Floating LNG Production

 
The Case for Floating LNG
 
Globally, there are more recoverable reserves of natural gas than oil on an energy equivalent basis with proven reserves of 6,100 trillion cubic feet. It is estimated that more than a third of natural gas reserves are stranded with half of these stranded gas reserves being offshore.
 
Floating LNG represents a fast track option to monetising these stranded offshore gas reserves which will help bridge the expected supply shortfall in worldwide LNG production.
 
These stranded reserves are held in approximately 2,500 gas fields containing 0.1 to 5 trillion cubic feet (tcf) of natural gas (2 million to 100 million tonnes of LNG equivalent).
 
Floating LNG production plants capable of processing these volumes of gas and condensate would typically have capacities of 1.0 to 2.0 mtpa as compared to up to 8 mtpa for the largest onshore plants. 

 
Technical developments such as LNG ship-to-ship transfer are at an advanced stage of development. A safety critical issue is the handling of LNG in partially filled storage tanks, due to sloshing of the liquid. LNG storage systems are now being classified for such operations.
 
Floating Production, Storage and Offloading (FPSO) for oil production is now widespread compared to only 15 years ago and experience in the design and operation of such facilities can be applied to LNG production.
 
These factors have led to increased confidence in the feasibility of Floating LNG and recent evaluations have indicated that the cost of Floating LNG projects could be competitive with those for onshore LNG projects, and the plants can be operational many months earlier. 

 
Process Technology
 
Recognising the increasingly attractive case for floating LNG, Costain has applied its considerable experience to develop a simple and inherently safe process design to meet the requirements of the market.
 
Applying optimisation experience and knowledge of different LNG processes, Costain considers that the double nitrogen expander cycle provides a highly effective solution. Costain’s confidence in the process design is based on its use in a number of low temperature gas liquefiers. Coarse safety studies undertaken by classification societies have shown that the simplicity of the process is ideally suited to the marine environment.
 
The following table compares the double expander with alternative processes proposed for floating LNG:

CRITERIA	CASCADE	MULTI COMPONENT REFRIGERANT	COSTAIN DOUBLE EXPANDER PROCESS
Uses proven technology	Yes	Yes	Yes
Avoids use of Hydrocarbon refrigerants	No	No	Yes (inherently safe)
Overall space requirement	High*	Moderate*	Low
Refrigerant storage hazard	Yes	Yes	No
Sensitivity to vessel motion	Moderate	Moderate	Low
Ease of modularisation and installation	Poor	Poor	Good
Low equipment count, footprint & weight	No	No	Yes
Simplicity of operation	Moderate	Moderate	High
Reliability / availability	Moderate	Moderate	High
Low maintenance	No	No	Yes
Ease of start-up/shutdown	Moderate	Low	High
Flexibility to feed gas changes	High	Moderate	High
Thermal Efficiency	High	High	Low
Total Capital Cost	High	Moderate	Low

 

 
Benefits of Costain Floating LNG
 
Portable facility

Facilities can be easily moved to new gas fields, extending the life of the plant and allowing smaller, stranded gas reserves to be monetised.

 

Simple, reliable operation of plant

Robust and reliable performance with simple start-up and shutdown ensured by the use of proven equipment, simple machinery configuration and control system.

 

Flexibility of design

Process flexibility allows the client to consider a variety of future gas sources without significant process design change. The design allows for space to add future systems, if required.

 

Inherently safe

Costain’s design is inherently safe due to the use of an inert refrigerant and minimisation of liquid hydrocarbon inventory within the process units. Costain carries out rigorous safety reviews of the design at all stages of a project to ensure emergent safety risks are eliminated.

 

Costain experience

Costain’s expertise in the design and supply assures reliable first time start, with specific experience of 17 nitrogen cycles and the design and construction of over 200 cryogenic plants over 50 years.