As demand for natural gas continues to grow, offshore liquefaction of natural gas (LNG) by floating production, liquefaction, storage and offloading (FPSO) vessels is becoming increasingly attractive. LNG FPSO solutions offer an economic and environmentally sound solution to exploiting the attractive stranded offshore gas fields and commercialising oil associated gas, reducing the need for flaring and giving an extended potential for oil production. For offshore applications different design criteria apply than for onshore plants and the Nitrogen Expander Cycle is meeting all these requirements. The "Aragon's Optimised Expander Cycle" has additional features that makes it highly applicable for offshore liquefaction.
The history of the development of large scale natural gas liquefaction plant started in 1960's with technologies based on either the classic cascade cycle (by Marathon/Phillips) or simple mixed refrigerant cycles.
At the beginning of this century, several new large scale processes have been demonstrated or proposed. These includes Air Products' AP-X process, Shell's propane pre-cooled Parallel Mixed Refrigerant process (PMR), Statoil/Linde's Mixed Fluid Cascade (MFC) and Conoco-Phillips' Optimised Cascade. Common for all these base load technologies are:
Ultra-large train size
High process efficiencies but variable overall thermodynamic efficiency
Large and heavy equipment
Requirement for large construction areas
These are features that are not advantageous for offshore applications. Through the last decades the industry has studied to find the most appropriate liquefaction technology for the offshore environment.
The paper gives an overview of the evaluations related to the engineering and design of an LNG FPSO topside. It discusses the different selection criteria that apply for selecting offshore liquefaction systems and gives an introduction to the Nitrogen Expander liquefaction technology and in particular the "Aragon's Optimised Expander Technology".
There is a wide range of applications where offshore LNG production is attractive:
Stranded Gas Field - These fields are typically located at significant distance from existing offshore or onshore production facilities or pipeline networks. Floating production units is a highly competitive choice compared to new build offshore production platforms and long distance pipeline tie-backs to onshore facilities.
Associated Gas Field Applications - As environmental regulation are getting stricter the presence of oil associated gas in locations remote from existing infrastructure is an increased challenge to Governments and the Oil and Gas Industry Economic development of many of these resources is difficult, particularly for the offshore fields. The result is that 110 billion standard cubic meter of natural gas is being flared annually in the world5. Production of this gas in form of exported LNG adds economic value to an existing project, and provides an additional benefit to the environment by reduced emissions.
Early Production System and Staged Development Applications - Floating liquefaction offers a fast-track project compared to an onshore plant. Current LNG FPSO projects show that the development time from project definition until first LNG is up to 50% of the time required for a traditional onshore LNG development. Hence the LNG FPSOs are capable of providing a viable alternative for an early production scheme generating revenues and creating value while the standard facilities are being approved and built.