by Amin Almasi
November 19, 2015

The difficulties with onshore liquefied natural gas (LNG) projects have renewed industry interest in offshore LNG production. The floating liquefied natural gas (FLNG) vessel can make the development of smaller and more remote offshore natural gas assets commercially viable. The technology developments in the offshore LNG storage and transfer market (particular the large extent of the LNG carrier market) have made the offshore FLNG vessel commercially viable at plant capacities of 0.5 to 5 million metric tons per year (mmtpy).

By floating the LNG facility, the FLNG avoids the needs for a traditional onshore development scenario that would include offshore platforms, long offshore gas pipelines and associated facility preparation works including coastal dredging. FLNG units should be deployed in natural gas fields as far away from the shore as feasible to compete with a pipeline-to-shore LNG facility. Ideally, a 300-800 kilometer distance from a shore could be a good option. Increasingly stringent no-flaring rules may prompt some existing offshore producers of giant volatile oil and wet gas fields to aggregate the gas from several such fields and develop a FLNG unit.

There are several hundred "stranded" natural gas fields in the world with sufficient reserves to support a FLNG vessel for up to 10 years. A FLNG vessel can be moved to a new gas field if the gas production declines, which may extend the service life to 30 or 40 years. The liquefaction of associated gas from the oil production is extremely attractive. The FLNG projects could (theoretically) demonstrate shorter time to the commercial production compared to onshore LNG projects, providing a more flexible solution for LNG sales. A FLNG vessel combines gas treatment, liquefaction, storage and offloading in a singular floating vessel. FLNG vessels would be among the largest floating object ever constructed. One FLNG vessel is approximately 490 meters (m) long and approximately 75 m wide. It has been designed and will soon be operated for a remote offshore natural gas reservoir in Australia.

Offshore Pumps

The design of a FLNG vessel requires the naval architects to coordinate the LNG plant design and the floating vessel engineering practice to ensure an optimal integration of the topside—including the processing plant, machinery packages, pump units and major utilities—with the vessel (hull, structures and supports). The design of LNG storage systems and hull structures should be optimized with the topside layout to reduce the overall weight and cost.

On any FLNG vessel, the space is restricted since the process facilities should be located away from the flare, buildings and other facilities. A FLNG vessel's plant and equipment design should be able to mitigate the effects of wave motions and wind loads. By designing the FLNG vessel to adapt to the wind, any tendency to roll could be considerably minimized.

All offshore facilities—including their pump packages—should be approved by a classification offshore society. Engineering and design teams should:

  • combine the best pump practices currently in use for LNG carriers.
  • use the existing standards for offshore pumps as far as feasible.
  • add specific LNG and leakage considerations.
  • use risk assessment for hazards.

The classification society could have a key role in producing safety assessments at different stages of engineering to identify hazards and risk mitigation measures.

Technical Challenges

The key technical challenges that the FLNG pump packages have to combat are:

  • Space and weight requirements: The floating structures are space limiting. Safety measures can also increase space requirements. High equipment density is needed to overcome space and weight constraints.
  • Ease of operation, start-up and shutdown: The ease of operation should always be respected. Also, bad weather may require a sudden shutdown, which means more shutdowns and starts compared to the land-based unit.
  • Efficiency: The efficiency of a FLNG unit should be at the equal level (or better) compared to onshore LNG plants; it is also applicable to pumps since a high efficiency is expected for all pump packages.
  • Flexibility: Because a FLNG vessel may serve different gas fields with varying gas compositions throughout its lifetime, facilities have to build sufficient flexibility into the gas treatment and liquefaction processes in terms of operating capacity and feed-gas quality. In this way, many pump packages should offer excellent flexibility, particularly good operational flexibility.
  • Safety: The safety in all operating modes and all foreseen feasible situations should be managed. The pumping of LNG at the cryogenic temperatures offshore (for example, LNG pumping from a FLNG vessel to a LNG carrier at sea) could be challenging. Also, the control of process-related hazards requires more robust pump package designs. For instance, the mechanical integrity of a pump train, ignition source control systems, explosion and overpressure cases should be considered. Pumping potentially large inventories of liquids and hydrocarbon refrigerants represents a major hazard, requiring added safety measures.
  • FLNG vessel motion: Vessel motion can induce large dynamic loading and movements on the pump packages.

Pump Package Design

Each specific pump train design in a FLNG vessel is mainly a function of the process technology and vessel size. Regardless of the preferred liquefaction process technology, important issues for the FLNG vessel lie with the equipment, machineries and interfaces. A FLNG vessel design should be optimized to limit the topside weight and minimize the capital cost.

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