34
LNG
INDUSTRY
SEPTEMBER
2014
fire resistant, thermally insulating materials. This material is
already attracting interest for constructing the type of high
performance safety walls between hazardous and
non-hazardous areas that are likely to become mandatory on
ships and offshore installations.
Prelude
FLNG
Many of the environmental protection cabinets needed for
housing the online process analysis instrumentation on Shell’s
Prelude
FLNG vessel are being supplied by Intertec. In total,
the company is supplying 90 cabinets for sample conditioning
systems and 30 cabinets for process analysers. Each custom
built cabinet incorporates a unique form of semi-passive
cooling technology and is designed for extended service life
in hostile climatic conditions (see Figure 4).
The cabinets are some of the most stringently specified
instrumentation solutions that Intertec has ever produced.
They are required to be capable of withstanding severe
Category 5 tropical cyclones with wind speeds in excess of
252 km/h and to have a minimum service life of 25 years –
with 50 years as a design aim. The cabinets also need to
provide a high degree of resistance to corrosion being caused
by the saline environment and the presence of sour or acid
gas. For the first 20 - 25 years of its life, the
Prelude
FLNG
vessel will be used in the Prelude and Concerto gas fields of
the Browse Basin, off the coast of Australia. After this time it
will be inspected, overhauled and possibly moved to a new
location, where it will be expected to provide a further
25 years of service.
All 120 cabinets, only three of which are the same size, are
constructed from Intertec’s proprietary GRP composite
material, produced using advanced manufacturing
automation. In order to meet the application’s high wind
speed requirements, each cabinet is equipped with special
built-in mountings (which are external to the thermally
insulated parts of the enclosure) to secure the top and base to
Prelude’s
deck structure. All external metal components are
fabricated from 316 stainless steel with a corrosion-resistant
protective coating that is specified for ship use, while the
external surfaces of the cabinets are protected by a thick layer
of UV-resistant gel coat.
For this particular tropical application, the electronics
equipment contained in each cabinet needs to be kept cool
without the use of special explosion proof air conditioning
systems, which incur high capital and operating costs. To
achieve this, the cabinets incorporate a unique form of
Intertec’s innovative semi-passive cooling technology.
The internal face of the rear wall of each cabinet is fitted
with a high efficiency heat exchanger, comprising one or more
aluminium cooling plates and stainless steel coolant pipes
connected to
Prelude’s
cold water supply system, which
obtains ‘cold’ water from the deep sea by a 150 m long pipe
below the vessel. Heat dissipated by the equipment in the
cabinets is absorbed by the water and transferred to the
vessel’s main water cooling system where it is dissipated to
the environment. The size of each cabinet depends on the
power dissipation of the contained sample conditioning or
process analyser system, which ranges from 140 W to 900 W.
This form of distributed cooling has enormous technical
and economic advantages, especially in hazardous areas or
where space and weight are at a premium. It provides an
efficient means of removing heat from the cabinets without
requiring local air conditioning systems, which would
necessarily need to be explosion-proof – and therefore
expensive. Furthermore, transferring heat from individual air
conditioners of below-deck cabinet installations to the
ambient environment would be a major challenge and involve
significant amounts of space-consuming air ducting. The
semi-passive cooling system is inherently safe for use in a
hazardous environment; at the cabinet level it requires little or
no electrical power of its own and is virtually maintenance-
free, making it ideal for long life-cycle applications.
The intended operating environment for Shell’s
Prelude
FLNG vessel poses significant instrumentation protection
challenges. The Browse Basin is very close to the equator,
which means that it has a tropical climate with an average
temperature of approximately 32˚C. This moist and hot
environment, combined with a salt-laden atmosphere, is
highly conducive to rust formation, making non-metallic
protection cabinets a sensible design choice. In some of
Prelude’s
more confined spaces, the ambient temperature
could reach more than 50˚C. Even under these extreme
conditions, all of the passively-cooled cabinets that Intertec is
producing for the project are designed to keep their internal
air temperature below 35˚C.
Conclusion
If the world’s first FLNG project lives up to expectations,
the growth in this new form of gas exploitation could be
significant and rapid. Some analysts predict that FLNG could
mirror the success of the floating production storage and
offloading unit (FPSO), which nowadays is used by most
leading players in the offshore oil and gas industry. Engineers
tasked with creating the instrumentation systems for FLNG
projects are unlikely to have time on their side; by offloading
the systems’ environmental protection to a specialist
company with a track record in the relevant disciplines,
they can help to ensure that performance and delivery
commitments are met.
Figure 4.
Semi-passive cooling technology simplifies system
integration and avoids the need for expensive explosion-proof air
conditioning equipment.
