Feature 5 | FLNG TECHNOLOGY Reaching for the remote
A growing global market for liquefied natural gas (LNG) is holding focus on the potential use of floating facilities to produce LNG from gas in remote offshore locations. Crondall Energy Consultants Ltd managing director Duncan Peace addresses some of the issues involved in the development of floating LNG (FLNG).
not be economically viable for a full-scale onshore LNG plant with associated offshore facilities and pipelines, and onshore infrastructure. The concept involves production and liquefacation of LNG onboard a floating facility moored directly over subsea gas-producing wells, aſter which the product is loaded onto LNG carriers for export to the growing global market. Crondall has been involved in a number
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of studies, advising clients on the use of such technology for potential gas developments in Africa, Australasia and South America. Our main focus in these studies is to ensure that lessons learned from the design and operation of oil-based floating production storage and offloading vessels (FPSOs) is properly transferred into the overall global design and configuration of
the FLNG
vessel. In the absence of any other reference points, the lessons learned from FPSOs provide a good starting point for the safe and reliable use of onshore gas processing and liquefaction technologies on floating facilities in remote and deepwater environments. FLNG requires the combination and
integration of complex LNG and FPSO technologies, requiring both the use of existing technologies in new applications plus the introduction of certain new technologies. FPSOs present a different set of design
challenges compared to the trading tankers from which they are oſten converted. FPSOs have to be designed to provide sufficient space and payload capacity to accommodate the oil production facilities and supporting utility systems, as well as sufficient capacity and strength to store, handle and offload the required cargo volumes over a continuous range of loading and offloading cycles. In addition to meeting the various
regulatory requirements for hull strength and stability, the FPSO must be moored on station in such a way that the motions of the
LNG technology offers an economic solution for the development of smaller gas resources, which might
the operating efficiency of the LNG process equipment, nor damage the LNG storage containment system when the tanks aboard the FLNG unit are partially filled. With no track record for the large-scale use
Crondall expects FLNG technology to be a significant feature of the oil and gas industry in the future, notes Duncan Peace.
vessel do not compromise the efficiency of the production equipment or the safety and operating capability of the crew. Usually designed to stay on station for the
duration of the production life of the field, the FPSO will not undergo regular dry-docking, therefore the design and maintenance of the hull structure, coatings and marine systems will generally be to a higher standard. Many of these FPSO challenges need to be
addressed in the design of FLNG facilities; the key focus areas are addressed below: Safety: As well as the usual major accident risks associated with the processing of hydrocarbon liquids and gases onboard an FPSO, an FLNG facility will have to deal with hazards associated with increased inventories of gas, some under significant pressure, plus cryogenic spills or releases. Size: Te equipment required to liquefy the gas, plus that required to separate and treat the gas and gas liquids before liquefaction adds up to a very large topside process plant. Depending on the throughput of the FLNG facility and the complexity of the gas pre-treatment, Crondall expects to see topsides weights in the range of 20,000tonnes to >60,000tonnes. To accommodate this and store the required amounts of LNG, the vessels will be huge. Motions: A major challenge will be to ensure that the vessel’s motions do not compromise
Offshore Marine Technology 1st Quarter 2012
of established LNG liquefaction technologies on moving structures, equipment will have to be designed to accommodate the impact of continuous wave-induced motions (and hull deflections) on rotating equipment, heat exchangers and other process equipment with liquid-free surfaces. Motions and mal-distribution of liquids can cause poor efficiency and additional inertial or thermal loadings, so these factors need to be clearly communicated between the vessel and process equipment designers. For liquefaction technology selection, Crondall expects that ease of marinisation of that technology might perhaps take precedence over outright system efficiency initially. Sloshing in FLNG cargo tanks is more
challenging than in LNG carriers: due to the cycle of loading and offloading, the tanks will inevitably go through periods of partial filling – possibly during bad weather. Te LNG cargo containment system needs to be robust enough to deal with these sloshing loads. Offloading: Finally, we must consider the design and layout of facilities for the safe offloading of a cryogenic liquid from the FLNG facility to a waiting LNG carrier. This involves the safe positioning of the LNG carrier in relation to the FLNG facility (by mooring or dynamic positioning) and a means of safely transferring the liquid. Initially we expect to see this carried out with the vessels side-by-side, with fluid transfer carried out via rigid arm devices. Te loading arms will be modified to accommodate the additional dynamic loads and ranges associated with ship-to-ship relative motions. As FLNG technology becomes more established, technology will become available for tandem offloading operations to modified and possibly dynamically positioned LNG carriers. OMT
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