Passively-cooled cabinets protect process analyzer instrumentation
High performance environmental protection cabinets have been selected for Shell’s ‘Prelude’ floating liquefied natural gas project. Designed for extended service life in hazardous areas, the shelters incorporate innovative semi-passive cooling technology to offer a stringently specified instrumentation protection solution.
Shell’s Prelude floating liquefied natural gas (FLNG) project is setting new engineering records. Designed to liquefy natural gas extracted from subsea wells by chilling it to -162 degrees Celsius, and to then store the liquid until it is offloaded to large LNG carriers, the vessel will be the largest in the world, with a length of 488 metres and a width of 74 metres. The hull of the vessel was constructed in South Korea and launched in December 2013. Due to enter service in 2017, the vessel will be towed to the Prelude and Concerto gas fields in the Browse Basin near Timor, about 200 km off the northwest coast of Australia, where it will be permanently anchored for about 25 years. 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.
Much of the on-line process analysis instrumentation on the Prelude FLNG vessel will be housed in purpose-built environmental protection cabinets supplied by Intertec. In total, the company is supplying 90 cabinets for sample conditioning systems and 30 cabinets for process analyzers, each designed to match the instrumentation content and layout precisely in order to optimise thermal performance.
As Intertec’s Commercial Director, Hans Geiger, points out, “Only three of the 120 cabinets that Intertec is supplying for this project are the same size. Our specialised design and automated manufacturing capabilities enable us to produce custom environmental protection products very easily, to optimise performance and match the space available on demanding applications like this.”
The cabinets that Intertec is supplying are required to be capable of withstanding severe Category 5 tropical cyclones with wind speeds in excess of 252km/h and to have a minimum service life of 25 years - with 50 years as a design aim. Additional requirements include a high degree of resistance to corrosion being caused by the saline environment and the presence of sour or acid gas, and the ability to cool electronics equipment without using explosion-proof air conditioning systems, which incur high capital and operating costs.
The environmental protection cabinets are constructed from a proprietary composite material comprising sandwich walls of long-fibre glass reinforced polyester (GRP) sheets, enclosing a core of polyurethane foam. The material has a similar strength to that of stainless steel but is about four times lighter, making it ideal for use on offshore platforms and floating structures, where every effort is made to minimise weight.
Other advantages include a very low thermal conductivity which facilitates the construction of efficient, condensation-free cooling cabinets, and an inherently high resistance to corrosion caused by salt or aggressive chemicals such as sulphur or chlorine. For this project, the external surfaces of the cabinets are further protected by a thick layer of UV-resistant gel coat. 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.
Each of the 120 Intertec cabinets incorporates a unique form of semi-passive cooling technology. The internal face of the cabinets' rear walls are 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 150m 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 analyzer system, which ranges from 140W to 900W.
This form of distributed cooling has enormous technical and economic advantages, especially in hazardous areas or where space and weight is at a premium. It provides a very efficient means of removing heat from the cabinets without requiring local air conditioning systems, which wouald 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 BrowseBasin is very close to the Equator, which means that it has a tropical climate with an average temperature of about 32 degrees Celsius. 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 degrees Celsius. 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 degrees Celsius.
