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FLNG to Subsea… are you following me?

Most onshore LNG plants and at-shore floating LNG barge units are fed from extensive pipeline networks which create a large gas inventory buffer such that any interruption event at either the LNG plant or the gas source is easily managed by the control systems. In contrast, a unique feature of most offshore FLNG facilities is the very limited gas inventory in the short flowlines and risers between the subsea wellheads and the inlet of the gas pre-treatment unit. This close coupling and small inventory can give rise to some significant dynamic interactions between the FLNG liquefaction plant and the subsea wells, particularly for wells with a high design pressure. As such the design and control of the subsea system and FLNG topside process must be considered together as an integrated system and not in isolation. It is recognised that the liquefaction capacity of an FLNG unit relates to the gas turbine (GT) power available to the compressors. GT available power varies with ambient temperature and hence LNG production follows both the diurnal temperature swing and the seasonal ambient temperature variations as the control system functions to maximise the LNG production at any time. As the LNG production fluctuates with ambient temperature so too does the feed gas supply. Without any active control of the subsea chokes the amplitude of the oscillations can be quite significant, especially in early morning and evening when the ambient changes most rapidly, causing packing and unpacking of the flowlines and risers as the topside pressure reduction valves operate to maintain a steady feed gas supply pressure to the inlet facilities. This paper discusses the interactions between the subsea system and the liquefaction unit and presents the results from an integrated dynamic model that demonstrates how stable operating conditions can be achieved under all scenarios.

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