The global demand for LNG in 2016 was ca. 260 MTPA.  This is projected to double to over 500 MTPA by 2030.  As the market grows, the composition and characteristics of demand are changing, providing greater opportunity for small to mid-scale LNG developments.  Costs for conventional onshore, base loads plants declined in the 1990’s as larger capacity trains were introduced, but this trend reversed in recent years, particularly in Australia where baseload LNG project costs have approached $4000/TPA due to high labor costs and “stick” construction in remote locations.

Floating LNG offers the potential to unwind the cost spiral by moving the construction work to shipyards and fabrication yards, while eliminating the cost for long pipelines to shore and extensive civil works and large scale construction housing on site.

By the end of 2016, four FLNG projects had been sanctioned, showing that FLNG is generally accepted in the industry as feasible.  The focus is now on technology selection, liquefaction & storage capacity, and safe operational considerations, particularly with regard to LNG offloading.

Concerns typically raised with proposed FLNG concepts include:

1. Size, weight and complexity of topsides
2. Topsides hydrocarbon inventory and impact on safety design
3. LNG offloading method and sea-state limitations
4. Overall project cost, schedule and uncertainty
5. Design for generic application/residual value

Additional general considerations, not unique to FLNG, include:

1. Method and cost for well drilling, completion, workover
2. Subsea architecture
3. Exposure to cyclonic storms or other extreme metocean conditions

FLNG System Configuration – Integrated Arrangement

Most FLNG concepts on the market utilize an integrated FLNG vessel that incorporates all functions into a single vessel including:

• primary processing
• gas treatment
• NGL fractionation
• Liquefaction
• storage of condensate/NGL/LNG
• offloading of each product from storage

This approach leads to very large and complex purpose built vessels.

Other cost and design factors in FLNG include the use of proprietary liquefaction technology (licensing fees), topsides weight, layout congestion & safety hazards due to hydrocarbon inventory/explosive overpressure, and limitations on LNG offloading operations except for the most benign metocean conditions.

Processing requirements for treating wellhead gas to LNG inlet quality vary considerably from field to field depending on

• condensate/gas ratio
• acid gas and nitrogen levels
• ethane, propane and butane content & LPG marketing
• method for hydrocarbon dewpointing

It is not practical to design a single generic topsides process scheme to deal with the wide range of conditions that may be encountered if the FLNG vessels has to produce two, three or more fields over its useful life.  However, once the gas has been treated and is rendered suitable for liquefaction , the design requirements for liquefaction, storage and offloading of the LNG are the same from one field to another, varying only according to capacity.

FLNG System Configuration – Split Arrangement

The LoneStar FLNG concept was designed to circumvent many of the cost and design concerns of the conventional, “integrated”, FLNG concepts.  The concept is based on de-integrating or splitting the system between:

1) a field specific upstream facility for primary processing, and gas treatment and

2) a generic LNG liquefaction/storage/offloading vessel

Splitting the overall FLNG arrangement between a conventional upstream host facility and a generic, standardized liquefaction vessel results in:

• an overall reduction in complexity and cost
• an increase in design flexibility and residual value for re-deployment.

Design features of the split system include:

1. Conventional, proven, “off-the-shelf” designs for both upstream host facility
2. Standardized, generic design for liquefaction vessel based on converted Moss LNG ship
3. Segregation of functions between processing/gas treating & liquefaction
4. Pre-cooled, Dual N2 expander liquefaction for compactness, safety and simplicity
5. DFDE prime movers on liquefaction vessel for improved efficiency
6. Multiple parallel construction paths, modularized and repetitive design/build

Segregating the FLNG functions between different facilities provides several benefits for cost, safety, project schedule and operations.

Processing requirements for treating wellhead gas to LNG inlet quality vary considerably from field to field depending on

• condensate/gas ratio
• acid gas and nitrogen levels
• ethane, propane and butane content & LPG marketing
• method for hydrocarbon dewpointing

It is not practical to design a single generic topsides process scheme to deal with the wide range of conditions that may be encountered if the FLNG vessels has to produce two, three or more fields over its useful life.  However, once the gas has been treated and is rendered suitable for liquefaction , the design requirements for liquefaction, storage and offloading of the LNG are the same from one field to another, varying only according to capacity.