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Jan-2019

Liquefaction technology selection for 
offshore FLNG projects

Development of a successful offshore floating LNG project depends on an appropriate liquefaction process that best meets project objectives

Saeid Mokhatab
Gas Processing Consultant
Article Summary
As global demand for LNG increases, interest grows in unlocking and monetising stranded gas reserves in deepwater locations. This makes floating liquefied natural gas (FLNG) technology a technically innovative solution and a commercially viable means of exploiting remote offshore gas reserves, while at the same time providing an economically preferable option to flaring associated gas at oil fields.

FLNG technology offers lower production costs, reduced time to first production, and less environmental impacts than land based alternatives. In addition, a potential advantage of a floating facility is that it can be moved relatively easily to an alternative offshore location as the original gas resources decline or economics change. This allows the operator to save money on future gas field developments or earn revenue by charging third parties to process their gas through the FLNG facility.

While principally aimed at remote offshore gas reserves, FLNG production technology can be considered for the development of nearshore gas fields with limited infrastructure. Alternatively, FLNG can be used to liquefy pretreated, onshore pipeline quality gas for export to markets that require small to midscale LNG supply volumes. It may also be used where the shoreline is too rugged and difficult to build infrastructure like LNG tanks onshore, for instance Western Canada.1

Initial offshore FLNG developments are focused on building large scale facilities that can move and process large quantities of LNG, typically 5 million t/y and up, that require significant capital investment. However, global LNG prices have recently dropped considerably and industry leaders are currently planning to mitigate project risks by developing small to midscale FLNG projects, limiting production capacities to 0.5-3 million t/y. One exception is Shell’s Prelude FLNG facility with a nameplate production capacity of 3.6 million t/y, located offshore NW Australia.

In large scale FLNG projects, the liquefaction facilities are mounted on a barge-like structure or a ship-shaped vessel (depending on the location) with the LNG stored in the hull underneath. In small to midscale FLNG projects, the liquefaction facility is built on a purpose-built vessel that is sized as a conventional LNG ship. Table 1 provides the key characteristics of both small to midscale and large scale FLNG facilities which are considered by project developers.

Offshore production challenges
Although the FLNG production concept has been the focus of research and development for decades, it is only in the last 10 years that any FLNG projects have progressed to the detailed design and construction phase. In fact, some special challenges exist in the design, construction and operation of an FLNG facility in the harsh offshore environment that require special solutions.

The key technical challenges, which influence the liquefaction process and equipment selection for a floating gas liquefaction facility, can be summarised as follows.

Space and weight requirements
Floating systems are space limited, requiring more compact and lighter equipment to fit the deck space. These systems have high equipment density and lower equipment count to overcome space and weight constraints. High equipment density substantially increases the potential for explosions (as opposed to deflagrations). In the event of gas release and ignition, this would result in higher impact severities, perhaps escalating to total facility loss. An FLNG facility must meet both fire loads and escape route requirements. Note should be made that due to the space constraint, equipment is designed with ‘fit-for-purpose’ criteria. This reduces the flexibility that may be desired. For example, gas turbine waste heat recovery may not be justifiable due to the additional weight and space or centre of gravity requirements.

Ease of operation/start-up/shutdown
Bad weather/extreme environmental conditions may require rapid shutdown of the FLNG facility. Thus, floating liquefaction facilities are generally considered for benign waters. Liquefaction process trains are most efficient when operated continuously with very infrequent shutdowns. However, an increase in operational interruptions should be expected offshore, and this will adversely impact operational efficiency.

Availability
The process technology options chosen for the FLNG facility should take into account maintaining the highest availability possible. Sometimes this may result in a less efficient design but will enable robust operations during changing sea states.

Flexibility and efficiency
Designing FLNG facilities intended to receive feed gas from multiple fields introduces the need for additional gas conditioning facilities when field gas quality differs. Gas processing equipment capable of handling well fluids with varying conditions will add to facility cost, equipment count, and operational complexity.

Safety
Design and operation of the FLNG facilities present a set of safety challenges as below:
• Potentially large inventories of hydrocarbon refrigerants (such as propane) represent a hazard which may require adding to safety mitigation in terms of separation or fire walls.
• Process related accidental hydrocarbon releases (both of refrigerants and partially processed natural gas or LNG) are also considered to be key safety risks.
• Control of process related hazards (for instance, mechanical integrity of process equipment, ignition source control systems, and explosion overpressure) require more robust designs and operating systems.
• Control of vessel collision hazards and other standard marine safety requirements add to the complexity of safety management and emergency response procedures of such facilities.

The potential for loss of containment must also be addressed when considering hull fabrication. A catastrophic tank failure could result in subsequent large scale discharge of LNG into the sea. This would be followed by a rapid phase transition, which could cause serious structural damage to the offshore facility, with possible stability loss. Prevention of such an occurrence is a key safety requirement of offshore designs. The use of concrete for the hull provides benefits in the storage of cryogenic fluids as it retains its structural integrity when in contact with LNG, but traditional steel ship designs are cheaper to build.

Vessel motion
Vessel motion due to wind and waves is the key limiting factor in deploying floating LNG facilities in harsh environments. Once a FLNG facility is in operation, moving decks may present major challenges on the operability and efficiency of process equipment with two-phase flow. The sea’s wave motions can also cause sloshing in the partly filled membrane tanks. As such, process equipment that is sensitive to vessel movement should be located close to the floating vessel’s centre line to reduce the forces exacted by movement in the six degrees of freedom. In addition, utilising hull storage in twin row tanks cuts down the sloshing issues while also providing a robust deck with support in the middle to help take heavy topsides loads.

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