A talk given by Robbie Sillars on 11th November 2024
Robbie introduced us to the FLNG Prelude, the biggest Floating Liquefied Natural Gas
platform built by Shell and completed in 2017, grossing nearly 500,000 tonnes
costing about $15-17 billion.
Floating platforms are used where land-based units are not suitable for such reasons
as political insecurity or proximity to final use of products.
This platform is moored some 300 miles offshore from western Australia
and collects natural gas from via four 12-inch risers from 250m below the sea. The
platform is anchored to the sea bed via 16 enormous steel link chains and is free to
swing with tides and wind. The platform is designed to meet all the sea and weather
conditions expected in the operational area taking into consideration wave, tide and
wind in multiple combinations and directions along with cyclones and tsunami.
The connection from the wells to the platform is via steel pipes whose length allows for
the flexing resulting from ship movement due to swing, roll and pitch. On board the
natural gas is passed through various refrigeration loops to condense it into its
various constituents and stored as liquid at virtually atmospheric pressure in
enormous insulated on-board tanks mostly as liquid methane and liquid petroleum
gas with some gas being used for process heat and gas turbines used to power
pumps and generate electricity. Excess unusable gas is flared off at the top of the
180m high flare tower required to minimize effect of radiant heat at deck level.
These cold liquid fluids are piped to tankers via 4 loading arms for LNG and 3 arms
for LPG that had to specially designed for the extra movement of a floating platform
and contained explosive breakaway joints to enable quick disconnect in the event of
an accident. The tankers transport the liquid gas in insulated tanks to shore based
regassification plants located adjacent to gas supply grids.
Some of the design conditions needed for this platform included wash down systems
to take care of liquid gas spills, provision of a SS skin any where pooling of LNG could
occur and refuges on deck in case of accident. Emergency lifeboats and rafts required
for the normal 250 crew and up to 500 during maintenance.
A major design constraint was for continuous operation with only 100year extreme
conditions allowable for shutdown.
Robbie finished with a short video (made by Shell) of the operation of the platform.
