets for gas globally continue to grow.

Mobil contends that an LNG facility located offshore near producing fields has distinct advantages:

25% reduction (or greater) in capital cost
25% reduction in time for construction and activation
Few connections and production handling pipelines
Parallel construction provides for fast-track projects
Enhancement in profitability of large gas fields and viability of smaller ones
Relocation of the LNG plant when production drops below commercial standards
No need for onshore site preparation and a harbor with loading facilities.

There remain some disadvantages to offshore designs:

Loading downtime could be less than onshore plants, because of the worst sea conditions.
Maintenance can be more difficult in a floating environment.

Fewer pipelines

In a traditional onshore LNG plant, a gas field offshore must move the gas at full volume to a production structure, where the gas is separated and dehydrated, and then send it to shore via pipelines, where it is liquefied and then loaded into LNG tanks in a harbor or enclosed area.

In contrast, the offshore LNG plant takes gas production directly from the producing wells, separates out liquids, dehydrates it, compresses it, and loads it directly onto an LNG tanker through an extended loading arm. Eliminated are all offshore-to-onshore pipelines and a separate production structure.

The designed proposed by Mobil is self-contained, requiring only a small base for helicopter movements and supply staging. In addition, the facility can accommodate up to 15% carbon dioxide content in the flow stream and associated liquids up to 55 bbl/MMcfg.

The design can be moored in water depths up to 4,000 ft, but designs for deeper waters are contemplated with new developments in mooring. Another feature of the design is the ability to take onshore gas from pipelines, convert it to LNG, and load it aboard LNG tankers

Hull and process

Since site preparation is not necessary, offshore LNG plant design allows for fast construction. The hull, LNG storage tanks, and topsides modules can be built concurrently.

The floating LNG plant hull is square, about the size of four connected barges inside a concrete hull, which offers great stability. During testing in model tanks, hull movements ranged from less than one degree in a one-year storm for an installation offshore Western Australia to six degrees in a severe typhoon. Wave heights reached more than 70 ft for the design storm.

For the LNG plant, heat is recycled from turbine waste heat, maximizing efficiency, minimizing carbon dioxide emissions, and adding to safety. Fired heaters are not necessary.

The liquefaction process selected by Mobil is less sensitive to motion than conventional versions. Closed-cycle refrigerant allows for immediate re-start following a shutdown. This features becomes more important when the facility is located in areas where rough weather is frequent.

Warehousing, chemical storage, and maintenance are located aboard the structure. The aero-derivative turbines aboard require a one-day maintenance shutdown every three years, compared with 12 days for conventional turbines, Mobil engineers point out. The normal volume of refrigerants is reduced, enhancing safety and efficiency.

Other features of the design include the following:

Space is provided for future booster compression as field pressure declines.
The two offloading facilities with cryogenic arms are located at opposite corners of the facility to accommodate loading in any direction.
Risers bring gas up through the moonpool, which also serves as the intake for cooling seawater.

Safety factors

Safety factors included in the offshore LNG design by Mobil are as follows:

Six anchor chains at each corner provides secure mooring for the facility.
Two floating flares would be positioned about one mile from the plant. The flares can burn up to 42 MMcf of gas per hour.
250-bed living quarters aboard the facility are located on the opposite corner from the highest risk LNG process facilities, and are separated from those facilities by a fire wall.
In the event of a collision with an external threat, the hull is protected by a double wall filled with potable, cooling, and ballast water.
Triple containment protects the processed and refrigerated LNG tanks in the lower levels.

Mobil's offshore LNG design is for a facility that can produce six million tons of LNG annually. The producer says the facility is the product of long experience in producing LNG. Mobil was one of the early developers of the Arun LNG project in Sumatra (Indonesia), and later the North Field off Qatar. Arun today handles 300 MMcfd and has not missed one cargo loading in 3,000 deliveries. The North field has the largest known reserves in the world. From both, Mobil says it has learned much about process and efficiency, as well as planning and financing.